Physicist demonstrates dictionary definition was dodgy
It is the defining moment that demonstrates a QUT physicist was correct in pointing out a 99-year-old mistake to one of the world’s most authoritative dictionaries.
QUT Senior Lecturer in Physics, Dr Stephen Hughes, sparked controversy over how a humble siphon worked when he noticed an incorrect definition in the prestigious Oxford English Dictionary.
In 2010, eagle-eyed Dr Hughes spotted the mistake, which went unnoticed for 99 years, which incorrectly described atmospheric pressure, rather than gravity, as the operating force in a siphon.
Dr Hughes demonstrated the science of siphons in a paper published yesterday in Nature Publishing Group journal Scientific Reports.
For Exploring the boundary between the siphon and barometer in a hypobaric chamber, Dr Hughes conducted an experiment in a hypobaric chamber, which simulates the effects of high altitude, at the Institute of Aviation Medicine at the Royal Australian Air Force’s Base Edinburgh in South Australia.
A siphon 1.5 metres high was set up in the chamber and when the pressure was reduced to an altitude of 40,000 feet a waterfall appeared at the top, but the water flow remained nearly constant.
At 41,000 feet, the siphon broke into two columns of water and, when returned to 40,000 feet, it reconnected as if nothing had happened.
Atmospheric pressure at 40,000 feet, which is more than 10,000 feet higher than Mount Everest, is about 18 per cent of the sea level value.
For the experiment, two buckets, one higher than the other and connected by tubing, were set up and a pool pump returned water from the lower bucket to the higher bucket.
“The fact that the water level in the upper and lower buckets is constant indicates that atmospheric pressure is not pushing water into the siphon,” Dr Hughes said.
“The stable water surfaces act like energy barriers between the atmosphere and siphon. For energy to be transferred from the atmosphere to the water the water level would have to go down, since the amount of energy transferred is equal to force times distance.
“If the water level is constant the distance is zero and therefore no energy can be transferred.”
Dr Hughes, whose previous research has taken him to Bhutan to examine how siphoning could prevent inland tsunamis, said siphons had been used since ancient times but how they work was still debated.
“If you think of a car, atmospheric pressure is like the wheels, it enables it to work. But gravity is the engine,” he said.
“It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm.”
The Oxford English Dictionary corrected the error and removed the reference to atmospheric pressure after Dr Hughes pointed it out. However, he said the new entry “unfortunately remains ambiguous”.
“This definition still leaves the question open as to how a siphon actually works,” Dr Hughes said.
“But at least the reference to atmospheric pressure has been removed. The vast majority of dictionaries of all languages still incorrectly assert that siphons work through atmospheric pressure and not gravity.
“I hope these findings are a useful contribution to the debate about how siphons work and will enable people to make more effective use of them.”
This is above my head, actually Anthony, so mistakes do happen and can be corrected by more modern research methodology. And taking into account some of the physics presented by alarmists, maybe they should revise their methodology too.
It is difficult to comment on this article due to lack of precision in
Specification of conditions.
Where are the inlets and outlets located in the two buckets. I assume that
Both tube outlets are located below the water surface.
What is meant by the statement that “flow splits into two columns”. Diagrams
Showing what happens here would be useful.
The OED is not a science textbook, as anyone reading it can plainly see. Well, plainly with the little magnifying glass that comes with it.
From Wankerpedia: “…Specifically, Pascal demonstrated that siphons work via atmospheric pressure (as Torricelli had advocated), not via horror vacui, via the following experiment…”
Empirical results from this engineer’s experience suggest that gravity will facilitate the entry of gasoline into my mouth if I fumble things a little. Gawd, how did anyone ever get on the atmo pressure kick? Some academic?
@jim.says April 24, 2014 at 10:50 pm
The descriptions to which you refer are puzzling as ypu point out, but if you follow the “paper” link [ http://www.nature.com/srep/2014/140422/srep04741/full/srep04741.html ] it goes into a much better explanation along with clarifying diagrams.
Here’s the paper.
Remember the water barometer? A siphon will not work if the height of the water column in the rise to the crest of the U is greater than 10 metres.
From the paper “Between 40 000 and 41 000 feet (17.87 kPa, 0.17 atm) the waterfall stopped and the siphon split into two columns” So it appears the author implies that SOME atmoshperic pressure is required to run a siphon(?)
One wonders what happens with a siphon of, say, two reservoirs of mercury at low pressure.
An error in the One True Dictionary! It is a sign of the End Times. We’re doomed.
The application to glacial lakes and prevention of inland tsunamis is brilliant. There is a lot of academic research into GLOFs (and a lot of environmentalist moaning about it), but this is the best practical solution I have seen, and generates electricity as an extra.
I am a home brewer and trying to siphon a batch of fresh beer or cider can have the same problems: a broke siphon. Very common.
I would imagine at 40K Feet, the water is outgassing and vaporizing and the agitation of the return pump is not helping.
I now use gravity feed from the primary fermentation tank into a set of five gallon soda (Cornelius) kegs and transfers after that are done with positive-pressure CO2.
Yaaay CO2 — what can’t it do?
I wonder…
Given this from the AGWers:
“Because of the small atmospheric pressure on Mars (less than one hundredth that on Earth), the spectral absorption lines of carbon dioxide on Mars are very narrow, and therefore act like a picket fence that lets most of the thermal radiation emitted by the Martian ground surface to escape directly out to space. This does not happen on Earth because of the atmospheric pressure that is exerted by the radiatively inactive nitrogen and oxygen, causing the spectral absorption lines of carbon dioxide and water vapor to be greatly broadened, making them more effective absorbers of thermal radiation. And, for comparison, it is the extremely high pressure on Venus that makes the carbon dioxide absorption of thermal radiation particularly efficient in the Venus greenhouse effect. On Mercury, there is no tangible atmosphere, and hence there is no opportunity for the greenhouse effect to operate.” {emphasis mine}
[Source: “Greenhouse Effect,” Andrew A. Lacis, NASA Goddard Institute for Space Studies New York, NY USA, p. 277; Link: http://cdn.intechopen.com/pdfs/32352/InTech-Greenhouse_effect.pdf%5D
A BONAFIDE QUESTION — Please, if you can, I would sure like to know the answer.
My Question:
What are the implications of this article for the AGWer’s CO2 driven greenhouse effect speculation? That is, does the fact that (assuming that Dr. Stephen Hughes is correct) gravity is the driver and atmospheric pressure only the relatively passive enabler of a siphon…
… make the above-quoted AGW conjecture even more implausible than we already know it to be? Or is Dr. Hughes’ gravity finding completely unrelated to CO2 greenhouse effect conjecture?
THANK YOU FOR HELPING ME!
Janice
(bedtime, here in PST land, so, I’ll acknowledge any help later zzzzzzzzzzzzzzzzz)
Experimental proof that siphons don’t run on atmospheric pressure, which is common sense to me! It also explains the “breaking into two columns” thing:
If you reduce the atmospheric pressure further the water will eventually boil and stop the syphon!
Thanks to TerryS for the link
“Figure 3 | Plot of flow versus time with the siphon at 39 000 feet and then
at 40 000 feet with the waterfall present. The pressure in the chamber was
reduced between points A and B over a period of about a minute. Flow
reduced but then recovered slightly.”
This is an awful set of results for a supposedly controlled lab experiment. It’s a farce.
His results (apparently a single run) indicate that pressure did have some effect. He then bushes that aside and publishes what he ‘knows’ to be the right answer.
If I got results like that I’d be embarrassed to present it as an under-grad project , let alone try to publish it.
Yet more proof that peer-reviewed science is now screwed. You can publish almost anything.
I thought it was the ether doing it.
“At 41,000 feet, the siphon broke into two columns of water and, when returned to 40,000 feet, it reconnected as if nothing had happened.”
This poorly explained bit seems to refer to the fact that a vapour bubble appeared in the syphon tube at low pressure.
The path of the syphon was so poorly designed ( well in fact it wasn’t designed at all) that this effect was poorly accounted for. This experiment should have been redesigned and re-run, not published.
Dr Stephen Hughes is a “Senior Lecturer in Physics” . Well if that is the state of work produced by those teaching hard science subjects in universities these days no wonder we are such a mess with climate science.
We are about to witness the fall of our science based civilisation and the dawn of a new dark age is upon us.
It’s worse than we thought.
Australian ex-PM, ju-LIAR Gillard, prompted the Macquarie Dictionary to redefine the word “misogynist” to reflect her broad and untruthful attack on the now PM of Australia, Mr Tony Abbott. Dictionaries will change definitions for the least plausible reasons in some cases.
I really don’t see the need to do an experiment on this in the first place. It is absolutely obvious that the difference in atmospheric pressure at the water surface in the top vessel is only infinitesimally different to that at the water surface in the lower one. There is NO air pressure difference to do anything.
The pressure difference in the tube between the two ends , which drives the flow, is due to the pressure head of the water column.
Jeezus , this is high-school science , you don’t need a hypobaric chamber.
Janice Moore: My Question:
What are the implications of this article for the AGWer’s CO2 driven greenhouse effect speculation?
===
The main implication is that peer-review science is screwed and those teaching science , even at university level in hard science subjects, no longer seem capable of doing a basic lab experiment.
“It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm.”
_______________________________________________________________________
That’s in fact quite inaccurate too. The longer downward arm can only pull the short arm up to tensile strength of the water column – above that the column breaks if there isn’t atmospheric pressure pushing equally from both sides of the column, making sure it continues to work.
So if we consider water column tensile strength negligible (which is correct for standard siphon sizes), we need to account for four forces – gravity pull on long arm, gravity pull on short arm, atmospheric pressure on long arm end, and atmospheric pressure on short arm end. Difference of gravity pull on long arm and atmospheric pressure on the same end results in decreased pressure at the top of the siphon and motion of the fluid down the long arm, while difference of gravity pull on short arm, atmospheric pressure at short arm end (atmospheric pressure is greater than gravity here) causes the fluid to move up the short arm, compensating decreased pressure caused by long arm.
And while it sure is nice to have Oxford dictionary corrected, many other popular places such as Wikipedia had apparently things right all along.
In common siphons, it IS atmospheric pressure that pushes the liquid up, after gravity has pulled the liquid down on the discharge side and lowered the pressure at the top. Gravity supplies the energy to create a pressure difference. Atmospheric pressure pushes the liquid up into this pressure difference. Even in this experiment Hughes notes that by 41000ft the siphon broke in two and when the atmospheric pressure was increased the water rose back to the top and reconnected. What was pushing the water back up to the top? The atmospheric pressure was pushing the water up as he raised said atmospheric pressure. What pushes the liquid up in a barometer? What pushes the liquid up in a drinking straw? In none of these cases can liquid cohesion explain the rise of the liquid. There is nothing else applying an upward force to the liquid when there is a gas or void at the top. The liquid cohesion theory also fails because in a typical not too tall siphon at near sea level, the liquids and gasses present in all parts of the siphon are at positive absolute pressure, and therefore the molecules are all repelling each other. There is no pulling.
But a siphon of liquid that doesn’t boil in vacuum can work in a vacuum. It has been published for more than a hundred years and has been demonstrated on video at youtube. Atmospheric pressure obviously cannot explain such a siphon. Liquid cohesion must be the explanation for siphons in vacuum. This confuses people because they think that two contradictory explanations can’t both be correct. They feel they have to choose. In fact both explanations are correct because they are not contradictory. A siphon in vacuum must be explained by liquid cohesion. A common siphon at sea level with a bubble at top, separating the two sides, CANNOT be explained by liquid cohesion between two bodies of liquid that AREN’T TOUCHING. There is no reason to believe that siphons can only work by one method.
Dr Hughes stubborn clinging and even compounding the mistake of his 2010 article, is a perfect example of part of the reason why climate scientists won’t change their mind about the effects of CO2. No matter how clear the evidence is, they will dream up any excuse they can to maintain their current belief and avoid the embarrasment of admitting their mistake. This is especially true because as they have said many times, they believe that even if they are wrong about CO2, it is worth it to stop fossil fuel pollution. They don’t even want to know if they are wrong.
Don’t make the mistake they’re making. Seek out your own errors. Be proud when you can overcome your own confirmation bias and admit your mistakes. Being able to admit your mistakes is a precious skill. Practice helps. Don’t dodge tough questions. Also try to account for the fact that you are very unlikely to be able to completely overcome your own comfirmation bias. You are probably wrong about some of the things you think you have lots of evidence and good reason for. I’m just glad there is so much evidence to support and confirm MY current beliefs. Seriously though, lets practice. Maybe Dr Hughes is right. Maybe the climate scientists are right.
“So it appears the author implies that SOME atmoshperic pressure is required to run a siphon(?)”
No, some atmospheric pressure is needed to prevent the water evaporating. Nothing to do with the syphon. Same would apply to mercury (except that the vapour is highly toxic).
If he’d cooled the water to the temperature of the atmosphere at 40,000 ft he would not have had a vapour lock problem. But heck the guy’s only _teaching_ science you would not expect him to actually know how to do it in practice, would you?
Read the paper. Nice didactically. But published in Nature – seriously? Basically, this (somewhat crude) study appears to be a bit of an overkill for showing that cohesive properties of water have some influence on (special situations in) siphoning. I bet there is a close connection between the vapor pressure of water and its cohesive properties… Moreover, pressure differences are still essential in the waterfall case since the ambient pressure is higher than the water vapor pressure in the waterfall region! Finally: no need for a fancy hypobaric chamber! Use a >10m high siphon instead and vary the reservoir-to-apex height to see the waterfall effect – much cheaper!
@ Greg – After the siphon broke and he raised the atmospheric pressure and the water started going back up to the top, from where was the force comming from to push the water up. The water vapor at the top would have been pushing down a little. Gravity would have been pulling down. Wasn’t the water being pushed up by atmospheric pressure, same as in a barometer and a drinking straw?
Greg says: April 25, 2014 at 1:42 am
No, some atmospheric pressure is needed to prevent the water evaporating. Nothing to do with the syphon. Same would apply to mercury (except that the vapour is highly toxic)
Exactly. See my previous post (link to YouTube video) above.
@ Reaumur – Siphons in vacuum do rely on liquid cohesion. But what about a siphon at sea level with the taller discharge side filled with liquid, but the top and entire short up side empty? If the liquid is allowed to fall from the discharge tube, and the up tube isn’t too tall, liquid will rise out of the upper reservoir despite there being no liquid cohesion between the sides of the siphon. Doesn’t this demonstrate that siphons at sea level use atmospheric pressure to push the liquid up, as in a barometer or drinking straw, unlike siphons in vacuum?
Actually the atmospheric pressure at the lower bucket is higher than that acting on the upper one so if air pressure was the driving force then the water would flow from lower bucket to the upper one. It flows from upper to lower so gravity must be the defining agent acting.
If atmospheric pressure is not pushing the liquid, then explain the operation of the common drinking straw.
@ johnmarshall – It’s not one explanation or the other, it’s both. Gravity supplies the energy to lower the pressure at the top and atmospheric pressure pushes the liquid up into the low pressure zone, as in a drinking straw. The almost identical atmospheric pressure at the entrance and exit don’t get a chance to cancel because of the greater weight of liquid above the exit, defeating more of the atmospheric pressure at the exit than at the entrance.
John S. says: If atmospheric pressure is not pushing the liquid, then explain the operation of the common drinking straw.
Where the ‘if’ ? A drinking straw is not a syphon. Incredible.
@Greg – A drinking straw is not a siphon but they share a major mechanism of operation. Understanding the drinking straw helps to understand the siphon. Do you agree that atmospheric pressure pushes the liquid up a drinking straw?
Mindbuilder: “What pushes the liquid up in a barometer? What pushes the liquid up in a drinking straw? In none of these cases can liquid cohesion explain the rise of the liquid. ”
OMG, another one.
OK in a drinking straw, the tongue applies a force causing a depression. The tongue exerts this force while moving a distance. Energy = force x distance . The muscles of the tongue convert chemical energy into kinetic energy which displaces the fluid against gravity thus increases the gravitational potential energy to the liquid being drawn into the mouth.
All this is relative to the background ambient atmospheric pressure, but it is not the atmosphere that provides the energy thus it is not the cause of the movement.
A syphon is just a kind of gravitational flow, it just passes through a tube. It is not air pressure that causes water to flow downhill, whether in a tube or not.
Consider an inverse syphon, like an irrigation canal that passes under a road. It is not the atmospheric pressure sucking it up the other side, it’s simply gravitational flow.
It is my understanding that both gravity and atmospheric pressure are needed to create a siphon. The fluid in the long tube creates a partial vacuum at the bend it the top of the tube from gravity trying to pull the fluid down. then the higher pressure on the fluid surface will push the fluid up the short tube to cancel out the partial vacuum. That seems pretty straight forward. What I find more interesting is how a rope siphon works.
