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.”
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“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??
@ur momisuglyferdberple: “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!
@ur momisugly 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.