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Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review

timothy posted about 5 months ago | from the suck-it-oed dept.

Science 360

knwny (2940129) writes "Peeved by the widespread misconception that siphons work because of atmospheric pressure, physics lecturer Dr. Stephen Hughes, [in 2010] wrote a mail to the prestigious Oxford English Dictionary(OED) pointing out the error. To back his claim, Dr.Hughes tested a siphon inside a hypobaric chamber to check if changes in atmospheric pressure had any effect on the siphon and demonstrated that gravity and not atmospheric pressure was the driving principle. [This week, the] paper detailing his experiment was published in Nature. The OED spokesperson responded saying that his suggestions would be taken into account during the next rewrite."

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corrected link (5, Informative)

Anonymous Coward | about 5 months ago | (#46841685)

This is the corrected link to the letter: http://www.theguardian.com/science/blog/2010/may/10/dictionary-definition-siphon-wrong

Actually it's both. (0)

Anonymous Coward | about 5 months ago | (#46841695)

A straw with a hole in it cannot siphon.

Re:Actually it's both. (5, Funny)

TechyImmigrant (175943) | about 5 months ago | (#46841707)

>A straw with a hole in it cannot siphon.

A straw has two holes in it.

A straw with only one hole can't siphon.

oh man (4, Funny)

dlt074 (548126) | about 5 months ago | (#46841745)

that sucks!

Re:oh man (1)

UnknownSoldier (67820) | about 5 months ago | (#46842219)

you blow! ;-)

Re:oh man (1)

s13g3 (110658) | about 5 months ago | (#46842315)

Notation that you create a vacuum. ;)

Re:Actually it's both. (2)

v1 (525388) | about 5 months ago | (#46841785)

and a straw with three holes in it might work as a siphon, depending on the size of the third hole (and other related factors such as the viscosity of air)

Re:Actually it's both. (-1)

Anonymous Coward | about 5 months ago | (#46841899)

your mom has three holes, and they all suck.

Re:Actually it's both. (3, Informative)

Agent0013 (828350) | about 5 months ago | (#46841925)

Actually, the wikipedia article on siphons shows an experiment done by Pascal where two beakers of mercury were positioned with a siphon between them. But in this version, the siphon had a third tube projecting upwards from where the top of the bend in the siphon is. The whole thing, excepting the end of the upward projecting tube, was positioned under water. So there is no ability for a vacuum to form in the siphon tube since it is open to air. The mercury still moved from the higher beaker to the lower from the pressure of the water. From this experiment, it would seem that this guy has it wrong and it is the pressure that pushes the fluid up and through the siphon.

Re:Actually it's both. (3, Insightful)

Noah Haders (3621429) | about 5 months ago | (#46842107)

i didn't realize there was any confusion about this. obviously it's because of gravity. it's like when you have a long chain suspended from above with both sides hanging down. when the two sides are the same length it is stable, but when one side becomes longer than the other then the weight of gravity pulls the whole chain down. duh?

Re:Actually it's both. (-1)

Anonymous Coward | about 5 months ago | (#46842303)

There's a big difference between a chain and a water column. A chain has very high tensile strength. A water column has very low tensile strength. This is why atmospheric pressure is needed for a syphon and not for a chain and pulley. If you replace the chain with a water-filled tube and reduce the atmospheric pressure to 0, the tensile stress at the top of the tube will break the water column.

In order to prevent this, atmospheric pressure is required to put the water column under compression. That is the role of atmospheric pressure -- it doesn't drive the syphon, but it prevents the water column from breaking.

Re:Actually it's both. (0)

Anonymous Coward | about 5 months ago | (#46842431)

This is why atmospheric pressure is needed for a syphon

Can't you at least read the title of the article?

Re:Actually it's both. (2)

Mente (219525) | about 5 months ago | (#46842127)

You just disproved your own point. What Pascal's experiment showed was that it wasn't a vacuum that created the siphon ( a vacuum would be a difference in air pressure), but when one beaker was placed higher, gravity caused the mercury to flow from the higher beaker to the lower beaker. Even without the vacuum normally associated with a siphon.

Re:Actually it's both. (4, Informative)

maird (699535) | about 5 months ago | (#46842131)

Lots of mistakes there. In the experiment you are referring to, the whole thing was NOT "positioned under water". In fact, the mercury siphon and both beakers of mercury were positioned in a larger container exposed to the air. The siphon tube has an extra pipe exposing the top of the bend to the air as well. The outer container that contains the siphon is "slowly filled with water". Since the two beakers that make up the siphon containers both contain mercury the siphon tube is then filled with mercury from the lower beaker before the higher one because of the weight of the water appearing on the lower one first. The extra tube at the bend in the siphon prevents any compression of the air in it. With properly selected heights of the two beakers of mercury the siphon pipe can fill from the lower one first, over the bend and into the higher one and the mercury will flow "upwards" due to the weight of the water only being present on the lower mercury. However, as soon as the weight of the water is present over both containers of mercury then the flow will reverse and go "downhill".

