Talk:Siphon/Archive 2

Archive 1

Archive 2

Archive 3

Archive 4

Archive 5

History

Any reason why Ctesibius was removed from the history? As for Greek fire, agreed, it probably wasn't a siphon but it was CALLED a siphon. That merits some mention. Samw (talk) 00:48, 1 June 2010 (UTC)

It appears from the Egyptian reference that Ctesibius wasn't the inventor of the siphon. It doesn't seem notable that Hero learned about the siphon from Ctesibius because Hero could have learned about it from anyone. Hero's treatise may be notable enough to mention though, so I left it in. While the Greek fire might have been projected through something called a siphon, it was obviously just something blowing liquid through a tube. Blowing liquid through a tube surely took place innumerable times before and since. Blowing liquid through a tube is a trivial invention that barely merits mention on Wikipedia at all, so I see no notable reason to include that particular instance, especially when it confuses the history of the interesting kind of siphon. The Greek fire itself was a notable invention, but not the way it was launched, except maybe in the context of the Greek fire article. It could reasonably be listed with lower prominence and less confusion down by the various other things called siphons. Mindbuilder (talk) 08:05, 1 June 2010 (UTC)

Strange Behaviour in Tall Inverted U-Tube

Andrew K Fletcher claims from actual experiments that water in a 6mm ID Nylon inverted U-tube will recoil up into the tube when the ends of the tube are removed from their reservoirs, and that blowing into one of the tubes will not result in the water being discarged out the other end. And that water will pull away from the top of a barometer exceeding 10m but will not from the top of an inverted U-tube. http://www.thenakedscientists.com/forum/index.php?topic=17612.msg200829#msg200829 Farther down he claims that his tall inverted U-tube won't siphon when the reservoirs are made unequal in height, rather the liquid will break and colapse to 10m. I wonder if water transitions into the expanded crystal structure of ice when put under negative pressure. Mindbuilder (talk) 19:57, 15 June 2010 (UTC)

The opening paragraph is still wrong

..."atmospheric pressure pushes the liquid up the siphon into the reduced pressure area at the top of the siphon that is caused by the liquid falling on the exit side."

Equally, atmospheric pressure pushes the liquid back on the other side - they effectively cancel each other out. It must be fixed.

I liked the experiments in the youtube video (reference 1)- this is good science. But wrong conclusions. The part 2 experiment - sealed source - should have been sealed on both sides, with an air tube connecting them. From another tube you could increase or decrease the "atmospheric pressure" on both sides (by sucking or blowing into the tube). The end result will be exactly the same flow. i.e. it is independent of the actual pressure itself. However, change g on both sides, and the flow rate changes (harder to do).

Although a valid siphon is one in which the pressures are different on entry and exit, it is not the typical case of a simple siphon. In a simple siphon, the pressures at entry and exit are at atmospheric pressure, with a minute difference based on pressure height, this can be shown to be negligible in the flow rate in a simple siphon.—Preceding unsigned comment added by Paulrho (talk • contribs) 00:58, 26 June 2010

Your objection relates to an issue that I've thought might need more clarification. When it is claimed that atmospheric pressure drives the siphon, many people seem to take that to mean that the difference in atmospheric pressure at the two ends drives the siphon. That can't be true generally because atmospheric pressure is actually very slightly higher at the lower end and thus that would tend to defeat the siphon. But what we actually mean when we say that atmospheric pressure pushes the liquid up the siphon is just that the pressure at the entrance of the siphon is higher than at the top of the siphon(on the inside). It's the effect of gravity on the liquid in the down side of the siphon that causes the pressure at the top of the siphon to be lowered. The atmospheric pressure at the outlet doesn't cancel the atmospheric presure at the inlet because the atmospheric pressure at the outlet is facing the additional pressure of the taller column of liquid above it. Perhaps you would like to propose a modification of the opening paragraph to make this clearer. However, one should keep in mind that the short opening paragraph can't be expected to address every confusion related to siphons. Maybe clarification in the theory section would be better. Dolphin51 recently dramaically changed the opening paragraph of the theory section, leaving out any mention of gravity. I've been planning to fix that up a little one of these days. Mindbuilder (talk) 02:25, 26 June 2010 (UTC)

I have now done some re-work of the offending sentence in the opening paragraph. I used the concept of hydrostatic pressure and cited Victor Streeter's book Fluid Mechanics. Wikipedia defines hydrostatic pressure as the pressure exerted by a fluid at equilibrium due to the force of gravity so the concept of gravity is now indirectly present in the opening paragraph.

It is true that in my re-working of the Theory section I deleted the expression gravity pulling down on ... This is not a scientific expression and therefore is unencyclopedic language in an article on a science subject. No in-line citation was ever provided to show that the expression gravity pulling down on was used in a reputable source on the subject. In contrast, my replacement wording is supported by citation of Streeter's Fluid Mechanics. Dolphin (t) 07:22, 26 June 2010 (UTC)

I don't object to using a more profesional style of wording, and I don't object to refering to hydrostatic pressure, but you have now removed the word gravity from both the opening paragraph and most of the way down the theory section. That is fine for me, you, engineers, and physicists, but the policy of Wikipedia is to write for a lay audience. Especially on a topic like this that has many practical, non-expert applications. We're trying to inform non-scientific readers of what a siphon is and how it works in *simple* terms. A more rigorous treatment is good and is provided farther down in the article. The problem with leaving out the word gravity is that it has been claimed that gravity and liquid tensile strength are what makes siphons work. I have tried to emphasize that typical siphons use air pressure to push the liquid up. But while I've emphasized air pressure as the lifting force of common siphons, I want to make sure it is clear that everyone agrees that gravity plays a critical role in all siphons and is the ultimate motivating force of the siphon. I don't think the word hydrostatic is commonly understood by the layman, and there is certainly a significant portion of readers that won't know what it means. There is nothing wrong or unprofessional about correctly using the word gravity in this context. If my wording was a little awkward, it is fine to fix it. Also, the opening paragraph of the theory section doesn't even use the word "hydrostatic" any more. It links to it, but the link word is "pressure". I actually removed hydrostatic from my previous version of the opening paragraph of the theory section, after I read the Potter article which explained that hydro*STATICS* don't apply to flowing siphons. Although I still think an understanding of the simplified hydrostatic case is a good introduction to help build a correct mental model of the siphon. In the opening paragraph of the theory section the first sentence speaks of why the liquid flows out the lower end. This creates needless complexity in the explanation. I think its intuitively obvious to anyone why the liquid flows out the bottom end. It's the same reason why the liquid falls out of a drinking straw when you stop sucking. Gravity just pulls it down. The second sentence says the same but for the free hanging tube case. The third sentence explains that the liquid goes up because of the pressures, but doesn't explain *why* the pressure is lower at the top of the siphon. That needs to be explained there. Please try to work the word gravity back in in a professional style. If you don't then I'll probably try to come up with something you can live with, but not tonight. Maybe in a few days or weeks. Mindbuilder (talk) 09:11, 26 June 2010 (UTC)

I will try to find a way to weave the word gravity into the opening paragraphs, and to find a suitable source to use as an in-line citation. — Dolphin51 , — (continues after insertion below.)

We don't need a special citation to say that gravity is pulling down on the water in the down column of the siphon and thereby lowering the pressure at the top. It's obvious and non-controversial and is supported by any of the references we already have that explain how a siphon works. Mindbuilder (talk) 21:19, 27 June 2010 (UTC)

You have written typical siphons use air pressure to push the liquid up. I gave my view on that idea on 15 June when I wrote: Potter says … more liquid, pushed in by the atmospheric pressure, enters at A. As you know, the pressure at A is not atmospheric pressure! The pressure at A can be considered to consist of two components – one component is atmospheric pressure, and the other is the contribution of hydrostatic pressure at depth h. Potter’s article would be a little more accurate saying … more liquid enters at A, pushed in by the pressure at A. However, the pressure at F is also atmospheric and this counteracts the atmospheric component at A, so it would be even more accurate to say liquid enters at A due to the pressure at depth h in the liquid. Dolphin (t) 13:15, 27 June 2010 (UTC)

OK, it's true that the pressure at the entrance end of the siphon tube is not really atmospheric but rather atmospheric plus some due to its depth in the reservoir. I usualy ignore the pressure from its depth in the reservoir because it cancels out as the liquid flows up the tube to a height equal to the surface of the reservoir. In other words, to simplify the situation, I often assume that the entrance of the tube is barely below the surface of the upper reservoir. Remember, the interesting and important part about the siphon is not why the liquid enters the tube, but why the liquid rises up above the surface of the upper reservoir. The liquid is pushed up above the surface basically by atmospheric pressure. That's the primary idea that people need to understand. The atmospheric pressure at the up side isn't completely canceled by the atmospheric pressure at the exit because the liquid on the down side partially cancels the atmospheric pressure at the exit, leaving the top of the siphon at less than atmospheric pressure. Mindbuilder (talk) 21:51, 27 June 2010 (UTC)

The opening paragraph is becoming more and more obfuscated. We have already said that the use of hydrostatic pressure as a term is acceptable in the definition, as such it should be easy to simplify the definition. Perhaps it should be a simple as:

'"The rate of fluid flow in a siphon results from the difference between the hydrostatic pressure (exerted due to the force of gravity) present at either end of a tube. An intermediate point of the tube may be higher (or lower) than either end. "'

Note that this does not state atmospheric pressure, nor is it present in the equation: $v_{C}={\sqrt {2gh_{C}}}$

There are limiting factors in a siphon that relate to the atmospheric pressure, but not the nominal case of an operating siphon. As a further example, we assume the temperature of the fluid is below boiling, as this is another limiting factor. Subsequent paragraphs should explain further, but keep it simple, it is simple.

I see the current definition has removed the atmospheric pressure reference, which is good, but also double-states the gravity reference by saying ..gravity causes the hydrostatic pressure.. which ignores the definition of hydrostatic pressure.

