Talk:Siphon

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How is a syphon depicted graphically in a drawing representing an hydraulic network? 88.72.2.55 21:03, 14 September 2007 (UTC)

Not all liquids boil under vacuum. That should be changed. It's true that water will. Ashi_Starshade

Can siphon also be used for U-bends that go below the level of the reservoir? Syd1435 01:27, 2004 Nov 25 (UTC)

If I understand you correctly, that's an inverted siphon described in the article and it's not a siphon at all.Samw 04:33, 25 Nov 2004 (UTC)

The article says:

The siphon was first used as a weapon by the Byzantine Navy, and the most common method of deployment was to emit Greek fire, a formula of burning oil, through a large bronze tube onto enemy ships. Usually the mixture would be stored in heated, pressurized barrels and projected through the tube by some sort of pump while the operators were sheltered behind large iron shields.

If this uses "pressurized" barrels and "some sort of pump", is this really a siphon?

See siphon bottle in the article. I didn't write this so I can't vouch for it's historical accuracy. But physically, it would be a siphon. The extra pressure in the barrel would make it a siphon bottle.Samw 04:09, 2 Dec 2004 (UTC)

The writer of this article has referred to vacuum in two instances. The first claims that flow of liquid creates the vacuum. later on gravity tends to draw the liquid down in both directions creating a vacuum. I would suggest that "flow of liquid" eliminates all the air and so the liquid flows. When the upper end of the pipe is exposed air immediately invades and breaks the siphon. Thus I believe that references to vacuum should be deleted. Jack hill 7 Jan 05.

You are correct, there is never a vacuum formed and there is never any air within the siphon or it will not work. I see how the current description can be misleading. The reference to a vacuum is simply explain how the siphon works. If you have a better description, feel free to rewrite that section. I tried without success. Samw 04:14, 8 Jan 2005 (UTC)

A lot of biology articles refer here, but the biological use of the word is really quite distinct. Unless anyone objects, I propose to take out the biological section (except for the discussion of the siphon effect in biology) and create a new entry for the siphon in invertebrate anatomy. Myopic Bookworm 17:23, 22 February 2006 (UTC)

I had a disambiguation at the top of the article just for that reason until it was removed on Nov 28, 2005. I would support you adding it back in. Samw 00:45, 23 February 2006 (UTC)

Contents

[edit] Mathematics

I originally added the math section at the end because the general reader of a general article like this isn't likely to be familiar with Bernoulli's equation. I'm merely applying the principle of progressive disclosure. Any objections to me moving the math section further down the article? Samw 03:19, 7 May 2006 (UTC)



I made an error in the discussion and erased my comment.

[edit] Siphon as technique to evacuate cellars from floodings

Dear Wikipedians!

I am new in Wikipedia and almost without experience as author.

Could You please give me Your help to establish this method to evacuate cellars from water after floodings? It bases on the siphon: [[1]] Between the flooded cellar and a deeper place outside a connection is being built, using a tube or some pipes. They are filled with water from the flooded cellar or through an intake valve. When the ends are beeing opened, the water flows through the pipe into the sewer or the river.

I practised it many times to help neighbours and others and would now like it to establish itself, so that many people in the situation of a flooding are less dependent on the help of the fire department, their resources and the availabilty of pumps and energy.

I can´t wait to see Your comments, best wishes, Daniel Schwanck __Daniel Schwanck 14:47, 28 January 2007 (UTC)

[edit] Inverted siphons

This is regarding the image captioned "Inverted siphon under a sink".

Inverted siphon under a sink.
Inverted siphon under a sink.