@ Greg – It’s true that your body provides the energy to lift the liquid in a drinking straw and it’s true that gravity provides the energy to lower the pressure at the top of a siphon. But it is also true that the atmospheric pressure pushes the liquid up. It is not a choice between atmospheric pressure or gravity. BOTH work together to make it happen.
When you start sucking on the straw no part of your body is touching the liquid. Your body does not exert any forces on the liquid molecules. Even after you have lowered the air pressure in the straw, the remaining air pressure is still pushing DOWN on the liquid, yet the liquid rises because the atmospheric pressure pushes up harder. Isn’t this true?
I sometimes wonder if these ‘contrarian physics’ posts are put up as some sort of test of WUWT readers. Who will applaud reflexively? Who will declare it all a plot by the physics establishment?
Water motion is dictated by pressure, and the pressure of a body of water under gravity is atmospheric at the surface and increases linearly with depth in proportion to depth, as it is the weight of the material above an area pushing down on it. This means water surfaces at different levels is at different pressures, and if connected by a channel full of water at positive pressure the pressure difference will drive water from the higher to the lower to even out the level.
This works even if the channel is a narrow tube, and even if the water rises above the surface of both exposed surfaces, so long as the pressure remains positive – in fact, it must remain above the vapour pressure of the water at it’s current temperature, or the water will boil and produce bubbles of vapour, or if things move fast enough even vacuum. The relevance of air pressure to the siphon is that it is only due to air pressure that the pressure can remain positive when above both the exposed surfaces. The pressure in the siphon is above zero but below atmospheric.
In vacuum, the pressure at the surface of a liquid would be zero, increasing linearly with depth. So long as the connecting tube stays below the uppermost of the two surfaces, it will have positive pressure extending from this surface and the ‘siphon’ will work. As soon as the tube rises above the surface, the fluid in the tube breaks. (There are complexities associated with vapour pressure and surface tension here that I am ignoring.) Water flows from the higher level in the tube to the lower level in the two exposed reservoirs.
So “air pressure” is *not* the answer to the question: “What drives water from one bucket to the other?” So long as the tube remains below the level of the water in both buckets, people find that intuitively obvious: the weight of water at a higher level pushes it through the channel. It is instead the answer to the question about the thing people really find surprising about siphons: “What force lifts water into the tube above the surface level in either bucket?”
The question “How do siphons work?” is vague and ambiguous – at the least it requires more than simple one or two word answers like “gravity” or “atmospheric pressure”. The classic answer is attempting to explain the most surprising feature of it, and the likely reason somebody has asked the question. But what people find most surprising depends on how people think things work in the first place.
@Greg From the article:
“If you think of a car, atmospheric pressure is like the wheels, it enables it to work. But gravity is the engine,” he said.
Agreed.
“It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm.”
Disagreed. Just as in a straw, it is atmospheric pressure moving water up the shorter arm, but it is gravity providing the “sucking” by falling down the longer downward arm. The water in the downward arm isn’t “pulling” it is “sucking” just as the mouth “sucks” on a straw.
Nullius in Verba says: April 25, 2014 at 3:29 am
As soon as the tube rises above the surface, the fluid in the tube breaks.
Do I really need to invoke the great god Feynman? As has often been quoted, he sayeth “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”
Please just watch the short video I linked to above – it shows a liquid siphoning in vacuum without the fluid ‘breaking’.
@ Reaumur – I agree that siphons in vacuum rely on liquid cohesion. Would you agree that the siphon in figure 4 of the Wikipedia siphon article relys on atmospheric pressure to push the liquid up, at least while it is starting?
@Reaumur Ah, the chain over the pully analogy of how a siphon works. Is the chain over the pully a “siphon?” I think not. Is the very carefully set up circumstances of “siphons in a vacuum” really a siphon? I think not.
Look again at your own video. At 1:17, as his initial demonstration depletes it’s source of water, behold the movement of liquid with bubbles in it. No direct contact of water from end to end, yet the bubbles and fluid continue to move until the lower column no longer maintains an airtight seal around the circumference of the tube.
Two similar looking setups, two completely different mechanisms.
“Please just watch the short video I linked to above – it shows a liquid siphoning in vacuum without the fluid ‘breaking’.”
Yes, it uses a special sort of liquid that is able to sustain negative pressure (i.e. can be put under tension). Most liquids aren’t like that, and for liquids that aren’t, atmospheric pressure *is* the explanation.
Feynman is absolutely right, the problem is usually in figuring out what the theory predicts the experiment should show. If you come up with a theory but then apply it wrongly, you can wind up rejecting the theory when you ought to be rejecting your own misapplication of it.
My theory predicts that fluids that cannot sustain negative pressures would break, but that hypothetical fluids able to sustain a negative pressure would still work. (As you’ll note, I did mention above that I was skipping over complexities due to surface tension.) So I’m still quite happy with it, thanks.
“Yes, it uses a special sort of liquid that is able to sustain negative pressure ”
There’s not such thing as negative pressure. Because a pressure is less than some arbitrary std like sea level pressure at 20 deg C does not make it “negative”. It is still finite and positive.
Funny indeed. This is basic Hydrostatics or Fluid Statics.
Surprisingly, water can, under some circumstances, sustain incredibly high negative pressures without vaporizing. See the very clever spinning Z-tube on Wikipedia for how this is measured. Negative pressures up to 280 atmospheres have been measured. Thus water can be comparable with rubber in tensile strength. There is a video on Youtube of a siphon operating up to 24 meters. Even more surprising is that the 24 meter siphon used nylon berage tubing, to which water doesn’t even adhere very well. If the siphon had used glass tubing it may have operated up to 2800 meters or more. To achieve such negative pressures, the water must be degassed and the tubing must be smooth and clean. Normal siphons of water at sea level break at 10 meters because the gas disolved in them promotes vaporization.
I was happy to leave my last comment to stand, as I just wanted people to see the experimental evidence – and the discussion seemed to have the makings of a flame war! – however, I have been asked two questions:
@ Mindbuilder: I haven’t seen the experiment in the Wikipedia diagram, but if it works as shown, then in that special case the descending water presumably causes a pressure drop in the air gap and allows atmospheric pressure on the top reservoir to force the liquid up. It is a different case and doesn’t affect my argument in any way. All I’m saying is that experiment shows that atmospheric pressure is not necessary to make the fluid move in a siphon, only a potential energy difference due to the different elevations of the two reservoirs.
@John S: If a “siphon in a vacuum” isn’t a siphon, then we are descending towards a semantic argument about the definition of “siphon”. If you discount the earlier mistaken Oxford Dictionary entry, most say something like “A bent pipe or tube with one end lower than the other, in which hydrostatic pressure exerted due to the force of gravity moves liquid from one reservoir to another.”
“Look again at your own video” – it isn’t my video, I just linked to it!
Again the behaviour of the liquid in air at 1:17 doesn’t affect my argument that experiment shows that atmospheric pressure is not necessary to make the fluid move in a siphon.
A common real world example of very high negative pressures in water is in the xylem of trees where pressures reach negative nine or ten atmospheres relative to pure vacuum.
There is a typo in my post above that should have read beverage tubing instead of berage tubing.
@ Réaumur – I agree that atomspheric pressure isn’t necessary for all siphons. However I would go farther and say that the liquid cohesion demonstrated in the vacuum siphon is not only not necessary, but that practical siphons under normal atmospheric conditions that are not too tall, always have atmospheric pressure pushing the liquid up, because all the fluids in the siphon are under positive pressure and therefore the molecules are all repelling each other and therefore they are not pulling over the top. Of course it is gravity lowering the pressure at the top.
“It is gravity that moves the fluid in a siphon, with the water in the longer downward arm pulling the water up the shorter arm.”
Surface tension? Capillary action? Pulling water? Pushing a rope?
BS meter is pegged on this one.
Hmm, an incorrect definition that made no difference to those not susceptible to the dictionary fallacy. Or that actually used manometers.
A large practical volume was stabilized, slightly pressurized (health physics considerations moot) and rate of depressurization carefully recorded. Then the test was repeated with various orifices installed, and the results compared to estimate the volume’s leak rate. In this case, conceptually radioactive air-borne contamination.
“When you start sucking on the straw no part of your body is touching the liquid. Your body does not exert any forces on the liquid molecules. Even after you have lowered the air pressure in the straw, the remaining air pressure is still pushing DOWN on the liquid, yet the liquid rises because the atmospheric pressure pushes up harder. Isn’t this true?”
Make your straw a 40 ft vertical pipe. 1 atm is about 32 ft head of water.
If you put a vacuum pump at the top you will be able to suck the water up to almost 32ft, no higher. It will be the pump which is excreting an upward force on the liquid via the air. It is you pump that is doing the work not the atmosphere. At some point before 32ft the water will be boiling at the surface and from then on you’ll be madly pumping out water vapour and the level will get not higher.
If there was not atmosphere this all falls apart because there’s no liquid water either.
In the upper U of the syphon there is a hydrostatic pressure less than surrounding atm pressure because of the weight of water in the descending column in the syphon tube. While this is greater than the vapour pressure of water at the ambient temperature there is no need to go any further. It’s the cohesion of the liquid (the force that binds the molecules into a liquid state rather than a free gas) that transmits this hydrostatic pressure to the opening of the tube, where the hydrostatic pressure is 1 atm.
The pressure in the lower basin is also 1 atm. Therefore the rate of flow of the water (or whether it flows at all) is not a function of the surrounding atmospheric pressure. It is NOT the air pressure making the syphon work.
Now if you want to ask why does the water continue to flow upwards at 42000ft when the water column breaks due to evaporation, we’re back to the 40ft pipe. It’s the pressure _difference_ that causes the water to rise , not simply the atm pressure at the bottom. And what causes the pressure difference is the pump, or your mouth , or the depression caused by the weight of the column of water …. which is caused by gravity.
So whichever way you play it, it’s gravity not atmospheric pressure that drives a syphon. Even one with a vapour lock.
Surface tension is capillary action and the force calculation trivial.
I was going to post my understanding, but Mindbuilder pretty much hit the high points. I see gravity, air pressure, and molecular cohesion working in concert.
Can cohesion work without air pressure? Using a special liquid that holds together ‘like magnets’ in the video you can siphon in low pressure… sure, same as throwing a chain over a pulley.
Can air pressure work without cohesion? I haven’t done the experiment but I don’t see why a siphon with a small air bubble at the top wouldn’t work, can’t explain that with cohesion.
All in all, I wouldn’t rate earlier explanations as wrong so much as incomplete.
Unbelivably funny. It’s the pressure gradient, caused by the height h of the fluid column (dp = ρgh). Again, this is basic fluid statics.
Jeez! I sussed this better when I was eight!
A is height of the the siphon rise above the liquid level each side of the “U” So is in balance.
B is the drop to the end of the pipe or the lower liquid level, whichever comes soonest.
A – (A + B) = B
Therefore it drops.
The only mitigating exceptions for engineers are:-
Gas pressure differences on the liquid at both ends. If the upper bowl was *very* high above the other, it experiences a lower pressure which reduces the siphon effect slightly. But in practice, ignore.
The other as, as told before on here is the pressure-temperature of the liquid at the top of the siphon tube. Too low a pressure-temperature will boil the liquid which might stop or stymie the siphon action.
“If you put a vacuum pump at the top you will be able to suck the water up to almost 32ft, no higher. It will be the pump which is excreting an upward force on the liquid via the air. It is you pump that is doing the work not the atmosphere. ”
Simple experiment then. Try your same setup in a vacuum and see how much upward force ‘work’ your pump does.
This is ridiculous!
Both pressure and gravity are vital components of a siphon. Cohesion (Réaumur) and surface tension (Mindbuilder, though there’s more to it than just surface tension) are secondary effects.
Here’s a better sort of a experiment:
Two buckets, one higher with water, one lower, empty.
Add tubing or piping from high bucket, up and down to the low bucket.
Vapor lock means nothing flows.
Add a tap at the top, connect that to a vacuum pump, start pumping out air.
Water levels will rise in each tube, and the distance between the level in one side of the tube and that bucket will match the same measurement on the other side.
Keep pumping air out, eventually the water in the high side will reach the crest of the tube.
At this point, gravity is pulling water back on each side, atmospheric pressure is pushing water up on each side. This happens during the entire experiment.
Suck more air out, now water can spill from the high side to the low side and the siphon operates.
Sucking more air out will increase the flow rate.
Now gravity is pulling back on water on both sides of the tubing, but on the high side it’s also pulling water down the low side.
This is a process called spilling well know to any baby since the development of drinking utensils.
Augh!
Note that normal atomospheric pressure can only push water up about 10 meters, so don’t make the tubing too high….
Formation of bubbles. Cavitation anyone?
The vapor pressure of water at room temp atmospheric pressure is around 2 kPa.
Pressure is about 18 kPa at the water surface. At the top of the siphon it will
Be 1 kPa lower for every 10 cm of height above water surface. There will some
Other reductions in pressure due to the bend and due to the inlet of the tube.
Therre will be a further reduction due to dynamic pressure of the water as a result
Of it’s moving. Static pressure decreases as fluid velocity increases. If the end
Of the u tube is discharging into air, reduction in pressure (for 1.5 m height difference) be
About 15 kPa due to velocity of water in the tube. The fluid velocity might be slightlyDifferent
If the end of the tube is in the bucket.
One needs numbers like water temperature, tube diameters, height of ubend above water
Surface, and radius of bends in tube to do a more detailed calculation. But the first rough
Estimates of the numbers are suggestive that the water is simply boiling due to a low
Pressure at the top of the siphon.
Note, I have not read the paper, just going by wot is here.
Ps, questions about cavitation at the top of siphons are a staple
In many fluid mechanics courses for engineering students.
Criminy, folks, don’t make me explain siphonage to some of you.
Reminds me of something I used to ask the other pilots back when I was a contributing member of society. We’d be cruising along and I’d ask what was keeping the passengers in their seats after we turned off the seat belt sign, Gravity, or the Upward Acceleration Generated by the Wings?
They’d inevitably say Gravity, so I’d propose that we had two switches in the cockpit, one that turned off Gravity worldwide (for fuel savings), and the other that Jettisoned the Wings (in case of emergency).
If I jettisoned the wings, they’d go flying off and the rest of us would start falling and everyone would float out of their seats, thus proving it was the upward acceleration of the wings that kept coffee in our cups and our bottoms in the seats.
I’d get a lot of skepticism, but no one could prove me wrong.
“At this point, gravity is pulling water back on each side, atmospheric pressure is pushing water up on each side. This happens during the entire experiment.”!
It the pressure _difference_ that produces a net force that causes movement and does work. It’s like electric potential (voltage) . It’s no good having an earth potential by sticking a rod in the earth, you need a potential difference to make a current flow.
With pressure you need to induce a pressure difference. and the static atm pressure is not going to do this for you. That’s why you need your pump.
It’s the pump which is doing the work to create a pressure difference that will make the water rise and eventually start the syphon.
Yes it is ridiculous. Atm pressure does not make a syphon work any more than the earth wire in your house makes the lights come on.
“Formation of bubbles. Cavitation anyone?”
That is the term use in the paper.
“Atm pressure does not make a syphon work any more than the earth wire in your house makes the lights come on.”
So you’re saying you don’t need a ground/neutral for your house wiring to work…. interesting take.
RyanP “Simple experiment then. Try your same setup in a vacuum and see how much upward force ‘work’ your pump does.”
Well I already said you can’t because the water would boil and there’d be no air to pump out either, by definition, in a vacuum. So you need to define what you mean.
But whatever way you define it , if you want to raise a mass of fluid by a certain height you will have to expend h.g.mass joules of energy to do it and the answer is exactly the same whether you are in the Netherlands or on the top of Everest (using local g) or inside a hypobaric chamber.
It has sod all to do with air pressure.
“So you’re saying you don’t need a ground/neutral for your house wiring to work…. interesting take.”
No I’m saying you need more that one wire. And it does not matter what the ground potential is, it’s the potential difference that makes things happen. That’s why we need a power company.
Like you need a pump ( or a mount or gravity feed ) to make a syphon work. The ambient pressure does not do any work, is not supplying energy and is not making anything happen.
Seems too many here have been reading OED to learn physics.
Enough for me today.
Greg has it almost right. It is the pressure differences caused by gravity that drives a syphon. Saying it is pressure or gravity is misleading, both are needed. The pressure at the top of the U-tube of the syphon is the surrounding gas pressure minus the hydrostatic head (weight per area due to height of the liquid column above the reservoir) of the long side of the syphon. Since this is a lower pressure than the hydrostatic head of the short side, the gas pressure pressure in the short side reservoir pushes the replacement liquid in to replace liquid exiting the long side. This of course ignores other effects such as capillary effects or strong bonding forces, which may allow special exceptions.