It is hard to keep up with the "current" definition, it keeps being changed, is there anyway that we can moderate this, and if so, who should we choose to be the moderators? —Preceding unsigned comment added by Paulrho (talk • contribs) 22:43, 1 July 2010 (UTC)

Paulrho has suggested The rate of fluid flow in a siphon results from the difference between the hydrostatic pressure present at either end of a tube. I disagree. If the upstream end of the tube dips one centimetre (or one inch) below the surface of the upper reservoir, and the down stream end is more than one centimetre (or one inch) below the level of the upper reservoir, the hydrostatic pressure at the downstream end is higher than at the upstream end. If Paulrho’s statement was correct the fluid should flow from the downstream end to the upstream end, but what happens is exactly the opposite. If and when we are writing about the rate of fluid flow it would be reasonable to write The rate of fluid flow in a siphon is dependent on the difference between the level of the surface of the liquid in the upper and lower reservoirs. If the siphon discharges into the atmosphere rather than into a lower reservoir, the rate of flow is dependent on the difference between the level of the upper reservoir and the end of the siphon.

The reason gravity is mentioned a number of times is largely because Mindbuilder suggested it was important. See Mindbuilder’s post above, dated 09:11 26 June 2010.

Paulrho has written that the current definition keeps changing. That is partly because interested Users see a need to suggest, and make, changes. Is there anyway we can moderate this? Why? This is Wikipedia. Dolphin (t) 23:21, 1 July 2010 (UTC)

I'm not convinced that hydrostatic pressure plays a part, and I think it's a rather technical term for the lead. It's not mentioned in the famous 2010 paper. Surely gravity causing water to exit the outflow end of the pipe and molecular cohesion drawing water in at the inflow end are all that is needed? This also agrees with the Guardian's Report. I fear that the substitution of 'hydrostatic pressure' for 'atmospheric pressure' is simply perpetuating the old error. The water isn't pushed; it's pulled. Mcewan (talk) 14:08, 29 July 2010 (UTC)

OK I see this gets discussed later in the article. Not convinced, but I'm no expert. We still 'rely on atmosperic pressure' in the vacuum siphons section though. Is that right? Mcewan (talk) 14:18, 29 July 2010 (UTC)

The concept of gravity causing water to flow is very popular (and with some justification) among non-scientists. (Hydrostatic pressure is defined at Fluid statics#Hydrostatic pressure - Hydrostatic pressure is the pressure exerted by a fluid at equilibrium due to the force of gravity.) The problem that is encountered is that when we go looking for high-quality sources among authoritative documents on the subject of fluid mechanics we find that fluid phenomena are explained in terms of mass, weight, density, acceleration due to gravity, pressure and viscosity. These six parameters are rigorously defined and are assigned units such as kilograms, newtons and pascals. In contrast, gravity is a more general and introductory concept, a bit like that other general and introductory concept inertia. Gravity and inertia are not assigned units of measurement so they are not measurable, and authoritative documents on these subjects make little use of them other than in introductory chapters. In contrast, fluid phenomena are explained, described and measured in terms of mass, weight, density, acceleration due to gravity, pressure and viscosity.

Mcewan has mentioned a role played by molecular cohesion. It is possible to initiate a siphon with a lengthy bubble in the tube. Despite the lack of cohesion between the two bodies of fluid either side of the bubble the siphon operates and the bubble is quickly swept out of the tube. This indicates molecular cohesion is not an essential consideration in explaining the phenomenon of the siphon.

Mcewan has also stated that the water in a siphon isn't pushed, it's pulled. That might be a popular view but it is incorrect. Newton's Second Law of Motion establishes that a body will begin moving (or decelerate to a slower speed) if, and only if, a resultant force acts on that body. Whether the resultant force is considered to be pushing from behind or pulling from the front is irrelevant to the acceleration of the body. And this is equally true of the flow of fluids. If a cube of fluid is subjected to a difference in pressure that is not balanced by its own density, that cube of fluid will accelerate in response to the force acting on it. When a cube of fluid accelerates all that can be stated is that a resultant force is acting on it - it cannot be stated that the fluid is being pushed or pulled because the acceleration is not simply in response to the fluid pressure acting on one face of the cube - it is in response to the difference in fluid pressures acting on opposite faces of the cube.

The section on siphons in vacuum contained a lot of material that was unsourced, and some was speculation about what might work in a siphon and what might not. This unsourced material is not appropriate in Wikipedia without a suitable citation to point to its origins and allow independent verification, so I have deleted it. Others are welcome to restore it if they have suitable in-line citations. Dolphin (t) 03:08, 30 July 2010 (UTC)

May I suggest a simple model? The fluid on the long low side of the siphon, because of its tensile strength, acts as a piston. When the siphon flows steadily, the acceleration is zero, and the net force on the "piston" is zero. The "piston" experiences the force of gravity (pulling down), atmospheric pressure at the bottom (pushing up), and a lower pressure at the top (pushing down). If there is an air bubble above the piston, then atmospheric pressure is needed on the short high side to make the siphon flow. If the fluid is continuous, then tensile strength alone can pull water "over the hump" and no atmospheric pressure is needed. The diameter of the tube on the short high side is irrelevant for zero viscosity because only a "tube" of water (diameter the same as the long low side) within the short high side needs to flow when the siphon runs.166.66.66.21 (talk) 17:56, 31 August 2010 (UTC)jwdooley

The article presently has the Chain Model. Until recently, it also had the Train Model. Models like this are useful for newcomers to a subject because they help the newcomer assimilate an unfamiliar topic by associating it with something much more familiar. However, the siphon is a simple application of fluid mechanics and it can be explained in purely scientific terms. Wikipedia is an encyclopedia so the siphon should be explained primarily in scientific terms. I don't doubt that the Piston Model, and the idea of a fluid having tensile strength, would help many newcomers to assimilate the idea of the siphon, but they are not scientifically rigorous.

Solids possess tensile strength. It can be demonstrated that fluids don't possess tensile strength. Consequently it would be inappropriate for the Wikipedia article on the siphon to mention the tensile strength of the fluid. Dolphin (t) 03:30, 1 September 2010 (UTC)

jwdooley - Under typical siphon conditions atmospheric pressure is squeezing all of the liquid in the siphon. The liquid molecules are slightly closer together than they would be otherwise and they are repelling each other. Since they are repelling each other there is no tensile pulling going on anywhere in the siphon in a typical siphon. However if the siphon is nearly as tall or taller than the barometric height of the liquid or if the siphon is operating in vacuum, then tensile strength (or whatever that property of a liquid that is demonstrated in the z-tube should be called) becomes an important part of the process. This is an interesting article from 1914 on a siphon in vacuum http://commons.wikimedia.org/wiki/File:Would_a_siphon_flow_in_a_vacuum_experimental_answers.pdf Mindbuilder (talk) 07:08, 1 September 2010 (UTC)

This seems to be a tangle. I'll come back next summer after I retired, but for now: Of course water has tensile strength; drops condense with no change in atmospheric pressure, it takes force to pull a drop apart, and that force measures tensile strength. Atmospheric pressure is needed to make an ordinary siphon work, and also to pour a glass of milk. It is not sufficient to drive the siphon - If it were, you could siphon sand. You need the tensile strength of water. 166.66.66.21 (talk) 14:32, 9 September 2010 (UTC)jwdooley

jwdooley - You may well be right that the tensile strength of a liquid is necessary for siphoning because it is necessary to hold the liquid into a cohesive unit. Certainly those cohesive tensile forces do still contribute in a normal siphon, to spite what I may have said above. But the point I was trying to make is that in a typical siphon, tensile forces do not contribute DIRECTLY to PULLING THE LIQUID UP the siphon. What's more, much of what appears to be attraction of the liquid to itself is, because of atmospheric pressure, actually not attraction but rather the reduction of repulsion. And a pair of molecules can both attract and repel each other depending on how close they are pressed together. If two water molecules are brought near to each other, they may at first pull together, but then if they are pressed even closer together, the net attraction will cease and they will begin repelling each other. Also, it is best to remember that we're not always talking about water when dealing with siphons. Other liquids considered include; gasoline, alcohol, mercury, oil, blood, dibutyl phthalate(a low vapor pressure liquid), tree sap, liquid helium, and more. And I don't know why you think atmospheric pressure is needed to pour a glass of milk. Can you not pour a glass of liquid on the moon (provided it resists evaporation like dibutyl phathalate)? Even milk chilled to just above freezing probably has sufficient cohesion to be poured on the moon before evaporating. Mindbuilder (talk) 04:07, 10 September 2010 (UTC)

jwdooley - By the way, have you read about the z-tube. Note that the z-tube must be spun up to a certain rpm before the measurement of liquid tensile strength even begins. Mindbuilder (talk) 04:15, 10 September 2010 (UTC)

still wrong

"A siphon operates to move liquid from an upper reservoir to a lower reservoir because gravity causes the hydrostatic pressure of the liquid in the discharge end of the tube to be greater than the surrounding pressure in the lower reservoir."

This says nothing about why the fluid in the upper reservoir is pulled up; only why fluid on the discharge side moves down. Somehow, the fluid on the discharge side must lower the pressure on the up-flow side, making that pressure below atmospheric pressure.

The bubble is a good example. To lower the pressure in the bubble below atmospheric pressure, the fluid on the discharge must pull down like a piston. It must cohere. You cannot siphon sand. 151.205.217.116 (talk) 21:56, 2 December 2010 (UTC)john dooley

Try this experiment. Insert a drinking vessel (tumbler, glass etc) into a bowl of water so that the vessel fills. Now invert the vessel and raise it partly out of the water. The pressure of the water in the vessel is now less than atmosheric pressure because the water is above the level of water in the bowl. And yet there is no down side or discharge side. Clearly a down side or discharge is not necessary to lower the pressure of the water below atmospheric pressure. All that is necessary is that the water is higher than the surface of the surrounding body of water. Hydrostatic pressure says it all.

In physics we talk about forces. Whether a force is pushing or pulling is usually not a good description of the force. Trying to explain the phenomenon of the siphon by relying on whether forces are pushing up or pulling down is unlikely to lead to a satisfying explanation. The siphon is amenable to a scientific explanation, and scientific explanations use the concept of force and the direction of application of each force. Pushing up and pulling down are not serious scientific concepts. Dolphin (t) 01:27, 3 December 2010 (UTC)

John Dooley - The liquid on the discharge side of a typical, non-vacuum siphon, doesn't pull down on a bubble at the top of a siphon. In fact it pushes up on it. When the liquid on the discharge side pushes up a little less hard, the bubble expands itself. The bubble is a gas under pressure and therefore wants to expand. The gas in the bubble is under a little less pressure than atmospheric pressure, but it is still a pressurized gas which is trying to expand. The only reason the bubble doesn't expand more is because the liquid on both the discharge and intake sides are pushing up on it, keeping it squeezed small. Atmospheric pressure in turn is what is pushing the columns of liquid up both sides. The weight of the liquid in the columns counters some of the atmospheric pressure, and since the column on the discharge side is taller, it cancels more of the atmospheric pressure. Since the bubble is being pushed up a little less hard by the liquid on the discharge side than on the intake side, the gas in the bubble moves toward the discharge side. Since the bubble is pushing down on both the intake and discharge column, and the atmosphere is pushing up on both the columns, both columns are entirely in compression. There is no tension and therefore no liquid tensile strength involved in raising the liquid.

