Talk:Siphon/Archive 4

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Could you provide a formula for the flow/volume of liquid displaced by a siphon?

First of all, I want to thank the editor/s of the mathematical section of this article (and of all the article in general) for the clear explanations and graphics you provide. Not only I found the mathematical answer for my questions, I found also the explanation using Bernoulli understandable.

Could you provide a math formula in the math section for the quantity of liquid displaced by the siphon? If my math does not deceive me, I believe that it should be something like:

$flow=v_{C}*A$

where $v_{C}$ is the velocity of the siphon, as explained in the article, and A is the area of the pipe

If this is the case, then, I suppose that the total volume displaced in time $t$ should be:

$volume=flow*t$

Many thanks

Pmronchi (talk) 20:48, 26 February 2011 (UTC)

This page from my bookmarks seems to have some equations of siphons I haven't seen elsewhere on the web:

http://instruct.tri-c.edu/fgram/web/siphon.html

Maybe you'll find it buried in there. Mindbuilder (talk) 22:41, 26 February 2011 (UTC)

Your equation $flowrate=v_{C}*A$ is valid for the flow of any incompressible fluid through a conduit of cross-sectional area A. It isn't an equation that is only applicable to siphons.

Similarly, your equation $volume=flowrate*t$ is applicable to any flow of an incompressible fluid, not just the flow through a siphon. A siphon can be constructed using a conduit of varying cross-sectional area. Consequently I don't see any reason for these equations to be published in the article siphon.Dolphin (t) 04:57, 27 February 2011 (UTC)

Mindbuilder: Thank you for the link. I tried to read the link you provided to me, but found it very confusing.

Dolphin: Thank you too, for your comment. It confirms to me that the formulas I needed are right.

BTW, I look at Volumetric flow rate, Incompressible flow, Flow measurement and Fluid dynamics and I couldn´t find those simple formulas in any place. I agree with you that those formulas would have a better place in another article. Anyway, with your useful remarks included, they could still have some use as a simple case example and, at least, hint to the solution of cases a little more complicated to people like me, lacking any knowledge more than very basic on the fluid dynamics topic. Consider it under this light, and give a second thought to their inclusion.

This is my final message regarding this topic. Thanks again, and congratulations to both for your contributions to this useful article.

Pmronchi (talk) 00:38, 28 February 2011 (UTC)

Fat Up Tube Siphon

174.56.112.3 wrote:

"The suction at the entrance to the narrow tube isn't equal to the entire weight of the water in the larger left column, but the ratio of the cross-sectional area of the tube to the surface area of the container. Since the tube is very small compared to the container, the suction on the tube is much smaller than the weight of the water. The siphon will operate if the weight of the water in the tube is greater than this suction."

What are you referring to when you mention the container? Is that the fat up section of the siphon or the reservoir that the siphon is pulling water from? I don't see how the surface area of the container comes into any calculation of the pressure in the siphon. One might say the pressure is related to the weight of the fat column divided by its cross sectional area. But that gets complicated by fat up tubes that have a varying cross section like the plastic coke bottle used in a youtube (not youtube) demonstration by some european professors(I think I'll add that video as a reference - actually I already put it in as reference 11 on the diagram). It's not quite accurate or at least clear to say that the weight exceeds the suction since weight and pressure do not have directly comparable units. Indeed the point of the fat up tube siphon demonstration is to show that the weight can be increased arbitrarily without increasing the suction because it is the height that determines the pressure. Another complicating factor your description runs into is the pressures in a flowing siphon compared to a static siphon. There was an article by Potter from 1971 referenced around here (search talk archive one for 1971 for the link) that goes into that a little more, though that article shouldn't be trusted completely because it starts out asserting something like the incorrect chain theory. I hope you will propose your further edits here before putting them in the main article. Mindbuilder (talk) 20:56, 26 March 2011 (UTC)

174.56.112.3:

You're correct; the container I referred to is the fat up-tube.

As it stands, there's no explanation at all for that section, which is an interesting case. My daughter's studying siphons in junior high and couldn't begin to make sense of the description as it currently stands. For people like her (which is 99% of the species) even a simplified analysis that documents its limitations is, I think, to be preferred over the implied 'magic happens' of the current treatment. (Tiny amount of water beats big amount of water!) So I put something up there hoping to irritate someone into posting an accurate explanation. ;-)

My point was that the down tube opening sees a force proportional to its small cross-sectional area when compared to the size of the larger up-tube. If you have a more rigorous analysis of the mathematics, that's cool. —Preceding unsigned comment added by 174.56.112.3 (talk) 23:07, 26 March 2011 (UTC)

To really understand the siphon requires a strong grasp of hydrostatic principles first. The questions of a novice like your daughter would be like gold in helping us to understand the weakness of our description. Ask her what questions she has about the description. Exactly what parts of which sentences are not clear and what is not clear about them? Understanding the siphon is probably a tall order for a jr high student since there is extensive misunderstanding among college physics professors about how they work. The professors often have a hard time grasping it even when it is explained and proofs are demonstrated. I didn't spend much time on an elaborate explanation of the failings of the chain model since it is a failed model. I had discussion of the chain model at the beginning of the theory section before it was insisted over my strong objection that it be demoted to the end of the section. But to spite my formerly placing it at the top, I only meant for it to be a lead in to the correct explanation of how the siphon works, which is that the pressure depends on the height of the columns, not their size or shape or on tension in the liquid.

Understanding the way pressure varies with height seems fairly intuitive to me. The higher stuff is piled up on top of you the more pressure you feel bearing down on you. Why an inverted glass raised up out of a sink full of water, brings the water up with it is much less intuitive. A demonstration that may help in understanding the siphon is of a siphon filled with liquid and having a closed valve at the top preventing flow. Understanding the variation of pressure with height will make it clear that there is a different pressure at each side of the closed valve at the top and if you know what the pressure is on each side, it should be obvious which way the liquid will flow If the valve is opened. If one understands that hydrostatic pressure varies with height regardless of the width or shape of the container, then it will be clear why the fat up tube siphon works just the same as a constant diameter tube siphon. One should also realize that the heavy weight of the liquid in the fat up tube isn't held up by the liquid in the skinny down tube like you would expect from the chain model, it is held up by atmospheric pressure. Imagine a bubble in the top of a siphon that has a fat up tube, so that there is liquid in the fat up tube and in the skinny down tube but not in the top of the siphon. In such a situation the skinny down tube will cause low pressure in the air at the top of the siphon and the liquid in the fat up tube will be pushed up into the low pressure zone at the top. It's important not to think of liquid or an air bubble at the top of a siphon as applying a pulling up force to the liquid on the fat side, because it is actually pushing down on it. The bubble is just pushing down less hard than atmospheric pressure is pushing up. I'm talking about practical siphons here. Siphons in vacuum are a different story. They involve the "negative pressures" or forces of liquid adhesion to the walls and "roof" of the siphon.

Most of my time on this article has been devoted to disproving the chain model and establishing the primary importance of atmospheric pressure along with gravity in practical siphons. You'll notice that the three diagrams in the theory section are of the chain model and two diagrams disproving it. It has been a struggle to get agreement on just that point. My explanations of how siphons work have been repeatedly and stubbornly butchered. I'm not likely to spend a lot of time repeating that effort. I've managed to get it stabilized in a not terribly wrong state with all the important ideas. Deciphering it is left as an exercise for the reader. You're lucky the Wikipedia article has the correct ideas it does, since few descriptions on the web or in books get it as good as this article. Mindbuilder (talk) 01:59, 27 March 2011 (UTC)

Velocity & Volume

The velocity of the liquid traveling through the siphon is well explained in this article, but there is no mention of the volume of liquid (as in gallons/hour, liters/second, etc.). How would the volume of liquid moved during a period of time be determined? To me, this seems like a big gap in the information the article presents. I would guess that it is a function of the size of the siphon (fatter tube = larger volume), but I don't know much about fluid dynamics.