I didn’t notice any discussion of a barometer. It is purely atmospheric pressure driven and the liquid would not go up the center tube at all if the barometer were in a vacuum.
http://hydraulicspneumatics.com/site-files/hydraulicspneumatics.com/files/uploads/2012/10/Vacuum%20Figure%201.png
For most practical siphons atmospheric pressure has to be the main operative force and explains why a water based siphon at sea level can only go up 10 meters before it comes back down.
At least for normal water the intermolecular binding power would be much weaker. The video experiment shows a very special liquid with a very strong intermolecular binding strength.
I wonder with that liquid how strong the surface tension is? Ie. how far above the top of a breaker could you go before the liquid overflowed? I’m betting a lot higher than with normal water.
Janice Moore says:
April 25, 2014 at 12:36 am
First, don’t take a physics class from Dr. Hughes. He may understand what’s going on, but he’ll spin it so fast you head will unscrew.
Second, the greenhouse effect doesn’t involve siphons. Well, if I spin it hard enough I could show pressure differentials pushing air against gravity and letting it fall again, but thunderstorms do that every day and I’ve never heard anyone call that a siphon. Dr. Hughes probably would.
As for the issue in the piece you quoted, I don’t have much to quibble with with Lacis’ comments except this one. This goes back to James Hansen’s early days, I think he was one of the original proponents of a runaway greenhouse effect on Venus.
The actual reason the surface of Venus is hot is much simpler and has to do with “adiabatic expansion” which show up any time you climb a mountain or a diesel fuel burns in an engine.
Venus’s atmosphere is so dense, pretty much no IR makes it off the surface, at least not at wavelengths CO2 blocks and clouds reflect. We know some sunlight does make it to the surface, thanks to the Venera spacecraft photos of the surface. That heats up the ground, that heats the air, and the most efficient way to get that heat away from the surface is via convection, think thunderstorms or “fair weather cumulus”. As the air rises, it expand and cools, eventually it gets high enough for IR to escape Venus. When it reaches typical Earth pressure, it has cooled so much that it’s comparable to Earth’s surface temperatures, after taking into account the increase sunlight due to being closer to the Sun and increased reflect due to all those clouds.
There are a couple WUWT pages about this, see http://wattsupwiththat.com/2010/05/06/hyperventilating-on-venus/ for one. (Warning – it’s a Steven Goddard post. those always became flame filled.) Most of my comments there are mild.
http://theendofthemystery.blogspot.com/2010/11/venus-no-greenhouse-effect.html is also good, the author, Harry Dale Huffman, sometimes posts here.
@ Greg – If you attach a vacuum pump to the top of a 40ft pipe and suck water up say about 20ft, the air pressure in the top of the tube will be about 1/3 of an atmosphere. Air at any positive pressure above pure vacuum will be trying to expand, thus the air at 1/3 atmosphere will be trying to expand both upward and downward and therefore exerting a downward force on the liquid in the tube. The vacuum pump doesn’t exert any force on the liquid through the air, it only takes away a force that was pushing the air and the liquid down. It’s like lifting a weight off the top of a spring. You’re not exerting an upward force on the spring causing the spring to move up, you’re just removing a downward force. If you were asked whether it was the steel of the spring coil pushing the spring up or is it your arm lifting the weight off the spring that is pulling the spring up, what would you answer? If you remove the cork from a champangne bottle or compressed air bottle, is the stuff inside pushing its own way out or are you pulling stuff out or just providing the energy to remove an impediment?
Would you also agree that in a typical, say 1m tall, practical siphon, at sea level, that all of the molecules are at positive absolute pressure, and therefore they are all being squezed together and repelling each other, and therefore no pull can be transferred over the top of the siphon? Isn’t it true that the liquid or gas at the top of the siphon is pushing down on both sides, just pushing down a little less on the up side?
Are you aware that the siphoning of carbon dioxide gas has been demonstrated? No cohesion necessary there.
Are you aware that small medium or even large bubbles so big that they completely disconnect the liquids on each side of a siphon, can pass through the siphon without hardly slowing it down? Any cohesion to pull is defeated by bubbles. Since those bubbles contain pressurized gas, they would be happy to expand very large if there was any pull at the top of the siphon. But they don’t expand much. They stay squeezed small.
If cohesion is the explanation, how would cohesion take part(other than to prevent boiling) in raising the liquid in the siphon of figure 4 in the Wikipedia siphon article, where it starts out with only air at the top?
A pressure difference is basically two opposing unequal forces. Imagine two children in a supermarket aplying pressure to a shopping cart in equal opposition to each other. Then imagine one child steps out of the way and takes his hands off the cart. Wouldn’t you say that the cart moves because the remaing child is pushing the cart rather than saying that the cart moves because the child that stepped aside with his hands not even touching, is pulling? A vacuum pump doesn’t pull on anything, it just clears out some space so that the remaining molecues have somwhere to push and spread on out to unopposed.
The vacuum doesn’t draw the fluid up the tube. A vacuum is not a force and as such cannot do any work. A vacuum is a lack of force. The weight of the air column above the top container generated by gravity provides the force. As for the experiment, since we cannot generate a complete vacuum the force of gravity will still generate a pressure differential as it draws the fluid down the long tube. The fluid will still flow with a bubble in the tube as long as the flow is fast ,enough to keep the bubble in the down leg of the siphon. The bubble is still a partial vacuum caused by the force of gravity acting on the fluid below the bubble.
“No I’m saying you need more that one wire. And it does not matter what the ground potential is, it’s the potential difference that makes things happen.”
Of course, and that is how gravity works in this case as well, you need to different heights in your reservoirs to get the gravitational potential. So it’s not really gravity that drives siphons then, (gravity and pressure and electricity just do what they do), it’s the person/scientist/force who lifts and holds one reservoir above the other, lol.
This is an interesting quote with regards to AGW.
“The stable water surfaces act like energy barriers between the atmosphere and siphon. For energy to be transferred from the atmosphere to the water the water level would have to go down, since the amount of energy transferred is equal to force times distance.”
How on earth is the heat hiding in the oceans then ?
Has he inadvertently proved the hypothesis that the heat is hiding in the oceans theory false ?
Do people arguing for atmospheric pressure as the driver of siphons, really think this siphon apparatus wouldn’t work in a vacuum? Imagine this siphon apparatus inside a vacuum tube (at the Earth’s surface). You wouldn’t claim the water in the upper bucket would fly out of the bucket to fill the vacuum because the pushing force of the atmosphere is removed, would you? No, of course not. Because gravity would still be there, pulling the water downward in the bucket. Gravity gives the water in the bucket its internal pressure. The internal pressure within the water itself pushes some of the water up into the siphon tube. I won’t dispute that atmospheric pressure adds its force to the water surface, but that explains only a tiny fraction of the internal water pressure in the bucket. It is not the driving force of the siphon.
I posted this on my e-mail list.
Interesting discovery: especially for a physicist. Nearly every dictionary printed in the past, until very recently, has miss-defined the word siphon.
The action of a siphon was attributed to atmospheric pressure rather than gravity. Of course A.P. does have a secondary effect. If you try to lift water more than 10 meters above the U a vacuum will form and the flow will stop; forming two separate columns, provided the second column is immersed in the second fluid container. Therefore the maximum amount of lift before the U is dependent on the S.G. of the fluid. As A.P. decreases the maximum lift also decreases before a vacuum forms.
The flow rate is controlled by the differential fluid level of the first and second container.
I would assume that at some maximum differential, greater than 10 meters for water, the flow rate would pulse do to intermittent vacuums forming in the flow line, provided the up flow is less than ten meters
A similar phenomenon takes place with cavitations of a propeller in fluid.
Greg, maybe this will help.
Imagine you get a siphon going between two buckets, then you raise the lower bucket until the water levels are at equal elevation and the flow stops. Even though the flow has stopped, obviously water remains in the tube, stationary above both buckets.
The water in the tube is at a higher level than the water in either bucket, and there are 2 open paths through the tube for it to flow down. What’s keeping it from doing that — flowing back down, half of into one bucket, half of it into the other? Gravity is still operating, so there must be some force stronger than gravity keeping the water up there.
Of course it’s because in order to flow down, the water would have to “break into two columns” and pull a vacuum in the tube, and yes, that potential negative pressure is stronger than gravity.
So a siphon and a drinking straw do work on very similar principles. In a straw the water flows up because your mouth creates a partial vacuum. In a siphon the water flows up because of the vacuum that would be formed if it didn’t.
But keep in mind, really the mechanism keeping the vacuum bubble from forming at the top of the tube is the atmospheric pressure on the water in both buckets. If you were to put both buckets in a vacuum, there would be no problem for the water in the tube to form a vacuum as well, and then it could yield to gravity and flow down both paths.
Because we feel atmospheric pressure all the time, sometimes we don’t realize how strong the air around us actually is. For example, in an atmosphere, people could theoretically use suction cups to climb a wall, but a similar idea couldn’t be used to help astronauts stay attached to the outside of a space station. The force that sicks suction cups to things is atmospheric pressure. It’s also atmospheric pressure that sticks the water together at the top of the tube in a siphon.
Ultimately, it’s gravity that makes water flow from one bucket to another, but air pressure is absolutely required as well. If it’s absolutely required, I think it has a bit more than “sod all” to do with it.
Both air pressure and gravity are required for a siphon to work. A siphon will not work in zero gravity. A siphon will not work with zero air pressure.
The “lift” section requires air pressure. The “fall” section must be longer than the “lift” section so that gravity provides the net energy to maintain the flow.
The higher the “lift” the more air pressure required. At sea level a siphon can theoretically lift water 10 meters, so long as the “fall” section is more than 10 meters. The size of the opening at the bottom end of the siphon must be reduced as the “lift” approaches the height of the “fall”, otherwise the water column in the fall may separate due to air entering via the bottom of the fall.
Nancy C says:
April 25, 2014 at 6:52 am
Ultimately, it’s gravity that makes water flow from one bucket to another, but air pressure is absolutely required as well. If it’s absolutely required, I think it has a bit more than “sod all” to do with it.
=============
correct.
Mickey Reno says:
April 25, 2014 at 6:49 am
Do people arguing for atmospheric pressure as the driver of siphons, really think this siphon apparatus wouldn’t work in a vacuum?
===========
a siphon will not work in a vacuum. the “lift” section cannot operate without air pressure.
You can however run a hose between two buckets in a vacuum and drain one into the other, so long as you attach the hose to the bottom of the buckets (and drill holes, etc). However you cannot siphon water (which involves lifting it over the edge of the bucket) in a vacuum.
of course the talk about siphoning water in a vacuum is hypothetical. In a vacuum water boils at room temperature, so your siphon quickly fills with steam, not water. If you sealed the containers and the siphon, in theory this steam would provide the pressure required to drive the siphon. Allowing you to perhaps build a siphon that could operate in a vacuum, so long as the apparatus itself was not open to the vacuum.
@ Nancy C “air pressure is absolutely required as well” I agree if you are only talking about water, but the (virtually) equal air pressure at each end isn’t providing energy for the flow, it is only pushing equally on both ends to keep the water ‘joined together’.
@ Fredberple “A siphon will not work with zero air pressure” ?
Have you seen the very clear experiment at http://www.youtube.com/watch?v=8F4i9M3y0ew which I have referred to several times? How do you explain it?
A siphon 1.5 metres high was set up in the chamber and when the pressure was reduced to an altitude of 40,000 feet a waterfall appeared at the top, but the water flow remained nearly constant.
41,000 feet, the siphon broke into two columns of water and, when returned to 40,000 feet, it reconnected as if nothing had happened.
Atmospheric pressure at 40,000 feet, which is more than 10,000 feet higher than Mount Everest, is about 18 per cent of the sea level value.
=========
about what one would expect. 1.5 meters is 15% of the 10 meters air pressure will raise water at sea level. it makes sense that the column would split at about 15% of sea level air pressure.
If you heat a bucket of water with a heat gun….
;-}
One last thought. This argument started because of a definition. What differentiates a siphon from any regular tube going from a high to low reservoir where water flows due to gravity (see, we don’t even have a special name for this) is the part where the water flows up due to pressure differences before it goes down the other side. The definition focuses on what is special in this case rather than the fact that gravity causes water to flow downhill (as it does in the longer leg of the tube)
Réaumur says:
April 25, 2014 at 7:21 am
Have you seen the very clear experiment at http://www.youtube.com/watch?v=8F4i9M3y0ew which I have referred to several times? How do you explain it?
============
the liquid is not water. you can build a device that will siphon in a vacuum so long as the molecular bonds of the liquid are stronger than the lift height. This would require a viscous liquid that does not boil in a vacuum.
So the science is not settled then?
Lots of long descriptions.
I’d shorten it all to…
The gravity driven falling water in the lower tube creates a vacuum in the upper tube sucking water into the other higher end producing a steady stream of falling water, vacuum & replenishment.
The real mystery is gravity itself.
http://chapelboro.com/columns/common-science/gravity-still-a-mystery/
“We are so accustomed to the effects of gravity that we fail to remember that although we can predict its effects, we don’t really know how it works. Really, we don’t.”
Compare this to alarmists pretending AGW scientists know how the climate works. I’ve heard some even suggest that denying AGW is like denying gravity?
It’s funny that such a comparison would be made because scientists do not really know how our climate or gravity works. Yet alarmists pretend that 97% of scientists know how out climate works?
Yeah sure, just like gravity?
I recently watched the series Wonders of the Universe S1 E3 Falling
It explored gravity from here through the universe.
I wouldn’t give these guy many marks at all for their work. Not impressed. They only experimented with a 1.5 m siphon. To be meaningful they should have experimented with a full range of siphon heights up to 10m.
Réaumur says:
I think you’re right that I shouldn’t have said “absolutely” required. But the video doesn’t demonstrate that a siphon “does” work in vacuum, it demonstrates that a siphon “can” work in a vacuum in very special circumstances.
Also, the youtube experiment would fail if they used a long enough, steep enough tube connecting the two containers. Eventually the weight of the liquid in the tube would overcome the ionic attraction between the molecules in the liquid, the liquid would break, and it flow down both sides of the tube defeating the siphon. So maybe atmospheric pressure isn’t absolutely required, but *some* second force beside gravity (and stronger than gravity) is absolutely required.
You could say that the difference in water pressure (not air pressure) causes the siphon. Gravity creates the pressure difference.
Yes gravity drives it but no, you still need atmospheric pressure to push the fluid up the inlet!
Consider that the elevation of a water siphon’s height above the inlet water surface must be less than ~32 feet, (@sea level). (A waste of hypobaric chamber time when all he needed was 75 foot garden hose and pulley mounted to some high rafter to get the same result.)
Réaumur says:
April 25, 2014 at 7:21 am
I know Fred has already answered this. I don’t think you understand what an ionic fluid is. That’s okay, I never heard of it either until that video. How do you explain what an ionic fluid is? How do you explain the decision to use an ionic fluid in the YouTube video? Are you aware that fluid does not behave at all like water? I strongly suggest we point out second order effects in future comments here and focus on the primary effects of fluids like water, tubing big enough to make surface tension and capillary effects insignificant, and concentrate on just air pressure and gravity.
> “The fact that the water level in the upper and lower buckets is constant indicates that atmospheric pressure is not pushing water into the siphon,” Dr Hughes said.
I take back any positive comments I may have uttered above or thought to myself. I hereby claim Dr. Hughes is the second coming Bill Nye.
Fred, perhaps I should not have used water as my example. Water will tend to form bubbles in a vacuum, and that could interfere with the pressure equalization within a siphon tube. You can siphon in a vacuum, as long as the fluid involved doesn’t form bubbles that break the siphon.
The point being, that air pressure is irrelevant as a force in the operation of a siphon.
By the way, for those people who suck on the siphon tube to siphon gasoline, that’s really dumb. To safely siphon gasoline, put the siphon tube into the gas tank. Blow into the tube and listen for bubbles, to make sure your tube is under the fluid surface. After you hear bubbles, use a rag to create an airtight seal around the siphon tube at fill tube (gas cap). Now BLOW! All you need is enough pressurized air in the gas tank to push enough gasoline into the tube to fill it. In this way, there’s no inhalation of gas fumes, no gasoline ever has a chance to enter your mouth.
Well the lower bucket is superfluous, unless you want to collect the water. To put it another way, the low end of the tube, only needs to be lower than the water surface in the upper bucket; it does not need to be under water in another bucket.
But try making the loop height higher than 34 feet above the upper water surface, and then ‘splain me how atmospheric pressure is not involved !!
Having siphoned plenty of gasoline out of my boat sitting on its trailer (emptying it for winterizing), I can attest to the fact, that the output end can be open to the air. It is better if it’s not, so that any flow interruption doesn’t let air in the tube.