The trap under a household sink is not an inverted siphon as defined in the article. The purpose of an inverted siphon is to carry the liquid under an obstruction. Per the article, it should be designed to maintain sufficient flow to keep solids in suspension. A trap does exactly the opposite. Can you please cite references for a trap being called an "inverted siphon"? If so, we should write that up properly in the main article. Thanks. Samw 01:28, 12 April 2007 (UTC)

Indicated is where siphoning and inverted siphoning take place.
Indicated is where siphoning and inverted siphoning take place.
The pictured contraption is first and foremost a means to keep sewer gas from entering the home. Second, when emptying a sink one expects that all solids in suspension, such a food particles, will freely and quickly flush down, unobstructed. That is may occasionally keep some small parts (jewelry maybe) from going into the sewer (i.e. trapping them) is not very relevant, however pleasant it may be. "Trapping" particles is not a primary function of this contraption. The name "trap" is really a misnomer considering the main functions. VanBurenen 07:53, 12 April 2007 (UTC)
To clarify where siphoning takes place, see picture Image:Siphon detail.png. VanBurenen 14:55, 12 April 2007 (UTC)
Thanks for the explanation. Agreed the trap is primarily for sewer gas. My issue though is calling it an "inverted siphon". Do you have references of people actually calling it an "inverted siphon" as opposed to a "trap"? (A quick search of Google for "inverted siphon" only shows it being used for sewers avoiding an obstruction.)
Your new picture raises another issue. You label a portion of the pipe "siphon section". While geometrically it is a siphon, it is my understanding that the siphon action is very weak or non-existent. Otherwise, the trap section would be emptied completely, defeating it's purpose of blocking sewer gas. All in all, I think it is problematic to label any portion of a sink drainage pipe a "siphon". Samw 00:08, 13 April 2007 (UTC)

[edit] Attached file not too precise

In Applications, the linked file "http://upload.wikimedia.org/wikipedia/de/b/bc/Hydrostatischer_Heber.pdf" (ref #8) is not too precise. The end of the pipe should be below the level of the tank, not below the bottom of the tank. The dashed line may confuse someone.

--Nicolaufg 16:12, 17 April 2007 (UTC)

[edit] Syphon

I wasn't sure if this should be added but thought I would mention it here. Maybe a bit on the ease with which a syphon can be made. I didn't want to include it due to low quality pic, and nosey background but if there is any interest I can retake the picture with a decent camera. It's a syphon I made from a runner bean stalk, making syphons from plant stalks is common in Kurdistan. Wolfmankurd 18:25, 12 May 2007 (UTC)

[edit] Siphon bottles?

Surely these so-called siphon bottles are simply pressurized bottles. According to the article, "It is a siphon in the sense that pressure drives the liquid through a tube." So if I suck liquid from a bottle through a straw (OK let's say the straw is an integral part of the bottle) that would make it a siphon? Of course not. I know these are popularly called "soda siphons" but surely that is a misnomer? A fountain (in fact any plumbing installation) also has pressure driving liquid through a tube, but that does not make it a siphon.--Shantavira|feed me 11:29, 20 May 2007 (UTC)

Feel free to clarify. The term "siphon bottle" appears to be a well-known historical term so I would object to the term being completely removed from the article. But by all means clarify that it is a misnomer. Samw 12:54, 20 May 2007 (UTC)
Not sure my O-level science is up to it, but I will consult elsewhere.--Shantavira|feed me 13:05, 20 May 2007 (UTC)

[edit] World Of Warcraft reference

Is it entirely necessary? A lot of the Wikipedia population likely do not know even what WoW is and I suspect a proportionally negligible amount of readers/editors will actually want to know about the clan. Thus is fails WP:NOTE. I suggest we either create a new article & disambiguation or remove it entirely; I vote the latter. --poorsodtalk 21:15, 13 October 2007 (UTC)

It looks like someone's already removed it. We can always have a dismabig page if someone wants to write a separate article on the WoW meaning. Samw 03:22, 14 October 2007 (UTC)

[edit] Gas collection at crest not desirable?

Really well written article.

The Applications section has the following

"The flow of the liquid moves bubbles thus the intake leg can have a shallow slope as the flow will push the gas bubbles to the crest. Conversely, the outlet leg needs to have a steep slope to allow the bubbles to move against the liquid flow. At the crest the gas can be trapped in a chamber above the crest. The chamber needs to be occasionally primed again with liquid to remove the gas."

It is good to mention how the gas can be collected and removed.

But I feel that most syphons with one crest are arranged to have the bubbles moved along and out.

In which case the outlet leg must have a shallow slope.