Also, maybe the velocity thing is oversimplified for the sake of the article, but wouldn't it be affected by friction or turbulence such as would be created if the liquid was flowing through a corrugated tube? – jaksmata 21:15, 29 March 2011 (UTC)

It is easy to determine the static performance of a siphon. This is the pressure difference between the fluid at the end of the pipe and atmospheric pressure outside the end of the pipe. Imagine a siphon filled with fluid and ready to flow but prevented from doing so by a tap at the end of the pipe - the greater the difference in pressure the greater will be the velocity and volume flow rate when the tap is opened. In the usual case of a siphon surrounded by atmospheric pressure, the difference in pressure is equal to the vertical distance from the surface of the fluid in the reservoir to the tap at the end of the pipe, multiplied by the density of the fluid, and multiplied by the acceleration due to gravity. This is a very simple equation and gives an exact measure of the static performance of the siphon, regardless of the dimensions or internal characteristics of the pipe. See Fundamental equations of the siphon.

The dynamic performance of the siphon is the velocity and volume flow rate of the siphon when it is flowing freely, such as when the tap in the preceding paragraph is opened. The dynamic performance is influenced by the pressure difference across the tap, but it is also influenced by the length of the pipe, the internal diameter of the pipe (assuming the pipe is circular in cross-section), the surface roughness of the interior of the pipe, and the viscosity of the fluid.

In an article of this kind, where we are talking about siphons in general rather than any one siphon in particular (as could occur in a laboratory,) it is easy to discuss the static performance of siphons, but not nearly so easy to discuss the dynamic performance of siphons. Dolphin (t) 21:34, 29 March 2011 (UTC)

Very interesting, thanks for the answer. – jaksmata 13:25, 30 March 2011 (UTC)

Cohesion modification of opening paragraph

@210.50.53.60 - If you think it is cohesion that pulls the liquid up in siphons at near atmospheric pressure, then how do you explain the siphon effect of the demonstration described in the theory section where the siphon starts out with air at the top? Have you skimmed the talk archive? Mindbuilder (talk) 17:53, 10 October 2011 (UTC)

Revision of wiki definition of the siphon

The experiment conducted by the team at the University of Nottingham (referenced above)is convincing experimental evidence for the gravity/cohesion model of the siphon. It clearly demonstrates that the chain model of the siphon is correct. The chain analogy is a very good way to get those who don't know much about physics to understand how siphons work. The work also clearly demonstrates that the siphons can operate above the height of the column of water that can be supported by atmospheric pressure. Can the gravity/coehsion model explain how a siphon can still operate when a bubble is present that separates the water in a siphon? Yes, it can. The reason that a siphon can still operate when a bubble is inserted into the ascening arm is because water flowing out reduces the pressure of the air in the bubble to below atmospheric pressure, enabling the pressure of the atmosphere to push water into the siphon. When the buble has been ejected from the siphon cohesion takes over. In effect, when a 'wall-to-wall' bubble is present in a siphon, water flowing out acts like the plunger of a syringe being pulled in order to fill the syringe with liquid. However, the fact that differential air pressure can be used to prime a siphon does not mean that this is how a siphon works in normal operation. An analogy to this might be to fill a drinking straw with water. Water can be ejected from the straw either by blowing or by orienting the straw vertically so water flows out by gravity - the same action but two different mechanisms. The Nottingham experiment demonstrates that liquid cohesion operates in a vacuum (a pretty good vacuum at only five billionths of atmospheric pressure). This being the case, why should the forces of molecular cohesion switch off at atmospheric pressure, as required by the atmospheric model of the siphon?

When water flows in a siphon, the lowest pressure is at the apex or crown of the siphon, therefore there is a differential pressure between the crown and air above the level of the water in the upper reservoir. However, there is an equal and opposite pressure between the crown and the air above the water in the lower reservoir. (This is not quite true - the pressure of the air above the lower reservoir is slightly greater than above the upper reservoir). Therefore there is a cancelling effect so there is no net contribution from atmospheric pressure - if anything the atmosphere slightly impedes siphon flow. (A siphon up a mountain will flow slightly faster than at sea level for a give siphon height). It is possible to devise an experiment in which the water level in the upper and lower reservoirs of a siphon are held constant (using a pump to return water from the lower to the upper reservoir). In this case the stationary reservoir levels act like an energy barrier between the atmosphere and water and therefore the atmosphere cannot be pushing water into the siphon. (In analogy this is like a seized piston in an internal combustion engine - if a piston is seized, the hot gases from combusted fuel cannot impart mechanical energy to the engine).

In my opinion, it would be good if the Wiki entry for siphon could be instrumental in getting the numerous incorrect "atmospheric" siphon definitions in dictionaries corrected. As far as I know, the only correct definition is that given by Oxford Dictionaries (http://oxforddictionaries.com/definition/siphon). StephenHughes (talk) 06:59, 12 October 2011 (UTC)

"The experiment conducted by the team at the University of Nottingham (referenced above)is convincing experimental evidence for the gravity/cohesion model of the siphon." You're right about the cohesion model with regards to siphons at low pressures, as is recognized as prominently as the opening paragraph of the current article.

"The chain analogy is a very good way to get those who don't know much about physics to understand how siphons work." I agree strongly. Unfortunately some editors here disagree on the grounds that while the chain model may be intuitive and correct with regards to siphons in vacuum, it is incorrect with regards to siphons at normal atmospheric pressures, and that an incorrect analogy should not be placed prominently in the article. Again, I disagree that it should not be included, and I agree with you that it is a good start to explain how siphons work.

"why should the forces of molecular cohesion switch off at atmospheric pressure, as required by the atmospheric model of the siphon?" Because atmospheric pressure is jamming all the molecules together in every direction. While the molecules will exhibit cohesion and resist being pulled apart, they will also exhibit repulsion if they are pushed together. All of the molecules in a typical siphon that operates near one atmosphere pressure, are at positive absolute pressures, even at the top, where the pressure is somewhat less than one atmosphere, but still positive absolute. Thus there is no tension between any of the molecules, only repulsion, and the chain model does not apply. Is this not correct?

"Therefore there is a cancelling effect so there is no net contribution from atmospheric pressure" The atmospheric pressure at the entrance is not canceled by the atmospheric pressure at the exit because the atmospheric pressure at the exit is also fighting the weight of the extra liquid in the down column. Thus some of the pressure at the exit never "makes it" to the entrance. This diagram in talk archive 3 shows this concept in the context of two guys pushing two trucks up a hill http://en.wikipedia.org/wiki/File:SiphonAnalogyTrucksOnCrestOpposingForcesNotCanceling.png Where one guy is pushing his truck up the hill even though it would seem his push had been canceled by the equal and opposite push of the other guy. Mindbuilder (talk) 19:10, 12 October 2011 (UTC)

Also, what about a siphon with a T fitting at the top where a bubble is permanently trapped throughout the siphoning operation? With a bubble at the top of the siphon, any cohesion would be defeated. Remember, the air bubble would be at positive absolute pressure and therefore it would be trying to push the liquid down, and therefore the liquid must be pushing up on the bubble. If there was any tension at the top of the siphon, the bubble would be happy to expand. So you have to admit that at least for some practical siphons, (e.g. http://www.youtube.com/watch?v=8lXl7tdJ7iY) the chain model cannot hold at any point in the siphoning process. Mindbuilder (talk) 20:05, 12 October 2011 (UTC)

It is interesting to ponder where this transition occurs between a siphon that relies on cohesion and one that has only repulsion. To a first approximation, ignoring drag effects, if you take a siphon that is operating in vacuum and start lowering it into the atmosphere, the liquid in the tube above the lower reservoir will be in compression and without tension if its height above the surface of its reservoir is less than the barometric height at that ambient pressure. The liquid above the barometric height will be in tension. Thus a siphon at low pressures or an extremely tall siphon, could be operating with BOTH the chain model AND the atmospheric model at the same time! But as the barometric height exceeds the crest of the siphon, as is typical in practical siphons, there would be no more tension, and the chain model would no longer apply. Mindbuilder (talk) 21:33, 12 October 2011 (UTC)

Whilst there is many discussions about gravity verses atmospheric pressure, it seems pressure difference gets forgotten along the way. Water moves in a siphon because of pressure difference. It could be argued that atmospheric pressure provides pressurised water, and that this water enteres a siphon, and exits it at a similar pressure, (Ignoring the minor increase due to the outlet being at a lower height), and thus atmospheric pressure can NOT be a nett contributor of energy.

It has been shown in past experiments that a siphon CAN operate in a vacuum. However, for a siphon to operate in a vacuum, some special conditions must apply. The water is usually boiled or degassed, and the pipe or tubing must have a smooth bore. Otherwise the water will boil as soon as it drops below it's vapour pressure.