You cannot siphon sand because air from the atmosphere will simply travel from the higher pressure atmosphere up between the grains of sand to what would be the reduced pressure area at the top of the siphon and raise the pressure there, until the pressure at the top of the siphon is the same as ambient pressure. With no pressure difference the siphon cannot work. In a vacuum, a sand siphon cannot work because sand has practically no tensile strength like many liquids do. Mindbuilder (talk) 23:33, 3 December 2010 (UTC)

Mindbuilder - water has no tensile strength. Does that mean a water siphon cannot work? If a water siphon is possible what does that imply for the significance of tensile strength? Dolphin (t) 01:26, 4 December 2010 (UTC)

If not tensile strength, what would you call that property of liquids that is similar to tensile strength and is demonstrated in the z-tube or a siphon in vacuum? Without that property, a liquid siphon couldn't work in vacuum, but it could still work under atmospheric pressure. Mindbuilder (talk) 04:35, 4 December 2010 (UTC)

Siphons in vacuums and Z-tubes are off-topic relative to this article. Siphon is about the humble siphon in the atmosphere, on the Earth's surface using common liquids such as water. I have no knowledge of what conditions allow a siphon to operate in a vacuum, but I do know that it is subject to a certain amount of controversy. This article is not about siphons in vacuums or other frontier applications of the siphon.

If a significant number of authoritative sources explain the functioning of the siphon in terms of the tensile strength of liquids it would be reasonable for Siphon to present that explanation, and to cite at least one of those authoritative sources. There are some authoritative sources that talk about a sort of tensile strength in liquids, but they aren't mainstream sources on the subject of fluid mechanics. I don't know of one fluid mechanics source that uses the concept of tensile strength to explain the humble siphon.

Mercury barometers (and very large water barometers - 30 feet?) operate to indicate atmospheric pressure because mercury doesn't display tensile strength of the conventional kind. I don't have any objection to a debate about the tensile strength of liquids, or whether siphons can operate in a vacuum, or what conditions are necessary to observe such. Nor do I have any objection to debate about the meaning of the Z-tube experiment. But I don't believe Siphon or this Talk page is the appropriate place. Dolphin (t) 12:06, 4 December 2010 (UTC)

I think you're right that the overwhelming focus of the article should be normal siphons one might actually encounter or use, rather than the theoretical curiosity of the vacuum siphon. And because of that, the contributions I have made almost ignore vacuum siphons except to note that they are an exception to how siphons usually work. But I can't imagine why you would think that siphons in vacuum would be off-topic in a siphon article. It's hard to think of anything more on topic than siphons in a siphon article. I see nothing in the title to suggest that this article is or should be limited entirely to siphons on earth's surface or common liquids. I don't think most readers of this article would want it to be limited like that. Even if there was another article devoted to vacuum siphons, I don't think it would be appropriate to completely eliminate discussion of vacuum siphons in the general siphon article. Vacuum siphons should be included for two reasons. One is that knowledge of them helps to form a complete and CORRECT understanding of how siphons work. The other is that vacuum siphons are a part of a big debate about siphon function. The article doesn't have to take sides in the debate, but the debate itself is important enough in this area that it would leave the article incomplete not to mention or explain it.

Z-tubes are on topic in the siphon article because the idea of liquid tensile strength is hard to believe at first, and the z-tube is the indisputable proof that such a property exists in liquids. And liquid tensile strength is on-topic because it plays a part in the debate and functioning of some siphons.

Hughes(2010), reference 9 in the article, describes water as having tensile strength. How do the mainstream sources on fluid mechanics describe that property of liquids described as "tensile strength" in Hughes and other articles? Do any of the mainstream sources on fluid mechanics explain the function of a siphon in vacuum? I agree that mercury doesn't display tensile strength of the conventional kind, but that doesn't mean it is incorrect to call it tensile strength of some other non-conventional kind. Words sometimes have narrow meanings and broad meanings. Just because a narrow meaning exists doesn't mean using it with a broader meaning is incorrect. For example, power has a precise meaning in physics. That doesn't mean it is incorrect to use power to describe an exponent or what a king possesses.

As far as what should be debated here on the talk page, it seems to me that we are debating what the article should or should not say. Many people have misconceptions of how siphons work. We have to explain the evidence and reasons here so that the misconceptions don't make it into the article as facts. That's exactly what this page is for. Mindbuilder (talk) 22:54, 4 December 2010 (UTC)

With reference to "tensile strength" in water: instead of calling it "tensile strength" consider that it means that watermolecules are pulling at each other, this is called van der Waals force. --VanBurenen (talk) 22:22, 4 December 2010 (UTC)

I don't think describing the tensile strength of liquid as van der waals forces is specific or descriptive enough. It doesn't effectively convey an understanding of the property to the reader. A reader who knows of van der waal forces still might not realize that the liquid has tensile strength. More importantly, I don't see any reason not to call the property "liquid tensile strength". I don't see that it causes any confusion, since everyone has handled water and will know that it is not a property quite like tensile strength in solids, though it shares important characteristics with solid tensile strength. Mindbuilder (talk) 23:05, 4 December 2010 (UTC)

Incomprehensible reply from Mindbuilder. Are you trying to decide for all visitors/readers what they should realize? Why should one follow your 'reasoning' on re-naming a physical property? Do you have powers to know that it won't cause 'confusion' when you do your POV thing here? How can we write a believable encyclopedia with contributions like that? --VanBurenen (talk) 13:36, 7 December 2010 (UTC)

VanBurenen, you are being too harsh on Mindbuilder. Avoid personal attacks or you might find your name mentioned somewhere like WP:WQA. Mindbuilder and I are not 100% in agreement, but we aren't too far short of it. This thread contains a valuable debate and it will ultimately lead to an improvement in the article. Dolphin (t) 22:35, 7 December 2010 (UTC)

VanBurenen, I can't blame you too much for finding that post hard to understand because I had a hard time finding a way to express those ideas. I probably did a poor job of it. But remember, I didn't invent the usage of 'tensile strength' to describe that property of liquids. I didn't re-name it. That usage was already common practice among several scientists (the large majority writing about that property in recent times, I expect). There is nothing wrong with the author of an encyclopedia deciding how to help readers realize important ideas. It is a decision which is impossible not to make in every sentence. The editors of this article should follow my 'reasoning' for the same reason they should follow yours, based on whether they find it persuasive. Mindbuilder (talk) 01:32, 8 December 2010 (UTC)

Instead of getting a little bit upset (as I did above) it's better for me to check back in a few months and see how this develops. I'll take this item off my watch list. --VanBurenen (talk) 14:14, 8 December 2010 (UTC)

I can summarise my thoughts as follows. Since 1911 the Oxford English Dictionary described a siphon as relying on atmospheric pressure. We are focussing on this topic now because in 2010 Stephen Hughes of Queensland University of Technology challenged this description, and argued that it is gravity that drives a siphon, not atmospheric pressure. Siphon currently reflects Hughes's view, and the opening paragraph talks about gravity and hydrostatic pressure. A discussion about whether siphons work in a vacuum, and if so why, seems to me to be relevant only in the context of being able to say "see, atmospheric pressure is irrelevant. Siphons work even when there is no atmospheric pressure." If the Z-tube and the fact that siphons work in a vacuum are being used to prove a point regarding the tensile strength of water then that proof has no place here. This is not an article about the physical properties of water, or any other fluid.

On the Earth's surface, a column of water can be about 30 feet above the free surface before the hydrostatic pressure at the top of the column reaches zero. Up to that point the hydrostatic pressure is positive and it is displaying the compressive strength of water. It is only when the column of water exceeds this height that we can speculate that the hydrostatic pressure is negative and the water is displaying its tensile strength. A water siphon that approaches 30 feet is one very large siphon, and is definitely not among the majority of siphons. I think it is safe to say the great majority of siphons are water siphons and are significantly less than 30 feet high so there is no grounds that I can see for saying the tensile strength of water is playing some part in the functioning of this great majority of siphons.

If the great majority of siphons clearly operate without approaching zero hydrostatic pressure, let alone negative pressure (ie tensile stress), we can't say that the tensile strength of water plays an important part in the functioning of these siphons.

Mindbuilder has described the vacuum siphon as a theoretical curiosity and I think this is a good description. So long as the vacuum siphon and tensile strength are given their proper place (not in the opening paragraph) I won't object too much. Dolphin (t) 02:46, 5 December 2010 (UTC)

The opening paragraph is now one written for Physicists and will mislead the layman. Most people will have to take the link to Hydrostatic Pressure, get re-directed to Fluid Statics, and then read past the opening paragraph to get to the explanation for Hydrostatic Pressure of "the only force acting on any such small cube of fluid is the weight of the fluid column above it" before then being left with the impression that the depth of the tube in the upper reservoir makes a difference and it's that pressure at the depth of the end that drives the siphon.

It is not true that atmospheric pressure is not involved. The hydrostatic pressure requires the addition of atmospheric pressure from the top surface. The pressure from the weight of water alone cannot raise the water in the tube above the surface. If the air pressure was reduced then the maximum height above the top reserviour would reduce, down to the point where the siphon failed in a vacuum (or virtually failed, I'm concentrating on the siphon on Earth in everyday use here). Moving to the Hughes viewpoint is a step backwards. It's his definition that caused all the confusion in the first place.

Put another way, the hydrostatic pressure definition is only correct if your reference point is somewhere in space so that the pressure in the siphon includes both air column and water column (ie: HP = absolute pressure). However, hydrostatic pressure is most usually used with the surface as zero, otherwise you may as well just call it 'pressure'. This is also how the Fluid Statics page is written.