Re:Actually it's both. (1)

gerf (532474) | about 5 months ago | (#46842201)

It's not atmospheric pressure, it's internally induced pressure due to buoyancy differences, which are normally created due to gravity and a connection that is rigid enough to withstand the internally induced pressure. If you have a closed system of two non-rigid containers connected by a rigid body, then the fluid will try to flow in the direction of its buoyancy. Helium balloons connected internally by a straw (even a curvy one) would try to fill the higher balloon, right?

So yeah, he's right that in the absence of gravity, a normal siphon will not work. But, if you took that siphon system on the ISS and put one end outside in space, and one inside, you'll have a siphon-like effect due to air pressure. Likewise, if you take two balloons of water with a rigid connector and submerge one in a pool of Hg, then that "siphon" will work against gravity. :D

Re: Actually it's both. (1)

Anonymous Coward | about 5 months ago | (#46841881)

Topology fail. Straw = 1 hole = donut = coffee mug.

Re: Actually it's both. (1)

TechyImmigrant (175943) | about 5 months ago | (#46842063)

I wasn't using the topology definition. There's no gravity or atmospheric pressure in topology.

Re: Actually it's both. (2)

bsdasym (829112) | about 5 months ago | (#46842165)

Oh thanks a lot. After reading this I tried pouring coffee into a donut hole to make a sort of coffee+donut breadbowl and it just made a mess instead. Topology fail is right!

Straws don't make good siphons anyways. (1)

CokeJunky (51666) | about 5 months ago | (#46841941)

A siphon (at least the kind in the article) generally means a u-shaped tube that pulls liquid up over the top and down again. I suppose a couple of bendy straws stuck together might work with a bit of tape, but holes are still a problem because it breaks the pressure seal and stops the slug of falling liquid from applying force to the container. I am sure in a couple of days we will all be able to see u-tube hypobaric siphon action on youtube.

Re:Straws don't make good siphons anyways. (3, Interesting)

TechyImmigrant (175943) | about 5 months ago | (#46842333)

I'll resist the temptation to carry on with the bad puns and innuendo, but....

u-tube plonked in water in an elevated bucket, one end outside the bucket.

1) You suck on the dry end. Water moves up to the apex of the tube.
        It's atmospheric pressure pushing the water up the tube as your sucking reduces the pressure in the tube.

2) Water keeps moving around the bend, past the apex.
          It's a combination of your sucking and momentum that keeps the water moving.

3) The water reaches a point lower than the surface of the water in the bucket. You stop sucking.
          It's the gravity (or the water seeking a lower energy state in a gravitational field) that keeps the water moving through the tube.

So all things are having an effect, which makes sense. Atmospheric pressure doesn't magically stop happening just because gravity is having a stronger effect.

 

Re:Actually it's both. (1)

sokoban (142301) | about 5 months ago | (#46842225)

A straw only has one hole. It just happens to be a very long one.

Re:Actually it's both. (1)

therealkevinkretz (1585825) | about 5 months ago | (#46841721)

Exactly. You beat me to it.

Re:Actually it's both. (1)

banda (206438) | about 5 months ago | (#46841725)

It can if the hole is below the level of the higer reservoir. Otherwise the hose itself becomes the higher reservoir in which case it still siphons, just not in the direction you want.

Re:Actually it's both. (3, Informative)

Luciano Moretti (2887109) | about 5 months ago | (#46841765)

Inside the tube it's not atmospheric pressure, as there is no gas in the tube of a proper siphon: it would be Fluid Pressure.

Re:Actually it's both. (1)

Anonymous Coward | about 5 months ago | (#46842033)

OK, here's what happens. There are two parts of tube to a siphon, the part where fluid flows upward and the part where fluid flows downward (maybe repeated multiple times, but let's stick to the simplest construction exemplifying the principle.) It is hardly surprising that gravity is what makes the fluid flow downward in the second part of the tube. The interesting bit is what makes the fluid flow upward in the other part, because fluids don't usually do that. The naive explanation is that the fluid in the other part pulls on the fluid in the upward part, but if that were right, then you could siphon with a half-tube, and that doesn't work. That's where pressure (but not necessarily atmospheric pressure) comes in. The fluid in the downward part would leave a "vacuum" where the two parts of the tube meet. This low pressure zone is the cause of the pressure difference, and the pressure of fluid in the upper reservoir pushes the fluid into the tube. Without a closed tube, this pressure differential couldn't form and the siphon doesn't work. The atmospheric pressure on the upper reservoir is part of the pressure which pushes the fluid into the tube, but it is countered by the atmospheric pressure at the lower end of the tube, which is actually bigger (because there's more air above the lower end of the tube). That's why atmospheric pressure is not involved, but pressure is. In the end it is of course gravity which causes the pressure, but the defining element of a siphon is that it requires a closed tube, and that makes pressure, not gravity, the key aspect.