What also seems to be forgotten in these experimental siphons in a vacuum, is that they all deal with very low heights. A practical siphon can operate up to 10 metres in height at sea level. An experimental siphon in a vacuum appears to be able to only operate up to around 8cm if I read early experiments correct.

All practical siphons operate under pressure or compression, not tension. In a full vacuum, and say with an 8cm lift, then the water is being subject to tension. However, it is not certain that the water has a tensile strength that is contributing. Why? If one is to remove an item from a space, then something must take it's place otherwise a vacuum will form behind it. A classic example is pulling on a syringe with your finger on the inlet. Release your finger and the air will rush in. A practical siphon in a vacuum will have the water boil and vapourize as soon as it drops below vapour pressure. However, what if you could prevent it from vapourising? What would happen then if water was to move out one end of the tube? Well, something has to fill the place of that water, and in the case of water in a vacuum, i.e. an absolute pressure of zero, then you can't lower the pressure further or remove the water without filling the space behind it. So water does indeed fill this space, up to a limit in siphon height of approx 8cm, before the vaporizing issue appears again.

If water could indeed sustain 10 metres of tension, drilling a hole in an operating siphon should not cause it to stop operating.

If water could could sustain 10 metres of tension, then it should be able to operate in a vacuum up to 10 metres in height.

The maximum height of a practical siphon is limited by atmospheric pressure. Why? Because it is back to the pressure difference. Put a practical siphon in a vacuum and the maximum height drops to close to zero. At altitude, the maximum height, it is reduced. Put it in a pressure chamber, and suddenly the maximum height is increased.

As stated above, pressure difference is a key issue. What ever atmospheric pressure (or air pressure if placed in a chamber) you have at the inlet/outlet, well your maximum height will be dictated by this. Tensile strength of water has nothing to do with it. — Preceding unsigned comment added by 58.165.131.2 (talk) 23:43, 12 October 2011 (UTC)

Andrew K Fletcher demonstrated a siphon, or at least an inverted U-tube of water, going up 24m http://www.youtube.com/watch?v=sz9eddGw8vg The Z-tube also demonstrates water at high negative pressures. Mindbuilder (talk) 00:08, 13 October 2011 (UTC)

Andrew K Fletcher himself confirmed that it was not a siphon. He also indicated that when he tried to make it operate as a siphon, it didn't work. So it can only be the inverted U tube. The video has at least 2 takes. Just past 2:07, they stop to fill the inlet jar around the 14 metre mark. At 3:22, a 2nd camera view starts showing and there is no stopping to fill the container when they go past 14 metres. 2 takes is not a crime, yet Andrew's own posting on youtube says the video is unedited. Not magic, just sound repeatable science he claims on youtube. So what went wrong with the 1st take. And why has no one else repeated the exercise. There is no footage of what is happening at the top of the tube. There is no clear footage of what is happening with the jars of water. Above 10 metre height, there will be quite a bit of negative pressure on that tube. Andrew advises it does not collapse, yet how does he know when there is no footage of what is happening. We would expect the water to turn to vapour above 10 metre height, again no footage. What if the water does indeed turn to vapour, that would still have water flowing out the exit pipe. From a siphon point of view, I think the idea from this experiment is that the water was able to have a height greater than 10 metres without breaking. From the video evidence alone, there is no clear proof of this, beacuse no one can see/know what is happening at the top of the tube, and there is not a clear indicatin as to when the tube stops flowing and why. I would suggest that this experiment would need to be repeated in a more scientific manner, with much robust proof before we can draw anything from this. You would also think that if the water didn't "break apart", then it should have worked as a siphon. Maybe that suggests that it worked some what like a siphon before 10 metres and somewhere beyond that, a vaccum occurred at the top and water still ran out the outlet tube.

In relation to Z tubes, they may be able to show high negative pressures, yet that is because of the very nature of the Z tube. The water can't escape as the centrifugal force has it heading outwards on both sides of the inner tube. So ofcourse a negative pressure will occur. The outward flow/direction of water is creating the negative pressure - and so the water has a weight density issue fighting against the creation of a vacuum. The weight/density of the water is winning out, yet cut the end off the Z tube to allow the water an opportunity to escape, and it has no tensile strength at all, and will run out the tube. So there is a clear difference between negative pressures and tensile strength. 123.211.208.229 (talk) 05:12, 15 October 2011 (UTC)Moonshine

Had a look at the youtube video on the vacuum siphon. How do they seal the rod going up through the top of the container? Under a vacuum, I would have thought air would leak past this point. How is it that he can easily pull that rod up? As he moves the rod up, part of the rod is going from the inside to the outside of the container. If it is already under an almost full vacuum, this would only increase the vacuum. It should not be so easy to pull that rod out, similar in a way to trying to pull a syringe with the end sealed. And he then takes his hand off it and it stays at the top. I would have thought the vacuum would pull it back in. They may have tested it under vacuum conditions, but I would swear when looking at the video clip, that the container is not under vacuum in this video.

They acknowledge that the liquid has a stickyness. It's a pity they don't demonstarte this. They advise atmospheric pressure has nothing to do with a siphon, It is the stickyness of the liquid, the surface tension they claim Yet water has a surface tension that can support only about 4mm You can overfill a glass by this much, and 4mm is about the biggest size of a water droplet before it breaks off. So how does a practical siphon involving water able to go 10 metres in height. So does atmospheric pressure just stop the water from boiling? Well it does that. However, atmospheric pressure also provides water with it's stickyness like property. Atmospheric pressure is pushing the water together. It is under pressure, not tension. However, in a siphon tube, it behaves like it has stickyness. If it was sticking together because of tension, then the fat upper leg siphon would win out on weight. However it is the pressure due to the height of the water in either leg, that causes pressure difference. And then water moves in the required direction to balance these pressure differences. If atmospheric pressure played no part, why do we see a diffence in the maximum height at different altitudes? 123.211.208.229 (talk) 05:12, 15 October 2011 (UTC)Moonshine

"why has no one else repeated the [AKF] exercise." Probably because the discovery has already been made and documented, so the effort to repeat it would be for nothing but verification. Nevertheless, myself and Micolich have both looked into repeating it. I doubt either of us will. I share with you a little doubt about the experiment, but there are reasons to believe it is real. There is the Z-tube and trees to demonstrate that high negative pressures in water are possible. However, when I put some tough questions to AKF, like how I was surprised at how well the polar water adhered to the non-polar Nylon tubing, he gave no answer. Did he just simulate what he believed must happen, but used the wrong kind of tubing? Or does water adhere better to Nylon than I thought?

"the water has a weight density issue fighting against the creation of a vacuum. The wight/density of the water is winnign out" I don't know what you mean by this. I would say the opposite. The inertia of the water tends to fling it outward and thereby encourage the formation of a vacuum in the middle, at least when it spins fast enough. If you cut the end off a Z-tube the water will come out, but it will probably maintain its cohesion and come out all in one "piece" or slug of water. It's true that liquid tensile strength and negative pressures are poorly defined and possibly confused, but the point is that it appears a water siphon *could* work above 10m, using whatever you want to call the property sometimes called liquid tensile strength or cohesion.

The rod into the vacuum container probably just has a rubber seal lubricated with some low vapor pressure liquid. It looks like the chamber was actually manufactured to have the ability to mechanically manipulate experiments under vacuum. If the cross section of the rod was for example one third of a square inch, the force to remove it from the container would only be about 5lbs (or 20N or 2kg) If the friction of the seal was more than that force, then it would hold its position when let go. Another trick that could be used is to run a rod in one side of the container and out the other. Then the atmospheric forces would balance at each end of the rod leaving no extra effort to push it back and forth. Although it doesn't appear that their container used that trick.

It would be nice if you'd sign your comments. You don't necessarily have to log on. Just type any consistent nickname at the end of your posts. This is useful because with just IP addresses that change, it can be difficult to keep track of the thread of the conversation and to know what responses are appropriate to which poster. If privacy is an issue, I'd say logging in is actually more private, because if you don't log in, everyone in the world gets your IP address instead of just Wikipedia. Mindbuilder (talk) 16:04, 13 October 2011 (UTC)

Finally: The Truth about Siphons!

It is simply "Pressure difference"

That's it, for both practical siphons and experimental siphons.

Of course, "Pressure difference" involves several ingredients: density, gravity, atmospheric pressure and the siphon tube. What it doesn't require or include, is liquid cohesion or tensile strength.