As you can tell, I don't agree with this new definition. It's too technical, requires specification of the zero pressure point to be correct, and the main problem is by removing atmospheric pressure it doesn't explain to the layman how his siphon is working. That water falls out of the end of a tube is not news. The magic of a siphon is that the water 'flows uphill', and the definition says nothing about that at all! Mike163 (talk) 19:12, 24 January 2011 (UTC)

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I agree with Mike163. Atmospheric Pressure is definately involved, and you have removed it. Please refer to PIA Australian Pipe Friction Handbook

Extracts From PIA Australian Pipe Friction Handbook 2.15 Discharge of Siphon Lines

Theoretically, the limit of possible flow up the inlet leg of a siphon is reached when the pressure Ps at the highest point in the system is absolute zero (-10.35 m of water). In practice, however, this limit would be less because when the absolute pressure Ps at the summit becomes equal to the vapour pressure of the liquid, cavitation begins. Consequently, vapour collects at the bend forming so called vapour locks and the flow stops. Hence, for satisfactory operation of a siphon, the maximum static suction lift allowable can be expressed by the following equation:

Static suction lift in metres < 0.102 x (Pa – Pv) – v squared/2g x (1 + Ks) – S

Where: Pa = atmospheric pressure in kpa

Pv = vapour pressure of liquid in kpa

Ks = the sum of resistance coefficients for the loss of head from inlet to the highest point of the siphon

S = a safety margin – a deduction of 1 to 2 m depending of the pipeline installation.

v = liquid velocity in m/s

g = gravitational constant = 9.8

End Extract

When a siphon is nearing it’s maximum height, the flow rate will be lower, along with the velocity and friction losses. At sea level, International standard atmospheric pressure is 101.325 kpa. Water at 20 degrees has a vapour pressure of 2.337kpa. So at zero flow, we can simplify the above formula to:

• Sea Level: Static suction lift in metres < 0.102 x (101.325 – 2.337) < 10.09 metres

1000m ASL: Static suction lift in metres < 0.102 x (89.88 – 2.337) < 8.92 metres

2000m ASL: Static suction lift in metres < 0.102 x (79.50 – 2.337) < 7.87 metres

• 2250m ASL: Static suction lift in metres < 0.102 x (77.07 – 2.337) < 7.62 metres

3000m ASL: Static suction lift in metres < 0.102 x (70.12 – 2.337) < 6.91 metres

3500m ASL: Static suction lift in metres < 0.102 x (65.78 – 2.337) < 6.47 metres

So, if a test to see the maximum siphon level was conducted at sea level, and another at the highest point in Australia, Mt Kosciusko, the results would have been approx 10 and 7.6 metres respectively. Why the difference? Atmospheric pressure! We need it to survive, but it gets in the way of the theory proposed by Dr Hughes.

To confirm that Atmospheric Pressure plays a part, rather than theory about Z tubes, tensile strength and siphon in vacuums, all that is required is a simple "maximum height of a siphon test" at sea level then repeat at a higher altitude.

The result: you will find indeed that atmospheric pressure plays a part. Yes, so too does gravity. However, gravity should not replace atmospheric pressure, as Dr Hughes would claim, rather gravity should be added as well as atmospheric pressure.

From Wikipedia Fluid statics (also called hydrostatics) is the science of fluids at rest, and is a sub-field within fluid mechanics. Yet a siphon is fluid in motion, not at rest. So, should you be using the hydrostatics definition in your siphon theory? Since a siphon is fluid in motion.

If we push a car down a road, why does it move? Because of some law of motion? Or, simply because we pushed it!

If water in a siphon moves uphill against the force of gravity, why does it move? Because of hydrostatics? Or because Atmospheric Pressure pushes it!

Hydrostatics is the theory and laws that help us to explain or understand something. However, atmospheric pressure is providing the push for the water to move uphill

Remember, the pressure at the top of a sihon is below atmospheric pressure. Why are you now aligning to Dr Hughes siphon theory that atmospheric pressure is not involved? Engineering and centrifugal pump text books confirm the role that atmospheric pressure plays in getting water to flow up a siphon or pump suction pipe. Dr Hughes's does not conduct any tests that proves his theory. In fact, if you read his paper, he confirms he did NOT conduct the one test he suggests would prove his theory correct!

And his chain analagy of the heavier weight water pulling the lighter weight water over the rise was also proven incorrect! Refer the article "The pulley analagy does not work for every siphon" by Gorazd Planinsic and Josip Slisko that was printed in the July 2010 Physics Education magazine, several months after Dr Hughes' appeared. This article includes building and testing a siphon to prove their results, not building a model of a siphon using a chain and pulley.

The Wikipedia opening explanation includes:

Practical siphons operate because gravity causes the hydrostatic pressure at the downstream end of the tube to be significantly higher than the surrounding pressure so fluid flows out of the tube into the atmosphere or into a second reservoir lower than the first.

This could describe a simply pipe from a tank or reservoir. It provides no explanation or understanding as to why water flows uphill agasint the force of gravity, which is the mystery of a siphon that people are looking for an explanation for. —Preceding unsigned comment added by 124.179.136.135 (talk) 02:29, 26 January 2011 (UTC)

IP 124.179.136.135 has successfully argued that the maximum height of a siphon is a measure of atmospheric pressure. That has never been in dispute - it is the principle behind the mercury barometer used for measuring atmospheric pressure. However, the debate here is not about the maximum height of a siphon. The debate here is about the best way for Wikipedia to explain the phenomenon of the siphon. Most siphons operate over a height that is nowhere near the maximum height for whatever fluid is being siphoned.

IP 124.179.136.135 has written If water in a siphon moves uphill against the force of gravity, why does it move? Because of hydrostatics? Or because Atmospheric Pressure pushes it! Certainly not because atmospheric pressure pushes it! Water in any conduit, siphon or not, moves uphill or downhill simply because the force generated by the pressure gradient exceeds the weight of the fluid and any viscous forces. Water will run out of a container, via a vertical conduit, out into the atmosphere if the pressure at the entrance to the conduit, minus the pressure outside the exit from the conduit, exceeds the weight of the fluid in the conduit. If the conduit is not vertical but horizontal it is even easier because the weight of the fluid is not relevant - fluid will run out of the container if the pressure at the entrance to the conduit exceeds the pressure outside the exit from the conduit. If the conduit initially carries fluid upwards, above the level of the free surface, before carrying it downwards to the exit we call it a siphon. If the upward movement doesn't reach the free surface we don't call it a siphon.

If the entrance to the conduit is at depth h₁ then the pressure at the entrance is h₁ times specific weight of water, plus atmospheric pressure. The pressure inside the exit of the conduit is h₂ times specific weight of water, plus atmospheric pressure. Atmospheric pressure cancels from both sides of the equation! What is left is h₁ - h₂, all multiplied by the specific weight of water. If the result exceeds the weight of the water in the vertical part of the conduit water will flow outwards because the pressure outside the exit from the conduit is atmospheric pressure - exactly the same atmospheric pressure as that which exists at the free surface of the water in the container. (If the result doesn't exceed the weight of the water in the vertical part of the conduit, air will be drawn into the conduit from the end exposed to atmosphere.)

So atmospheric pressure cancels from both sides of the equation. Even if atmospheric pressure was zero (a vacuum) the fluid would flow out of the siphon, or back down the conduit into the container, based solely on the considerations I have described above. A siphon works because atmospheric pressure is the same at the free surface as it is outside the exit of the conduit, even though the two are at different heights.

For a siphon to work, the density of the working fluid must be significantly greater than the density of the surrounding atmosphere. This is the driving force of any flow of a fluid out of a container via a conduit, including the flow of a fluid through a siphon. If the density of the working fluid and the density of the surrounding atmosphere were identical, or even similar, a siphon wouldn't operate. The atmosphere only plays a part insofar as its density must be so low compared with the density of the working fluid that atmospheric pressure can be cancelled from the pertinent equation. Dolphin (t) 05:47, 26 January 2011 (UTC)

@124.179.136.135 - I agree that the last sentence of the first paragraph is rather pointless. As mike wrote, it basically says that water flows out the end of a tube. I have been reluctant to change it because it was hard to get it to stabilize at its current version which at least has the virtue of not being too incorrect.

But it does not appear to be the case that atmospheric pressure is definitely always involved in the operation of a siphon. See cite 31 Minor(1914). And for a modern demonstration of a siphon exceeding barometric height see the Youtube video by AndrewKFletcher of a siphon reaching 24 meters. Though AndrewKFletcher's video is poor proof, the Z-tube research and research on trees supports the plausibility of it. I've said before and I still feel that the article should concentrate on the importance of atmospheric pressure in nearly all siphons, but that concentration shouldn't be exclusive. Vacuum siphons appear to be real and are notable. Mindbuilder (talk) 06:17, 26 January 2011 (UTC)

The significance of atmospheric pressure is that it cancels from both sides of the pertinent equation. That can hardly be described as importance. Dolphin (t) 06:20, 26 January 2011 (UTC)

The first diagram in the theory section and Micolich's video shows a siphon that starts with air at the top. Such a siphon cannot start without atmospheric pressure. If it won't work without it, I'd call that important. Mindbuilder (talk) 18:51, 26 January 2011 (UTC)

Thanks for drawing that to my attention. Yes, the text and the caption accompanying the first diagram both talk about atmospheric pressure pushing the liquid up. It is pure original research, and not expressed in particularly scientific language. I will replace it with something more encyclopedic. Dolphin (t) 00:51, 27 January 2011 (UTC)

Remember that Wikipedia is not just a scientific reference work. It is meant to be understood by readers of an average education level, not just physicists. The wording of basic sections is not expected to be in rigorous scientific terms if that is not helpful to the understanding of readers of the target audience. The article has a section for the highly technical explanations down below.