I’m thinking OED and the physicist blew it again.
I don’t believe there can be many engineers that didn’t know a siphon works by gravity – the long column pulling water up the short column. To stop a siphon but maintain the tube filled, one only has to raise the lower bucket up to level with the upper. This in itself cries out gravity. Surely they weren’t thinking that the upper liquid surface, being higher had a different atmospheric pressure – it does, but the upper level has (infinitesimally) LOWER atmospheric pressure, and the siphon would then work the other way! Perpetual motion machine anyone? Probably the mistake was perpetuated by the fact of the prestige enjoyed by Pascal if it were he who did indeed (shame, shame) state this.
Despite correcting this, Dr. Hughes shouldn’t come off totally unscathed in all this! He didn’t seem to understand that although having nothing to do with air pressure per se, a very low pressure causes ‘boiling’ of water and this will break the siphon in the tube when the bubbles become numerous enough.
Have the people thinking the atmospheric pressure will drive the liquid above the short end of the tube ever used a siphon? If that was the case, one would only need to put the empty tube in the liquid and the liquid would spill automagically over. The air pressure pushing the liquid is also pushing inside the tube, annihilating the force. The 10 m water column of a barometer happens because the atmospheric pressure in the tube doesn’t exist (vacuum in the column).
Having never read the definition of a siphon, I was never confused by the reference to atmospheric pressure. It is pretty obvious that a siphon would not work in the space station with atmospheric pressure but not gravity, but would work on the moon (in a closed system to prevent the liquid from vaporizing) with gravity but now atmospheric pressure.
This is a bit ridiculous. Water flows are well explained by the Bernoulli equation: p/w+V^2/2g+z. By inspection, it is a simple matter to determine that pressure, fluid density, and gravity all play a role. To the extent what forces are more dominant is determined by the situation (boundary conditions). In a classic siphon, gravity is indeed the dominant force. However, it is NOT the only force.
Janice Moore says:
April 25, 2014 at 12:36 am
“Because of the small atmospheric pressure on Mars (less than one hundredth that on Earth), the spectral absorption lines of carbon dioxide on Mars are very narrow, and therefore act like a picket fence that lets most of the thermal radiation emitted by the Martian ground surface to escape directly out to space. This does not happen on Earth because of the atmospheric pressure that is exerted by the radiatively inactive nitrogen and oxygen, causing the spectral absorption lines of carbon dioxide and water vapor to be greatly broadened, making them more effective absorbers of thermal radiation. ”
Ah. Recently somebody wanted to tell me that H2O and CO2 do not cannibalize each other’s IR photons because their lines are not at the exact same places. So that guy was wrong.
“It is pretty obvious that a siphon would not work in the space station with atmospheric pressure but not gravity, but would work on the moon (in a closed system to prevent the liquid from vaporizing) with gravity but now atmospheric pressure.”
That is less than obvious to me. At last check, gravity alone does not force stuff up tubes.
So if both pressure and gravity are needed, siphoning on the moon would not work??
@ferdberple: “the liquid is not water” – I didn’t say that it was, but neither did you specify water when you said “A siphon will not work with zero air pressure” – the experiment shows that a siphon works in (very near-) zero air pressure!
@ Rik Werme: “How do you explain the decision to use an ionic fluid in the YouTube video?” It is clearly explained in the video! Ionic liquids don’t evaporate in vacuum and have extremely low vapour pressure. Are you aware that fluid does not behave at all like water?” Water is a fluid.
Wikipedia article on Siphons cleary states: “atmospheric pressure pushes the liquid up the tube”. See there, settled science.
Mickey Reno says: April 25, 2014 at 8:14 am “The point being, that air pressure is irrelevant as a force in the operation of a siphon.”
Rubbish. The ionic fluid in the youtube video is sophistry. It’s no different than having a chain draped over a pulley then have one end slightly lower than the other which pulls the shorter end up and over purely by gravity alone. A siphon of ordinary liquid requires atm pressure at the inlet to function as a siphon. As I and others repeatedly point out, ~32 feet is the maximum height of a water siphon. Explain how that could be true if pressure is “irrelevant”?
To get the siphon started you are decreasing the air pressure in the tube and the outside air pressure is pushing the liquid up. Correct? I think that should be stated.
I always thought that is was a combination of both gravity and air pressure that kept it flowing.
The outflow of water that is a result of the gravity on the water counteracts the pressure out of the siphon’s outlet. The only certain way to find out is to do it in a complete vacuum which I know doesn’t exist. But if it was just air pressure then in a gravity free environment the siphon would still work? That’s if it was just air pressure keeping it going.
Not one scintilla of actual technical knowledge was involved in this post.
Thank you, Greg, for honoring my question with a useful answer.
Thank you, Ric Werme, for so kindly taking the time not only to answer, but to explain. MUCH appreciated.
Thank you, Dirk, my ally for truth in science, 😉 for making sure that I was not hoodwinked by AGW speculation.
I can tell from two of you guys that you strongly suspect that I give some credence to the quoted Goddard material — just for the record I do not. That quote is a bunch of JUNK SCIENCE — just wondered if the siphon principles made it even MORE implausible.
What a JOY it was to not have my question ignored! That happens so many times… .
THANK YOU, THANK YOU, THANK YOU!
Janice
And… Way — to — go, all you science giants above! GREAT (if to you very basic) education and bona fide SCIENCE on this thread. WUWT is the best blog in the world — thanks to people like YOU! Thank you.
Patrick Schlüter says: April 25, 2014 at 8:35 am “The 10 m water column of a barometer happens because the atmospheric pressure in the tube doesn’t exist (vacuum in the column).”
You annihilated your own argument. That’s the whole point, the pressure at the top of siphon is less than the atm pressure at the inlet. If you were to put a valve at the very top and stop water, the pressure at the valve on the inlet side, the side of the SHORTER column will be greater than the pressure on the outlet side of the valve, the TALLER column. The difference in pressure across the closed valve is from the height difference, (extra tubing) of the mass of fluid between the inlet and outlet within a gravitational field.
Dr. Hughes demolishes his own credibility with this quotation in the QUT article:
“The column of water acts like a chain with the water molecules pulling on each other via hydrogen bonds,” he said.
If hydrogen bonds were central to the siphoning process, you would have to know something about them in order to predict how high a siphon water can climb; but in fact all you need to know is water’s density. Mercury has no hydrogen bonds, will it not siphon at all? Would you have to know something about mercury’s interatomic forces in order to predict its siphoning limit? No, you only need to know its density . . . and the local atmospheric pressure.
H2O siphons work due to the cohesive properties of water.
Gravity and/or air pressure surely only serve to assist that cohesive property.
Cause me thinks cohesive H2O siphons of greater that 100 vertical feet are commonplace in nature that have no dependence on either gravity or air pressure.
Cause me thinks that is the mechanism that trees use for transporting H2O up to the topmost portion of their canopy.
Sequoia or redwood trees are up to 379 feet (115.5 m) in height
Cheers
Gary Pearse says: April 25, 2014 at 8:31 am “I don’t believe there can be many engineers that didn’t know a siphon works by gravity”
No… gravity AND the atm pressure needed to drive the water up the tube. Proof? Completely seal the inlet reservoir and lower the air pressure. At some lower pressure of the inlet than atm – the siphon will STOP thus disproving that pressure is not needed.
And.. that difference in pressure will be exactly equal to the pressure difference from the additional column height of the outlet below the inlet.
Samuel C Cogar says: “H2O siphons work due to the cohesive properties of water.”
(You’re probably just being cute but…)
Siphoning is NOT a capillary process. The strength of the capillary effect of water in the cross section of a garden hose amounts to about the height of the minicus.
Mickey Reno says:
April 25, 2014 at 6:49 am
“…Imagine this siphon apparatus inside a vacuum tube (at the Earth’s surface). You wouldn’t claim the water in the upper bucket would fly out of the bucket to fill the vacuum because the pushing force of the atmosphere is removed, would you?”
Actually, I would. The water would boil and leave the bucket.
I strongly suggest that commenters who think only gravity is necessary for a siphon never try this experiment at home. They might easily drown.
A water siphon can work at over 10 meters, though its delicate and and the water needs to be degassed first. this one got to 24m http://www.youtube.com/watch?v=sz9eddGw8vg
Tests of the tensile strength of water in a Z-tube suggest that in a glass tube a water siphon could reach more than 2800m
And there is no reason to limit ourselves to a discussion of water siphons. Siphons are often used on gasoline and alcohol for example. And I expect a siphon of motor oil would work in a vacuum as does an ionic liquid.
Nature? Seriously? And using an airplane instead of a hypobaric chamber?
A standard (raised) siphon works the same way an inverted siphon (picture a tube connected to the bottom of two fluid reservoirs) does. Once the connecting tube is filled, flow is driven by the pressure differential between the two ends, ρgΔh, where ρ is the density of the liquid, g gravitational acceleration and Δh the difference in height between the surface of the two reservoirs. Here atmospheric pressure is relevant only to the extent there is a significant difference between pressure at the two surfaces levels (insignificant for air) and at any rate the effect is of opposite sign to the pressure differential in the tube: the lower liquid surface is at a higher pressure than the upper one.
Atmospheric pressure plays a role if the siphon tube is initially filled by suction. And the maximum height of a standard raised siphon is limited by the (negative) pressure at which the liquid column separates, which is controlled by the liquid’s surface tension, boiling point (a function of pressure), and/or the tube diameter. An inverted siphon tube is under positive pressure throughout and is not susceptible to separation of the liquid column.
Siphons will work under very low atmospheric pressures (presumably some experimenters have used low-vapor-pressure vacuum pump oils, assuming any thought the experiment worth doing) but not under zero gravity or in free fall.
Coming to this rather late in the day, surely the essential features of a syphon are that (a) at some point(s) the syphon tube goes upwards, above the surface level of the outflowing container (if the syphon tube always lies below this level gravity just takes its course and the container flows away naturally) but (b) the eventual outlet of the syphon tube must lie below the surface level of the outflowing container: if the outlet were above that level, the flow would have to be blown or driven at all times.
So there are actually two requirements: the flow (to a level above the outflowing container) must be initialised, and the eventual outlet must be below the outflowing container. While gravity indeed drives the flow after initialisation, the syphon won’t operate at all unless it is first initialised by some external (non-gravitational) force – generally gaseous pressure: blowing or sucking, of course.
So (like all good legal issues) the answer is compound – while gravitation indeed drives steady syphon flow, gravitation alone can’t start the flow. Yes, it’s a funny old world!
Here is a link to the ingenious Z-tube which is used to measure liquid tensile strength. It is a Z-shaped tube of glass nearly filled with liquid and spun on a table to high RPMs.
http://en.wikipedia.org/wiki/Z-tube
Mindbuilder says (April 25, 2014 at 6:19 am): “Are you aware that the siphoning of carbon dioxide gas has been demonstrated? No cohesion necessary there.”
Interesting! That reminded me of a couple Youtube videos of sulfur hexafluoride, a gas nearly six times denser than air:
http://www.youtube.com/watch?v=1PJTq2xQiQ0
Presumably a siphon would also work with SF6.
Peter Pearson says (April 25, 2014 at 10:44 am): “Dr. Hughes demolishes his own credibility with this quotation in the QUT article…”
Yeah, that “hydrogen bond” explanation pegged my BS meter. As Mindbuilder has pointed out, one air bubble in the line demolishes the whole argument.
Although Dr. Hughes is technically correct about “gravity” being the entire explanation, since without gravity there would be no atmospheric pressure to drive the upward leg of a typical water siphon.
BTW, in the “ionic liquid in vacuum” siphon, what happens if a vacuum bubble is introduced in the middle of the siphon line? Does the siphon still function?
I think I get it now, the tricky science of siphons is still being debated but, climate science is settled. Those who doubt atmospheric pressures role in siphons should be silenced or put to death.
“””””……Mindbuilder says:
April 25, 2014 at 11:14 am
A water siphon can work at over 10 meters, though its delicate and and the water needs to be degassed first. this one got to 24m http://www.youtube.com/watch?v=sz9eddGw8vg…..”””””
If that is supposed to be a scientific experiment, it explains why climate science is in such disrepute.
But the characters fit the play. Face and head full of unruly hair, the mark of the academic.
Looked like organized chaos to me.
No explanation of what was supposed to be going on.
I saw a cameraman, being extremely careful, to NEVER show the top of the “siphon” up there at 24 meters, nor did he ever show any liquid actually flowing anywhere.
So the guy picks up the flask and tips it; anybody see him get any fluid out of it.
What a total screw up; we see some not very adept characters pulling a tube up in the air to god knows where, but we never see any siphonic evidence whatsoever.
So WHAT was their scientific explanation of what was happening. The chap with the facial hair never said a word that I could hear, above the seagulls. And what was with the dame shrieking ??
“”””””…..Gary Hladik says:
April 25, 2014 at 12:20 pm
Mindbuilder says (April 25, 2014 at 6:19 am): “Are you aware that the siphoning of carbon dioxide gas has been demonstrated? No cohesion necessary there.”
Interesting! That reminded me of a couple Youtube videos of sulfur hexafluoride, a gas nearly six times denser than air:…..”””””
If I’m not mistaken (I was once), SF6 is used to insulate UHV (oltage) transmission components; transformers and the like.
I believe there was a misstatement WRT the Ionic Liquid in Space. While the Ionic Liquid might not boil away (loose cohesion) in the hard vaccuum of space, If you left it there (exposed in Outer Space) for any length of time, the liquid would freeze solid
“””””…..Mike M says:
April 25, 2014 at 11:05 am
Samuel C Cogar says: “H2O siphons work due to the cohesive properties of water.”
(You’re probably just being cute but…)
Siphoning is NOT a capillary process. The strength of the capillary effect of water in the cross section of a garden hose amounts to about the height of the minicus…..”””””
Actually the strength of the surface tension effect is pretty much limited to the height of the meniscus, regardless; or irregardless, as the case may be, of the tube diameter.
As a rough seat of the pants guesstimate of the capillary effect, the pressure difference inside a bubble (droplet) due to surface tension, is given by delta P = 2t/r, where t is the surface tension (Newtons per meter) and r is the surface radius. That’s the result for a small (spherical ) droplet or a vapor bubble inside the liquid.
For a soap film bubble it is 4t/r, as there are two surfaces trying to collapse the bubble.
Surface tension is trying to minimize the surface area by reducing the radius of the bubble; that increases the pressure inside the bubble which eventually stops the collapse.
Students can prove the relation I just gave by employing the principle of virtual work. Assume a small change in bubble radius, and compare the work done by the surface tension, to that done by the excess pressure.
Soif you make the capillary very small diameter, and the liquid wets the tube, the capillary height can be quite substantial. Tall trees depend on it, among other things.
I’m an expert at the physics of siphons and this guy Hughes is wrong. No one ever believed that atmospheric pressure was the driving force for a siphon. Atmospheric pressure is required to maintain the siphon, and this idiot proved it in his experiment. It behaved exactly as one would expect when you lower the pressure and the siphon breaks at the top when the pressure is lowered. You cannot in fact use a siphon at 1 atmosphere that is higher than 33 feet precisely because water at that height will exert one atmosphere of pressure downward. Were gravity stronger the siphon would fail even sooner (at the same atmospheric pressure). Try siphoning mercury over a three foot high barrier. It can’t be done.
One of my pet peeves is when a scientist doesn’t understand his own field and thinks he’s made some great discovery but everyone else already knew about it.
Starting with the wiki article on siphons,
http://en.wikipedia.org/wiki/Siphon
I found an article contradicting Dr. Hughes’s hydrogen bond or “molecular chain” explanation of a typical water siphon:
http://www.phys.uhh.hawaii.edu/documents/TPT-final.pdf
I didn’t find a free version of the carbon dioxide siphon article, but I did find a pretty good Youtube video (the CO2 siphon starts at about 6 minutes):
Interestingly, after siphoning the CO2, the presenter then siphons a CO2/water mist mixture. The tiny water droplets of course form because of surface tension, but remain separated by air/CO2, and so can’t possibly “pull” each other into the receiving flask.
Charlies Nelson, “If you reduce the atmospheric pressure further the water will eventually boil and stop the syphon!”
Precisely correct which is exactly what has happened in his “waterfall effect”. He essentially has a barometer on the down tube while the up tube still is short enough that atmospheric pressure is strong enough to keep the water height above the point where it goes over the apex. The atmospheric pressure is not what is making the water move (that’s caused by gravity) but it is what allows the siphon to be maintained.
george e. smith says (April 25, 2014 at 12:41 pm): “If I’m not mistaken (I was once), SF6 is used to insulate UHV (oltage) transmission components; transformers and the like.”