So all practical siphons move water because of pressure difference. During priming or operation, there is no switching of modes to cohesion. A siphon with a bubble will still work, proving there is no cohesion. Also, a multi bubble siphon will work, i.e. where you continully let small bubbles in the inlet side. The pressure differenc model explains this, since there is no change in mode.

If you siphon a chain through a tube, it will work because of tensile strength Drill a hole in the tube, and it will still work, because the tensile strength is not effected by drilling the hole Siphon water through a tube, and if cohesion and tensile strength was at play, drilling a hole in the top of the tube should have no effect.

Water needs atmospheric pressure to keep it as a liquid. So a siphon needs atmospheric pressure, one to keep the water as liquid. However, it also sets the limit on the pressure difference. So the atmospheric pressure is not a nett source of energy. However it sets the limit to the pressure difference and thus the maximum height. At sea level, you have the equivalent of 10.3 metres (water) height of air pressure. If water gets below 0.24 metres, it will vapourize. So we can allow our water that is pressurised to 10.3 metres absolute get almost to 0, before the siphon stops. At altitude, our starting pressure is lower, and thus our maximum height is lower. If tensile strength was the limiting factor for maximum height as Dr Hughes claims, then there would be no change to the maximum height, and drilling a hole in the top of the siphon tube would not stop the flow.

What about in a vacuum. Notice the first thing that is changed or modified is the liquid, usually by degassing it. This prevents it to some degree from vapourizing. Thus we no longer need the atmospheric pressure to keep it in liquid state. And removes to some degree the height limitation before the liquid vapourises. However, the removal of atmospheric pressure doesn't change the ability to have pressure difference.

Hughes suggests "The experiment conducted by the team at the University of Nottingham (referenced above)is convincing experimental evidence for the gravity/cohesion model of the siphon."

Not the case at all. It doesn't prove cohesion is present. Drill a hole in the top of that siphon tube and that siphon will stop operating. The drilling of the hole would not change the cohesion/tensile strength of the liquid if it existed. It would change the pressure difference.

It appears possible to have negative absolute pressures in certain circumstances. The top of the University of Nottingham siphon would be an example. And thus pressure difference still occurs and causes the liquid to move

The following link shows a non tube siphon. Now clearly this has some cohesion/tensile strength, unlike water. http://www.youtube.com/watch?v=j65Emw9yRWw

So cohesion and the chain model in practical siphons is dead.

Suggest that this line in the opening paragraph needs to be reviewed. That siphons can work in a vacuum does not provide proof about tensile strength. Pressure difference again can likely explain that.

"In the laboratory, some siphons have been demonstrated to work in a vacuum,[1][2][3][4][5] indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures."

123.211.208.229 (talk) 05:12, 15 October 2011 (UTC)Moonshine

Negative pressure is just another way to describe cohesion or liquid tensile strength. The reason a siphon in vacuum won't work if you drill a hole at the top is because the liquid must adhere to the tubing. For example, experiments in the Z-tube show that if the inner surface of the Z-tube is coated with oil, the water in the tube will demonstrate no liquid tensile strength. The Z-tube will also fail if you drill a hole in the middle, but there must be some cohesion in the Z-tube as well as in a siphon in vacuum. Mindbuilder (talk) 04:03, 15 October 2011 (UTC)

Negative pressure and cohesion/liquid tensile strength are 2 entirely different beasts. Nature abhores a vacuum. People mistake the reluctance/difficulty creating a vacuum with liquid in a tube to mean that it has tensile strength. How can the liquid be adhering to the sides of the siphon in the vacuum when it is flowing?

In the siphon in the vacuum experiment, assume that the up leg is 5cm height, and the downleg is 10cm. When they are first moved into these positions, before the liquid starts to flow, the pressure on the liquid surface of the 2 containers is the same. Travel up the up leg, and it reduces by 5cm. Travel up the downward leg, and the pressure reduces by 10cm. At the top of the siphon, there is now a 5cm pressure difference. You can't have a pressure difference with fluid in that situation, so the fluid balances out the pressure, moving from the higher pressure area to the lower pressure area, i.e mfrom the upleg side to the downward leg side. Drilling a hole in the top finds an alternative solution to the pressure difference, allowing them to seperate and go down their own legs. If the liquid was indeed sticky like they claim, if it had tensile strength, it could easily pull the 5 cm side up, and thus the chain and pulley analogy might have some merit.

Cohesion is not a required item in all the different experiements shown so far. There is no cohesion shown when you reach in and take a handful of water out of a bucket. Yet put it in a tube like a siphon, and now we have people arguing that it has tensile strength. It can only support about 4mm of it's own weight. Inside a tube, it is the difficulty of creating a vacuum that is misleading people.

As stated above, pressure difference is the key that explains the operation of all siphons, both practical and experimental. What is surprising in the latest video on the vacuum siphon, is from the outset they advise you can't put water in the vacuum as it will simply boil. So right there, what they have acknowledged, without admitting it, is that a practical siphon needs atmospheric pressure. Near the end, the 2nd presentter claims that atmospheric pressure is not required. Did he not listen/talk to the first presenter. Did he not give any thought to why we have a maximum height of 10 metres at sea level, and a reduced level at elevation. Did he not give any thought to pressure differences.

People are grabbing one experiment and then saying it prooves their theory. When you look at all the evidence, and the different views expressed along the way, there is only one model that explains the operation and fits in with the results of all these experiments. Note also if one has a siphon and increases the pressure above the outlet container, then due to pressure diffewrence, the liquid will flow in the opposite direction, i.e uphill against the forces of gravity.

123.211.208.229 (talk) 05:12, 15 October 2011 (UTC)Moonshine

The idea that nature abhors a vacuum has been obsolete in physics for many years. Nature abhors a vacuum just as much as the pressure of the liquids or gasses trying to push their way in, and no more. Take the examples of the barometer and a piston in a cylinder. Nature abhors the vacuum in a barometer just as much and not more than the atmospheric pressure. There is no particular difficulty creating a vacuum at the top of a barometer. The siphon in a vacuum is just a barometer until the levels of the reservoirs are changed. A piston in a cylinder, such as the plunger of a syringe, is another example where creating a vacuum is only as hard as overcoming the atmospheric pressure pushing the plunger in. In fact nature has so little resistance to creating a vacuum within the siphon in vacuum, that one has to be careful with the siphon or it may "break", for example with a gentle tap. Nature also has no problem with the vacuum created within a Z-tube coated with oil and filled with water. It would be an odd coincidence if nature only abhored a vacuum within a tube completely filled with a sticky liquid like water that can adhere to the walls of the tube.

Liquids often stick to things when they are flowing. If you hold your arm at a 45deg angle and pour water on the upper part, the water will likely stick to your arm as it flows down the 45deg angle instead of just running around your arm and falling straight down. Oil sticks to your skin even more tenaciously, even when flowing, requiring soap and water to get it off.

Although siphon liquids can easily pull up 5cm, the tensile strength they use to do it is an odd kind of tensile strength, considerably different than the tensile strength of solids. It's so different in fact that some persons like Dolphin have even argued that it should not be called tensile strength. I'm not entirely unsympathetic to that view. A term other than tensile strength would be desirable, except that there is a certain usefulness to the familiar term tensile strength when explaining the idea to people who didn't realize liquids could do such things. The term cohesion is often used instead of liquid tensile strength.

"There is no cohesion shown when you reach in and take a handful of water out of a bucket. Yet put it in a tube like a siphon, and now we have people arguing that it has tensile strength." Yes, that counterintuitive idea that turns out to be so well supported by the evidence is what makes this subject so interesting. It takes a while to wrap your head around it.

They were probably mistaken when they claimed water couldn't be siphoned in a vacuum, at least if it is near ice cold. And there is reason to believe, including the Z-tube and Andrew K Fletcher's experiments, that water can be siphoned to a height considerably more than 10m. Naturally if you have bubble forming gasses like air or carbon-dioxide, dissolved in the water, it is much less likely to be possible. Mindbuilder (talk) 16:42, 15 October 2011 (UTC)

My apologies. My reference to "Nature Abhores a Vacuum" was in relation to Negative Absolute Pressure. Of course, this was not the original use of term. As you indicate, they original thought vacuums couldn't exist. This was also partly to do with ensuring one survived. The absence of any substance meant then the absence of God. The religious nutters who controlled the world would have nothing to do with that, and researchers risked their life reviewing the topic.