What is original research about it? That it is atmospheric pressure that pushes the liquid up? When the liquid falls down the down tube, the pressure at the top is lowered enough that atmospheric pressure can push the liquid up the up side. If there is only low pressure air at the top of the siphon then what else is there besides atmospheric pressure to get the liquid up? There is nothing pulling the liquid up. The liquid isn't pulling itself up, so it must be being pushed. I don't think this is original research, I think nearly all the sources say this except the ones that claim tensile strength pulls it up, but those are obviously wrong in the case of a siphon that has an air bubble at the top. You could say that the lifting force is a combination of the hydrostatic pressure in the reservoir and the atmospheric pressure, but that would just be adding several unnecessary complicating steps to the simplified caption explanation when it is ultimately the atmospheric pressure where the force is coming from. And if the inlet is barely touching the water, not really submerged then any hydrostatic pressure at the inlet would be insignificant. We could talk about the electrical forces between molecules and even the nuclear forces if we wanted to make sure every part of the chain of forces was described. Is that the kind of scientific rigor you are looking for? Do you really think it is not atmospheric pressure that pushes the liquid up a typical siphon? Mindbuilder (talk) 05:20, 27 January 2011 (UTC)

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That the maximum height of a siphon is a measure of atmospheric pressure also confirms the role that atmospheric pressure plays. At sea level, it is 10.09 metres. At 2200m ASL, it is 7.87m, and when we reduce atmospheric pressure to less than 2.337 kpa, the siphon will stop working. So it is highlighting not just the issue about the maximum height, but that atmospheric pressure is essential for the siphon to operate. There is a direct link to the reduction and ultimate removal of atmospheric pressure, and the siphon no longer operating.

So even when a siphon is not operating at maximum height, it is still utilising atmospheric pressure to work. And the current definition has removed atmospheric pressure from playing any role.

The formula provided earlier from the PIA Australian Pipe Friction Handbook is not just an issue about maximum height. It confirms that in order to get any siphon to operate, you must provide a pressure at the inlet that equals or exceeds the height of the siphon up leg plus the friction losses, including velocity head, and after allowance for vapour pressure.

If you reduce the entrance pressure (atmospheric pressure) to zero, then clearly the siphon will not operate. That the outlet atmospheric pressure is also zero doesn't change the fact that the siphon will not operate, since there is no longer any pressure to cover the up leg height or friction losses

Re "Certainly not because atmospheric pressure pushes it!"

It IS because Atmospheric pressure pushes it. That is the whole point. Hydrostatics is part of the theory, formula and understanding. Atmospheric pressure is part of the source of energy. Atmospheric pressure is part of the practical reality at the siphon site.

The reason that the pressure gradient allows water to move up the siphon, then down and out is that the entrance to the siphon is at atmospheric pressure, and the top of the siphon is at sub atmospheric pressure due to gravity moving water on the downward leg. If our inlet pressure was reduced to absolute zero (compared to absolute 10.33m which is atmospheric pressure), then how can we move the water up the siphon since we can't have a pressure gradient less than absolute zero further along the siphon?

That atmospheric pressure is the same at the outlet (or slightly higher) does not change the relationship of the pressure gradient between the inlet and the top of the siphon. There is no need to compare the inlet to the outlet atmospheric pressure.

If atmospheric pressure was zero (a vacuum), there would be no fluid flow, since everyday water turns to vapour once you reduce atmospheric pressure to below the vapour pressure of the liquid.

"A siphon works because atmospheric pressure is the same at the free surface as it is outside the exit of the conduit, even though the two are at different heights." ????

The AndrewKFletcher siphon video is of very poor quality, does not involve standard water and if watched closely, may have been filmed twice, as when they switch cameras, it doesn't match the earlier camera footage. I understand that the test involved boiled saline water solution.

Z tubes also involve boiled water solution and are a special device that in effect prevents water from escaping. The boiled water is to prevent the water from vapourising.

Atmospheric pressure IS involved in the standard siphon.

Vacuum siphons were raised to highlight the point, "look, a siphon works in a vacuum, thus atmospheric pressure is not required". Except, Dr Hughes used this to support his theory, even though he acknowledges he didn't conduct the test. And earlier tests by Nokes in 1948, well I haven't seen the paper, but I understand that they involved special liquids and that there is not even certainty of the tests conducted and the results. The tests have not been duplicated. And if he/she did get a siphon to work in a vacuum using special liquid at a low height, doesn't that confirm that he/she couldn't get a standard siphon using everyday water to work without atmospheric pressure.

Any test that utlises special liquids or treated water only serve to highlight the limitations and issues that everyday siphons face with standard everyday water.

So what tests would be required to prove the role of atmospheric pressure? or prove that it didn't play a part?

i.e. if any one on this discussion page could request a test, what would you want? And what would the test parameters be? And what would the expected results be? And what would be the conclusions/arguments drawn from those expected results?

1: Heat water and watch it boil/vaporise at 100 degrees celcius? Proving that water has a vapour pressure that varies with it's temperature. And that atmospheric pressure is required to keep it as liquid. And if you exceed the vapour pressure requirement that atmospheric pressure can provide, the water will turn to vapour.

2: Set up a siphon at sea level and determine it's maximum height? Result: 10.09 metres. Confirms the maximum height of a siphon at sea level as being approx 10 metres. Would not confirm if the maximum height was due to either atmospheric pressure or so called tensile strength.

3: Set up a siphon at elevation and determine it's maximum height? Result: Depending on elevation but will be less than that at sea level. Would confirm that the maximum height is directly related to atmospheric pressure.

4: Set up a siphon in a vacuum chamber and reduce the pressure to less than 2.337kpa? Result: Water will vapourise proving atmospheric pressure is essential for water to stay as a liquid. Also proves that atmospheric pressure is essential for a siphon to operate, since without it, water will not exist as a liquid.

5: Set up a siphon in a vacuum chamber, but reduce the pressure to just above 2.337 kpa? Say 5kpa. This is to avoid the water turning to vapour. Then test a siphon with a lift of 0.5 metres. Result: siphon will not operate, as there is not enough pressure at the inlet, to push the water over the siphon rise.

Preceding unsigned comment added by 124.179.136.135 (talk) 10:46, 26 January 2011 (UTC)

Here is a simple experiment that will demonstrate my thesis. (My thesis is that the driving force of a siphon is the significant difference in density between the working fluid and the surrounding atmosphere. If the working fluid and the surrounding atmosphere are of the same density, no siphon will operate.)

Set up a container with a pipe up and over the lip of the container, so that the pipe will carry fluid as a siphon. (The container may need a lid with a small hole in it). At the downstream end of the pipe there is a tap to prevent the siphon from operating until the tap is opened. Fill the container and the pipe with colored water to make it easy to see. Allow the siphon to operate and remove all air from the pipe, and then close the tap. Ensure the container is filled with colored water, right up to the top of the lid, until some comes out the small hole in the lid.

Place the apparatus, all primed with colored water and ready to go as soon as the tap is turned on, on the bottom of a dry tank or similar chamber. Slowly and carefully fill the tank with clean water so that it covers the apparatus, and then keep adding clean water until the water in the tank is at least a few centimetres above the top of the apparatus. The apparatus is now surrounded by an atmosphere of clean water! The atmosphere is of the same density as the working fluid in the apparatus.

Carefully reach down (or use a tool) and turn on the tap. What happens? Does colored water flow out of the siphon? Of course not. The entire apparatus and its watery contents are surrounded by an atmosphere of water. Nothing moves. A state of equilibrium exists throughout all the water in the tank.

Even though the "atmosphere" of clean water is of significantly greater pressure than 101.325 kPa, colored water does not flow in the siphon. The siphon does not operate because the density of the "atmosphere" (clean water) is equal to the density of the working fluid (colored water). Dolphin (t) 11:21, 26 January 2011 (UTC)

@124.179.136.135 - The Wikipedia article on siphons is not only about pure water siphons. Practical siphons are also used with gasoline, oil, seawater, possibly blood and tree sap, and others. Siphons in vacuum may be an academic curiosity, but they are also properly called siphons. The definition in the opening paragraph should take these different kinds of siphons into account. More detail about the kinds and functioning of various siphons should be, and is, given in the sections of the article below the opening paragraph.

I see little reason to suspect the AndrewKFletcher video of being fraudulently edited. Given what is reported about Z-tubes, tree capillaries, and vacuum siphon experiments, his results are plausible. I have not heard of any reports that boiled water siphons of more than 10m have failed to function. The purpose of boiling the water is not to change the vapor pressure of the water but to remove dissolved air contamination. Boiled and cooled water has the same vapor pressure as un-boiled water. The Z-tube experiments show that water doesn't necessarily vaporize at the pressures and temperatures you would expect it to. Though Fletcher adds salt to part of the siphon, it doesn't seem likely that that makes much difference to the height of the siphon, and even if it did, saltwater siphons are practical and common.

The Pipe Friction Handbook cannot be considered a reliable source on this issue. There are a lot of misconceptions among physicists about the workings of siphons. There is no reason to think that the authors of the Pipe Friction Handbook have carefully studied the issue or are aware of all the evidence. Looking at the evidence, it appears the Pipe Friction Handbook is, strictly speaking, wrong. Though perhaps it is right about most practical siphons.

Both Nokes and Minor reported successful vacuum siphons, so there is corroboration. Given the research on Z-tubes, tree capillaries, and AndrewKFletcher's demonstration, I see little reason to doubt Nokes or Minor. I looked into the possibility of a water siphon in vacuum. It looks to me like the vapor pressure of water near freezing is low enough to be contained by the surface tension. I think a pure water siphon could be made to work over a small height in vacuum near freezing. Mindbuilder (talk) 19:52, 26 January 2011 (UTC)

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Comments from Andrew K Fletcher at the following link that might be of interest. In the 1st paragraph, he advises it was not a siphon. thenakedscientists.com/forum/index.php?topic=1982.0

Dave, the water at Brixham was not siphoned, as you well know a siphon will not work at those heights. In fact, to prove it was not a siphon that was taking place, I lowered one of the bottles in my experiment to see if siphon would occur, and because there was no saline solution at the centre of the loop of tubing no circulation took place, therefore disproving that we were looking at a siphon. ......... Andrew

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Interesting. Well at least his experiment shows that water will not necessarily vaporize at heights above 10m and pressures below the vapor pressure in an inverted u-tube siphon like device. I'm perplexed why the liquid wouldn't flow. Did the water expand to a warm but non-liquid ice like solid crystal structure that only flowed when the salt was present to "melt" the warm ice? Mindbuilder (talk) 05:33, 27 January 2011 (UTC)

The purpose of the salt was to create a liquid of different density to standard water, so as to cause the water to be heavier than the non saline side. Moonshine124.185.100.36 (talk) 03:05, 28 January 2011 (UTC)

Additional comments:

"I think a pure water siphon could be made to work over a small height in vacuum near freezing."

