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

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."

corrected link

[Anonymous Coward]

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

Not "Nature", a lesser journal of the Nature group

[cpotoso]

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.

Re:Actually it's both.

[Luciano Moretti]

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.

[Agent0013]

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.

Plot twist:

[LordLimecat]

Atmospheric pressure is actually due to gravity.

Re:Actually it's both.

[maird]

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:wrong

[Anonymous Coward]

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:Actually it's both.

[rabtech]

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.

[Tiger4]

No. A water column height is proportional to temperature and pressure. Under standard conditions, you can get a column about 32 feet long before the water breaks to form a void. It is called cavitation, but in effect it is a local boiling effect. Boiling is when the vapor pressure of the water is at or above the local atmospheric pressure. Water vapor bubbles jump out of the water liquid. If that happened in the siphon tube, it would break the siphon, but again, the column would have to be pretty long before it happened

Re:Actually it's both.

[Nethemas the Great]

The liquid within the siphon tube acts in a manner similar to a pump's piston. For the siphon to work a sufficient amount of liquid is required to be drawn by gravity to the receiving end to overcome the head pressure on the end from which the liquid is being drawn. Atmospheric pressure contributes only in as much as it can vary the head pressure that needs to be overcome for the siphon to operate. The idea of a pump's piston may be extended to the vessel from which the liquid is being drawn. Consider for a moment that this vessel is sealed but for the point from which the liquid is siphoned. As the liquid in the vessel is drawn down a vacuum is created which resists the drawing of liquid. The more liquid siphoned away the greater the vacuum and thus the greater the head pressure that must be overcome for the siphon to operate. Of course this could operate in the reverse fashion were a substance to be pressed into the vessel from which the liquid is drawn. As the pressure increases against the liquid the head pressure is reduced.

Re:Actually it's both.

[michelcolman]

Nope. Strange how many people get this wrong, it's really not that complicated.

The water doesn't work like a chain, the cohesion of water is only just enough to hold a drop of water together, certainly not enough to pull a whole column of water along through a siphon. The motion is caused by gravity BUT atmospheric pressure is needed as well (as shown in the actual experiment [nature.com] that was referenced in the Slashdot summary and described in more detail in Nature). Here's how a siphon acually works:

Suppose you have a source reservoir and a destination water reservoir, with the water level of the destination lower than that of the source. The reservoirs are connected by a tube that goes from the source reservoir up to an apex above both water levels and then down into the destination reservoir. The tube is filled with water (you have to start the siphon somehow by filling it with water before it can work).

Now, if you would calculate the pressure at the apex starting from the inlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the source reservoir level. On the other hand, if you calculate the pressure at the apex starting from the outlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the destination water level. If the destination water level is lower, the latter value for the pressure at the apex is lower than the former. Of course there can only be one pressure at the apex, which will be in between these two pressures. It is lower than what you would expect when calculating from the inlet, and higher than what you would expect when calculating from the outlet, so the pressure gradient will suck water in from the inlet and push it out of the outlet.

But note the two times I wrote "MINUS" in bold capital letters. You can't go below zero pressure. When the atmospheric pressure is too low to push the water from the source reservoir up to the apex, the siphon breaks up.

That's exactly what happened in the experiment described in Nature. They tested it with a 1.5 meter siphon in a pressure chamber. The water in the siphon broke up when they reduced pressure to below 0.18 atmosphere, which makes perfect sense because at that point the pressure at the apex would start to approach zero. The siphon actually turned into a double barometer with vacuum (or a bit of water vapour, actually) in between.

So yes, the motion is caused by gravity but you DO need atmospheric pressure or it simply won't work. In fact, if you look at it a certain way, it's not even wrong to say that atmospheric pressure is pushing the water up to the apex and therefore making the siphon work.

Re:corrected link

[michelcolman]

And if you look at that link, you'll see it's a letter from 2010. Which is yesterday by Slashdot definitions, of course.

By the way, the letter was nicely debunked here:

http://www.theregister.co.uk/2... [theregister.co.uk]

Gravity is responsible for lowering the pressure in the outbound leg of the siphon, but you still need atmospheric pressure at the inlet to push the water over the top of the siphon. You can't siphon up more than about 10 m under normal atmospheric pressure.

So in a way, it's not really wrong to say that atmospheric pressure is pushing the water over the top. Just like atmospheric pressure pushes liquid into a drinking straw as well. Would you say that a drinking straw has nothing to do with atmospheric pressure?

Of course the exact value of atmospheric pressure doesn't matter much as long as it's enough, the flow rate will only depend on the difference in height between the two water levels, but without enough atomospheric pressure the siphon stops working. Which was clearly shown in the experiment described in Nature as well. In fact that experiment dispoves rather than proves his point.

Re:No. Try it with no air pressure, like TFA did

[mysidia]

It bears mentioning that atmospheric pressure is created by gravity in the first place. The wall of air above any patch of ground has a mass which gravity gives a weight. The sum of the weight of this mass creates the atmospheric pressure: without gravity, this gaseous atmosphere would spread and disperse into space until pressure was eventually that of the vacuum.

The mechanism of a siphon does rely on fluid pressure to work just not atmospheric pressure. As some liquid pulls out and follows the force of gravity; a suction is created, and water molecules that are adhering follow the flow this creates.

If pressure is reduced by 80%, it stops working at all. See the article for details.

After pressure is reduced by 80%; the substance ceases to be a proper liquid -- in essence, it loses the properties of water.

You can also accomplish this by increasing temperature as well, until the liquid begins to vaporize.

This does not mean that a siphon is something caused by low temperature.

The siphon is something caused by gravity, that relies on some properties of the liquid to work that have some sensitivity to pressure/temp.

For a siphon to work; the liquid needs to have certain properties that water does have. Including surface tension/adhesion, and specifically -- capillary action.