In a practical siphon, when the pressure gets below the vapour pressure, the water turns to vapour. The next question is then what if we could prevent it vapourizing? With the vacuum experiement, that is the 1st thing that has to be done, either by boiling/degassing the liquid, or changing to something other than water.

Now consider what are the consequences of such a set-up when it reaches and then attempts to exceed zero absolute pressure (by exceed, I mean try and go to negative absolute pressure).

Negative absolute pressure should not be possible, how ever what if we create circumstances that could do this. Like what? Well the vacuum siphon shown in the Youtube video. If the pressure at the inlet/outlet is zero absolute, then the pressure at the top of the siphon tube must be below this by the height of the tube above the inlet(and any friction loss).

If negative absolute pressures are not suppose to be possible, then shouldn't the water just fall down partly on either side, and a void created at the top. Therein lies the proble, we just can't create a void of nothing, something has to fill that area. If the liquid has been treated/selected so it can't vapourize, then we simply can't get a void, because we are already at absolute zero pressure.

What we still have, is the fact that water finds it's own pressure balance. Example, fill one side of a tank and you can't have more pressure on one side. In the siphon, if we create a situation where water is at a lower pressure to the water next to it, it is going to find it's balance and water will move in the direction required to even out the balance. Whether the absolute pressure is postive or negative shouldn't matter from one point of view, i.e., we have a pressure inbalance, it needs to be evened up.

I suggest that this reluctance to create a void (absolute negative pressure) at the top of the siphon, and the requirement to balance out the pressure is what people are mistaking for tensile strength.

I suggest that the view that atmospheric pressure pushes the water into the inlet leg creates some issues when we remove atmospheric pressure. Once atmospheric pressure is removed, i.e in the vacuum experiment, then you have to put something in it's place, i.e tensile strength.

I suggest that whilst atmospheric pressure is critical to the operation of a practical siphon, and sets the limit on maximum height, it should not be seen to be pushing the water into the inlet. Once you see that water moves inside a siphon because of pressure difference, you realize we don't need changing rules for when there is or is not a bubble, whether we are or are not in a vacuum, whether you raise or lower either side of the siphon, or whether you add pressurised air to the outlet side, so the siphon flows backward.

Degassing the liquid, to prevent vapourizing means that we can extend the maximum height. With the difficiulty as noted above of creating a void of nothing in a siphon, the potential height could be much greater in theory, still without the need of tensile strength.

So in theory, Fletchers experiement could work. However, I would express 2 additional concerns about it. One, plastic pipes, and in particular plastic tubing has very little resistance against collapsing in below atmospheric pressure situations. A pipe that has a 60 metre pressure rating, might only be good for 6 metres vacuum (4 metres absolute). So I thinl his tube would be expected to collapse. He indicates he was using only water, with some salt added. It was degassed, however earlier experiements by others showed that it was difficult to get a siphon operating much above the original height limitation without the vapourizing issue returning. — Preceding unsigned comment added by 124.180.90.11 (talk) 04:58, 16 October 2011 (UTC)

"we just can't create a void of nothing, something has to fill that area" I see no basis for that idea. And I've never heard that idea asserted by modern physicists. Mindbuilder (talk) 09:04, 16 October 2011 (UTC)

Okay. If we remove everything from a room, furniture and air, we have created a vacuum. We then can't go back a 2nd time and remove more, there is nothing to remove. In relation to my comment "something has to fill that area", I was not suggesting that we can't create a vacuum. Or that someting then has to fill that vacuum. I was relating to our attempt to go back for a 2nd grab of stuff. So in relation to the siphon in a vacuum, once we get to the point of going into absolute negative pressure, we are going back for that 2nd bite. So if the water on the downleg is trying to, for the want of a better word, pull the water apart at the top, how can we seperate the water and leave a void if we have already reached absolute negative pressure. And it's the difficulty of trying to remove something after we have already removed everything, i.e the difficulty of creating an absolute negative pressure as to why we don't get a void at the top of the siphon in the vacuum experiment. The pressure difference between the water means it is easier for the water to move across (via balancing the pressure difference) rather than trying to create a absolute negative pressure void.

Moonshine124.180.90.11 (talk) 09:43, 16 October 2011 (UTC)

"how can we seperate the water and leave a void if we have already reached absolute negative pressure" I don't see the difficulty there, the liquid simply separates and leaves a void at zero absolute pressure. You also appear to be equating the reduction of pressure below zero with the removal of something. Pressure can be reduced to zero without removing any material. Pressure is like a train car being pushed by other cars. If the car is pushed on less and less until it is no longer being pushed on at all, then we could say the pressure has gone down to zero. If the car is then pulled, we could call that negative pressure. If the car is being pulled and the coupling breaks, i.e. the atoms fail to maintain sufficient cohesion, the "pressure" goes from negative to zero. It is not as if taking away all the push on a car means that negative pressure can't be reached due to there being no more push to be taken away. When all the push is taken away, it can then just transition to pull. Mindbuilder (talk) 10:26, 16 October 2011 (UTC)

A question: One syringe. No liquid. Flat end to plunger and end of inside syringe Pushed fully in. Seal inlet end. Put in a vacuum chamber (like the one in the recent youtube vacuum experiment)

Could you open the syringe in the vacuum? i.e could you pull back that plunger easily?

Moonshine124.181.94.9 (talk) 02:19, 17 October 2011 (UTC)

Yes, I expect it would be just as easy as it would be when opening a syringe under normal one atmosphere conditions, (when the inlet is not sealed so that air can enter), because typically plungers don't have much adhesion to the cylinder end. On the other hand, if there were a little sticky liquid like that used in the vacuum siphon experiments filling the syringe in vacuum with a plugged end, and no air or vacuum bubbles or other features on the inside of the syringe that cause or ease separation, then I expect it might be fairly hard to pull the plunger out.

I thought of another way to come at this problem. It is said there are only four forces of nature (or three nowadays?). I think we can ignore the strong and weak nuclear forces. In the situations we are considering, gravity only pulls down. In the siphon in vacuum experiment, some force is holding the liquid at the top of the siphon against the pull of gravity. Of the forces of nature, electromagnetic forces are all that could be holding up the liquid. The electromagnetic forces we are concerned with here are basically only significant over short ranges, basically only between the atoms and molecules as they touch each other. There is no "nature abhors a vacuum" force or "nature resists negative pressure" force. There is only the electromagnetic forces as the molecules touch each other. And of these electromagnetic forces there is basically only repulsion and attraction. The liquid molecules at the top of the siphon are not being pushed up by repulsive forces from the molecules below(at least not in the vertical sections of the tube). Thus the only force left to account for why these liquid molecules at the top of the siphon don't fall, is the attractive forces between the liquid molecules and the other liquid molecules and the walls(including the "ceiling") of the tube. These attractive forces between the molecules and the tube are what we call negative pressure, liquid tensile strength, or liquid cohesion. Pressure differences are not an alternative explanation to cohesion or liquid tensile strength for siphon operation, because the negative pressures and pressure differences themselves demand an explanation. That explanation IS cohesion or liquid tensile strength. Mindbuilder (talk) 06:15, 17 October 2011 (UTC)

This article is wrong.

This is a new development by experiments at the University of Nottingham have shown that siphons do not need atmospheric pressure to allow a siphon to work. Paper Here: [1]. Instead when put in a vacuum siphons still manage to function, and it is now thought that molecular cohesion and gravity drive the ability for a siphon to work. There is an interesting video for anyone who wants to see here: [2]. --MarcZimmer (talk) 04:44, 12 October 2011 (UTC)

Why is the article wrong? At the end of the introductory paragraph there is the following sentence: In the laboratory, some siphons have been demonstrated to work in a vacuum, indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures. (The sources for this sentence are suitably identified using in-line citations.) Dolphin (t) 05:04, 12 October 2011 (UTC)

This article is wrong as it still uses atmospheric pressure as the main driver of a siphon, which is not the case it is clearly gravity. --MarcZimmer (talk) 17:26, 12 October 2011 (UTC)

Your objection is perplexing to me. As we were drafting the current version, I tried to make sure that we emphasized that gravity was indeed the main driver of BOTH siphons in vacuum AND typical siphons operating near atmospheric pressure. Consider these quotes from the opening paragraph. "powered by the fall of the liquid as it flows down the tube under the pull of gravity" and "The reduced pressure is caused by liquid falling on the exit side."

