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Cohesion is necessary to lower siphon pressure

The statement

"Furthermore, since common siphons operate at positive pressures throughout the siphon, there is no contribution from liquid tensile strength, because the molecules are actually repelling each other in order to resist the pressure, rather than pulling on each other."

is incorrect in light of the earlier, correct, statement that

"A practical siphon, operating at typical atmospheric pressures and tube heights, works because gravity pulling down on the taller column of liquid leaves reduced pressure at the top of the siphon..."

Unless the liquid in the lower part of the taller column coheres, it cannot act as the piston pulling down to lower the pressure at the top. Once that pressure is lowered below atmospheric, then the flying droplet siphon is possible. By the way, the fact that those droplets form is evidence for the surface tension associated with cohesion. 72.70.165.31 (talk) 01:06, 10 September 2016 (UTC) jwdooley@aol.com

Then how is it that you think a carbon dioxide gas siphon can work? I think the increased density of the CO2 gas, along with gravity, inhibits the lighter air from penetrating down into the reservoirs and up into the siphon, without cohesion being needed there at all. Likewise the increased density of liquids, inhibits the entry of air into liquid siphons the same way, without needing cohesion. Although some dissolve, air molecules largely can't get between the smaller liquid molecules the way they can between sand grains that have the rigidity to hold each other apart.
Of course surface tension has a role in the flying droplet siphon, but even without cohesion, it would work fine. It would just be a squirting stream siphon instead of flying droplets.
The liquid in a siphon operating at atmospheric pressure doesn't act as a piston pulling down the pressure at top, there is no pull from the "piston" on the molecules above. The piston pushes up on the molecules above! The piston is pushed down in turn by the molecules above it. The piston is also pulled down by gravity, and the piston pushes down on the atmospheric pressure that pushes up on the piston from below. The pressure in the top lowers itself, as the repulsion between the molecules at the top is free to spread the molecules out a little when they are relieved of some of the squeeze of atmospheric pressure coming up from below.
That question, "[If cohesion is required] then how is it that you think a carbon dioxide gas siphon can work?" is not just a rhetorical question. It would be good to answer it to help us reach an understanding. Mindbuilder (talk) 04:50, 10 September 2016 (UTC)

The statement "The pressure on the top lowers itself..." is not physics but is anthropomorphizing matter; assigning motive to material which decides to lowers it's own pressure. The force of gravity pulls the water column "piston" down, and that falling piston lowers the pressure. In the absence of tensile strength, fluid must ( as usual) flow from higher pressure to lower pressure, so for the CO2 "siphon," the column of CO2 in the upper reservoir must provide the extra pressure. 108.55.68.5 (talk) 23:30, 15 September 2016 (UTC) jwdooley@aol.com

By saying the pressure lowers itself, I'm just saying that as the pressure coming up from down below is partially relieved, the thermal motion of the fluid molecules at the top, pushes the molecules slightly apart (very very slightly if it is a nearly incompressible liquid like water) and its pressure drops naturally as it expands. There is no need for anything to pull on the fluid at the top to get its pressure lower. Gravity does pull down on the water column piston, but atmospheric pressure pushes up on it, typically much harder than gravity pulls down unless the siphon approaches the barometric height. So although gravity cancels some of the upward push of atmospheric pressure, atmospheric pressure, pushing through the piston, still pushes up hard on the fluid in the top of the siphon.
In a CO2 siphon, there does not need to be a taller height of CO2 in the upper reservoir to push the CO2 up the up column of the siphon because gravity acting on the CO2 in the down column lowers the pressure at the top of the siphon enough for just the atmospheric pressure of the air above the upper reservoir to push the CO2 up the up side. Remember, the pressure at the top of a siphon is lower than the ambient atmospheric pressure. That enables the lighter air to push the heavier CO2 up in the same way that lighter air can push up heavier water in the flying droplet siphon.
There is however a difference between a CO2 and liquid siphon due to the CO2 siphon's lack of cohesion. In a liquid siphon you can fill the siphon and submerge the ends in equal height reservoirs and the liquid will stay up in the siphon for an indefinite time. But if you do that with a CO2 siphon, molecules of air will probably occasionally diffuse through the CO2 reservoirs and accumulate at the top of the siphon until eventually the CO2 will be displaced down to the level of the reservoirs. But if the siphon is flowing, then any air that reaches the top will be flushed out by the flow of CO2 just like air bubbles are flushed out of a liquid siphon. This is an example of how a siphon without cohesion can work even if it is very leaky.
But in a normal siphon, I'm not completely clear what you think the "piston" is doing. Do you think the top of the piston in the taller down column is pushing up on the fluid in the top of the siphon or do you think it is pulling down? If you think it is pulling down, then do you think the fluid pressure at the top of the piston is positive relative to pure vacuum or negative relative to pure vacuum? Mindbuilder (talk) 02:59, 16 September 2016 (UTC) and again at 06:14, 16 September 2016 (UTC)

If this were presented as a speech it would be a “Gish Gallop,” full of physics words but very little physics.