Will a taser spark in a SF6 atmosphere? See for yourself (at about 1 min):
The first figure and caption mention “the hydrogen bond”. What would that have to do with a siphon? Wouldn’t, say, carbon tetrachloride much the same way?
@ Brian Macker – How ironic that you claim to be a siphon expert and complain that other siphon specialists don’t understand their own field, yet you don’t realize that water siphons have been demonstrated to 24 meters and mercury siphons have been demonstrated to more than 15cm above the barometric height of mercury(and that in glass tubes to which mercury adheres poorly – much higher is likely possible in say a copper tube). The Xylem of tall trees is another example where water resists vaporization at negative pressures several times atmospheric pressure.
Furthermore, while everyone agrees that gravity is the ultimate energy source of a siphon, it is nonetheless ALSO true that it IS atmospheric pressure that pushes the liquid up in typical siphons after gravity has lowered the pressure at the top. Atmospheric pressure doesn’t just prevent vaporization. See my links and comments above. Be extra careful when you call people idiots.
Brian Macker is exactly correct. Every engineering hydraulics book for well over 100 years has correctly analyzed the siphon via Bernouli’s energy equation, and the driving force is gravity.
The only role of pressure is to prevent dissolved gases from coming out of solution at the top of the siphon and to prevent water boiling, which also breaks the siphon by putting vapor at its top.
Prof. Hughes could have found the answer in any freshman physics book or any engineering hydraulics book, but it would not have been as much fun.
@ bob skyes – If the only role of pressure is to prevent bubbles, then how do you explain Figure 4 in the Wikipedia siphon article, where the siphon starts out with only air at the top? Is not pressure needed to push the liquid up?
Brian Macker says:
April 25, 2014 at 1:11 pm
Amen! to that.
Reference number 2 in the Wikipedia siphon article (Would a siphon flow in a vacuum – Minor 1914) reports siphoning mercury 30cm above the barometric height.
It is hoped that this paper may assist in correcting the common misconception that the operation of a siphon is dependent on atmospheric pressure. In view of the extensive search made of online and offline dictionaries, it is possible that every English dictionary in the entire world needs to be corrected.
.Talk about delusions of grandeur!
Let’s face it Dr. Hughes, you are simply wrong. If ordinary water needed the cohesive properties you require for the function of an ordinary siphon you would never be able to get out of a swimming pool because you’d be trying to pull out all the water behind you. That your feeble idea passes as science is frightening.
If you do not believe that atmospheric pressure is required at the inlet Dr. Hughes then simply just try taking it away and see what happens.
==============================================================
Mr. Layman here.
It would seem to me that gravity pulls the liquid (and the air) down then the liquid rises to fill the vacuum.
What would happen if there was also a vacuum above the upper liquid….? No siphon?
@ Mike M wrote:
“If you do not believe that atmospheric pressure is required at the inlet Dr. Hughes then simply just try taking it away and see what happens.”
It’s been done. It worked. The atmospheric pressure was not required. There is a Youtube video of it linked above. Not all siphons require atmospheric pressure. Some do require it. Some don’t require it but use it anyway.
=====================================
DUH!
If the vacuum was only over the upper liquid the “siphon” would reverse … but then it wouldn’t be a siphon.
“Giant sucking sound” indeed.
Gary Hladik says: April 25, 2014 at 1:24 pm
Exactly right, thanks for the links:
http://en.wikipedia.org/wiki/Siphon (which I found to be well written) and
http://www.phys.uhh.hawaii.edu/documents/TPT-final.pdf
.. both of which annihilate Dr. Hughes’s hydrogen bond or “molecular chain” explanation of a typical water siphon.
The latter stating:
The chain model predicts that, as the gravitational driving force has not been altered, the siphon will continue to operate. Bernoulli’s law predicts that, once enough fluid is drawn out of the top bottle, pressure of the gas trapped inside it will lessen until at some point the siphon stops operating. We observed that the walls of the plastic bottle caved in as the pressure within the bottle decreased, as seen in Fig. 3(b); once the pressure inside the top bottle was low enough, as shown in Fig. 3(c), siphon operation stopped.
Stop the presses Dr. Hughes.
Réaumur, There is still a height above which even the ionic fluid will not siphon. Guess what. Adding atmospheric pressure would make it work again. That tiny chamber in the video cannot test the heights required.
The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber. Once the flow is established it will keep flowing. Air pressure will actually tend to counter this as the lower chamber has a higher surface pressure. I am a simple Engineer am I missing something?
Mindbuilder says: April 25, 2014 at 2:38 pm ” Not all siphons require atmospheric pressure. ”
Yes they do or they are not “siphons”. The chain over a pulley model is NOT how a siphon works. The youtube video is a chain over a pulley – actually pulling the mass behind it purely by gravity.
Reduce the pressure at the inlet of a REAL siphon, (i.e. garden hose and tap water), and the siphon will stop. In the wiki entry they correctly point out that in an ordinary siphon the fluid is in compression, (pressurized) at all points along the way. That is NOT true with a highly cohesive fluid which is in tension – the exact opposite.
Mr. Layman wrote:
“It would seem to me that gravity pulls the liquid (and the air) down then the liquid rises to fill the vacuum.
What would happen if there was also a vacuum above the upper liquid….? No siphon?”
Right. The question is where does the force come from to make the liquid molecules rise to fill the partial vacuum. Since the partial vacuum of air at the top of the siphon is pushing DOWN on BOTH sides of the siphon, and gravity is pulling DOWN on all the liquid, the only force left to make the liquid molecues rise is atmospheric pressure. Same as in a drinking straw. If there was vacuum above the upper liquid, there would be no siphon effect in a siphon starting out with air at the top.
Fascinating, just fascinating. Thank you Anthony, not for the post itself but for all the discussion it has generated. This has been a real learning experience in quite unexpected ways.
For your next trick, try posting something about how wings generate lift!
BeauGatun,
“The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber. ”
No, the pressure at the surface of the higher chamber is lower than the pressure at the surface of the lower chamber. You don’t even need any chambers for a siphon to work as long as you have a thin enough tube, or a movable plug in the uphill end of the tube. All the water will flow out the lower end. The pressure differential is in the tube, not the chambers.
If the pressure is the same at both ends of the hose, then it can’t be atmospheric pressure. My father taught me that it was gravity when I was a kid.
“””””…..Gary Hladik says:
April 25, 2014 at 1:28 pm
george e. smith says (April 25, 2014 at 12:41 pm): “If I’m not mistaken (I was once), SF6 is used to insulate UHV (oltage) transmission components; transformers and the like.”
Will a taser spark in a SF6 atmosphere? See for yourself (at about 1 min):…..”””””
You’d never guess in a million years, where I first learned that.
It was in my first year in high school, about 1948-9, and I was a “lab assistant” in the “elec and mag” lab.
Which meant basically that I came in before class or labs, an put out the equipment that we were going to be using to do whatever experiments we were assigned, or what the teacher wanted for class. That day, we had been introduced to the Whimshurst machine.
After class, I had to return the machine to the teacher’s office, where it belonged on top od some high book cases, alongside: believe it or not, a mercury filled barometer.
Naturally, I fooled around with things, including the barometer, and ended up spilling a lot of the mercury on the wooden floor. Swept it up with a dustpan and brush, and got most of it back in the tube.
While I was in there I was looking at his tech journals, which he received regularly. One of those was the Brown-Boveri Technical Journal. I think, that’s some giant European Electric company (maybe Swedish ??).
And I read an article in there about using SF6 to encapsulate > 100 KV transformers, and in testing those giant long ceramic insulators, on transmission lines.
I have no earthly idea why I remembered that event.
Well actually, I spilled the mercury while standing on a stool playing with the Whimshirst machine, and I accidently touched one of the balls, while furiously cranking on the handle.
The resulting blast knocked me A over kite off the stool, and a flailing arm knocked over the mercury barometer. I landed on my back on the floor, with all that glass in my lap, so it never broke; but the mercury all went flying.
Yes I did catch hell from the teacher, and he made me clean up all that mercury in alcohol or something, to get the dust and crud out of it. I even kept my lab job.
So many of these explanations are vitiated by considerations that water might boil at low pressures etc. Explanations that a syphon are like a chain and rely on cohesive forces are rubbish. All this is ‘noise’ that gets in the way of thinking about what is going on. Why so?
Consider a thought experiment: we have a syphon with both ends immersed in their respective reservoirs filled with a hypothetical liquid that has the same density as water but is incompressible and has zero tensile/cohesive strength and infinite boiling point/zero vapour pressure. No longer can the argument be used that liquid gets ‘pulled’ uphill since there is zero tensile strength. And remember, vacuums don’t suck, they don’t do anything.
So, will this syphon work? Yes, like an ordinary water syphon if there is atmospheric pressure. No, if there is no atmospheric pressure. How does the liquid get up the tube from the upper reservoir? By air pressure. Gravity is simply not going to make the liquid ascend from the upper reservoir, neither can it be ‘pulled up’ by any putative vacuum, nor by liquid cohesion (which in our thought experiment is zero).
Extend the thought experiment further: would the syphon work if the reservoirs were filled with an ideal gas that was very dense (compared to air) rather than a liquid? Here there is no possibility of cohesion/tensile strength. It will still work as a syphon if provided there is atmospheric pressure above the reservoirs. What makes the gas ascend (ignore diffusion) from the higher reservoir? The atmospheric pressure.
Simply stated you can start a siphon with properly differentiated air pressure but it maintains flow due to gravity. You shouldn’t need a paper to establish that.
Water will flow along a pipe from a reservoir with an elevated phreatic surface to a reservoir (or outfall) where the phreatic surface is lower. If the full length pipe alignment is below the hydraulic grade line* there is no siphon involved – it’s ‘just a pipe’! And (if the atmospheric pressure is the same at both upstream and downstream ends) water will flow down the pipe by gravity – as ball-bearings would (if friction was negligible).
But a siphon is not ‘just a pipe’. A siphon is where the pipe rises above the hydraulic grade line. If you were to allow a ball-bearing to roll along the pipe it would not get over a siphon. Gravity alone may explain normal pipe flow (also open-channel flow) but it does not explain a siphon.
At sea-level a siphon will not flow if it rises more than 10m above the hydraulic grade line. Both gravity and atmospheric pressure are needed. But the role of atmospheric pressure is the defining difference between a siphon and other gravity-driven liquid conduits.
Also, water has no cohesion or tensile strength. (It does have viscosity which is related to velocity but is not relevant to a basic siphon.) So the chain over a pulley analogy is flawed.
* Hydraulic grade line is a line running downhill from the upstream phreatic surface to the downstream phreatic surface level. It must always run downhill – the only exception is pump locations.
Overlooked constituative properties.
Wobble,
“If the pressure is the same at both ends of the hose, then it can’t be atmospheric pressure. My father taught me that it was gravity when I was a kid.”
What did your father tell you about siphons that are maintained when the water on both sides of the tube are at the same height. The siphon will be maintained in this situation even though there is no difference in height. In fact this is the whole mystery of the siphon. It seems to defy gravity. It works on the same principle that keeps water in an inverted bottle in your automatic pet waterer. It’s the atmospheric pressure that keeps it up.
The definitive experiment to show that Dr. Hughes’s hydrogen bond or “molecular chain” explanation is false is the siphon fountain. A siphon fountain is designed to maintain an air gap at the top of the siphon. There is no continous chain of water molecules between the two water reservours in that case. Heck you can prove this with a simple tube. Start with two vessels of water at the same height and a six foot long flexible tube raised into a three foot high inverted U shape. Have the tube filled with water except have an air gap at the top. Now lower one side and raise the other till the water starts to siphon. It will siphon even though there was not a continous chain of water from one vessel to the other.
REPLY: Sorry, no. Actually the definitive experiment to show that siphoning works by gravity alone has been done, see this siphon made with bead chain. Gravity and inertia is all it needs, and it wouldn’t work if the beads weren’t connected, just like water molecules are with bonds. – Anthony
Jorge, thanks, I understand that the boiling begins immediately. I was getting at the notion that not all of the water would instantaneously turn to vapor, and enough liquid water would remain in the bucket to move through the siphon tube (if you can hypothetically keep the bubbles out) to prove that atmospheric pressure is not the pressure pushing liquid water up into the tube.
Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose. That’s what I thought I was arguing about.
The siphon fountain also shows that the chap in the ionic fluid video is wrong. A water siphon is not maintained by cohesion, nor by gravity as he summarized at the end. I also think he is wrong in the case of the ionic fluid. Mere cohesion to itself would not keep it from flowing out of the tube in a vacuum. There would also have to be a strong cohesive force with the wall of the tube, or some other factor he did not control for like high viscosity. It might be that his liquid is more like a gel. I haven’t worked with the stuff so I can’t tell you. I’d like to see him repeat the experiment with a valve at the top of the tube which he could open, or with a tube of a substance to which the ionic fluid is phobic.
If it doesn’t involve the evacuation of a gas from a tube, the replacement of the gas with a liquid, the “raising” of the liquid in the tube to a height greater than the higher reservoir, and then the flowing of the liquid of the liquid into the lower reservoir, then it is not a siphon, by definition. If you disagree, then you and I have a disagreement about the definition of the word “siphon,” not a disagreement about physics.
Why do we assume that the video at the top of this article and the one referred to by Mindbuilder (water “raised” to a height of 24M) are examples of siphons, just because the creators of those videos say so? In both videos the tubes are pre-filled with a liquid *before* the tubes are raised above the height of the higher reservoir. That is not a siphon, as we commonly refer to one in everyday life. Put a mouse in the vacuum chamber (with an O2 tank, of course) shown in the video and have the mouse try to “suck” the liquid from the higher reservoir to the lower reservoir through the *evacuated* tube that rises above the higher reservoir then down into the lower reservoir. Will the mouse be able to siphon (as classically defined) the liquid to the lower reservoir?
I prefer Merriam-Webster’s definition of a siphon: “a tube bent to form two legs of unequal length by which a liquid can be transferred to a lower level over an intermediate elevation by the pressure of the atmosphere in forcing the liquid up the shorter branch of the tube immersed in it while the excess of weight of the liquid in the longer branch when once filled causes a continuous flow.”
If we accept the new definition in the Oxford English Dictionary, bedazzled upon its editors by a respected PhD, then the water system at my home could be termed a “siphon.” I do not accept such a *change* in definition.
This post sucks.
😉
Mickey Reno, “Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose.”
We just don’t have enough information. The guy in the video did not understand the proper explanation for how a siphon works in the first place, so he did not eliminate all the possible differences about an ionic fluid that would not make it a proper model of a siphon. Capillary wicking can be used to create something similar to a siphon which is not actually a siphon. How do we know this ionic fluid doesn’t strongly attract to the sides of the tube he used.
Try wetting a dishtowel in a almost full bucket of water, then hang it over the side of the bucket so that it is in the water but also lower than the water level. It will wick all the water out of the bucket down to the whichever end of the rag is high. The water drips off the end of the rag. There is no tube involved, and it is not a siphon although it acts like one.
Seems like the ionic fluid does in the experiment may not be acting like a siphon or a wick. We can’t tell because he had no ability to try other things. Like what if he were to open a hole in the top of the tube? A small diameter hole might not cause the siphon to break as it would with a normal siphon. A large diameter hole might cause the attraction to the side of the vessel to collapse. I’d also like to see an ionic liquid in a inverted jar under vacuum.
I’d like to see this experiment instead: Have a vacuum chamber. Inside a deep pool of ionic liquid and above that an inverted jar. The inverted jar is lowered till it is half submerged. Since there is no gas inside it should fill half way up with liquid so that it is level with the outside liquid. Now raise it. The liquid should not be pulled upwards. Now lower the jar all the way down into the liquid until it is completely full. Since the liquid is not touching the inside bottom of the jar there are one of two options when you pull it out. a) It has no attraction to the jar surface and just falls out. b) it is strongly attracted to the jar and as you pull it out the jar remains full. Of course even in case b) there will be a limit to how high you can pull (and how long the jar is) before the weight of the liquid overcomes the attraction of those molecules clinging to bottom of the jar from inside.
Mindbuilder says:
April 25, 2014 at 2:03 pm
Umm, can you explain that a bit more clearly? Exactly what do you mean by negative pressure? I can understand relative negative pressure up to -1 atm, but something several times that, e.g. -5 atm seems as problematic as reaching Kelvin temperatures that are several times ambient, e.g. -1000K. That would either be really cold or negatively hot.
Back when I was in high school, our biology teacher talked about root pressure and transpiration pull for getting water high into trees. I also considered that redwoods grow in foggy environments a rather important point….
Anthony, Did you even bother to look up what a siphon fountain is. Even with an air gap in the bottle the water will remain very high in the tubes on both sides. As I said you can have a plain old straight tube with a air gap at the top and the water will remain very high in the tube. As high as you want it to. Try doing that with your bead chain. Cut it at the tapex where it hangs over the side of the cup. It will NOT remain suspended vertically. Both sides will fall down.