But you have to admit that at least in some siphons, such as a siphon starting out with only air at the top, it is atmospheric pressure that pushes the liquid up, at least at first (of course such pressure difference being ultimately caused by gravity acting on the down leg). Mindbuilder (talk) 17:50, 12 October 2011 (UTC)

I thought this was wrong too (per the above source), but now I'm not sure. It's this sentence that has me wondering: "In practical siphons, atmospheric pressure pushes the liquid up the tube into the region of reduced pressure at the top of the tube". Is a "practical siphon" a particular kind of siphon for which atmospheric pressure is a relevant force? Or something like that? As written, that introductory para certainly gave me the impression that siphons rely on atmospheric pressure, though that impression was quickly dispelled by the rest of the article. Harry Metcalfe (talk) 01:14, 22 October 2011 (UTC)

It has been often claimed that no siphon has ever exceeded the barometric height. So by practical siphons we mean practically all of them. Of course now we have finally gathered together a few references and a video of siphons operating at heights exceeding the barometric height and even in near perfect vacuum. But if you try to do something useful with a siphon at heights exceeding the barometric height, it would probably be a rare exception if you did not fail. A siphon exceeding the barometric height is always on the edge of failure, barely holding on. It takes very little to break it, a tiny air bubble, dissolved gas, a rough spot in the tube causing turbulence, a tap, or possibly even a loud noise. I would say siphons do rely on atmospheric pressure. I could easily be wrong, but I doubt if there would be a single useful application of siphons without the assistance of atmospheric pressure to keep the delicate siphons from breaking. Mindbuilder (talk) 05:41, 22 October 2011 (UTC)

Assumption in explanation using Bernoulli's principle

The section Siphon#Explanation using Bernoulli's equation makes the tacit, and I think bold, assumption that the velocity of the fluid is constant at all points during its flow through the siphon. Can someone please provide a justification for this assumption? Noldorin (talk) 03:23, 13 November 2011 (UTC)

Hi NoldorinElf, Fluid velocity in a siphon will only be constant if the cross-sectional area of the tube is constant along its length and the siphon height is constant. From conservation of material, siphon inflow must equal outflow - the expression "what goes up must come down" also applies to a siphon. However, there can be variations in flow velocity along the length of a siphon. Flow (F) in a tube is defined as F = v x A, where v is the average fluid velocity and A is the cross-sectional area. Therefore if the cross-sectional area of a siphon varies, the velocity must also vary. I've just done a quick experiment in my kitchen (while the rest of the family are asleep in bed) to demonstrate this is the case. I found a length of 6 mm clear PVC tube in my garage (I've got a supply of tubing at home for siphon experiments when I'm away from my university laboratory) and used a clothes peg to produce a constriction in the middle. I got out a plastic microwave container, filled it with water and ripped open a tea bag and emptied the tea leaves into the water. I put the two ends of the tubing together under the tap to prime the siphon with water and then put one end of the tube into one of the containers next to the sink and the other end above an empty container in the sink. I then ran the siphon and could see the tea leaves speed up as they entered the constriction and slow down on the other side. There may also be variations in flow within a cross-section. Normally in a tube (e.g. a blood vessel) flow is fastest in the centre and zero at the walls. However, I'm not sure about this in a siphon. I will do some experiments using a medical ultrasound scanner and see what the flow pattern looks like and get back to you. StephenHughes (talk) 14:21, 13 November 2011 (UTC)

I agree with StephenHughes. In real flows through closed conduits (see pipe flow) there will exist a variation in fluid velocity from a maximum at the center of the conduit to zero at the wall of the conduit. This variation is due to viscous shear forces. However, Bernoulli's principle assumes an inviscid fluid; it assumes there are no shear forces operating within the fluid; and it assumes there is no loss of energy or input of energy to the fluid. Consequently Bernoulli's principle assumes no variation in velocity from the center to the wall of a conduit, and Bernoulli's principle can't be used to calculate the variation that does exist in the velocity profile of flow of a viscous fluid in a conduit. Despite this, Bernoulli's principle is a powerful tool in predicting and explaining the variation of static pressure and flow velocity in flow patterns where boundary layers are thin and influence only a small proportion of the fluid. Dolphin (t) 21:54, 13 November 2011 (UTC)

Tensile strength necessary

The statement "common siphons can easily be demonstrated to need no liquid tensile strength at all to function." is incorrect. Tensile strength is necessary on the long leg. You can't siphon sand. see http://www.physicstoday.org/resource/1/phtoad/v64/i8 209.158.255.111 (talk) 16:33, 7 August 2011 (UTC)john dooley aug 7, 2011

The siphon is a phenomenon of fluid mechanics. Sand is not a fluid so the sand analogy is not pertinent to the subject. Please explain why you believe tensile strength is necessary on the long leg. Dolphin (t) 22:45, 7 August 2011 (UTC)

@john dooley - The liquid in the downward long leg is under compression in the demonstration described, not tension. If you think the liquid in the long leg is under tension, from what object or source does the tension force come? Remember, in the demonstration we're talking about a siphon that starts out with air at ambient pressure in the top. Also remember that for a siphon in vacuum, tensile strength is indeed necessary. But once the apparatus is submerged in our atmosphere, all the fluids in the siphon transition to being in a state of compression, all the molecules squeezed together, repelling each other, not in tension.

As pointed out in the article, you can also introduce a large bubble into a flowing siphon, and if any part of the siphon were in tension, the bubble would break the siphon at the point of tension on its journey from entrance to exit.

Finally, the fact that you can't siphon sand is not because the sand has no tensile strength, rather it's because the sand allows air to leak through, dissipating the pressure differences that make typical siphons work (pressure differences which were caused by gravity). It's also because sand has much higher friction to flow. If the friction of sand were low enough, the sand could be siphoned, despite some pressure losses to leakage and despite it's lack of tensile strength. Mindbuilder (talk) 08:37, 8 August 2011 (UTC)

FWIW, following up on Mindbuilder’s suggestion, you could in principle siphon sand (or round balls, say ping pong balls, which also don’t have tensile strength) by an experiment similar to Pascal’s with mercury and water – have two beakers with sand and a siphon between them, then pile heavy substance that doesn’t leak (larger balls, sticky tar, etc.) on top – the pressure would (if sufficient to overcome the friction needed for sand to flow) push the sand up into the siphon and over, just as in an ordinary siphon. As MB notes, it’s the leakiness of sand and its flowing friction that cause the issue, not its lack of tensile strength.

—Nils von Barth (nbarth) (talk) 09:05, 23 December 2011 (UTC)

Also, as cited in Siphons, Revisited (Richert), one can siphon carbon dioxide (shown experimentally), which has negligible tensile strength.

—Nils von Barth (nbarth) (talk) 18:01, 23 December 2011 (UTC)

12m Siphon

Dr. Hughes, if you've got tubing in your garage, do you by chance have enough to do a 12m siphon? I'd love to see video of that, whether it succeeds or fails. I'm especially interested to see someone replicate AndrewKFletcher's claim that the water would not flow at the top of his siphon without salt, and that the water would recoil up into BOTH sides of the tube if it was removed from the reservoirs. Did the water expand and crystalize at negative pressure? Glass tubing would have the best chance of success due to water's strong adhesion to it, but it is unwieldy. You could perhaps do just the top 2m of the tube, that is at negative pressure, in glass, and the rest below in plastic. You could also perhaps adapt a couple garden hoses to the bottom of the 2m of glass tube to save purchasing that much tubing, except they might not be able to resist atmospheric pressure with near zero pressures inside. Of course hoses wouldn't be as nice as clear tubing all the way up for the video proof. Mindbuilder (talk) 07:18, 14 November 2011 (UTC)

Hi Mindbuilder, I've got 30 m of 12 mm ID PVC tubing at work. I will try what you suggest. I could use clear armoured hose at the top that might resist the crushing of atmospheric pressure. I won't be able to do it for a few weeks since I need to gain access to the university theatre that has 15 m of vertical height and I've got piles of exam papers to mark. StephenHughes (talk) 14:01, 14 November 2011 (UTC)

You should be warned that AKF found that unfortunately his siphon would break if he put a T-fitting at the top. So pressure readings may be hard. A pressure gauge with negative scale readings may be hard to find anyway. You might find vacuum gauges for use under increased atmospheres that go to less than 1 atm vacuum. AKF used, I think, 6mm ID Nylon beverage service tubing. I was surprised the water would adhere to hydrophobic Nylon to 24m. I would expect PVC to adhere a little better, but I'm just guessing. I think beverage service tubing has the advantage of being very clean inside from the factory, for reasons of health and purity of flavor. Other tubings may have manufacturing lubricant residues left over. Experiments with the Z-tube show that a lubricated tube will not support any negative pressure. It may be necessary that all the tubing above 10m be smooth and continuous inside. Of course the water must be boiled. The better it is chilled, the lower the vapor pressure will be. I probably would initiate functionality with clear water and then put in some food coloring and record it on video making its way to the exit. Have fun. If you have a hard time getting 12m or more without breakage, try adding some soap to the water, to encourage adhesion to the hydrophobic tubing or contaminates. Mindbuilder (talk) 02:19, 15 November 2011 (UTC)

Barometer & hydrostatic pressure

This appears to be a frequently confusing topic, presumably because there are a few things going on.