I'm not confident to write this elegantly, so would appreciate someone else putting it in if they agree.Polypipe Wrangler 03:43, 4 November 2007 (UTC)

I've added. Be bold next time and add it yourself! BTW, do you have a reference for this alternate design? Samw 12:07, 4 November 2007 (UTC)


[edit] Siphons in Nature

I removed the section on "naturally occurring siphons" which described the ability of water to travel over obstructions for short distances, Hydraulic jump. By definition, a siphon includes a tube.

I have also renamed the section on biological siphons to siphons in nature. Greenknight21 01:27, 6 November 2007 (UTC)

[edit] Direction of siphon flow in tank to be siphoned

I haven't seen this addressed in the article, or elsewhere, so hopefully someone here can help me out. When siphoning from a tank does the siphon draw the liquid down or up? To put it a different way, say you're siphoning a volume of oil floating on top of water. You want to siphon off all the oil while avoiding as much of the water as possible. Do you place the (intake) end of the siphon tube below the oil layer (for it to siphon some water & the oil from above down into the siphon tube), place near the top of the oil layer (for it to siphon the oil & water up from below - until it hits air, then having to frequently move the siphon) or place it at the bottom of the oil layer but above the water (for it to siphon the oil below it up, lowering the oil level until its all up)? Thanks AllanHainey (talk) 12:22, 4 March 2008 (UTC)

I'm not sure I really understand the distinction. The siphon draws whatever liquid is at the intake. So if you place it horizontally at the bottom of the oil layer, theoretically it will siphon off all the oil until air hits the intake and no movement of the siphon intake is required. Samw (talk) 01:25, 5 March 2008 (UTC)
The distinction is where it isn't possible to place (or retain in position) the siphon in a horizontal position. Where it has to be verticle I think the direction of flow would be relevant, though I may be wrong. AllanHainey (talk) 08:52, 6 March 2008 (UTC)
OK so if I understand correctly, you're proposing 2 positioning: 1) opening of the tube pointed down; or 2) opening of the tube pointed up (& snaking down and up again out of the container). Siphon flow rate should be the same in both cases and the orientation doesn't matter. That's a key point of the equations: only the height difference matters. If someone wants to draw a diagram, it would be a good addition to the article; not sure I want to explain this in words! Samw (talk) 01:03, 7 March 2008 (UTC)
Yeah, it's a little hard to get the head around but I undestand now. Thanks AllanHainey (talk) 13:09, 7 March 2008 (UTC)

[edit] "sadly this is not true"

Fix that! looks tacky for an encyclopedia! —Preceding unsigned comment added by 208.126.51.37 (talk) 21:17, 25 March 2008 (UTC)

[edit] Vacuum

This sentence has been in the article for many years:

Some (notably Encyclopedia Britannica[2]) argue that theoretically, "a siphon will work in a vacuum".

I agree Britannica is probably wrong. But given that Britannica is so prominent, I believe it is notable enough to mention. Samw (talk) 22:00, 6 May 2008 (UTC)

I think that the Britannica probably isn't wrong. Liquids have some obscure properties such as tensile strength (that's why trees can be more than 32 feet tall- they actually do suck up the water further than 1 bar of pressure difference- counterintuitively to me when I first learnt of this). Water can take negative pressure without necessarily cavitating). This tensile strength would be present in a vacuum, and in ideal conditions, siphoning should work.- (User) WolfKeeper (Talk) 22:08, 6 May 2008 (UTC)
OK point taken. So I think the old sentence is a succinct summary. Any objections if I put it back in? If there are references for the other concepts (trees, etc), by all means we should add that as well. Samw (talk) 03:10, 7 May 2008 (UTC)
I don't personally consider EB to be a reliable source (I don't consider the wikipedia to be either), so no, not without another source.- (User) WolfKeeper (Talk) 04:16, 7 May 2008 (UTC)
I still think it could be done though; particularly if the height of the reservoir you're siphoning from is around 32 feet (to avoid bubbles entering the pipe from the boiling liquid), and even if the siphon is a bit taller than that.- (User) WolfKeeper (Talk) 04:16, 7 May 2008 (UTC)