What does this prove about everyday siphons? i.e. it doesn't rule out everyday siphons using atmospheric presssure. Why the need for only a small height? Why not set up you siphon for a 1 or 2 metre rise in a vacuum?

—Preceding unsigned comment added by 123.211.226.205 (talk) 01:55, 27 January 2011 (UTC)

It proves that Mindbuilder believes a water siphon could operate in very low atmospheric pressure. I think he is right. If the rate of flow of water through a siphon was shown to be independent of atmospheric pressure, all the way down to very low pressure, it would confirm that atmospheric pressure is not the driving force behind the siphon.

This would not be surprising. Even the most elementary analysis of the siphon indicates the driving forces are the high density of the working fluid, the low density of the surrounding atmosphere, and the strength of gravity. (On the Earth's surface, strength of gravity is not a variable so it can be ignored when discussing the common water siphon.) Dolphin (t) 02:26, 27 January 2011 (UTC)

It doesn't prove anything about everyday siphons. I hope I have made myself clear that I think atmospheric pressure is generally *indispensable* to the operation of everyday siphons. My point in bringing up water and mercury and oil siphons in vacuum is that atmospheric pressure may be indispensable to virtually all practical siphons, but SOME siphons don't use atmospheric pressure at all, and therefore the definition of siphon in the opening paragraph shouldn't ignore that fact.

Actually I think you're right. A low temperature water siphon wouldn't have to be limited to a small height. I was thinking about the surface tension providing the "pressure" to prevent vaporization. But the Z-tube experiments show that the water at negative pressure in the tube still wouldn't vaporize and the water on the surface of the reservoirs wouldn't be at negative pressures so it shouldn't vaporize. 05:52, 27 January 2011 (UTC) — Preceding unsigned comment added by Mindbuilder (talk • contribs)

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Why the need and insistance for the SMALL height? I have no issues with using a low atmospheric pressure in the test. But clearly the reason for the small height is the fact that without some atmospheric pressure, the siphon will not work in the vacuum. Removing atmospheric pressure, then also removing the siphon lift doesn't provide the required proof. Suggested test conditions.

1: Everyday water (not boiled before hand, or any other liquid) 2: Vacuum of 5kpa, so that the water stays as a liquid 3: Siphon lift height of at least 1 metre

The purpose of the above is to ensure the only change is the removal of atmospheric pressure. And thus the test results relate directly to everyday siphons.

Note: We already know that the RATE OF FLOW is generally independent of atmospheric pressure. i.e The RATE OF FLOW is related to the HEIGHT DIFFERENCE OF THE INLET/OUTLET (or outlet surface level if the outlet is submerged), along with the pipe internal diameter and pipeline length. However, Atmospheric pressure must still exceed the sum of the siphon lift and friction in the upward leg and velocity head.

If a siphon can lift 10 metres at sea level, should it not also be able to lift 10 metres in a vacuum if atmospheric pressure played no part?

—Preceding unsigned comment added by 123.211.226.205 (talk) 03:12, 27 January 2011 (UTC)

The reason Mindbuilder has nominated a small height is a very practical one that is unrelated to the functioning of a siphon. At the low pressure existing at the top of the siphon the water is likely to BOIL! When water reaches its saturation pressure it boils. With progressively reducing atmospheric pressure, the minimum pressure in the siphon gets closer to saturation pressure and so the water gets progressively closer to boiling. Mindbuilder is suggesting avoiding this practical difficulty by using very cold water and low siphon height. An alternative is to find another working fluid - mercury perhaps.

These are practical considerations and in no way invalidate the demonstration regarding functioning of a siphon at very low pressure of the surrounding atmosphere. Dolphin (t) 04:48, 27 January 2011 (UTC)

But it IS related to the functioning of a siphon. The water will boil since we no longer have enough pressure. And that pressure is normally supplied by atmospheric pressure. You are confirming that a siphon will no longer lift 10 metres, not even 1 metre when atmospheric pressure is removed. You are confirming that an everyday siphon utilises atmospheric pressure to operate, both to keep the water as liquid, and to move water to the higher point. Using an alternative fluid is not the appropriate answer, as it is getting away from the everyday water siphon that needs to be discussed first, before you move to special applications.

Water in a reservoir has a pressure at the surface of not zero but 10.33 metres head absolute. In you have a siphon pipe in the reservoir, as you head upwards, the absolute pressure in the siphon pipe reduces, by both friction loss and static LIFT. This is based on hydraulic principles. If we start with 10.33 metres, then this sets the limitation on what the total of friction and static lift can be. In this calculation, the lower reservoir level and it's difference to the upper level has no relevance. Once we get the inside pressure close to zero, yes the water will boil. This is a basic limitation of a siphon, and it is the siphon operation that is causing this reduction in pressure. If our starting pressure is reduced, so to will our total allowance of friction loss and static lift. 123.211.226.205 (talk) 05:22, 27 January 2011 (UTC)

You have written an everyday siphon utilises atmospheric pressure to operate, both to keep the water as liquid, and to move water to the higher point. You are repeatedly focussing on the function of atmospheric pressure in keeping the water as liquid, although that is not in dispute. You have provided nothing to support your idea that atmospheric pressure works to move water to the higher point. Would you please try to write something in support of the latter of these two? If you don't, we will have no choice other than to assume nothing can be written in support.

Would you also care to comment on my description of an experiment - given here? Thanks Dolphin (t) 06:25, 27 January 2011 (UTC)

Why do you concentrate on water that has not been boiled? Practical siphons have air dissolved in the water, but I've stated repeatedly that I agree with you that practical siphons rely on atmospheric pressure. But not ALL siphons use atmospheric pressure. When I talk about water siphons in vacuum, I am referring to PURE water because all bets are off when you start dissolving gases into the water. The purpose of boiling is to remove the air. Imagine a high concentration of Carbon Dioxide being dissolved in a water siphon and then attempting to operate it in a vacuum. It would bubble over like a bottle of soda. That doesn't prove that ALL siphons rely on atmospheric pressure. Mindbuilder (talk) 06:05, 27 January 2011 (UTC)

Re water that has not been boiled. Because people will take the results from special tests, and use them to explain/justify their view/position on standard everyday siphons. You agree that practical siphons rely on atmospheric pressure, however many others do not. And atmospheric pressure has been removed from the definition. (A number of my contributions were related to comments by others).

If the majority of everyday siphons, be it water or petrol utilise atmospheric pressure, shouldn't the Wikipedia description include atmospheric pressure. If there is proof that a siphon can operate without atmospheric pressure due to special circumstances, such as low height, special liquid, capillary action or other forces, then these should be included as special cases further down in the description.

The current description appears to be trying to be political correct to offend no-one, and thus doesn't really explain how the majority of siphons work.

I understood that the original reason for including vacuum siphons was, as noted earlier, "Look, a siphon will work in a vacuum, thus atmospheric pressure is not required".

It appears that vacuum siphons are now included for other purposes, i.e to try and explain and cover all types of siphons.

It would appear that the actual vacuum tests need to be done and fully documented. However, it seems that people will still draw their own conclusions, some of these incorrect, from the results. Which is why I asked what tests people wanted. —Preceding unsigned comment added by 123.211.226.205 (talk) 06:38, 27 January 2011 (UTC)

Please sign your posts at least with a nickname even if you don't log in, because it's hard to tell the difference between the authors of posts that are signed by IP addresses. Most people have a dynamic IP address so you may be thought to be the same person as another person who posts with only IP addresses. If you're worried about privacy then you should realize that it is actually more private to log in because then your IP address, which can likely be traced to your address, is hidden and only your arbitrary nickname is revealed.

I don't care if you add atmospheric pressure to the opening siphon definition, but I don't recommend that it imply that ALL siphons use it. It might be appropriate to leave discussion of atmospheric pressure to the theory section.

It would be nice if modern tests of vacuum siphons would be fully documented and on video. But it doesn't matter much what tests we want because by Wikipedia policy tests done by Wikipedia editors are prohibited original research. I have argued that one test of the first diagram in the theory section should not be rejected on original research grounds because it can be carried out by anyone with a garden hose in a matter of very few minutes so that it has almost the ultimate in verifiability,(and we have a video by a phd physics professor from a major university) but I can only really claim a very narrow exception to the original research policy there. Any tests done with equipment like a vacuum pump, could not qualify for an exception to the original research prohibition.

@Dolphin - My previous response to you was a hasty and embarrassingly poor response to your thesis. I was paying too much attention to other things and let too much time elapse between reading what you wrote and commenting on it, so that I forgot what you had wrote. I'll have a fuller response tomorrow. For now I'll mention that I think it is the conventional view that atmospheric pressure does push the liquid up the tube of a barometer. The functioning of a siphon has similarities to that. Of course you could say that it is the pressure in the liquid of the reservoir that pushes the liquid up, and that would be strictly true, but the important part of the pressure in the reservoir comes from the pressure of the atmosphere above the reservoir. Without the pressure of the atmosphere (i.e. in a vacuum) the liquid in the reservoir would not rise up the barometer but would sit flat in the reservoir. On the down side of the siphon, the weight of the liquid assists in overcoming the atmospheric pressure at the outlet thereby allowing the pressure at the top of the siphon to drop. The pressure of the atmosphere doesn't cancel at the two ends of the siphon tube because part of the atmospheric pressure is opposed by the greater weight of liquid above the exit of the siphon. Mindbuilder (talk) 09:30, 27 January 2011 (UTC)

@Mindbuilder - thanks. I look forward to your views. Dolphin (t) 10:43, 27 January 2011 (UTC)

Re Dolpin51's test. Yes, the density of water and air is important for water to flow. You can do the same with a small reservoir of water and tube out the bottom which is set at a lower height than the reservoir. Water will flow out the end of the tube from the reservoir. Then put the whole lot inside a larger reservoir, and the water will stop flowing in the tube. So density is an important part of the whole set-up, however this only covers water flowing downhill, not uphill.

The density issue applies also if we attempt to mix air and water in a container, water will sink to the bottom, air rise to the top. Put oil and water into a container, water will sink, oil will rise. Put mercury and water, mercury will sink, water will rise.

In a siphon, the mystery is why water rises, against gravity.