What seems the easiest way to understand for me (and how it was figured out historically, i.e. by Torricelli and Pascal in the mid-17th century) is to start with a barometer, and note that water can rise to about 10 meters but no higher, and thus discover atmospheric pressure (if you’re Torricelli). This is (by no coincidence) the same as the maximum height that one can siphon, which shows pretty clearly that they are working by the same principle (i.e., atmospheric pressure), and explains clearly why water goes up into the siphon. (The other theory, by Galileo, say, is that the vacuum at the top pulls the liquid up, and also used a model of a cord not being able to hold up weight. This was disproven by Pascal, via the mercury siphon under water pressure with air in the tube – no vacuum in sight.)

The issue of why liquid flows through the siphon seems more confusing. As I understand it, this is due to hydrostatic pressure (which is ultimately what’s underlying the barometer as well), which is a bit abstract and confusing. I’ve had a shot at explaining the siphon using this (in this edit), specifically by first analyzing the two vertical tubes separately, noting that the pressure at the top of the longer tube (from bottom basin) is lower, and thus that liquid flows across the horizontal section from high pressure (top of tube from top basin) to low pressure (top of tube from bottom basin). This models a siphon as being two connected barometers, which seems to clarify things: understand two barometers separately, connect them, and realize that there’s a pressure difference at the top, hence flow.

More direct is following the pressure gradient up the first tube, across, then down the other tube, and get that the pressure at the bottom of the bottom tube is higher that atmospheric pressure, hence liquid flows out of the bottom tube. This is better for more sophisticated analyses and necessary when the siphon isn’t a barometer on both sides (i.e., second end isn’t connected to lower basin), but is a bit trickier to understand because you have to buy the notion of pressure changing across the siphon, which isn’t immediately visible.

Exposition-wise, I think it’s key to emphasize the connection with the barometer, stating early and often that they work for the same principles – that’s why the maximum height of a siphon is the barometric height, and that is how this was discovered historically. To explain the flow, ultimately hydrostatic pressure must be understood – the key point is that the liquid is under pressure, not tension and is being pushed through, not pulled. A diagram (showing the changing pressure) would help, and how to arrange exposition can take some work. I’ve had a shot, and this article seems heavily edited (and discussed), so I’m sure edits and discussion will continue – hope this helps!

—Nils von Barth (nbarth) (talk) 18:59, 23 December 2011 (UTC)

Siphon Coffee?

The operation of "siphon coffee" doesn't involve a siphon and should be removed from this page entirely. Windsor (talk) 00:10, 21 January 2012 (UTC)

I concur. Siphon coffee uses the word in the much broader, almost meaningless sense of liquids flowing through tubes, just like the "siphon bottle" discussed below. Discussion of it in the theory section seems to add more confusion than clarity. It does demonstrate the phenomenon of downward pressure pushing liquid up, but I think a barometer shows that just as well, without adding complications of high heat and vaporization. I say go ahead and move that section down by the siphon bottle section. Mindbuilder (talk) 04:38, 21 January 2012 (UTC)

Banu Musa Double Concentric Syphon

Why was this cited as verification needed? Was there a dispute that the Banu Musa invented the double concentric syphon? They certainly were the first to write about it, as I have verified with my copy of Hill's translation. I have removed the little note to the citation.

Actually there is no mention of the concentric syphon, perhaps I will write a little section about this useful embodiment of the syphon. — Preceding unsigned comment added by Celephicus (talk • contribs) 22:07, 30 January 2012 (UTC)

Deleted ````

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Additional factors?

"The maximum height of the crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure."

Should this list of factors not include the value of g, the acceleration of gravity, as well as the degree of rigidity of the tubing with respect to pinching off? What would the maximum height of crest be for a siphon operating with water on the moon in a habitat with normal earth atmospheric pressure? NitPicker769 (talk) 02:16, 5 August 2012 (UTC)

I've reworded. That wasn't the point of that paragraph. Samw (talk) 04:28, 6 August 2012 (UTC)

Some additional research http://reu.eng.hawaii.edu/harp/sites/reu.eng.hawaii.edu.harp/files/mcguire_finalpresentation.pdf — Preceding unsigned comment added by 58.168.139.95 (talk) 02:50, 24 November 2012 (UTC)

'Open Syphon Effect'?

Is this any different to a normal syphon? Why does it seem to have a more specific name? --عبد المؤمن (talk) 16:06, 5 January 2013 (UTC)

The Straight Dope

At http://www.straightdope.com/columns/read/2372/how-does-a-siphon-work the Straight Dope makes some claims about siphons, which are likely to result in multiple edits by well-meaning new editors. To those editors, your contributions are welcome, but please read WP:V and WP:CONSENSUS first. --Guy Macon (talk)

Here are some interesting links on the topic:

http://www.guardian.co.uk/science/blog/2010/may/10/dictionary-definition-siphon-wrong

http://www.phys.uhh.hawaii.edu/documents/TPT-final.pdf

http://eprints.qut.edu.au/31098/

http://clui.org/ludb/site/los-angeles-aqueduct-jawbone-canyon-pipe

http://www.youtube.com/watch?v=8F4i9M3y0ew

--Guy Macon (talk) 20:11, 15 May 2013 (UTC)

Should explain why altitude is "0" in Bernoulli's equation

I think there should be a brief mention as to why "y" in Equation 1 is "0". It is skipped completely and taken for granted. Maybe a full explanation is not required but a link to gravity gradients of some sort. — Preceding unsigned comment added by ‎ Pihmpdaddi (talk • contribs) 14:19, 27 June 2013

I think the explanation is clear. At the beginning of the section it states "Let the surface of the upper reservoir be the reference elevation." In the preamble to equation 1 it states "Apply Bernoulli's equation to the surface of the upper reservoir." So equation 1 applies Bernoulli's equation to the reference elevation.

Equations 2, 3 and 4 apply Bernoulli's equation to three other elevations of interest so y is given the values d, h_B and h_C. I think it is sufficiently clear. Dolphin (t) 23:55, 27 June 2013 (UTC)

Newton's beads

Seems that the idea that air pressure is still the accepted explanation despite this being proven wrong many many times. Newton's beads may help get peoples heads around the concept, why is this article still promoting a patently wrong modus operandi? 118.208.233.178 (talk) 03:50, 1 July 2013 (UTC)

Whenever a parcel of fluid moves in a pipe it is caused by, or resisted by, the pressure gradient in the pipe and the weight of the fluid parcel. It isn't possible to ignore either one of these and get a satisfactory answer. In the case of fluid in a siphon, the weight of the fluid and the pressures at either ends of the pipe (or two different positions in the pipe) must be taken into account. There are people who argue that a siphon is due entirely to the weight of the fluid and not at all due to air pressure; and there are others who argue that it is due entirely to air pressure and not at all due to the weight of the fluid. Both groups of people can claim to be half right, but none can claim to be completely right. It is possible to describe the function and static performance of a siphon beginning with a simple free-body diagram; if the free-body diagram is to be drawn correctly it must show forces due to the weight of the fluid and forces due to air pressure; leaving out either of them will inevitably lead to an incorrect answer or explanation. It is true that the air pressure at one end of the pipe is likely to be the same as the fluid pressure somewhere near the other end of the pipe, and therefore its effect will cancel, but the fact that the two pressures cancel is an important part of the explanation of the siphon. Whether a siphon will operate or not is dependent on the aggregate effect of forces due to gravity and forces due to air pressure. (When considering the dynamic performance of a running siphon it is also necessary to take account of friction forces between the fluid and the wall of the pipe.)