I oppose the restoration of the Britannica quote. If you scroll down to the last equation at the bottom article, you will see that in theory the maximum height of a siphon is proportional to the ambient pressure. So in a vacuum, the maximum height would be zero, and that's not much of a siphon. Surface tension may draw water up a narrow tube in a vacuum, as in transpirational pull, but then it won't release it at the other end as a siphon would. I can't verify what Britannica said since the referenced link doesn't work for me, but it had to be either wrong or misinterpreted. There are lots of mistakes in Britannica, as in many other sources, and I don't think it serves any purpose to copy them. Now that Wikipedia is so prominent, should Britannica articles start discussing our mistakes?--Yannick (talk) 06:02, 10 May 2008 (UTC)

Do you have a reference to the EB being wrong, or is this your OR?- (User) WolfKeeper (Talk) 06:43, 10 May 2008 (UTC)
Do you have a reference to the EB even saying that in the first place? A broken link is not verifiable.--Yannick (talk) 05:17, 11 May 2008 (UTC)
Yes, a few moments of googling gave: [3]. FWIW I would recommend the use of search engines before asking such trivial questions in wiki talk pages. Incidentally you can see the beginning of the quoted phrase at: [4]- (User) WolfKeeper (Talk) 06:28, 11 May 2008 (UTC)

So courtesy of other Wikipedians, I have: http://www.iop.org/EJ/abstract/0143-0807/29/3/003

It starts: "But the embarrassing matter of this humble instrument is the question: ‘does this device work in vacuum?’. In some references, the answer is ‘yes’ and in others the answer is ‘no’."

It cautiously supports that a siphon does work in a vacuum. Can we now agree to but back the original statement with this reference? Samw (talk) 03:42, 12 May 2008 (UTC)

That paper confirms that a siphon does not work in a vacuum. It cites multiple sources that have shown experimentally that siphons do not work in a vacuum. It shows that Pohl's cohesion theory agrees that a siphon will not work in a vacuum when compressibility is considered. And although it doesn't point it out explicitly, Potter & Barnes hydrodynamic theory would predict impossible negative pressures at the crest if the apparatus was in a vacuum. So under all theories and experiments discussed, siphons do not work in a vacuum. Thank you for finding the perfect citation to support my point.--Yannick (talk) 02:29, 14 May 2008 (UTC)
Ummmmmmm:
Another paper [2] explicitly claims that a siphon does work in vacuum. In a very nice experiment, the author uses two glass reservoirs connected by a sealed glass tube having a side connection with a stopcock (for air evacuation); he uses mercury and water, boiled long enough to exclude air bubbles.
[2] Nokes M C 1948 Vacuum siphons Sch. Sci. Rev. 29 233
Nokes M C 1948 Vacuum siphons Am. J. Phys. 16 254 (Reviewed)
So Nokes actually did the experiment successfully, in a vacuum, and published, again, successfully, in a refereed journal.- (User) WolfKeeper (Talk) 22:48, 14 May 2008 (UTC)
That experiment was not performed in a vacuum. The siphon was evacuated in order to prime it, as they always are, but there is no indication that the reservoirs were exposed to vacuum.--Yannick (talk) 15:21, 17 May 2008 (UTC)
That's not the question, the question is whether the top of the siphon tube is negative pressure or not.- (User) WolfKeeper (Talk) 19:28, 20 May 2008 (UTC)
No, the question is whether or not a siphon will work in vacuum. I'm saying it will not, because you need atmospheric pressure to act on the reservoir. The Nokes experiment was not performed in a vacuum and did not attain negative pressure at the top of the siphon tube.--Yannick (talk) 14:43, 24 May 2008 (UTC)
I'm saying that that's your OR, that the inlet to the siphon itself is never vacuum pressure anyway due to hydrostatic head, and that negative pressures in liquids are pretty well known to be possible anyway.- (User) WolfKeeper (Talk) 13:12, 7 June 2008 (UTC)
But in any case the question is whether the references support the contention. They do. End of. If you have references that attempt to show that this does not work, then we can add them as well. Removing references and material where there is contention is NOT NPOV and isn't the wikipedian way; you ADD material.- (User) WolfKeeper (Talk) 13:07, 7 June 2008 (UTC)

I'm not sure the paper clearly concludes that siphons do not work in a vacuum. I also like pointing out Britannica's flaws. So how about this statement in the main article:

Some (notably Encyclopedia Britannica[5]) argue that theoretically, "a siphon will work in a vacuum". The latest research disagrees[6].