An initial note on the role of atmospheric pressure. Atmospheric pressure is equal to approx 10.3 metres of head pressure at sea level. The weight of the air is sitting ontop of our upper reservoir. And thus it applies 10.3 metres of pressure onto the surface of the water. So our upper reservoir of water at the surface level, has a constant pressure (assumming no high or low weather patterns), not of zero, which is guage pressure, but 10.3 metres absolute. Before we even get to a siphon, it needs to be understood that the upper reservoir is actually a container of pressurised water. If the reservoir was quite deep, and we went down inside it by 10 metres, our absolute pressure at this point becomes now 20.3 metres.

From a siphon point of view, the only pressure of interest is the surface level pressure, since if we put the tube down inside the reservoir by 1 metrre, although the pressure at the inlet is 1 metre higher, we actually lose this by the time we get back to the surface level inside the tube.

The role of atmospheric pressure in a siphon is simply that stated above, i.e. it provides us with a container of pressurised water, that is always at 10.3 metres absolute (weather patterns aside). The upper reservoir is also always at this 10.3 metres absolute, regardless of what we do downstream of the siphon inlet.

In a way, atmospheric pressure plays no further part in the siphon (Putting aside the density issues listed above on exit of the siphon). Assumming the siphon inlet remains in the water, then the atmosphere never gets to put any air into the siphon. It is always just water that is going in. And the water molecules inside the siphon tube, say 1 metre in, no longer have any communication with the outside atmospheric pressure.

So maybe you guys should comment on the above to ensure we are all in agreeance.

After this, water falling down the long arm of a siphon creates a vacuum, or lower pressure at the crown of the siphon.

And now our water inside the siphon tube, that has an absolute pressure of 10.3 metres when at a similar height to the upper reservoir surface level, will head towards the lower pressure area at the top of the siphon crown.

Now, inside our siphon, it is not an issue of atmopsheric pressure. It has already done it's role to provide the pressurised container of water. It is one of pressure difference. Pressurise a container of liquid, then give it a lower pressure area to escape to, and it will head there. If the pressure difference is great enough, it will even be able to head uphill. But only uphill as much as we have pressurised it.

Moonshine121.222.239.180 (talk) 01:41, 28 January 2011 (UTC)

This thread has become too long so I have started a new one titled Atmospheric pressure, immediately following this one. I suggest all new edits should be posted in the new thread. Dolphin (t) 02:24, 28 January 2011 (UTC)

Atmospheric pressure

The previous thread has become too long so I will start a new one. The recent conversations in the previous thread have been about atmospheric pressure and its relevance to siphons, so that is the name I have given this new thread.

The latest edit in the previous thread was from Moonshine. Here is my reply.

@Moonshine: You have written In a siphon, the mystery is why water rises, against gravity. There is no mystery. For every gram (or pound) that enters the siphon, moving upwards against gravity, there is one gram (or pound) that leaves the siphon, moving downwards. It is a bit like a wheel on a moving vehicle – part of the wheel is moving upwards, but another part of the wheel is moving downwards so the gravitational potential energy of the wheel remains constant. There is no mystery.

However, the down-leg of the siphon is longer than the up-leg. At first glance, we could say the speed of water through the siphon must be steadily increasing as predicted by the work-energy theorem, but that obviously isn't so. The extra work done by gravity on the water descending down the down-leg matches the losses due to viscous forces on the water as it moves through the siphon.

It is true that the air pressure on the free surface of the water is 10.3 metres (or 101 kPa), but don't over-state the significance of that. The same air pressure applies at the outlet from the siphon. It is easy to express the situation in a math equation and show that atmospheric pressure cancels from both sides of the equation. From that point on, atmospheric pressure plays no further part in the math.

Moonshine, why not create an account for yourself? Moonshine would be a great User name. User:MoonShine and User:Moonshiner are both taken, but User:Moonshine isn’t while ever it appears as a red link. It costs you nothing and allows us to communicate with you directly via your User Talk page. It also provides greater personal security than exists using your IP address for identification. Dolphin (t) 02:22, 28 January 2011 (UTC)

There is no mystery? There is no mystery? Then why is there so much discussion and varying opinions!

Re: It is true that the air pressure on the free surface of the water is 10.3 metres (or 101 kPa), but don't over-state the significance of that.

You need to move beyond atmospheric pressure at the surface, and just below the surface so that you are dealing with the pressurised water. The significance of the pressurised water needs to be over stated, especially when people start comparing the outlet pressure. Water is made up of droplets or molecules that are not formally linked. So the water in the inlet that is moving to the top of the siphon has no idea about the outlet pressure, nor does it have any idea about formulas cancelling each other out. It is moving from one pressure area, 10.3 metres head, the siphon inlet, to a lower pressure area at the top of the siphon. Moonshine124.185.100.36 (talk) 02:55, 28 January 2011 (UTC)

... at the top of the siphon where the pressure might be 9 or 10 metres head, and then it moves down to the outlet where the pressure is again 10.3 metres.

Why is there so much discussion and variety of opinion? It is my understanding that, for many years, the Oxford English Dictionary explained the siphon as working due to atmospheric pressure. Engineers, physicists, and science students have known for decades that atmospheric pressure would not be a good explanation of the working of a siphon but they didn't know about the explanation in the Dictionary. In the last year or two a Queensland academic blew the whistle on the folks at the Dictionary and pointed out that the functioning of a siphon owes nothing to atmospheric pressure and everything to gravity. See Siphon#Oxford English Dictionary. Some people have sided with the folks at the Dictionary and others have sided with the academic. Since then there has been plenty of debate and argument wherever people gather to talk about things like the siphon. Dolphin (t) 06:34, 28 January 2011 (UTC)

Well, the Queensland academic clearly got it wrong in reference to "owes nothing to atmospheric pressure". The maximum height of an everyday siphon being limited by atmospheric pressure clearly proves that. And he would be wrong if he said it owes everything to gravity, the key issue being the use of the word "everything". Except, he didn't claim it was everything to do with gravity, rather it was gravity, tensile strength and hydrogen bonds. Gravity's involvement is correct, but not tensile strength and hydrogen bonds. His chain analogy about the heavier side pulling the water over was subsequently proven to be incorrect. (A lighter downside still had water move to it's side from a heavier upside)

... at the top of the siphon where the pressure might be 9 or 10 metres head, and then it moves down to the outlet where the pressure is again 10.3 metres. ???

Yes, the outlet pressure is also 10.3 metres! On the downward leg,it would appear that the water is moving from a low pressure (9 metres) to a higher pressure area (10.3 metres) which contradicts my earlier statement. Except the water has pressure added as it reduces it's height down the siphon leg. This is called potential energy at the top of the siphon. And this potential energy, say 4 metres at the top of the siphon, is added to the absolute pressure at the top of the siphon, and is higher than the 10.3 metres at the siphon outlet. The flow rate will increase then settle at a rate that burns up this difference in friction.

An everyday siphon operates due to a combination of water being a liquid, the different densities of water and air, gravity, atmospheric pressure, pressure differential and hydraulics. It is not tensile strength and hydrogen bonds, otherwise our water would be a jelly like substance if it had a tensile strength of 10 metres. But even jelly, which is mostly water, doesn't flow.

Moonshine124.187.18.17 (talk) 08:09, 28 January 2011 (UTC)

@Dolphin - Still don't have time for a full response, but let me quote the Wikipedia article on barometers: "Torricelli ... proposed that air had weight, and that it was the weight of air (not the attracting force of the vacuum) which held (or rather, pushed) up the column of water. He thought that the level the water stayed at (thirty-four feet) was reflective of the force of the air's weight pushing on it (specifically, pushing on the water in the basin and thus limiting how much water can fall from the tube into it)." When we say that it is atmospheric pressure we know and we expect that it will be understood that the atmosphere is acting through the liquid in the reservoir. But we still say it is the atmospheric pressure that pushes up the liquid because the atmospheric pressure pushing down on the reservoir is the source of the pressure and therefore force that pushes the liquid, not just up the entrance of the tube, but above the level in the tube that is at the same height as the free surface of the reservoir. If you set up a siphon like the first diagram of the theory section (which I'll call the "air top siphon") you will find that the maximum height that the air top siphon will be able to bring liquid above the upper reservoir, will be limited by the atmospheric pressure, because it is the atmospheric pressure that pushes the liquid up. The air top siphon or a siphon with a bubble in it cannot use liquid tensile strength like Hughes theorized or like a siphon operating in vacuum does. Nor does the hydrostatic pressure of the reservoir contribute to propelling the liquid up above the surface of the reservoir. To see a little more clearly that it is the atmospheric pressure that pushes the water up, consider trimming the part of the siphon tube that is submerged in the upper reservoir until it just barely touches the surface of the reservoir. In such a case the pressure due to the liquid in the reservoir will be tiny, but the liquid will still be pushed up by atmospheric pressure. Normally the pressure at the top of the siphon pushes down on the liquid as much as the atmospheric pressure pushes up. But when gravity acting on the falling liquid counters some of the atmospheric pressure at the outlet and lowers the pressure at the top, the atmospheric pressure can push the liquid up. Saying that it is the liquid pressure at the entrance of the tube that pushes the liquid up rather than atmospheric pressure, is like saying that it is the glove on my hand that is holding up this steel weight rather than my hand. It's strictly true, but we don't normally speak like that. The liquid in the reservoir is like an intermediary that the atmospheric pressure pushes on. It's also like saying that it's not my legs that propel the wheels of my bicycle, it is the chain. Again, the chain and the liquid in the reservoir are just the intermediaries. Mindbuilder (talk) 09:17, 28 January 2011 (UTC)

I should make clear that it is gravity that powers all siphons, but in air top and practical siphons, the gravity acts to lower the pressure at the top of the siphon which then lets the atmospheric pressure take over and push the liquid up the siphon. Mindbuilder (talk) 09:30, 28 January 2011 (UTC)

@Mindbuilder. If the article says atmospheric pressure pushes liquid from the reservoir up the siphon readers could be forgiven for thinking siphons are self-priming; that an empty U-tube only needs to be immersed in the reservoir for atmospheric pressure to take over and push the liquid up the siphon. As you and I know, siphons are not self-priming. Priming must be done manually by an operator - atmospheric pressure doesn't push the liquid up the siphon.