Re:Actually it's both. (1)

Noah Haders (3621429) | about 5 months ago | (#46842161)

no way man. if you have a siphon that is 2 inches tall, there is no way there's a meaningful difference in atmospheric pressure between the top and the bottom. if that were the case you could hold a straw vertically and wind would rush through it.

it's like a chain hanging from a ladder, just gravity.

Re:Actually it's both. (0)

Anonymous Coward | about 5 months ago | (#46842397)

Nope. The weight of the air in the straw creates the additional downward force to counter the higher atmospheric pressure at the lower end. Atmospheric pressure differences are not as small as you seem to believe. Although there are certainly bigger forces at work in small parts of disturbed atmosphere, the pressure difference due to the weight of the air alone is about 1Pa per 8cm at sea level. In other words, air at sea level weighs about 1.2g per liter.

Re:Actually it's both. (1)

BasilBrush (643681) | about 5 months ago | (#46841789)

A straw with a hole in it cannot siphon.

It cannot be atmospheric pressure, given that he demonstrated that a syphon works in a vacuum.

It seems to me that surface tension is enough to keep the liquid in the tube, even when the equipment is in a vacuum.

Re:Actually it's both. (3, Insightful)

jeffb (2.718) (1189693) | about 5 months ago | (#46841907)

He demonstrated no such thing. In fact, he demonstrated that the siphon stops working at sufficiently low atmospheric pressure:

When the pressure was reduced further the siphon broke into two columns - in effect becoming two back-to-back barometers.

You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.

Saying "siphons work due to gravity, not atmospheric pressure" is like saying "fire works due to oxygen, not fuel".

Re:Actually it's both. (1)

locofungus (179280) | about 5 months ago | (#46842213)

The claim in the paper (linked from one of the first comments) is that it's the tensile strength of water that allows the siphon to work.

For the case of water I think that's garbage. Water doesn't have enough tensile strength to support more than a very low siphon. It's air pressure that allows siphons of usable height. Because of it's relatively high vapour pressure while a liquid it's going to be hard to prove anything either way using water though.

Mercury would be a better bet. It has a very low vapour pressure.

If you set up a siphon so that no mercury is flowing (source and destination reservoirs are at the same pressure) then you can make it flow either way by lifting or lowering the reservoirs relative to one another. (You can do this with water too)

I predict that if you were to then move the apparatus to a vacuum chamber, the mercury in the siphon tube would come out due to gravity and there would be a vacuum in the tube too. Raising and lowering the reservoirs would then not cause any mercury to flow either way.

Re:Actually it's both. (1)

BasilBrush (643681) | about 5 months ago | (#46842267)

He DID demonstrate such a thing. However he also demonstrated what you describe.

Below the height at which two barometer columns form, the siphon works.

You can't pull on one end of a column of liquid and drag the whole column up.

But you can have a column of liquid higher than the pool is comes from, without any atmospheric effect. It's called capillary action. My reference to surface tension should have given you the hint.

Re:Actually it's both. (1)

wagnerrp (1305589) | about 5 months ago | (#46842305)

You can't pull on one end of a column of liquid and drag the whole column up. Something has to push it from the bottom, unless its own inertia can carry it.

If you have a fluid with high intermolecular attraction (like water), yes you can.

Re:Actually it's both. (2)

K. S. Kyosuke (729550) | about 5 months ago | (#46841965)

In vacuum, there wouldn't be any water, only vapor. So a water siphon actually can't work in vacuum.

Re:Actually it's both. (1)

BasilBrush (643681) | about 5 months ago | (#46842135)

Who said anything about water? I didn't, the person I responded to didn't, and the scientist in the story DID perform a siphoning experiment in a vacuum.

It's perfectly possible with mercury.

Ummm... (0)

Anonymous Coward | about 5 months ago | (#46842145)

Water doesn't suddenly boil in a vacuum, it would still be liquid water. Water, in liquid form, is not compressible, so its size would even expand. In the absence of ambient air, if left long enough, the energy in the system should dissipate, and after a few minutes the water turns in ice at an astonishing rate. You can find many youtube videos of people using vacuums to freeze water.

The article actually says that in a vacuum the siphon breaks and you are left with two water columns.

Re:Actually it's both. (1)

WalksOnDirt (704461) | about 5 months ago | (#46842183)

Pure water, without any dissolved gasses, has a substantial tensile strength. It is not theoretically stable, but in practice it is. Enough so that a siphon will work in a vacuum.