Samw (talk) 03:31, 14 May 2008 (UTC)

I don't have access to that, and given that trees have negative pressure at the leaves, I'm not that impressed. And, read this, before claiming 'impossible negative pressures'. Pressures below -9 bar have been done.- (User) WolfKeeper (Talk) 03:33, 14 May 2008 (UTC)
If anyone needs a copy of [7], email me at samwsamw at gmail.com Samw (talk) 00:47, 15 May 2008 (UTC)
If you want to talk about capillary siphons, such as a towel in a sink, then yes you can have negative pressures. (BTW, I find the following web page more helpful than the AIP one: [8].) A capillary siphon still would not work in a vacuum though, because the same tensile force that pulls the liquid out of the reservoir will also prevent its release at the outlet. In any case, that is not what most people mean when they talk about a siphon. They mean a u-shaped hose or pipe, where negative absolute pressures are impossible. The capillary siphon should only be presented as a special case.--Yannick (talk) 15:21, 17 May 2008 (UTC)
But negative pressures are not impossible. People hang heavy weights on water filled pistons, and the water doesn't cavitate. Water can take MINUS 9 bar or lower. The water molecules are attracted to each other, that's why it's a liquid, not a gas. Only if you have nucleation sites and dissolved gases does it do that. And any bubbles take time to grow. These factors mean that the siphon can work even at very low pressures where the u-bend is at negative pressure.- (User) WolfKeeper (Talk) 20:11, 17 May 2008 (UTC)
Water can take -1400 bar or lower according to your own reference (backed up by this cite,) but not in a normal real-life siphon. Your AIP source says water can take minus 9 bar in a xylem, and I can believe that because of surface tension. But if you hang a heavy weight on a water filled piston, the water will certainly form bubbles. Again, your own reference says so in the second paragraph. You can call it cavitation, but most people call it boiling. That's what happens at the top of a barometer. These bubbles will form when the absolute pressure of the liquid falls below its vapor pressure, which is about +0.03 bar for water at room temperature. These bubbles would form an embolism in a siphon, even if it could be primed to begin with. Siphons cannot work in a vacuum.--Yannick (talk) 14:36, 24 May 2008 (UTC)
What you're forgetting, but what the source points out, is that metastability is very common with water, for example, I routinely superheat water in a microwave oven by degassing it by boiling it, letting it cool and repeating the process. But stick in some granules (with a wooden spoon), and it instantly boils. And that's in a not especially clean mug. The source says that a water filled piston with sufficiently clean walls, degassed and with no nucleation sites is in a metastable condition, and will not break down to -9 bar. That is easily low enough for it to siphon in a vacuum.- (User) WolfKeeper (Talk) 21:55, 29 May 2008 (UTC)
The perfectly clean piston is imaginary. The microwaved water has nothing to do with negative pressure. And neither cases are siphons. I do not wish to argue the plausibility of negative pressure with you - I've already conceded that they may exist in special situations due to surface tension/capillary action. The issue is that siphons do not work in a vacuum. Can you come up with a common example of a vacuum siphon?--Yannick (talk) 02:44, 30 May 2008 (UTC)
I don't have to. Given the references, you have to come up with references to an opposing POV and add them to the article.- (User) WolfKeeper (Talk) 13:14, 7 June 2008 (UTC)
The mug example is really simple and easily demonstrated example where the saturated vapour pressure of water is higher than ambient- but it does not boil until you dump in nucleation centres. Experiments have demonstrated this down to -9 bar, in real world experiments. I'm sorry if you don't understand or believe this, that is not the wikipedia's problem.- (User) WolfKeeper (Talk) 13:19, 7 June 2008 (UTC)

I have deleted the following "Vacuum siphons" section:

However, the above assumes that a liquid cannot take a negative pressure which normal liquids with dissolved gases are unable to do. In practice, liquids such as water, mercury exhibit a property known as tensile strength and are able, under certain conditions to take negative pressures. One example is in tall trees, where the water is pulled up from the roots further than 9.81 meters. Strangely to many, experiments have indeed shown that siphons can operate in a vacuum, provided that the liquids and surfaces are very clean and pure, and are degassed.(Historical and pedagogical aspects of a humble instrument S Ganci et al 2008 Eur. J. Phys. 29 421-430)(Nokes M C 1948 Vacuum siphons Am. J. Phys. 16 254)