Similarly, atmospheric pressure doesn't push mercury up an evacuated tube to create a barometer. The tube is laid on its side, or even inverted, in a bath of mercury and mercury runs into the tube, displacing the air. Then the tube is raised to the erect position where the mercury is in equilibrium - the pressure gradient at every point exactly balancing the specific weight of the mercury. The average pressure gradient is equal to atmosheric pressure minus saturation pressure of mercury at the relevant temperature, divided by the vertical height from the top of the mercury column to the free mercury surface in the reservoir. Language of this kind is not perfect but it is more encyclopedic than saying atmospheric pressure pushes the mercury up the tube.

There is a significant difference between a barometer and a siphon. The former is a static application whereas the latter is dynamic with fluid flowing from the reservoir, through the siphon out into the atmosphere. A column of mercury has atmospheric pressure at one end, and a very low pressure at the other, and that is sufficient to explain why the column of mercury is in equilibrium. A siphon has atmospheric pressure at both ends and yet the liquid flows from one end to the other. The presence of equal atmospheric pressures at both ends of the siphon confounds our attempts to explain why the liquid flows in one direction only, and the flow can be of significant velocity despite the resistance of viscous forces.

If we were to use language about atmospheric pressure pushing water up the up-leg of the siphon the astute reader is entitled to say to himself "The atmospheric pressure must also push water up the down-leg of the siphon. Although clearly that doesn't happen because water flows down the down-leg." I hope readers can now see that trying to weave atmospheric pressure into the explanation of operation of the siphon leads us nowhere. Dolphin (t) 12:03, 28 January 2011 (UTC)

It's a good point to recognize that readers may make the mistake of thinking the siphon is self-priming. We should make it clear that the atmospheric pressure takes over after it is primed. Though in the air top siphon, the up side need not be primed, only the down side, and it will function.

Filling the barometer with mercury and then putting it in the reservoir is the common practical way to prime a barometer, but it also works to attach a vacuum pump to the top of the barometer and remove the air. Then the atmospheric pressure will push the liquid up the barometer. Again, it is true that the pressure gradient in the reservoir will balance the weight of the mercury, but the source of that pressure is atmospheric pressure. That is of course the primary use of the barometer, to measure atmospheric pressure. As the barometer is taken to different altitudes, the mercury is pushed up the tube to a height proportional to the atmospheric pressure. We don't say the mercury reservoir at the bottom pushes the mercury up the barometer because although it could be technically considered true in a sense, it wouldn't explain the operation of the siphon because the hydrostatic pressure alone of the mercury reservoir cannot push the mercury up the barometer any higher than the level of its own surface. It will be obvious to any reader that atmospheric pressure is not acting directly to push the liquid up, but rather through the liquid in the reservoir. We don't have to worry about that misunderstanding.

In the encyclopedic language of Britannica online, it is stated that in a barometer "atmospheric pressure balances a column of mercury". Of course in a siphon the atmospheric pressure doesn't balance the liquid, it pushes it right over the top. But again, the encyclopedic language doesn't mention the pressure gradient in the reservoir at the entrance of the barometer tube. Wikipedia is a little different than a traditional encyclopedia in that there is more room to go into more detail and explanation about the subject of the article. Encyclopedias explain things similarly to textbooks, but traditionally they do so more briefly, because although the many volumes of an encyclopedia are much bigger than any single textbook, encyclopedias cover the ground of thousands of textbooks. Wikipedia is less space constrained and so can give more explanation in both technical terms and simpler terms. Wikipedia is not meant to be a textbook. This is not turning Wikipedia into a textbook, because we are not going all the way to being like a textbook.

If the static nature of the barometer obscures the difference between it and the dynamic siphon, consider the siphon where the surfaces of the reservoirs are at the same level and the siphon tube is full of liquid, except for a bubble at the top which separates the two columns of liquid. The siphon will not flow, but the liquid will not drop out of the tube. What then is supporting the liquid in the tube but atmospheric pressure? Surely not the hydrostatic pressure of the reservoirs which could not support the water up the tube any higher than their surfaces. It can also be seen in this case that atmospheric pressure does push the liquid up the down tube (though not really the down tube if it's not flowing). The usual flow of the siphon in one direction can be explained by the fact that the siphon is net flowing down hill. In a flowing siphon, the down flow is caused by gravity and retarded by atmospheric pressure. The up flow is caused by atmospheric pressure and retarded by gravity.

You have pointed out that the atmospheric pressure cancels from some equation of the siphon. But I think you are using the wrong equation. The equation you are referring to is probably an equation about the flow rate at the ends or something. Atmospheric pressure may cancel in THAT equation, but an equation that relates how high the liquid can be pushed up above the surface of the reservoir, must not drop or cancel the value of atmospheric pressure (except in the rare cases where the liquid can maintain tensile strength). In the static siphon with an air bubble, described in the previous paragraph, the equation of the height of the liquid above the surfaces of the reservoirs will vary with atmospheric pressure, and atmospheric pressure will not cancel. In the equation you have been referring to, there is an unstated simplifying assumption, that there is sufficient atmospheric pressure (or tensile strength) to push (or pull) the liquid to the top. If that assumption is not met, then elements must be added to the equation to keep the equation valid, and atmospheric pressure will no longer cancel. Mindbuilder (talk) 18:04, 28 January 2011 (UTC)

@Dolphin, As far as I can tell, you are the only one arguing against the reversion to an explanation including atmospheric pressure. We need to reach some kind of consensus here to avoid a ping pong definition.

To say that readers would be confused because atmospheric pressure is also pushing up the down leg is to say they would also be confused by water leaking out of a hole at the bottom of a bucket since the hole also has atmospheric pressure against it. We don't need to go into details for this normal behaviour, namely, water falling out of an open pipe. It's the water rising against gravity that is unusual, and this is essentially what the Siphon entry has to explain.

I don't know what maths you are using where the atmospheric pressure has no effect. Whilst it might cancel out across the fluid, it clearly has an effect on the potential lift height of the siphon. So if you have eliminated any effect of air pressure on the water surface then I respectively suggest you have cancelled too far. I suspect that rather than eliminating it, you have incorporated its effects into your hydrostatic pressure.

That priming must be done manually is not an argument that atmospheric pressure doesn't push the liquid up the siphon. It is everyday experience that siphons are not self priming, so I don't see a great problem here. It is easily fixed with, for example, "atmospheric pressure pushes liquid from the reservoir up the siphon when the pressure inside the tube is reduced".

There seems to be general agreement in the way fluid pressure varies in order to drive the siphon. The debate is over the language to describe it. Might I suggest, if you are more in favour of the Hughes description, that you add your version into the OED section?Mike163 (talk) 17:56, 2 February 2011 (UTC)

Hi Mike. You say I don't know what maths you are using where the atmospheric pressure has no effect. See the following thread, titled Fundamental equations of the siphon! I look forward to seeing your maths on the subject. Dolphin (t) 21:25, 2 February 2011 (UTC)

Hi Dolphin. I'm not a theoretical physicist although I do have two physics based degrees. I've no desire to enter into a maths competition because it's going to come down to the definition of some of the terms. It is clear that both inlet and outlet have atmospheric pressure on them so the net static pressure will cancel out. It is also clear that the siphon would not start without atmospheric pressure on the upper reservoir. This argument denying the importance of atmospheric pressure is the same as the one about siphons operating in a vacuum, which was removed from the main definition as being exotica and not normal everyday experience. Even the textbook link for siphons in a vacuum agrees. This detailed debate about the language and this desire to get more and more clever with the definition has caused the article to loose sight of the prime explanation required for a siphon: that the liquid flows up. If you read the current title paragraph, all it says is that liquid will fall out of the end of an open pipe! Ask your friends and family to read it: it's completely useless for the task of explaining how a siphon works. As I said previously, hydrostatic pressure alone, as defined by the WP Fluid statics page with Z0 as the fluid surface, can't raise the fluid above the height of the top reservoir. It requires additional pressure from the atmosphere to do that. We have had enough debate here to see that neither side will convince each other, and unfortunately I'd have to say your view is the minority. Please consider including a separate section with your views or including them in one of the technical sections. It is not acceptable that we have an explanation for siphon that says nothing about the point most will want to understand: why does the liquid flow up. Thanks. Mike163 (talk) 14:24, 6 February 2011 (UTC)

Hi Mike. Seeing you have no wish to present your own maths on the subject I would still appreciate it if you comment on mine. See the next thread Fundamental equations of the siphon. So far only one person has commented on my math and that was Mindbuilder who made a very relevant comment so I offered to add a comment to my opening equation. Mindbuilder agreed that my comment would be appropriate. (Note that my math addresses the overall performance of a simple siphon, not the narrower question of what drives liquid up the up-leg of the siphon.)

Wikipedia's first priority is to explain what a siphon is. Explaining how it works is a second priority. Everything in the article must be supported by references and citations that show the information comes from reliable published sources and can be independently verified. (See WP:Verify.) If one or more reliable published sources can be found saying atmospheric pressure is what drives a siphon then that is what Wikipedia should say. If one or more reliable published sources can be found saying something a little different, then that is what Wikipedia should say. But if both views can be found in reliable published sources then there is a task to determine whether Wikipedia should present both, or whether the sources for one view are more authoritative than the sources for the other. That is actually what we are doing here - trying to determine which view withstands critical scrutiny better than the other.

You have said that my view is in the minority and this should see the matter resolved. This does not apply at Wikipedia. See WP:Polling is not a substitute for discussion.

The significant feature of a siphon is not that liquid rises upwards against gravity. (That happens in lots of apparatus that aren't siphons - for example a coiled garden hose.) The significant feature of a siphon is that the liquid flows through the crest where the pressure of the liquid is sub-atmospheric. In a siphon this is usually achieved because the liquid rises above the level of the reservoir's free surface. (In some siphons there is no reservoir.)

If you fill a U tube with water and place it inverted with both ends immersed in the same reservoir of water, no water flows and it doesn't function as a siphon, even though atmospheric pressure is acting on the surface of the reservoir. My position on what causes the liquid to flow upwards in a siphon is that it is driven upwards as the result of a pressure difference. It is the difference between atmospheric pressure and the sub-atmospheric pressure at the crest of the siphon. Note that two pressures are involved. As far as I am aware no-one has challenged that position. Dolphin (t) 22:21, 6 February 2011 (UTC)

I think the old consensus has collapsed and a new, at least partial one, has formed, so I revised the opening paragraph. Lets discuss this at the new section of the talk page at the bottom. Mindbuilder (talk) 23:04, 6 February 2011 (UTC)