Such pure water is hard to find, though.

Re:Actually it's both. (4, Informative)

rabtech (223758) | about 5 months ago | (#46842415)

They cover that in the paper and videos. At 40,000 ft equivalent atmospheric pressure, water begins to cavitate or boil inside the siphon, but the momentum of the water pulls the bubbles past the apex before they can stop the flow, resulting in a "waterfall" inside the tube. Slightly lower pressure decreases this effect, slightly higher increases it.

At some point around 41,000 ft equivalent pressure the bubbles form too quickly and touch all sides of the tube at or slightly before the apex, resulting in the flow stopping. However if you then increase the pressure again at a certain point (around 30,000 ft IIRC) the flow resumes. They discuss attempting the experiment in the future with an ionic liquid that won't vaporize.

If you think about it, this is the same phenomenon as the ball chain flowing out of a container (https://www.youtube.com/watch?v=_dQJBBklpQQ). Gravity pulls on the first ball, which pulls on the next, which pulls on the next. As soon as that pull is strong enough to lift the chain from the surface to the apex, a siphon effect begins that will empty the entire container.

IANAP, but it appears that water siphons work the same way. Once enough water flows over the apex sufficient that the force of gravity on that water exceeds the weight of the water prior to the apex the siphon will flow. The big tell-tale sign that any explanation involving the air pushing down on the surface of the liquid is wrong is the flow rate - it is almost completely independent of atmospheric pressure.

The one question I still have is why the flow stops at 41,000 ft. I would have expected a kind of spring effect, followed by the lower portion of the siphon slowly descending as water vaporizes off the pre-apex portion, allowing the water in the lower part to descend while maintaining the same vapor pressure. I'm sure it is my failure to understand, so if anyone can offer a better explanation please do so!

Re:Actually it's both. (0)

Anonymous Coward | about 5 months ago | (#46842051)

Wouldn't that depend on where the hole is? Which brings us back to gravity. Dummy.

Re:Actually it's both. (1)

ShanghaiBill (739463) | about 5 months ago | (#46842175)

A straw with a hole in it cannot siphon.

If the liquid has sufficiently high viscosity and surface tension, the siphon may still work. If the liquid has sufficiently low viscosity and surface tension, or if the siphon is too tall, the siphon will not work even without the additional hole.

Gravity! (5, Funny)

JustOK (667959) | about 5 months ago | (#46841697)

You win again, gravity!

Hm... (0)

Anonymous Coward | about 5 months ago | (#46841715)

Have they ruled out Peer Pressure as well? I wonder if Peer Review can help there.

Intuitive (1)

Iniamyen (2440798) | about 5 months ago | (#46841749)

I'm not very smart, but it seems to me that the difference in potential energy between masses even at small differences in height would be vastly greater than the work that the negligible delta pressure between those same two heights could do, so isn't this kind of obvious?

Re:Intuitive (0)

Anonymous Coward | about 5 months ago | (#46841851)

I think before people jump to conclusions, they should read the wikipedia article, specifically about what Pascal was saying and his experiment (which involved mercury in a siphon under water, with the middle of the siphon tube having a hole exposed to the air. I think what Pascal was saying was different from what people assume is meant by air pressure being involved.

Re:Intuitive (1)

Immerman (2627577) | about 5 months ago | (#46841931)

That was my thought. It sounds like the issue may be largely confined to dictionaries though - the 1911 Oxford dictionary got it wrong, and it and most every other dictionary since has continued to do so. Presumably because dictionary editors are not typically skilled in the hard sciences, and anybody that knows the science is unlikely to look up siphon in the dictionary. The resulting tiny set of people aware of the error probably just didn't include someone concerned enough to send in a convincing bug report until now.

Re:Intuitive (1)

lgw (121541) | about 5 months ago | (#46842227)

Well, there are also tubes that people might loosely call "siphons" that do work by atmospheric pressure involved in some clever pumpless perpetual fountain designs, where the high air pressure in one chamber pumps water up through a pipe. But if we want to change even the non-technical definition of siphon to exclude that, that also seems reasonable. "Reverse-siphon" maybe, since it goes against gravity?

Not "Nature", a lesser journal of the Nature group (3, Informative)

cpotoso (606303) | about 5 months ago | (#46841753)

The 2012 Impact Factor for Scientific Reports is 2.927. For comparison, that of Nature is 38.597. Still impressive, but please lets be precise.

I'm Confused (2)

rotorbudd (1242864) | about 5 months ago | (#46841755)

Does it suck or blow?

Re:I'm Confused (1)

Anonymous Coward | about 5 months ago | (#46841863)

It sucks to blow!