Ganci et al 2008 does not support the claim, and says about Nokes: "Nokes seems to report an accurate experimental work demonstrating that not siphoning in vacuum is due to a breach in continuity caused by expansion of an air bubble, but this reading key is open to readers' criticism." The Nokes citation is actually just a short digest[9] of Nokes, M C, 1948, Vacuum Siphons School Science Review 29 p. 233, which is a journal for school teachers.[10]--Yannick (talk) 12:50, 7 June 2008 (UTC)

Uh huh. I've reverted your edit. The contention is supported by the references if you bother to read them carefully.- (User) WolfKeeper (Talk) 13:07, 7 June 2008 (UTC)
FWIW both cited journals are refereed journals for university professors as well as other professional physics teachers. How on Earth is that supposed to not be notable?- (User) WolfKeeper (Talk) 13:53, 7 June 2008 (UTC)
I have read Ganci's paper carefully. Nokes's 1948 paper is a little hard to get a hold of, although I'm willing to read it if you can provide a copy. Ganci provides 4 references saying that siphons do not work in a vacuum. He then reconciles Pohl's cohesion theory by showing how it would also stop in a vacuum.--Yannick (talk) 13:57, 7 June 2008 (UTC)
I have Ganci's paper, and that does not appear to me to be what is stated. Please quote the piece that you think says that, and I will you show that that is not an accurate precis.- (User) WolfKeeper (Talk) 15:20, 7 June 2008 (UTC)

Let's try to look for common ground. Do you at least agree that a siphon would only work in a vacuum if special precautions are taken to remove and prevent nucleation sites?--Yannick (talk) 16:39, 7 June 2008 (UTC)

See Phase_diagram#Pressure-temperature_diagrams and Vapour_pressure#Water_vapor_pressure: in vacuum, i.e. at zero pressure, the water will go into gas state. So siphons will not work in vacuum, since vacuum is zero pressure by definition. Normally, siphons operate at atmospheric pressure. Normal tap water cannot siphon anymore below about 0.3 times the atmospheric pressure, due to the dissolved other gases. So in practice you cannot pump water higher than about 7 metre by suction pumping or by using a siphon[11]. Crowsnest (talk) 11:50, 8 June 2008 (UTC)
Unfortunately, due to the existence of superheated water that does not boil, that phase diagram is not sufficient to deal with this.- (User) WolfKeeper (Talk) 14:59, 8 June 2008 (UTC)
  • Hi folks. Interesting debate. I have learnt quite a bit. I would summarise it like this: Under steadily decreasing pressure, most liquids boil long before a pressure of zero is reached. Negative pressure in liquids is not something normally seen in practical situations. Therefore, with a siphon in conditions of steadily decreasing pressure the liquid eventually boils and the siphon ceases to operate before zero pressure is achieved. We say a siphon won't work in a vacuum. However, in certain circumstances, a liquid can remain in the liquid state when the pressure reaches zero, and even beyond when the pressure becomes negative (and there are sources that can be cited to support such a statement). Therefore, theoretically if these certain circumstances could be replicated in a siphon, the siphon would work in a vacuum. We say a siphon will work in a vacuum (but don't try it at home using pipes you bought from the hardware store. It won't work). If there is a source that can be cited to support a statement that a siphon has been shown to work in a vacuum, then the statement can be made in Wikipedia and the source must be cited. However, if a real siphon has never actually been demonstrated to work in very low pressure approaching vacuum, the statement would be more misleading than revealing. If the sources simply say that liquids can remain liquid under negative pressure that is not the same as saying the humble siphon will work in a vacuum, and the statement should not be made in Wikipedia. Dolphin51 (talk) 13:31, 8 June 2008 (UTC)
The problem is that a liquid at slightly above its boiling temperature is in a state called 'metastability'- it requires energy to push it over into actually boiling, or the presence of 'nucleation centers'. It takes energy to form bubbles. But like the superheated water in a mug that has been microwaved two or more times, these are not always present.- (User) WolfKeeper (Talk) 14:53, 8 June 2008 (UTC)
And a liquid, even at zero applied pressure ("a vacuum")actually sticks together, you know, like surface tension, so provided the liquid is cool enough, the partial pressure of the gas isn't enough, on its own, to actually make it boil. It will evaporate, but not boil. Under these conditions, in the absence of bubbles, it will syphon.- (User) WolfKeeper (Talk) 14:53, 8 June 2008 (UTC)
Yes, that seems very plausible. Also, searching for "negative pressure" and "water" on Google Scholar gives tens of thousands of hits. But as stated by you and by Dolphin51, the kitchen sink siphon with tap water will not work in vacuum. On the other hand, there seem to be many instances where real negative pressures (may) occur (and siphons will work). A point of care is the definition of "zero pressure" which is used, in order to define what negative pressure is. In many engineering applications it is custom to take "standard" atmospheric pressure as zero, while here it is vacuum. -- Crowsnest (talk) 15:47, 8 June 2008 (UTC)