REPLY: Yes I did, and that’s the point, if you cut either the water connectivity or the bead chain it won’t work. Connectivity, be it water or chain has everything to do with it (maintaining momentum), air pressure has zero role.
If you break connectivity of the water in the hose at the top, no siphon.
– Anthony
The bead chain rises above the edge only because of the fact it has a minimum bend radius. I played with those things as a kid. If you think this is at all analogous then how come when I spill water over the side of a glass it doesn’t fly up in the air like this draining the entire cup. This has just about zero in common with a siphon.
With your bead chain example. Did you even bother to look up what a siphon fountain is. Even with an air gap in the bottle the water will remain very high in the tubes on both sides. As I said you can have a plain old straight tube with a air gap at the top and the water will remain very high in the tube. As high as you want it to. Try doing that with your bead chain. Cut it at the tapex where it hangs over the side of the cup. It will NOT remain suspended vertically. Both sides will fall down.
You can in fact start a siphon where there is no water whatsoever in the tube which is above the water level. Start with two deep 5 gallons buckets of water on a table next to each other. Place a long tube from one to the other, and make it rise say four inches above both. Have a t joint at the top with a valve so you can let out all the air that is below the water surface on both sides. Now lower one of the buckets over the side of the table to the floor. The water will start flowing up out of the top bucket even though there was no connection between the water in the buckets.
REPLY: Sure, but again, it is about connectivity of the fluid. The siphon won’t start unless the water is connected. Your valve trick just connects the water so it is a single column, and when it is a single column, then it can act as a siphon. – Anthony
Hey, it looks like the commenters didn’t read the Wiki page all the way to the bottom where it says:
@ Ric Werme
“settled science” then. LOL!
“Your valve trick just connects the water so it is a single column”
No it doesn’t. The tube is four inches above the bucket and that is all air. Eight full inches of air in the tube with no connection. You then close the valve and lower the one bucket, leaving the apex of the tube four inches above the top bucket and the siphon will still start. I’ve worked with siphons quite a bit. I know way more about them then any of these scientists. This nature article needed some serious peer review by someone who understood the subject.
REPLY: OK, thank you for the clarification. Your previous explanation sans diagram wasn’t clear to me. In that case air is connecting the two water columns in the tube, and because the air is resisting stretching/decompression it acts to connect the two as a fluid. Atmospheric air pressure, as a force of action, has absolutely nothing to do with that. It’s all still gravity, inertia and connectivity that is moving the fluid. Any fluid between the two water columns would act the same.
You can believe whatever you want, but I’m perfectly fine with the explanation given in the article. – Anthony
I just build the apparatus I described except instead of having a valve I just put the air bubble in manually. If I start with both buckets on my deck and raise one up to the railing the water rises in the tube of the bucket I’m lifting in what looks like a contrary to gravity fashion. The siphon starts with water pouring over the apex in this “waterfall effect” that he describes. There is nothing mysterious about it. It happens at 1 atmosphere if there is already air in the tube. Plus it should have been a gigantic hint that the water wasn’t being pulled by some chain effect. It’s dark out and I have plans to go to a gardening show tomorrow morning, but I’ll post a video of it if I get a chance.
BTW, my position is that Oxford English Dictionary is wrong, that Dr Stephen Hughes is wrong, and the summary by Brady Haran in his video is wrong.
I remember an experiment I did in fluid dynamics class. It was a syphon and we measured the flow rate as the long part of the outflow arm was increased in length. Conclusion; the flow output increased as the length of the outflow pipe increased up to a certain length then did not increase any further even as the length of the output arm increased and I think even decreased due to the friction increase in the extra pipe. would a syphon work in zero gravity of the space station.
@ Paul
Question
Kurt Larson asked:
Would a siphon work in space?
Answer
No it wouldn’t, because you need gravity to make that siphon work. If you imagine the fluid at different points along the length of that siphon, at some points the siphon will be going uphill, and at other points it will be going downhill, and there’ll be gravitational force on the fluid inside your siphon. And where the siphon is going uphill, the force is pulling the fluid backwards along the pipe. When it’s going downhill, that force is pulling the fluid forwards along the pipe and what you need is more bits of length of your siphon going downhill with those forward forces than you have going uphill. And that means that because of the water pressure inside that siphon, the places where the force is going downhill is dragging fluid along the whole siphon from the beginning to the end. On the International space station, you have no gravitational force so there’d be nothing to drive the fluid along the siphon.
http://www.thenakedscientists.com/HTML/questions/question/3177/
@ Rick Werme – Pressure is a push, negative pressure is a pull. The ability to resist negative pressure is like tensile strength. The purpose of a rope is to resist negative pressure. Water can resist negative pressures about as well as a rope made of rubber. The ability of liquids to resist negative pressures is not quite like tensile strength though because liquids need tube walls to adhere to or else they will not exhibit any tensile strength. A rope needs no tubes.
@ Anthony – An air bubble trapped at the top of a siphon with no water from each side touching, does not “connect” the two sides of the siphon. Said air is pressurized(though less pressure than atmospheric) and wants to expand and therefore is pushing DOWN on the water on BOTH sides. The air at top cannot pull the water up. Gasses cannot pull. The water goes up for the same reason the liquid goes up in a drinking straw – because atmospheric pressure pushes it up when the pressure at top is lowered (by gravity in the siphon, and by your lungs in a drinking straw).
REPLY: Your description and my visualization of the description don’t match. Somebody should draw a diagram. But it doesn’t matter, a siphon can’t work without gravity. A drinking straw is NOT the same setup, and is not self sustaining like a siphon. – Anthony
Take a look at the diagram and description of figure 4 on the Wikipedia siphon page. And yes, you’re right that gravity is the energy source that lowers the pressure at top to enable atmospheric pressure to take over. It’s gravity AND atmospheric pressure that makes a typical siphon work.
Aren’t there materials that allow air to pass through but not water. If so, then another experiment could be done that would minimize the influence of atmospheric pressure by making a siphon tube out of such a material. If air can pass freely through the walls of the tube, a vacuum would not be able to form anywhere in the tube. It would then be interesting to see if the siphon still worked using gravity and molecular cohesion alone. My suspicion is that it might work for small distances but not when the bend in the tube is several feet higher than the source.
The idea that it is gravity rather than atmospheric pressure that makes a siphon work seems a bit of a red herring to me. Isn’t it gravity that creates atmospheric pressure in the first place, at least in normal earthly circumstances? Without gravity we would have no atmosphere. So atmospheric pressure in an open system is just another manifestation of the force of gravity.
It seems to me that without gravity you have no atmospheric pressure. Sure you could have a pump that creates artificial pressure, but that isn’t a real world condition, so it seems to me that the people who say you have to have gravity and atmospheric pressure are correct.
Gads you would think this would be settled science.
Mickey Reno says: April 25, 2014 at 6:14 pm Mike M. accuses the video vacuum experiment of sophistry. How so? Are you saying that what happens in the video is NOT a siphon? The video demonstration works exactly like a siphon in an atmosphere, but without any atmospheric pressure on either side of the hose. That’s what I thought I was arguing about.
*********************
The fluid in the video is transferring like a chain over a pulley and that is not how a siphon works. Ordinary water in an ordinary garden hose does NOT have the cohesive property to be “pulled” through the hose. When you suck on a soda straw it is NOT being pulled up the straw it is being PUSHED up the soda straw by the pressure differential you created by lowering the pressure at the top.
Now take two soda straws, one twice as long as the other, each with the bottom end slightly immersed in water and the tops of the straws level. Suck the water to the top of each one and seal it. Atmospheric pressure is pushing the water up both of them. The pressure at the top of either straw must be less than atmospheric but, additionally, the pressure at the top of the longer straw will be LESS than the pressure at the top of the shorter straw by virtue of the difference of their respective water column heights.
Because the pressure is lower at the top of the longer one than the top of the shorter one, if you then connect their tops together the water will flow from the higher pressure to the lower pressure creating a siphon.
Jim says:
April 24, 2014 at 10:50 pm
“Diagrams Showing what happens here would be useful.”
Agreed. The paucity of pictures made the article difficult for me to follow. However it is intuitively obvious that Dr Hughs’ claim is correct for well-behaved liquids.
The hypobaric stuff appears to be overkill. On the other hand, a good way to put a false scientific meme to rest is with an experiment.
Feynman notwithstanding . . . Better still if the experiment involves expensive bells and whistles. /sarc
The paper states “At 41,000 feet, the siphon broke into two columns of water and, when returned to 40,000 feet, it reconnected as if nothing had happened.” In fact the siphon had completely stopped. Elsewhere in the paper they note that they had to switch-off the return pump or it would have run dry and burned-out. This indicates that atmospheric pressure IS requisite for the operation of a siphon.
They also state, “flow remained more or less constant during ascension indicating that siphon flow is independent of ambient barometric pressure,” (I love the words ‘more-or-less’!) This key statement should have been substantiated with a graph of Flow vs Ambient Barometric Pressure. But this was not done. Why not? The only graph in the paper is Flow versus Time. There is no graph of Pressure vs Time.
In other gems they state “In the second run, the ascent was slowed to allow outgassed CO2 to dissipate.” What the …? Were they using soda water? Did they think to take samples of the outgassed susbstance?
This paper is a travesty! And the journal ‘Nature’ published it – amazing! They have already got a poor reputation and this won’t help. It will be interesting to see what response it gets when serious physicists read it.
Why all this talk about connectivity? I reiterate that getting the liquid to the crest of the tube is the key requirement in starting the flow from the higher container to the lower. About the only connectivity that doesn’t work is vacuum, and then only if you ignore atypical effects like capillary tubes and ionic fluid.
Thanks, A. This is a superb post.
I think it is gravity that makes a siphon work, not atmospheric pressure that is somewhat lower at the high reservoir.
J Calvert N(UK) says: April 26, 2014 at 6:00 am “This paper is a travesty! And the journal ‘Nature’ published it – amazing! They have already got a poor reputation and this won’t help. It will be interesting to see what response it gets when serious physicists read it.”
Reply: Completely agree! As I wrote above, I find it frightening.
Ric Werme says: April 26, 2014 at 6:45 am “Why all this talk about connectivity? I reiterate that getting the liquid to the crest of the tube is the key requirement in starting the flow from the higher container to the lower. ”
Reply: Yes, precisely so. Perhaps another succinct way to challenge those who refuse to accept that fact is for them to ruminate two questions:
(1) Why does water go up a soda straw when you drink from it?
(2) Why would the explanation for #1 magically change to “connectivity” when we exchange another but longer column of water for a mouth/lips?
Greg says April 25, 2014 at 6:05 am
“So you’re saying you don’t need a ground/neutral for your house wiring to work…. interesting take.”
No I’m saying you need more that one wire.
Ah, strictly a “DC” (Direct Current) man I see. Tesla would disagree with you as would I …
.
re: Tom Trevor says April 25, 2014 at 9:57 pm
It seems to me that without gravity you have no atmospheric pressure. …
Minimum requirements: Two different fluids (or a gas and a fluid) of sufficiently differing densities in a given, fixed gravitational field such that at least one of the fluids may be ‘corralled’ and encouraged to occupy specific volumes defined by, say, a pair of (upper and lower) buckets …
.
Anthony: “A drinking straw is NOT the same setup, and is not self sustaining like a siphon. ”
A drinking straw is indeed exactly one half of the setup Anthony. There is pressure on each end of the straw. The pressure at the top is lower than the pressure at the bottom. To get the water to the top the difference in pressure must exceed the pressure exerted by the water at the bottom. The higher the column, the greater will be the pressure at the bottom, the more differential will be needed to drive it to the top. If you don’t believe that try sucking water up ~10 feet vertically.
REPLY: Yes 1/2 but not the same. Take the typical “siphoning gas out of a gas tank” problem. Suction is applied at one end of the hose to get the liquid “over the hump”. Once that is done, suction is terminated, and flow self-sustains due to gravity. As pointed out upthread, without gravity, such as in the space station, a siphon won’t work even though there is air pressure. – Anthony
No Anthony, the suction is still there! The column of descending fluid is providing the SAME suction that you provided for initiation.
Put a shut-off valve at the discharge end and shut off the flow. What is holding up the water on the other side, (shorter column)?
Anthony: “If you break connectivity of the water in the hose at the top, no siphon.”
Not true. You CAN have a bubble of air pass through a siphon. I do that when I change my waterbed water every couple years. It’s a siphon to get the water over the window sill which is higher than the top of the waterbed. There is usually a quantity of air in the waterbed and I push it over to the hose inlet and let it gulp air intermittently but quite a bit at a time because the hose is draped out the window and down the ground two stories below thus generating a lot of suction.
Those bubbles break your “connectivity” altogether thus disproving you.
This has been fun. Nothing wrong with reexamining what you think you know.
I used to keep a small garden. Just 4 to 6 plants. I didn’t want to drag the hose out to water it everyday so I took a trash can and drill a hole about an inch or two below the top. The hole was the diameter of some aquarium airline tubing I bought. I ran a line from the bottom of inside of the trash can to the ground a gang valve I bought. Then I ran a tube from the valve to each plant.
I’d fill the trash can above the hole and a siphon would start. Then all I had to do each day is adjust the valves so I had a slow drip and check the water level in the can periodically.
Mike M says: April 26, 2014 at 9:01 am “Put a shut-off valve at the discharge end and shut off the flow. What is holding up the water on the other side, (shorter column)?
Mike M says: April 26, 2014 at 9:18 am “You CAN have a bubble of air pass through a siphon.”
*********************
Anthony – Putting those two points together have an experiment for you. Construct my ‘extreme’ waterbed siphon, the short inlet side column is only 6″ and the discharge side column is 20 feet, (5/8 garden hose). Have shut-off valve on the discharge end below in the open position. Get the siphon going then shut off the valve.
Now remove the inlet from the water and allow all the water to pour out (allowing a bubble to rise to the crest). The short 6″ column now has only air, no water in it at all.
Now re-immerse it into the water and open the valve – the siphoning will resume.
“REPLY: Sorry, no. Actually the definitive experiment to show that siphoning works by gravity alone has been done, see this siphon made with bead chain. Gravity and inertia is all it needs, and it wouldn’t work if the beads weren’t connected, just like water molecules are with bonds. – Anthony”
Analogies can be tricky, especially if the analogy has little in common with the concept in question. If the bead chain were a good analogy to a water siphon, you wouldn’t need the water siphon’s tube, would you? If you must use a bead analogy, make the beads very small individual ball bearings, and you’ll be closer to the actual case of your typical water siphon. Oops, now it doesn’t work…
I gave three links earlier (April 25, 2014 at 1:24 pm). As Mindbuilder has pointed out, the Wiki article on siphons is excellent, with several examples showing why atmospheric pressure is necessary (but not sufficient) for the typical water siphon. If still not convinced, read the second link, Richert & Binder’s reply to Dr. Hughes, complete with photos. If still not convinced, watch the CO2 siphon video. See? No chain required.
Misconceptions aside, this is an excellent article and discussion, which illustrate the misconceptions that bedevil even real life honest-to-goodness scientists, let alone us poor benighted laymen. I’m reminded of the commenters who couldn’t wrap their minds around concepts like “the steel greenhouse”
http://wattsupwiththat.com/2009/11/17/the-steel-greenhouse/
or Dr. Spencer’s “Yes, Virginia” article
http://www.drroyspencer.com/2010/07/yes-virginia-cooler-objects-can-make-warmer-objects-even-warmer-still/
or Anthony’s videotaped “backradiation” experiment
http://wattsupwiththat.com/2013/05/27/new-wuwt-tv-segment-slaying-the-slayers-with-watts/
(Remember some of the comments on that thread, Anthony?) Sometimes I despair for the future of the human race.
But it’s kind of funny, too. 🙂
PS OMG, how could I forget the tar pits thread?
http://wattsupwiththat.com/2014/04/10/the-la-brea-tars-pits-gets-themselves-in-a-sticky-wicket-over-climate-change-and-adaptation/
REPLY: Thanks, if you used highly magnetized ball bearings, to simulate the molecular bonds of water, instead of bead chain, it might work just the same. – Anthony
Anthony,
Interestingly, that statement brings us full circle – back to the dictionary from whence this whole hubbub began.
Your statement is incorrect. The dictionary demonstrates that quite nicely. It does that not by telling us how a siphon works. Dictionaries are not the proper reference for that question. Rather, the dictionary does its proper job by telling us what a siphon is. If one uses ellipsis to pull the OED’s nose out of the realm of the encyclopedia that it is not, the dictionary that it is defines a siphon as:
Fundamentally, that is what a siphon is. Thus, it is not correct to say that a siphon will not work in zero G. Rather, a siphon cannot exist in zero G, because a “siphon” is undefined in zero G. Irrespective of its disputed role as the force that operates a siphon, it is without question that it is gravity that provides the “up” and the “over” that fundamentally define what a siphon is. No gravity, no up. No up, no over. Without gravity, there is no siphon, “working” or not. There is just a tube with a bend in it.