Somebody thought it was atmospheric pressure? (3, Interesting)

Anonymous Coward | about 5 months ago | (#46841795)

Seriously? If atmospheric pressure had any influence, it would do the opposite: The pressure at the lower end of the tube is higher than at the other end, so the fluid would flow upward. Obviously this doesn't happen.

Re:Somebody thought it was atmospheric pressure? (1)

K. S. Kyosuke (729550) | about 5 months ago | (#46841905)

I think that aerostatic pressure differences are not the point here.

Re:Somebody thought it was atmospheric pressure? (0)

Anonymous Coward | about 5 months ago | (#46842207)

I think it is. You couldn't siphon air up- and then downwards in an "atmosphere" of water, precisely due to the pressure difference of the surrounding medium. With a sufficiently heavy atmosphere (denser than the material you want to siphon), the siphon doesn't work. Atmospheric pressure works against the siphon, not in favor of it.

Re:Somebody thought it was atmospheric pressure? (1)

mark-t (151149) | about 5 months ago | (#46842159)

If the pressure difference is great enough, then yes... it will flow upward. Have you ever used a straw? That's pulling liquid upward entirely through difference in air-pressure

However, there is still a maximum height that can achieved even if the higher end were in complete vacuum, and for something like water, that distance is not even 35 feet (it's even less for heavier liquids, like mercury). The difference in atmospheric pressure with so little difference in altitude is not gong to be sufficient to pull liquid upward, which is why you don't ever see it happening.

And which is why the output of a siphon must be positioned at a lower altitude than the input unless you provide artificial pumping to pull the liquid upward. But with a regular siphon, between the input and output, the siphon can certainly go higher than its input, making it appear as though the liquid is defying gravity. But while siphoning water, for instance, no point on the siphon can be greater than about 35 feet higher than the input because of the maximum vertical height that water can be drawn from only 1 atmosphere of pressure.

Re:Somebody thought it was atmospheric pressure? (1)

BitZtream (692029) | about 5 months ago | (#46842369)

That's pulling liquid upward entirely through difference in air-pressure

Wrong. Thats atmospheric pressure PUSHING the liquid up through the straw due to the lower pressure in the part you're 'sucking' on.

The reason you can't 'suck' water up past 30 feet or so is because the water column becomes equal to atmospheric pressure, so even when the upper end is in a 'vacuum', the atmospheric pressure is in equilibrium with it and it stops flowing upwards.

You can not pull water up, only push.

Likewise, those people who taught you that airplanes fly because a vacuum on the top edge of the wing 'sucks' it up are equally as wrong. A vacuum or lower pressure never does any work, its the positive pressure on the other side that is ALWAYS pushing.

You can only pull on solids, never on gases or liquids (unless you want to get pedantic and start throwing in high viscosity fluids or amorphous solids).

Re:Somebody thought it was atmospheric pressure? (1)

danbert8 (1024253) | about 5 months ago | (#46842421)

Right, basically a siphon can only work if the pressure at the high point is above the vapor pressure of the liquid. If you pump a vacuum, then your water will vaporize which will kill your siphon. Gravity has nothing to do with it aside from being the driving force behind atmospheric pressure in the first place. As long as you have an unbroken liquid in your siphon, atmospheric pressure is what drives it. If "tension" between water bonds were what drove it, it would behave like a chain fountain and you wouldn't need a closed pipe to continue the siphon.

Misattributed article (0)

Anonymous Coward | about 5 months ago | (#46841797)

The article discussed in the paper was not, in fact, published in Nature. It was published in Scientific Reports, which is an online journal published by the Nature Publishing Group, the publisher of Nature and many other journals.

Like duh! (0)

Anonymous Coward | about 5 months ago | (#46841817)

Seriously, people thought that ?

Obviously members of the "the world is flat" club.

gravity shuts off after 33 feet (0)

Anonymous Coward | about 5 months ago | (#46841849)

I didn't know that

wrong (0)

slashmydots (2189826) | about 5 months ago | (#46841859)

We learned in grade school that it works because a lot of liquids, especially water, stick together. The water going downward pulls the water upwards because the whole amount in the hose is bonded together. THAT is how it works.

Re:wrong (1)

jeffb (2.718) (1189693) | about 5 months ago | (#46841957)

I'm very sorry that your grade school taught that.

If "the whole amount is bonded together", how do drips happen?

Re:wrong (1)

Payden K. Pringle (3483599) | about 5 months ago | (#46842065)

Uh...

It is bonded together with Covalent Hydrogen bonds. Which are very weak. Thus, liquid. It doesn't mean they aren't bonded. It means the bonds are easily broken and have little effect (i.e. the substance doesn't change).

Re:wrong (4, Informative)

Anonymous Coward | about 5 months ago | (#46842223)

First off, a hydrogen bond is not covalent.