Newton's beads (also known as the "chain model") is not particularly relevant to this debate because the chain is a solid and forces on the chain due to air pressure are insignificant. When a chain is suspended vertically, the normal stress is tensile and it is at a maximum at the top of the chain; with a fluid in a vertical column the normal stress is compressive and it is at a maximum at the bottom of the column. Newton's beads and the chain model do not persuade scientists that air pressure is insignificant in a fluid siphon. Dolphin (t) 06:28, 1 July 2013 (UTC)

Siphon Pump

@98.102.207.60 - Doesn't the siphon pump pictured in reference 20 (pat#136809) work by basically the same principle as the ram pump? I haven't had the time yet to figure out the siphons in reference 21(pat#5358000). Do they not use the oscillating water hammer effect also? Mindbuilder (talk) 23:21, 10 August 2013 (UTC)

Regarding the hydraulic ram, it takes in water at a high flow-rate and outputs water at a higher hydraulic head and lower flow-rate using a water hammer effect to develop pressure to continually lift a portion of the input water higher than the source. The Siphon Pump Having a Metering Chamber of US patent #5358000 does not work like a ram pump. The Siphon Pump is a cyclical operating system and it stops siphon flow momentarily allowing for timing and operation of an air inlet valve atop a Metering Chamber which empties the canister contents and then is closed back airtight. The start/stop operation is a continuous movement, but with simultaneous mechanical valve motion. After the siphon pump runs for a sufficient time to purge the air from the system at the long leg outlet, then the siphon flow can be stopped again to allow for further withdrawal above or away from the source at the metering chamber canister. — Preceding unsigned comment added by 166.137.12.79 (talk) 20:03, 29 December 2013 (UTC) The speed of the liquid within the siphon pump is only related to conduit size and atmospheric pressure. The AUTOSIPHON company has expanded siphon technology to include the capability of the Siphon Pump and the EMAIL info@autosiphon.com may be of some help.

I see now that the patent 5358000 siphon pump does not rely on the water hammer effect. It appears to drain some water from the top of the siphon by allowing air to enter, and then rely on the speed and capacity of the down leg of the siphon to flush the resulting air bubble out. The process requiring a manual or automatic repeating cycle of valve actuations. That is an interesting alternative method I haven't seen elsewhere, of raising and actually discharging water at a higher level using the fall of water as power. Is there a reference showing a completed and functioning real world example of this device? Mindbuilder (talk) 00:48, 15 January 2014 (UTC)

The Patent #5358000 expired in 2013 and this invention is now in the public domain for everyone to freely use.— Preceding unsigned comment added by 69.92.116.241 (talk) 20:58, 19 January 2014‎

Aqueduct siphon

I was reading the Massachusetts Water Resources Authority's Water System Master Plan (2006) yesterday and noted that they describe a number of structures on their aqueducts as "siphons" that were not obviously the same things as described in this article (no additional explanation is provided). The MWRA aqueducts in question are gravity-fed, so changes in elevation should be possible entirely on the basis of hydrostatic pressure. By contrast, the Quabbin Aqueduct actually is a siphon (the whole thing) as the high point in the tunnel is at the Ware River Diversion, about halfway between the two endpoints. 121a0012 (talk) 02:14, 8 July 2013 (UTC)

I agree that, from your description, it looks like the MWRA is misleading readers of its Master Plan by suggesting parts of the aqueducts are siphons. As our article indicates, the feature that constitutes a siphon is that in at least one region of the conduit the fluid pressure is less than atmospheric. The Quabbin Aqueduct appears to qualify as a siphon because the Ware River Diversion is about 136 feet above the level of water at the inlet. Dolphin (t) 08:19, 8 July 2013 (UTC)

At Dolphin. Can you explain how the Quabbin Aqueduct siphon is able to operate with a 136 feet high point above the inlet, whereas practical siphons are supposed to be limited to 33 feet? i.e. when the high point of a siphon exceeds the inlet water level by 33 feet, the absolute pressure in the pipeline at the high point drops below the vapour pressure of water and thus the water turns to vapour and prevents the siphon from operating. In an experimental siphon, where the water is degassed, it is possible to have the water drop below the vapour pressure and for the water to not turn to vapour, although it is considered unstable and can turn to vapour with just the slightest turbulence. However for practical siphons using everyday water, siphons are not known to work beyond 33 feet. Why is this one able to claim 136 feet? Is the 136 feet the difference between the high point and the pipe inlet and there is additional water in the reservoir above the pipe inlet? I understand that the Ware River Diversion is used as a priming source but once the siphon is up and operating, the Ware River Diversion is isolated from the siphon. Something just doesn't add up with the 136 feet. — Preceding unsigned comment added by 124.187.39.17 (talk) 23:34, 5 January 2014 (UTC)

Examining Quabbin Aqueduct and reading between the lines, I think this is how it operates. The water surface in the Ware River Diversion is about 136 feet above the water surface in the Quabbin Reservoir but there is no suggestion that water flows from Quabbin, uphill to Ware River by siphon action. Water from the Ware River Diversion is allowed to flow (downhill) into both the Quabbin Reservoir and Wachusett Reservoir, thereby priming (filling) the conduits joining the three systems. When all air has been flushed from the conduits and they are completely filled with water, the outlet from Ware River Diversion is closed, allowing the pressure in the conduits to fall and establish a siphon. Water from Quabbin Reservoir only flows uphill 25 feet and then flows downhill into the Wachusett Reservoir. (The article states When the Wachusett branch begins to create sufficient suction as it fills, then the Ware River Diversion inlet is closed and water flows from the Quabbin to the Wachusett Reservoir as a natural siphon. The article also states however the water head is only about 25 feet (8 m) on the suction side of the aqueduct.)

It is helpful to imagine an inverted capital letter "T" - the two ends of the horizontal branch indicate the conduits entering Quabbin Reservoir and Wachusett Reservoir; and the top of the vertical branch indicates the conduit entering the Ware River Diversion. Water only flows down the vertical branch of the inverted T (out of Ware River Diversion) for the purpose of priming the system, removing all the air, and establishing the siphon. Once the conduits are filled with water, the vertical branch out of Ware River Diversion is closed. Dolphin (t) 05:37, 6 January 2014 (UTC)

@ Dolphin. Yes, The upside down T would make sense and resolves my concerns. They are using the riser pipe to prime it however the maximum siphon height is much less than the 136 feet. — Preceding unsigned comment added by 203.51.211.142 (talk) 03:32, 15 January 2014 (UTC)

Wrong statement

Can anybody else see the flaw in this sentence in the 'Operation' section of the article? 'The second issue, why liquid flows up, is due primarily to atmospheric pressure (in ordinary siphons), and is the same mechanism as in suction pumps, vacuum pumps, and barometers, and can be replicated in the simple experiment of placing a straw in water, capping the top, and pulling it up (leaving the bottom tip submerged).' 118.208.233.178 (talk) 05:02, 1 July 2013 (UTC)

Please tell us what flaw you can see in the sentence. Dolphin (t) 06:11, 1 July 2013 (UTC)

One way to describe the flaw is to note that the air pressure is the essentially the same on both ends of the siphon (a little higher on the output side), but the sentence seems to assume zero atmospheric pressure on the output side. 72.95.50.148 (talk) 16:33, 9 February 2014 (UTC)jwdooley@aol.com

I got rid of the Operation section. We don't need to waste space in this article explaining that water flows down hill. The first two sentences of the Theory section describe the theory and deserve more prominence than the parts between the operation and theoy headings. Emphasizing that the liquid goes up for the same reason as in a barometer or in the straw example, could be useful, So I stuck it down lower.

It is true though that atmospheric pressure is what makes the liquid go up. The molecules on the up side of the siphon are not acted on --directly-- by any forces at the exit of the siphon. The molecules at the up side are only acted on by the pressure at the exit indirectly through the molecules going up over the top. The pressure at the top is lowered by the action of gravity on the liquids in the siphon, especially the liquid in the taller down side. When the pressure at the top is low enough, atmospheric pressure can push the liquid up the siphon entrance. It is an over simplified mental model we use that says that the virtually identical pressure at each end will cancel, but actually it will not, because of intervening forces between the entrance and exit. If you don't think atmospheric pressure makes the liquid go up then how do you explain Figure 4, where there is an air bubble at top completely separating the liquid on each side? Mindbuilder (talk) 23:15, 9 February 2014 (UTC)