[edit] More on siphons in vacua

I accept Wolfkeeper's point that liquids don't always boil as the pressure is progressively lowered. Therefore I have no objection to this fact being posted in appropriate articles on Wikipedia. For example, there are probably articles on liquids, and boiling point and nucleation sites and so on. The question in my mind is "has a siphon ever been actually demonstrated to work at such a low pressure that it is reasonable to describe it as a vacuum?"

If the answer is "yes" and there exists a citation to comprehensively support that statement then it should go ahead in Wikipedia.
If the answer is "well no, not actually been demonstrated as such, but theoretically it is definitely possible" then the statement should not go ahead in Wikipedia. In the latter case I can understand Wolfkeeper and many others objecting to a statement in Siphon that liquids cannot exist in a vacuum. In the latter case there needs to be a debate on this Talk page about how to describe the principle of the siphon, and the lower limits to its effectiveness, without resorting to a statement that liquids cannot exist in a vacuum. It may be sufficient to say liquids cannot normally remain liquid in a vacuum except in special circumstances such as trees. Dolphin51 (talk) 00:24, 9 June 2008 (UTC)
The only source proposed to support the statement is Nokes, M C, 1948, Vacuum Siphons School Science Review 29 p. 233, but Ganci et al noted that this source was open to reader's criticism. I have not read it myself. I would argue that even if Nokes's vacuum siphon has demonstrated the point, it is still a unique case which should not be given undue weight.--Yannick (talk) 02:55, 10 June 2008 (UTC)
I certainly agree that it should not be given undue weight, OTOH it does seem to be notable in this context, one way or another.- (User) WolfKeeper (Talk) 03:04, 10 June 2008 (UTC)
There's also the point you I think made earlier, in the case of capillaries, even at very low/no external pressure, the surface tension provides considerable pressure due to the small radius of curvature. In that case, siphons very clearly will operate.- (User) WolfKeeper (Talk) 03:04, 10 June 2008 (UTC)
In the cas of capillaries, I can see how the surface tension could pull a liquid up and over the crest of a siphon. But wouldn't that surface tension then also prevent the liquid from being released at the outlet?--Yannick (talk) 03:23, 10 June 2008 (UTC)
It's not that the capillary forces the liquid over the crest (although depending on how the liquid wets the surface that may happen), it's more that the pressure in the liquid is way above vacuum due to the compression effect of the surface tension at each end.- (User) WolfKeeper (Talk) 12:27, 10 June 2008 (UTC)
Not if it outlets below surface in a container with the same liquid. Or if the outlet is gradually widened to let gravity or gravity-induced instabilities "win" from surface tension. Crowsnest (talk) 11:38, 10 June 2008 (UTC)
If the head at the exit is greater than the pressure due to the surface tension then it should be fine. You'd expect viscosity to be strong, but that just slows it, you should get a drop that appears, grows and then falls. I guess in a vacuum it may tend to freeze or boil at the exit and that would complicate the behaviour at the exit, but the siphoning itself should be working.- (User) WolfKeeper (Talk) 11:59, 10 June 2008 (UTC)