Secondly, hydrogen bonding has nothing to do with the ability to siphon a liquid. If it did, you couldn't siphon gasoline, as, being a hydrocarbon, gasoline doesn't have any hydrogen bonds.

Re:wrong (1)

danbert8 (1024253) | about 5 months ago | (#46842429)

Mod Anon Informative... Siphoning works with all liquids within their vapor pressure limits regardless of surface tension or cohesion properties.

Re:wrong (0)

Anonymous Coward | about 5 months ago | (#46842049)

Did you go to grade school in Texas?

Re:wrong (1)

MozeeToby (1163751) | about 5 months ago | (#46842155)

And what, pray tell, causes the water to go downward?

Re:wrong (4, Funny)

BasilBrush (643681) | about 5 months ago | (#46842343)

Pessimism.

Re:wrong (1)

locofungus (179280) | about 5 months ago | (#46842285)

We learned in grade school that it works because a lot of liquids, especially water, stick together. The water going downward pulls the water upwards because the whole amount in the hose is bonded together. THAT is how it works.

But if you fill a large diameter pipe with water then the water falls out of the pipe even if you keep the top end closed. Put a piece of card across the low end though and air pressure will hold the water in.

Based on looking at a drip, I'd guess that water doesn't have enough tensile strength to support anything more than a couple of mm of itself.

Still need atmospheric pressure to syphon (0)

penguinoid (724646) | about 5 months ago | (#46841869)

Gravity pulling on the liquid creates a pressure differential -- but only if there's atmospheric pressure.

Re:Still need atmospheric pressure to syphon (0)

Anonymous Coward | about 5 months ago | (#46841969)

You can't even have liquid water without atmospheric pressure.

Re:Still need atmospheric pressure to syphon (1)

jbrandv (96371) | about 5 months ago | (#46841971)

It cannot be atmospheric pressure, given that he demonstrated that a syphon works in a vacuum. Please RTFA before you post.

Re:Still need atmospheric pressure to syphon (0)

Anonymous Coward | about 5 months ago | (#46842115)

He did not demonstrate that a syphon works in a vacuum. He demonstrated that it still works at 0.18 atmospheres. Please RTFA before you post.

Re:Still need atmospheric pressure to syphon (0)

Anonymous Coward | about 5 months ago | (#46842147)

Actually, he demonstrated that the siphon stops working once the atmospheric pressure reached the equivalent of 41,000 feet, at which point the flow stopped due to the atmospheric pressure being insufficient to support the water column(s), and the device the acted as a pair of linked barometers. Good job with that article reading before posting, BTW.

Re:Still need atmospheric pressure to syphon (1)

fizzup (788545) | about 5 months ago | (#46842039)

You are completely incorrect. The liquid may need vapour pressure to remain a liquid, but a siphon manifestly does not require any pressure to run. All you need is a full U-shaped tube and a downward force. Gravity is convenient. The U-shaped tube is often filled by using atmospheric pressure to start the siphon, but this is not a necessary condition. The way the tube gets filled in the first place has no impact on the steady state operation of the siphon.

Re:Still need atmospheric pressure to syphon (1)

Payden K. Pringle (3483599) | about 5 months ago | (#46842109)

Then that brings up the question of how the siphon actually pulls the liquid up and over.

Gravity pulls the liquid down on the back end, but the front end needs to be pulled up by something and that's the pressure differential penguinoid mentioned. So you are incorrect, as a siphon requires the pressure created by the gravity pulling the liquid down to pull the liquid up the front end. Or it can't siphon.

This can work in an environment without an atmosphere but with gravity, obviously, but not the point. You still need pressure. Just not explicitly atmospheric pressure.

Re:Still need atmospheric pressure to syphon (1)

BasilBrush (643681) | about 5 months ago | (#46842373)

Gravity pulls the liquid down on the back end, but the front end needs to be pulled up by something.

For low heights that can be surface tension.

Re:Still need atmospheric pressure to syphon (1)

necro81 (917438) | about 5 months ago | (#46842123)

Liquid pressure exists entirely independent from atmospheric pressure. This can be demonstrated from first principles. A siphon can be operated just fine in a total vacuum, although not with water, which would boil like mad.

One can also make a perfectly workable siphon using two immiscible fluids - e.g., oil and water.

Re:Still need atmospheric pressure to syphon (0)

Anonymous Coward | about 5 months ago | (#46842173)

Yes, because if liquid separates inside the tube, it will create vacuum (and the outside pressure acts against it). This is why siphon doesn't work for water column higher than 10m.
If a liquid is highly cohesive, it can also add to the effect, so the siphon can work without the atmospheric pressure.