🙂
Ric Werme says: April 26, 2014 at 6:45 am “Why all this talk about connectivity? I reiterate that getting the liquid to the crest of the tube is the key requirement in starting the flow from the higher container to the lower. ”
Reply: I have to slightly walk back my earlier reply, (my fault for not thoroughly digesting your comment). That is not quite correct, the liquid has to be beyond that point, brought up and over the crest then back down to be below the level of the of the feed water in order for the siphon to start functioning.
Well If one defines a “siphon” as a device that continuously (ally) transports a material over a rim at a higher altitude without artificial assistance, then I believe the chain bead demonstration, establishes, that “pure” siphon action requires only gravitational energy to run.
In that sense, I believe that OED and the good Dr. are correct in deleting the atmospheric pressure as the mechanism.
Obviously, the bead chain experiment will work perfectly in a complete high vacuum, but it won’t work, absent gravity.
But when applied to the more common case of a liquid, it would seem to be necessary for most ordinary liquids to maintain positive pressure (>0) in the entire upward side of the loop, or at least a pressure greater than the vapor pressure at the prevailing Temperature. But I’m not totally sold on that restriction yet. The upward momentum thing needs consideration.
From an experimental point of view, I have siphoned liquids, at least water, and gasoline, by totally immersing the tubing, in the liquid, so it fills, and excludes all bubbles of air. Then with my thumb blocking the outlet end of the tube, that end is lifted over the rim, and brought below the liquid level, before releasing. The other end of course must remain submerged. That has always worked for me. No I don’t steal gas, but if you have a boat with a 75 gallon fuel tank, you often use a siphon to remove gas for winter storage for example. In my case, said gas, went into my car to be used up.
Now in that case, I can’t get my hand in the gas tank, to use the above process. Well of course, I suck on it, but you clamp the tube end, before the gas gets to your mouth; so use a transparent plastic tubing.
I have to think some more about how the momentum thing allows for an increased loop elevation.
Now just for kicks, having dealt with bead chains and gasoline.
I wonder if you can siphon empty a vat full of sulphur hexafluoride gas, by just swilling some over the edge (no tube).
PS. Don’t have the boat anymore; got stolen out of my yard.
G
I’m going to invest in some bead chain, so my kiddiegarden teacher can demonstrate pure siphon physics to her munchkins; all of whom, were not responsible for being here.
“””””…..Mike M says:
April 26, 2014 at 9:49 am
Mike M says: April 26, 2014 at 9:01 am “Put a shut-off valve at the discharge end and shut off the flow. What is holding up the water on the other side, (shorter column)?
Mike M says: April 26, 2014 at 9:18 am “You CAN have a bubble of air pass through a siphon.”…..”””””
So Mike; you let in an air bubble that completely fills the up side of the siphon AT ATMOSPHERIC PRESSURE.
Now you immerse, and open the valve.
Water starts flowing, and air bubble too, which starts expanding, and bubble pressure drops as volume expands. Air has low mass, so low momentum, so very little turbo boost from falling water column, coupled through air bubble , to get the upwater going again.
Pretty much falls to ambient air pressure to send water up the upside, so it would seem that upside tube length must be less than the 10 meter limit, and down tube would need to be much longer, to allow for air bubble expansion. For a 9 meter uptube, you’d need of the order of 90 meters of downtube to refill the uptube with water.
This problem is getting totally out of hand; somebody better call a halt, before we all go nuts.
“REPLY: Thanks, if you used highly magnetized ball bearings, to simulate the molecular bonds of water, instead of bead chain, it might work just the same. – Anthony”
It might indeed, but again, if water acted like that you wouldn’t need the tube. You can even build 3D structures with the magnetic balls; try that with liquid water.
http://www.tealco.net/bucky_balls_buckyballs.html
There probably is more going on here than a “siphon” but take an empty glass and place it next to the sink. Take a strip of paper towel and place it so one end is at the bottom of the glass and the other end is in the sink but lower than the bottom of the glass. Fill the glass with water and be sure the towel is also wet all the way to the sink end. The glass will empty.
Reply to Brian Macker
BeauGatun says:
April 25, 2014 at 3:00 pm
The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber. Once the flow is established, it will keep flowing. Air pressure will actually tend to counter this as the lower chamber has a higher surface pressure. As a simple Engineer, am I missing something?
Brian Macker says: April 25, 2014 at 3:35 pm
BeauGatun,
“The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber.”
No, the pressure at the surface of the higher chamber is lower than the pressure at the surface of the lower chamber. You don’t even need any chambers for a siphon to work as long as you have a thin enough tube, or a movable plug in the uphill end of the tube. All the water will flow out the lower end. The pressure differential is in the tube, not the chambers.
Brian Macker from BeauGatun
Yes, you are quite correct air pressure has nothing to do with it. I was being a bit simplistic when I said “The pressure per unit area in the higher chamber is greater due to the greater potential energy of the water than that of the lower chamber” I was not referring to air pressure or a closed system of chambers. What I meant and should have made clear was the potential liquid pressure once the siphon has been established, the difference being between the ends of the tube. The siphon has to be established, it is not spontaneous, which seems to be the thought of some posters. Although I am sure, someone has an example of it. Think of the fun you have establishing the flow of gas out of the car for the lawn mower, that bit uses differential air pressure but only to fill the tube, there are many ways to establish a siphon. The water is going from a higher to a lower potential, that is what all matter will do if it is given the chance, all thanks to four-dimensional space-time aka gravity.
george e. smith says: April 26, 2014 at 12:35 pm:
** So Mike; you let in an air bubble that completely fills the up side of the siphon AT ATMOSPHERIC PRESSURE. Yes
** Now you immerse, and open the valve.Yes
** Water starts flowing, and air bubble too, which starts expanding, and bubble pressure drops as volume expands. Yes, yes, yes & yes. The bubble expands as it’s pressure drops. As it’s pressure drops the differential pressure between it and atmospheric at the inlet increases so water immediately begins going up the inlet the moment the bubble begins expanding.
**Air has low mass, so low momentum, so very little turbo boost from falling water column, coupled through air bubble , to get the upwater going again. Yes. Momentum has some influence on how quickly everything responds but has no affect on the steady state flow once it is established.
** Pretty much falls to ambient air pressure to send water up the upside, Not exactly certain what you are stating here? It started at ambient so now it’s well below ambient as you pointed out above.
** ..so it would seem that upside tube length must be less than the 10 meter limit, Yes, 6 inches is less than 10 meters. 6″ was chosen for demonstrable proof; no need to test limits.
** .. and down tube would need to be much longer, to allow for air bubble expansion. Yes EXACTLY SO! The bigger the bubble, the greater the volume that bubble will have to expand to to reach a low enough pressure for the water to get up to the crest and back down to below the inlet elevation – that’s WHY I picked a very short column going up and a very long one going down – to be certain it would work.
** For a 9 meter uptube, you’d need of the order of 90 meters of downtube to refill the uptube with water. Yes and no. Again, such are longer than needed for demonstration purposes. My description was solely an experiment to disprove the contention that a siphon cannot work with a bubble going through. That’s all it was meant for – not to examine limits.
Hmmmn.
With all of this erudite and deep “thought experiment” discussion ..
1. Has any one actually done the experiment with a pressure gage mounted at the high point between the two ends of the siphon? What is the measured change in rise height pressure w/r height of rise?
2. Across different rise-heights (top of suction fluid level) to (height of rise point), what is the plot of internal pressure to external pressure (gage d/p) at the top of the siphon tube?
3. For different fluids of different vapor pressures at the (room temperature) time of the experiment, what is the d/p with respect to different vapor pressures?
4. For a given height of the siphon rise point, and a single fluid type (ie, a constant vapor pressure) ,what is the relationship between rise height and existing bubble size? (At what point will a given size bubble break the siphon?)
5. What is the relationship between bubble size and tube diameter? Does it matter?
6. For a given rise height and fluid type and fluid vapor pressure, does tube diameter affect rise height?
RACookPE1978 says: April 26, 2014 at 2:11 pm ” 5. What is the relationship between bubble size and tube diameter? Does it matter? ”
No it does not matter at all because having ANY bubble at all disproves the contention that some magical cohesive property in water is dragging the water up and over the crest like a chain going over a pulley. Atmospheric or some other amount of pressure is required at the inlet in order to push the fluid up the ascending tube for a true “siphon” to work. Yes there are cohesive bonds but so trivially insignificant in strength that they are no factor in any discussion of ordinary water siphoned with a garden hose as proven when a bubble goes through.
I have enjoyed all the above. Many of the contributions have failed to clearly differentiate the “powering force” of the siphon from the physical effects taking place when it is running under different conditions. That causes a lot of misunderstanding and digression.
Mike M is correct when he in effect asks for a definition of a siphon to exclude chains-over-pulleys. He is right to do so thus some putative ionic siphons are disqualified.
it was not sufficiently clarified that multiple siphons in series will not work for water if the total “up” exceeds 10 metres which points to the need for air pressure to be present, but the air pressure is not the source of “work” as many have stated.
The comment about the pipeline being above the hydraulic gradient is incorrect, full stop. It applies if the pipeline goes above the source point *only*. Pipelines above the HG work but at a lower flow rate than the starting and endpoints alone indicate.
Mike M, I am inclined to agree with your narrower definition of a siphon as a device that requires at least some atmospheric pressure to function. The others are chain-over-pulley devices some of which look like siphons. Appearances can be deceiving.
Take a container of water. Fill a tube full of water. Place fingers over ends of tube. Place an end of the tube at bottom of the water and other end below the water level in the container and open ends of tube. Water will flow at a speed proportional to pressure of water at the depth that the outlet is relative to surface of the water. That is where gravity comes in. A dense fluid in the tube does not allow separation of the molecules of the fluid in the tube, therefore the fluid need not consist of polar molecules. Therefore, the water siphon does not depend upon any kind of hydrogen or other kind of bonding and only on pressure due to gravity and the dense fluid.
I find it useful to distinguish between a barometer and a siphon.
A water barometer made out of a capped clear flexible pipe will read some 10 meters of atmospheric pressure.
Then lower the capped end below the barometer water container, remove the cap and the pipe is now a siphon that will drain the barometer water container.
The barometer works because of atmospheric pressure, but the siphon works because of gravity.
Of course gravity is the cause of atmospheric pressure. And of course, atmospheric pressure is lower at the barometer water container than at the siphon discharge. I can not imagine how the atmospheric pressure difference might not work against the siphon effect, much less being its cause.
“””””…..Mike M says:
April 26, 2014 at 2:01 pm
george e. smith says: April 26, 2014 at 12:35 pm:
** So Mike; you let in an air bubble that completely fills the up side of the siphon AT ATMOSPHERIC PRESSURE. Yes….”””””
Mike; I was not in any way challenging your assertion that it would work with an air bubble.
I was stepping through your process, for the benefit of other readers, who might not see it, to show them how to figure the maximum up tube height versus the size of the bubble. You stated (correctly), that a bubble could pass on through the siphon. But you didn’t specifically say it wouldn’t change size doing so. I merely used your description to make that point for other readers.
Obviously, a small bubble (short) serves to disconnect the water column, so it isn’t all connected, an that kills most of the momentum transfer, because of the much lower mass of the air bubble.
No, I had no quarrel with your post; but since you gave a blow by blowof your sequence, I just used that to step through and deal with the bubble expansion. Yes, if the up tube height approached the 10 meter atmospheric limit, it will take a very large bubble expansion to get it running again.
G
@Mike M the atmospheric pressure is at both ends of the tubes and annuls itself each side. It isn’t the force that maintains the flow during the siphoning. It is true that it is this pressure that will kickstart the motion when one creates a depression one the other side of the tube, but that action is only removing the counteracting pressure from the the other side of the tube.
The point is that changing one of the pressure on one side (augmenting the pressure on the start side is called a pump) or the other side (then it’s called a straw) will start the motion, but the motion itself is maintained by gravity. That’s why you need a difference of height of the 2 basins and the sipĥon always works from the high basin to the low basin.
TL;DR
The atmospheric pressure is the same on both side and is in equilibrum, therefore not intervening in the siphon.
PS: as for the bubbles, they don’t rupture the flow when under atmosphere is because of this pressure. Under vacuum, where the air pressure is absent in “pushing from both sides” there is no force to maintain the liquid cohesive, that’s why you need a ionic liquid where the internal cohesion of the liquid is strong enough to compensate for the absence of air pressure on both ends, exactly like a chain on a pulley. But the motion of the liquid in the tubes is still due to gravity.
george e. smith says: April 26, 2014 at 11:00 pm “No, I had no quarrel with your post; …”
Sorry, I never had any intention of implying that you did. I found your post to be an opportunity to further expound on details supporting my contention so I used it.
Patrick Schlüter says: April 27, 2014 at 3:24 am
“the atmospheric pressure is at both ends of the tubes and annuls itself each side.” Ultimately, yes it does but … so what?
“The atmospheric pressure is the same on both side and is in equilibrium, therefore not intervening in the siphon.” Just because the pressure is in equilibrium does not preclude it’s necessity for functionality at column “A”
If you were correct – that pressure is no longer needed to push the fluid up the “A” column once the siphon begins flowing – the crest could be hoisted up to any elevation, miles high, and it would continue to work – no it will NOT!
There is a maximum height beyond which the siphon will cease to function and that height is solely dependent on the pressure at the “A” side inlet. Lowering the pressure will decrease the maximum height, raising the pressure will increase the maximum height.
Work is being done on the water raising its elevation in column “A” by virtue of a pressure differential between the bottom and the top. The ONLY way to get it up there is to PUSH it up!
I can put a valve at the bottom of column B and throttle the flow to a trickle and it will not matter.
The water dropping in column “B” is the substitute for lips drawing the water up in column “A”.
“exactly like a chain on a pulley. ” – is not a “siphon” because it is in tension and therefore does not require pressure/force to push it up the “A” column. It WILL work in a vacuum and to any height, (within the tensile limits of the material.) The bubble scenario eliminates the “chain over the pulley” description because it is a missing link unable to pull the water up column “A”.
Not only will a siphon work with an single air bubble but will work with a continuous injection of air bubbles at the intake pipe that separate the water into uniform individual segments. All that is required to make the water move in either direction is that one side has an additively deeper column of water in the pipe. Furthermore, if the two sides are not in balance with regards to the the amount of air vs. water then the column will shift in the siphon even if the water level in the to reserviors are at the exact same height. I find it amazing that people who have obviously never experimented with siphons are making claims about how they work when they haven’t a clue.
the pumps that were installed in New Orleans at the beginning of the 20 th century are a combination of syphon and propeller pump and are still in use to clear the city of rain water.
Patrick Schlüter, A response of TL:DR is a quite a bit offensive considering he’s right and you are wrong. I read all of Mike M’s comments and he, so far, has gotten everything correct.
I can’t believe the simple concepts here that people are not getting like the fact that air bubbles are under pressure even when at the top of a siphon. Air if not confinded in some way will just keep expanding because the molecules will just drift apart via inertia. Mike has shown and understanding of these concepts.
Patrick Schlüter says:
Redundantly incorrect.
Unclear as to what “rupture the flow” means, but it does not seem to address the fact that a standard siphon can function just fine with air bubble(s) in the tube that completely disconnect the water column.
You are confusing concepts. Cohesion is not a function of external pressure. To the contrary, cohesion is exactly the opposite of external pressure. Cohesion is the internal attraction of the liquid molecules to each other.
What happens to an ionic liquid siphon under vacuum, if you introduce a small but tube-spanning bubble into the tube? It stops.
What happens to an ionic liquid siphon under atmospheric pressure, if you introduce a small but tube-spanning bubble into the tube? It continues to operate.
Of course it is.
It is gravity that causes the liquid in the descending leg of the siphon to fall, lowering the pressure within the tube to something less than atmospheric.
And it is gravity that creates the atmospheric pressure that pushes the liquid from the upper reservoir toward that lower pressure and up the tube.
This dispute is largely semantic. Ultimately (and by definition), it is the difference in gravitational potential between the upper and lower ends of the tube that drives a siphon. In a typical siphon (constructed in an atmosphere, from a non-ionic liquid with dissolved and entrained gas) the difference in gravitational potential can and most frequently does operate via the action of (also gravitationally induced) atmospheric pressure.