To keep things simple, I am assuming that the lower catch basin of the siphon is empty, and remains empty.

BY SAYING THE PRESSURE LOWERS ITSELF, I'M JUST SAYING THAT AS THE PRESSURE COMING UP FROM DOWN BELOW Pressure does not “come up.” It does not move; this is not a weather pattern with high and low pressure regions moving around. The pressure outside the siphon is static at atmospheric pressure. Pressure in different places is different. The pressure at each point within the working siphon can be calculated with Bernoulli’s equation, and does not change with time a given location.

IS PARTIALLY RELIEVED, “Relieved” is a legitimate engineering term which is clearly misused here. There is no relief valve in this system. The pressure in the chamber is lower because the falling water (a) seals the chamber from below and (b) pulls down like a piston – creating a “suction” in the same way that a syringe can. 

THE THERMAL MOTION OF THE FLUID MOLECULES AT THE TOP, PUSHES THE MOLECULES SLIGHTLY APART (VERY VERY SLIGHTLY IF IT IS A NEARLY INCOMPRESSIBLE LIQUID LIKE WATER) AND ITS PRESSURE DROPS NATURALLY AS IT EXPANDS.

This slight expansion is the stretching caused by the falling water pulling on the water above it.

THERE IS NO NEED FOR ANYTHING TO PULL ON THE FLUID AT THE TOP TO GET ITS PRESSURE LOWER. In fact, there is no other way to get the pressure lower.

GRAVITY DOES PULL DOWN ON THE WATER COLUMN PISTON, BUT ATMOSPHERIC PRESSURE PUSHES UP ON IT, TYPICALLY MUCH HARDER THAN GRAVITY PULLS DOWN UNLESS THE SIPHON APPROACHES THE BAROMETRIC HEIGHT. “Much harder” is irrelevant, since only pressure *differences* exert force on an object. In this case the pressure difference is between the room atmospheric pressure and the pressure at the top of the siphon. In a working siphon, the velocity of the falling water is constant, i.e. the acceleration is zero. This means that the sum of the gravity force plus the pressure *difference* force plus the force by the walls adds to zero.

SO ALTHOUGH GRAVITY CANCELS SOME OF THE UPWARD PUSH OF ATMOSPHERIC PRESSURE, ATMOSPHERIC PRESSURE, PUSHING THROUGH THE PISTON, STILL PUSHES UP HARD ON THE FLUID IN THE TOP OF THE SIPHON. Pressure does not “push through.” It is exerted at a particular place. In this case atmospheric pressure is applied to the bottom of the falling column and to the surface of the upper basin. The atmosphere does not “push” at all on the interior of the siphon. To repeat the previous comment: The net force on the column of falling water is zero.

IN A CO2 SIPHON, THERE DOES NOT NEED TO BE A TALLER HEIGHT OF CO2 IN THE UPPER RESERVOIR TO PUSH THE CO2 UP THE UP COLUMN OF THE SIPHON BECAUSE GRAVITY ACTING ON THE CO2 IN THE DOWN COLUMN LOWERS THE PRESSURE AT THE TOP OF THE SIPHON ENOUGH FOR JUST THE ATMOSPHERIC PRESSURE OF THE AIR ABOVE THE UPPER RESERVOIR TO PUSH THE CO2 UP THE UP SIDE. No physics here, only assertions. The physics is this: Fluid will not be forced to flow unless there is a pressure difference. If you think a utube video shows forced flow, it is your problem to explain how the necessary pressure *difference* is created.

REMEMBER, THE PRESSURE AT THE TOP OF A SIPHON IS LOWER THAN THE AMBIENT ATMOSPHERIC PRESSURE. Of course it is, if it is really a siphon. The question is what physics caused the lower pressure.

THAT What physics is “that?”

ENABLES THE LIGHTER AIR TO PUSH THE HEAVIER CO2 UP IN THE SAME WAY THAT LIGHTER AIR CAN PUSH UP HEAVIER WATER IN THE FLYING DROPLET SIPHON.