Re:Still need atmospheric pressure to syphon (1)

fermion (181285) | about 5 months ago | (#46842387)

In the first run there was little change in flow until the siphon reached 25 000 feet (37.60 kPa, 0.37 atm), when the siphon became choked with bubbles and stopped

So what we call a siphon, which is just a simple hose, does not work below a couple hundred torr. What is proved here is that a specially constructed siphon can work at low pressure. What we need to see for the gravity hypnosis is that a specially constructed siphon cannot work at low gravity.

My take on this is that as gravity pulls water down out of the exit of the siphon, creating a vacuum in the tube, that then pulls water up from the reservoir. It is a compelling and reasonable theory. More experimentation is needed.

Re:Still need atmospheric pressure to syphon (1)

Demonantis (1340557) | about 5 months ago | (#46842443)

No that is incorrect. He showed that the atmospheric pressure value dropped out of the flow equations. The flow is caused by the difference in pressure generated directly from gravity acting on the columns of water. From what I understood, this is why the predicted cavitation, and break down of the siphon action, occurred when the pressure, not the pressure difference, at the siphon apex dropped to the vapour pressure of water.

I would believe this if... (0)

Anonymous Coward | about 5 months ago | (#46841973)

The pipe/tube that is the siphon, rose to a height of more than 10 meters. 10 meters is the height that you cannot "suck" water above, because of it weight is greater than pressure pushing. Basically, the water would turn to a gas crosses the apex. If it still flowed, then I would "believe" more fully. Based on what I understand, if gravity is the sole component, then moving water over a mountain would be easy.

Re:I would believe this if... (0)

Anonymous Coward | about 5 months ago | (#46842005)

Only if you ignore the role gravity plays on the water you're moving.

Re:I would believe this if... (1)

orgelspieler (865795) | about 5 months ago | (#46842345)

Moving water over a mountain is easy in a pipe. Say you have a reservoir at height, like a mountain lake, and you want to pump it to a city in the valley below. You need only get it over the ridge. Once the flow to the lower height starts, it will continue. The problem with your suggestion is that you can't get the siphon started. All this guy is saying is that the flow continues due to gravity. Which makes good sense. The atmospheric pressure at the lower basin is actually slightly higher than at the higher basin, so it's clearly not atmospherically driven.

OED? (1)

Obfuscant (592200) | about 5 months ago | (#46842025)

I'd be more impressed if the Oxnard English Dictionary accepted the change.

vac pump can't raise liquids > atmo pressure (2)

bzipitidoo (647217) | about 5 months ago | (#46842061)

No doubt the confusion comes from the fact that raising water with a vacuum pump does require pressure. People learned some centuries ago that atmospheric pressure can't raise water more than about 10 feet. Simple siphons are commonly started with vacuum pumping.

If the top of the siphon is too high for a vacuum pump, some other method must be used, but the siphon action will work at much greater heights because, as the article points out, the siphon action itself does not depend on pressure. What are the height limits, I wonder? Redwood trees are about as tall as trees can get with the capillary action method they use to raise water. I expect siphons work at much greater heights than that.

Re:vac pump can't raise liquids atmo pressure (1)

Overzeetop (214511) | about 5 months ago | (#46842423)

I believe the limit on height is the pressure at which water turns from liquid to a gas at the ambient temperature. If it were to remain liquid at all pressures, then the water column could be lifted the height where the weight of the water equals the pressure of the atmosphere (which would be roughly 33' at STP).

Plot twist: (5, Informative)

LordLimecat (1103839) | about 5 months ago | (#46842071)

Atmospheric pressure is actually due to gravity.

Re:Plot twist: (0)

Anonymous Coward | about 5 months ago | (#46842281)

This.

Also, nobody ever claimed that atmospheric pressure was supplying the work/energy needed to move liquid from point A to point B.
These "scientists" are just attention whores.

tl;dr: Anybody that has ever thought about it for 2 seconds already knew that gravity was doing the work.

Re:Plot twist: (1)

HaeMaker (221642) | about 5 months ago | (#46842325)

True, but would a siphon work in orbit where there is artificial atmospheric pressure but microgravity?

Re:Plot twist: (0)

swillden (191260) | about 5 months ago | (#46842419)

True, but would a siphon work in orbit where there is artificial atmospheric pressure but microgravity?

No.

Siphons work because of atmospheric pressure differentials, which exist when atmospheric pressure is created by gravity. In microgravity the artificial atmospheric pressure would be uniform.

Re:Plot twist: (1)

Overzeetop (214511) | about 5 months ago | (#46842451)

It does work, with some caveats. I was about to post that it wouldn't, and then remembered the first shuttle payload I worked on : http://istd.gsfc.nasa.gov/cryo... [nasa.gov] which transferred helium between to dewars without pumps in a microgravity enuvronment. It's been a couple of decades, and I wasn't on the principal investigators team (I was carrier support), so I don't remember the details of how the transfer worked.