Brian Macker says:
April 27, 2014 at 8:47 am
“All that is required to make the water move in either direction is that one side has an additively deeper column of water in the pipe. ”
That is not true. Try placing the inlet tube to the bottom of the water tank. Then the siphon works with the mouth of the outlet tube just below the water’s surface level and the flow rate from the siphon is proportional to the pressure of water above the outlet of the tube. In this case the mass of water in the tube is much greater than the mass of water in the outlet tube since the outlet tube is shorter than the inlet tube which reaches to the bottom of the water in the water tank.
Adding gas bubbles may or may not stop the flow depending upon whether the gas volume is able to increase too much as pressure decreases on the bubbles as water flows down the outlet tube.
Mike M says:
Not true. The maximum height is also dependent on the properties of the liquid in the siphon.
JJ says: April 27, 2014 at 12:07 pm ” Not true. The maximum height is also dependent on the properties of the liquid in the siphon.”
Pedantic much? For any given fluid … “There is a maximum height….” Of course it’s going to be something less for mercury than for water, I think everyone understands that, thanks for sharing.
JJ says: April 27, 2014 at 11:45 am “This dispute is largely semantic. ”
That’s true and several others above have stated the same thing. The dictionary definition describes the apparatus and a general idea of it’s function without getting into exceptions involving cohesive fluids, air bubbles and chains over pulleys. Such issues are the domain of physics books not dictionaries IMO.
I think Dr. Hughes is a notoriety seeker who has deluded himself into believeing that he alone has discovered something “wrong” with the dictionary definition which appears to me to be thoroughly adequate as written:
“a tube bent to form two legs of unequal length by which a liquid can be transferred to a lower level over an intermediate elevation by the pressure of the atmosphere in forcing the liquid up the shorter branch of the tube immersed in it while the excess of weight of the liquid in the longer branch when once filled causes a continuous flow.”
There is one aspect of this debate that is not semantic – the “attraction” between water molecules, i.e. water possessing tensile strength in an amount capable of “pulling” molecules up the siphon. If water did possess tensile strength anywhere near this proportion, you would feel this when you removed your hand from a pool of water. If there is any molecular attraction there, it is so small that you can’t feel it, and certainly not substantial enough to have a material effect on the operation of a siphon.
Thank you Brian Macker and the several others who agree with you for helping me maintain hope that scientific intellect is not dead. I am deeply disturbed and saddened by what I have read here, particularly from people whom I had come to respect for their scientific acumen.
Here is a simple experiment I would like to see someone carry out, perhaps I will buy the stuff and do it myself but hopefully someone has already done it somewhere saving me the effort. Set up two containers of water, say around a gallon in size, and a rigid pipe in a “U” shape to form a siphon tube, something that will not collapse under atmospheric pressure if a vacuum were drawn in it, perhaps 1/2″ schedule 40 pvc pipe, with elbows to form a U shape, and a valve in the middle of the U to turn flow on/off. Set up water column barometers to read the pressure, one for each tank. At normal barometric pressure you will need a column around 35 feet high for this. Or, seal each container, with a tube connecting them going to a vacuum pump and draw a vacuum to reduce the height required for the water column barometers, if you can draw a vacuum down to around 2 ft of water it obviously makes the experiment easier to setup. The actual vacuum doesn’t matter, what matters is that it is the same in both containers. Now, here is the critical part of this experiment. Set up the upside down U tube so that it’s center is above the water level in the water column barometers, say perhaps a foot or two above. Fill the inverted-U tube with water with the valve shut off to prevent water flow, and the ends of the U are submerged below the water line in each respective container. If pulling a vacuum obviously this will all need to be sealed, or do the experiment at regular atmosphere pressure with an inverted U thats close to 40 ft high. Either way is a lot of work, but certainly do-able. Lot easier to see results if using a vacuum. This would be so easy to sketch but I don’t know how to do that here so bear with my word description.
Now here is the key part. The two water columns barometers should be same height showing pressure is the same on both sides, and the inverted U is filled with water with each end submerged in water of the respective container. One container is set perhaps a foot or so higher than the other, enough to produce a significant siphon flow. If the top of the inverted U is lower than the water column level in the barometers, and the valve is opened, water will flow. But, if the U is higher than the water columns, what will happen? I say, the amount that the U can be raised above the water columns is a measure of the “tensile strength” of the water, i.e. the strength of the “chain”. And I say, this level, if not zero, is close to it. I’m not sure what it would be, but I predict it wouldn’t be more than an inch or so, which could be attributed to surface tension and capillary effects and not molecule attraction.
So, anyone ever done anything like this, or game to try? Bets anyone?
Mike M says:
Not pedantic at all. You did not say “For any given fluid” and among the fluid properties that differ and matter to the maximum height of a siphon is the fluid’s molecular cohesion. You will note that bit is at the crux of the discussion.
This link from a comment above:
“A water siphon can work at over 10 meters, though its delicate and and the water needs to be degassed first. this one got to 24m http://www.youtube.com/watch?v=sz9eddGw8vg”
This is interesting experiment, similar to what I attempted to describe in my post above, however not near carefully controlled enough for my taste. I may try this myself when I get some time. It wouldn’t surprise me if the tubing above the level of barometric water column was crushed from atmospheric pressure which would distort the experiment, just like a straw crushes when sucked to hard on a thick milkshake, one atm pressure actually has a lot of crushing force, tubing needs to be strong, I would suspect even fairly rigid hdpe tubing would crush under full atm pressure. Crushing would decrease the diameter and make surface tension and capillary effects more pronounced. I will say it again, surface tension and capillary effects are real, but I am highly doubtful of the presence of any significant molecular attraction leading to a “chain over pulley” effect with water. I am very interested to experiment with this, if there are any significant “chain pull” effects with water siphon (more than maybe an inch) I will happily eat my words.
LRshultis,
“That is not true. Try placing the inlet tube to the bottom of the water tank.”
I said depth, not mass. When the tubes go deep below surface The numbers you need to add depend on depth to surface. Water depth in the tube below the respective surface are added as zero, and air as negative. A siphon can move water in one direction, either direction, before stopping, depending on how the air bubbles are situated in the tube. Their are aquarium filters that operate by injecting bubbles into tubes to move water up a siphon and over the edge that is higher. This requires that the outlet it the filter (where the water level is higher ) not be below the water line too far so the bubbles can escape.
When I wrote my previous posts I had not read Dr. Hughes papers themselves, only this WUWT blog post about it. Now I have read Dr. Hughes papers themselves, and all I can say is OMG. Here are links:
http://www.nature.com/srep/2014/140422/srep04741/full/srep04741.html
http://eprints.qut.edu.au/31098/25/31098.pdf
http://eprints.qut.edu.au/31098/
It appears the hyperbolic chamber experiment is pretty much the same as what I was trying to describe above, about pulling a vacuum. What he didn’t have in his experiment that I would use in mine, are the barometric water columns for reference, one in each of the two water tanks. It appears to me that his results actually disprove the molecular bond “chain” theory he is promoting, I’m happy to get into details of why if anyone is interested. I’m not a regular poster here (or anywhere for that matter) but I do enjoy reading WUWT. The whole climate science/activism things we have today makes my fur bristle, because I have a pretty good BS detector, and it frequently goes off with the CAGW chicken-littles but it usually doesn’t go off at WUWT, keep up the good work! This siphon thing, though, it has my fur bristling, not because I don’t believe in freedom of speech, but rather the tone of this Dr. Hughes publicly claiming these dictionary definitions are wrong, based on what I see as incorrect interpretation of his own experimental results. And what really gets me going, is the credentials of this guy – he is the expert in the field, no less. Think about this. IF I am right (which I may not be, but on this one I’m pretty sure I am), then this means a senior university PhD in a published paper(s) in peer reviewed journal(s) has made claims that are incorrect, and can easily and repeatably be falsified through lab experiments. If I am wrong on this I will tuck my tail between my legs and skulk off in shame. I’m pretty dang sure I’m not wrong on this, though, and I desire to prove it.
Consider for a moment, IF I am right (and that is still a big IF at this point) then this is an example that could, and maybe should, be used to show what can go wrong with science. To me this is a direct parallel with the problems with climate science, professors not skeptical enough of their own opinions to discern between reality and what they may believe is real.
Here is my interpretation of Dr. Hughes hyperbolic chamber experiment (link above):
1. The siphon quit functioning and turned into a pair of water column barometers when the pressure was lowered to the point where a reference “real” water barometer would show a water level at about the same level as the top of the siphon hose – i.e. exactly what I would expect to happen when barometric pressure is what is responsible for “pushing” the water up the column, and NOT being “pulled” by molecular attraction.
2. The “waterfall” effect he describes is exactly what I would expect to happen as the pressure is lowered such that the reference water barometer (that he should have included in the experiment) shows a level just slightly above the height of the siphon tube. The reason for the “waterfall” is that the pressure differential pushing the water up the tube becomes small enough such that the friction effects of water flowing in the tube become noticable. There is no longer enough flow to fill the tube, and thus it trickles over the top with the tube only partially filled.
3. The height of the “pool” at the bottom of the “waterfall” is approximately where the water level of the other reference barometric water column installed in the lower tank is at – i.e. above the “pool” it is a near complete vacuum, thus corresponding with a reference water barometer placed next to it.
I would appreciate comments or debate on this topic. I don’t personally know this Dr Hughes, and have no personal beef with him. However, if I am correct, this is a big deal to me, and should be to you as well.
One more thing (sorry if I’m becoming a nuisance) Wikipedia has what is IMHO a great writeup on this topic, with a debunk of the chain analogy.
http://en.wikipedia.org/wiki/Siphon
Mike
Unfortunately Dr. Hughes’ explanation is inadequate. Gravity alone cannot completely explain how siphon works. For example, cover the higher or lower reservoir to make it air tight and the flow stops. Why? Decrease the diameter of the tube and flow stops. Why? Gravity cannot explain these because the weight of the fluid is the same.
The more complete explanation is fluid flows when the hydrostatic pressure in the tube is greater than the static pressure of the fluid in lower reservoir and the friction force due to viscosity. Covering the higher reservoir, the air expands and air pressure decreases. This decreases the hydrostatic pressure. Covering the lower reservoir, the air is compressed and air pressure increases. This increases the fluid static pressure. Decreasing the tube diameter increases the fluid friction force.
Dr. Hughes’ study is of interest to hobbyists and amateur scientists. But it is trivial to engineering because the physical principles are well known to students of fluid mechanics. There is nothing new here that is not described in fluid mechanics textbooks.
Spicy Mike, Your assessment is identical to mine.
The mistake in dictionaries is the term “atmospheric pressure.” It should be “hydrostatic pressure.” How does a siphon work? In plain language, fluid flows when the difference between hydrostatic pressure and pressure loss due to fluid friction is greater than the difference between air pressure in low reservoir and high reservoir. In mathematical form:
(Ps – Pf) > (Pl – Ph)
Where: Ps = hydrostatic pressure; Pf = pressure loss due to fluid friction; Pl = air pressure at low reservoir; Ph = air pressure at high reservoir. Fluid flows when the inequality is satisfied. In Dr. Hughes’ experiment, Pl and Ph are equal so it doesn’t matter what the air pressure is because their difference is zero. Ps is always greater than zero therefore water flows.
Had he change Pl = air pressure at sea level and Ph = air pressure at 40,000 ft. Water will flow in the reverse direction – upward. Now that is something gravity alone cannot explain. This is proof that siphons work by differences in hydrostatic pressure and air pressure.
If it were possible to make a water siphon taller than the barometric height then you could discount the atmospheric effect, especially if the two reservoirs were only separated by a few inches. This would also conclusively prove the cohesion theory.
The Vacuum Siphon on Youtube is in effect taller that the barometric height. However water has surface tension and cohesion limited to about 3 to 4mm. You could have a 9.5 metre upleg on a siphon, the bottom 9m could be 2m diameter, the last 0.5m could be 5mm tube and the downleg could be 5mm tube. The up leg would contain approx. 28 tonnes of water. The siphon would work. However, don’t try and tell me that a water column just 5mm in diameter and 500mm long could support 28 tonnes because of cohesive or tensile strength. You are dreaming. The source of energy for the siphon is gravitational potential head, i.e. the height difference between the inlet and the outlet and the water moves due to the circumstances that create a pressure gradient. The different densities of the two mediums, i.e. water and air also come into play.
3 Blind Mice
That is not quite true. What you are forgetting is the bouncy force of the water on itself. I know that this is quite unusual so direct you to http://www.youtube.com/watch?v=_dQJBBklpQQ . As you can see the beads actually lift from the surface. In effect a cylinder of water does not just hang from the top as a weight on a pulley. At each level the liquid is pushing down but it is also pushing up. If you do the maths you can see that in fact it doesn’t matter that the volumes are different, with or without air.
You can see that this is true by imagining the experiment set up with an ionic liquid in the way you describe only with the bottom of both tubes in the same reservoir. e.g a u-tube barometer with one fat leg. Now put the whole device in a vacuum chamber and take away all the air. If the larger volume drags down one side you have in effect created a perpetual motion machine. Clearly this is not possible.
@ Adrian
What is not quite true?
You mention “The bouncy force of water”???
Then you add a link with something that has no water at all, rather beads!
And exactly what are you trying to say in your message, you are just rambling.
Maybe you are one of the 3 blind mice
@ 3 blind mice
Oh! I’m sorry that my two short paragraphs were too long for you to follow, and please feel free to ad hominem. The point was that at no point does the water need to support 28 tonnes.
@ Adrian
My comment re “The Vacuum Siphon on Youtube is in effect taller that the barometric height” was based on the revised barometric height because atmospheric pressure within their chamber has been reduced to zero.
My original comment re tensile strength was directed at those that argue that siphons work only because of gravity and tensile strength, and use the chain analogy that the down leg pulls the up leg with it. Under such circumstances, then it would have to support 28 tonnes.
You wrote that what I said was not quite true but then go on to agree that it doesn’t have to support 28 tonnes.
I have no idea what the beads have to do with water siphons. They are physical attached to each other so it is a chain and expect it would incorporate issues such as minimum bending radius and centrifugal force.
Yes I did follow your original argument and I disagree that atmosphere plays a significant role in the siphon once it is running. (Obviously initiating the filling of a siphon by sucking one end does require barometric pressure).
The criticism of the chain analogy is false however, as I tried to explain earlier. Putting aside the beads (which in fact if you had followed the explanation video and paper does not rely on centrifugal force alone) the fat leg siphon does not lead to the collapse of the chain model, as you suggest. Rather than taking the case of a 9m long 2 m diameter tube which is an extreme example, try considering a more plausible case.
Imagine a 10 cm high siphon in a vacuum using the ionic liquid as the fluid. In this case the tensile strength is not at issue as it will not break. Now construct this siphon with one fat leg, say twice the diameter of the other e.g. one 10mm diameter and one 5mm diameter. This fat leg siphon should still work as a siphon, i.e. the liquid will still flow without breaking as it is easily with the tensile strength of the liquid to support this difference. However, as I’m sure you will agree, one side is heavier than the other.
Now set the reservoirs of this siphon at the same height. In this case there is no atmosphere and the tension in the liquid is able to support the difference in weight. By YOUR analogy the liquid in the fat leg will pull down on the other thin leg thus causing the liquid to flow from the thin side to the fat, creating an imbalance in the level in the reservoirs.
My contention is that that cannot happen. If it were the case then putting this siphon in the same reservoir would lead to liquid flowing up the thin leg and down the fat. (This is the same as connecting the reservoirs via a second direct tube). If the fat leg worked as YOU suggest there would be a continual flow of liquid around the tube i.e. a physical impossibility.
@ Adrian
Re your comment: “…….and I disagree that atmosphere plays a significant role in the siphon once it is running”, I never ever claimed this so you must be disagreeing with others.
I viewed the chain bead video but never followed the explanation further because it is not a siphon, does utilise liquid and does not contain a tube. I didn’t claim it works by centrifugal force alone, just suggested it might have been one of the issues involved.
The chain analogy is put forward by others, not by me. With the chain analogy, it is others who are claiming the heavier weight of the down side is pulling the up leg over the siphon crest. The fat leg siphon rules out the heavier weight winning out on a siphon. This means that the chain analogy is not a good one to use.
Re: “Rather than taking the case of a 9m long 2 m diameter tube which is an extreme example, try considering a more plausible case.
The 9m diameter tube is realistic, it can be built. You can buy 2m diameter pipes. There is no need to change to something more plausible.
Re: “By YOUR analogy the liquid in the fat leg will pull down on the other thin leg thus causing the liquid to flow from the thin side to the fat, creating an imbalance in the level in the reservoirs.
I am not claiming the fat leg will pull down the other leg of a siphon. And it is not MY analogy. If one built a chain with a heavier short side compared to the longer side, the shorter side would win out. But in a siphon, the diameter and weight are not relevant to the working of a siphon. And I am NOT suggesting there would be a continually flow of liquid.