Re:Plot twist: (1)

wickerprints (1094741) | about 5 months ago | (#46842425)

I know you know this, but just to be sure no one else gets confused: A causes B and A causes C does not imply that B causes C. So for instance falling rocks are also caused by gravity but they don't have anything to do with siphons.

Duh (1)

jhswope (716605) | about 5 months ago | (#46842111)

Actually combination of gravity and surface tension of the fluid. Gravity pulls the fluid down the tube, surface tension makes it want to stay together so it's friends go along for the ride.

Atmospheric pressure also caused by gravity (1)

flyingfsck (986395) | about 5 months ago | (#46842143)

Well, obviously, since it is all due to gravity.

a mail? (-1)

Anonymous Coward | about 5 months ago | (#46842153)

Has this really come full circle? People used to say this wrong in the 90's with "e-mail" and now it's worked its way back to "mail" ?

Subbie:

You send mail.
You send a letter.
You send a message.
You send e-mail.
You send bright blue colored mail
You send a bright blue colored letter.

Write it down if you can't remember.

It's semantics, not a new revalation (0)

Anonymous Coward | about 5 months ago | (#46842245)

For those who didn't read the article well: the paper actually does show that the flow stops when there isn't enough pressure. The water column still needs to be supported, and this happens by a combination of atmospheric pressure (the dominant force at 1atm) and molecular cohesion.

Also, NO, this paper does NOT show a water siphon working in a vacuum. (Reference is made to another study, but not at similat water column heights)

The key point being made here is that although atmospheric pressure is required to maintain a certain siphon height, the force causing the water to flow is due to the potential energy difference.

Many years too late... (1)

ChrisKnight (16039) | about 5 months ago | (#46842293)

Huzzah! If only my high school physics teacher was still alive. We frequently argued this point.

siphon wars (0)

Anonymous Coward | about 5 months ago | (#46842295)

this was in the news 4 years ago with arguments going back and forth about whether it was gravity or atmospheric pressure...

See this register article entitled siphon wars
http://www.theregister.co.uk/2010/05/17/siphon_retaliation/

Published in the journal Duh? (1)

Jeff Flanagan (2981883) | about 5 months ago | (#46842307)

Who didn't understand that siphons used gravity to move fluids?

Pedantic (1)

tom229 (1640685) | about 5 months ago | (#46842309)

Of course it's both forces (pressure and gravity). This is simply a pedantic attack at the way the dictionary defines the process.

Dictionary definition:

"A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe."

This definition is correct as atmospheric pressure differences start the process. However the dictionary doesn't explain that gravity eventually takes over. Dr. Hughes sums up:

As any petrol thief knows, to get the liquid over the "hump" of the tube you have to suck the other end or, more pedantically, lower the pressure in your lungs to beneath atmospheric pressure by expanding them. Once the liquid has passed the highest point in the tube, the continuous chain of cohesive bonds between the liquid molecules in the tube, and the force of gravity, do the rest.

An even simpler experiment (1)

wisebabo (638845) | about 5 months ago | (#46842341)

Since he had to go to some length describing the troubles he had because the low pressure formed bubbles due to cavitation, etc. (remember he could not perform this at zero atmospheric pressure because the water would boil), why use water?

Why not use a liquid that will not boil in a vacuum, like (I think) mercury? That would very easily prove that atmospheric pressure is not required to make a siphon work (because there's no atmosphere!).

Take a flexible tube and dunk it in a bucket filled with mercury letting it fill up. Now, sealing the ends, keep one end in the bucket while lowering the other end to another bucket positioned substantially below the first. Pump all the air out of the chamber and unseal the ends. If the siphon works, it is definitely solely due to gravity (remember there's no air!).

Actually, not knowing what the intermolecular bonds are like between mercury molecules, will the siphon still work? If mercury molecules have little or no attraction between them (unlike water which has very strong intermolecular bonds as seen with its high surface tension and high boiling point), perhaps it would behave like discrete particles and there would not be any siphon effect. For example, imagine the bucket and tube to be filled with sand. Would there be a siphon effect? I don't think so because the grains of sand wouldn't "pull" on each other so the sand in the tube would just run out in both directions from the high point in the tube.

Another way to think of the intermolecular bonds is to think of a coiled chain which is held aloft. If a part of it is pulled over a pulley and a substantial length is allowed to dangle down the other side, the rest will be pulled up to the pulley and then down. Of course if all the links in the chain are broken (no intermolecular bonds) then the chain will simply fall away from the pulley on both sides.

So chain fountain and siphon are the same? (1)

jernejk (984031) | about 5 months ago | (#46842379)

Both work due to gravity. The difference is, in chain fountain, it's the link between beads that's pulling the next bead down and in siphon it's vacuum between molecules in the tube?

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