THERE IS HOWEVER A DIFFERENCE BETWEEN A CO2 AND LIQUID SIPHON DUE TO THE CO2 SIPHON'S LACK OF COHESION. Right, and you must explain the cause of the alleged pressure difference when there is no cohesion.

IN A LIQUID SIPHON YOU CAN FILL THE SIPHON AND SUBMERGE THE ENDS IN EQUAL HEIGHT RESERVOIRS AND THE LIQUID WILL STAY UP IN THE SIPHON FOR AN INDEFINITE TIME. BUT IF YOU DO THAT WITH A CO2 SIPHON, MOLECULES OF AIR WILL PROBABLY OCCASIONALLY DIFFUSE THROUGH THE CO2 RESERVOIRS AND ACCUMULATE AT THE TOP OF THE SIPHON UNTIL EVENTUALLY THE CO2 WILL BE DISPLACED DOWN TO THE LEVEL OF THE RESERVOIRS. No physics here, just supposition.

BUT IF THE SIPHON IS FLOWING, THEN ANY AIR THAT REACHES THE TOP WILL BE FLUSHED OUT BY THE FLOW OF CO2 JUST LIKE AIR BUBBLES ARE FLUSHED OUT OF A LIQUID SIPHON. THIS IS AN EXAMPLE OF HOW A SIPHON WITHOUT COHESION CAN WORK EVEN IF IT IS VERY LEAKY. No physics here, just supposition. BUT IN A NORMAL SIPHON, I'M NOT COMPLETELY CLEAR WHAT YOU THINK THE "PISTON" IS DOING. I’ve included a sketch of how the siphon works.

DO YOU THINK THE TOP OF THE PISTON IN THE TALLER DOWN COLUMN IS PUSHING UP ON THE FLUID IN THE TOP OF THE SIPHON OR DO YOU THINK IT IS PULLING DOWN? It is pulling down of course. If it were pushing up, the siphon would run backwards.

IF YOU THINK IT IS PULLING DOWN, THEN DO YOU THINK THE FLUID PRESSURE AT THE TOP OF THE PISTON IS POSITIVE RELATIVE TO PURE VACUUM OR NEGATIVE RELATIVE TO PURE VACUUM? In a working siphon, the pressure at the top is less than atmospheric, and greater than the vapor pressure of the fluid.

MINDBUILDER (TALK) 02:59, 16 SEPTEMBER 2016 (UTC) AND AGAIN AT 06:14, 16 SEPTEMBER 2016 (UTC)

108.55.68.5 (talk) 13:07, 17 September 2016 (UTC) jwdooley@aol.com Here is astep by step example of how a siphon works. Refer to wiki/siphon, Figure 2, the flying droplet siphon. Begin with the principle that fluid will flow from a high pressure region to a low pressure region. It is this pressure difference that forces the flow. I will call the column of water on the right the “falling column” and the column of water in the tube that makes the nozzle, on the left, the “rising column.” For simplicity, suppose that the basin on the right is also empty, with a hole in the bottom so it never fills. 1) Suppose for a moment that the column on the right has no water in it. Then air from the room can freely flow in the column on the right. In that case, the air pressure in the chamber at the top is the same as the air pressure in the room, and no fluid will flow in the “rising column.” 2) Now, fill the falling column with loose dry sand. Let the sand fall. The volume of the upper chamber grows down into the “falling column.” Because the sand is loose, air from the room flows up past the grains of sand so that the air pressure in the chamber remains at atmospheric pressure. With no pressure difference, no water flows in the rising column. 3) Now fill the falling column with water, and arrange for the pressure in the chamber to be atmospheric pressure. Let the water fall from the falling column. Because the water holds together (just like water drops do) and because it presses against the walls of the column, no air from the room can enter the chamber. The falling of the water increases the volume of the chamber, and because it is sealed off, its pressure is lowered below atmospheric pressure. 4) Now the atmospheric pressure on the upper basin and the rising column provides a pressure difference with the room and the chamber. This pressure difference causes the fluid to rise in the rising column. (There is a similar pressure difference in the falling column, but in a working siphon, the weight of the falling water is greater than the weight of the rising water, so the water in the falling column continues to fall.) 5) If the pressure difference is great enough (the falling water has fallen far enough) then water rises high enough to squirt out of the nozzle and we have a working siphon. The squirting water replaces the water that leaves the bottom of the falling column, keeping the pressure difference constant. 108.55.68.5 (talk) 13:13, 17 September 2016 (UTC)jwdooley@aol.com


Comment on jwdooley's point 4 above re weight, i.e. (There is a similar pressure difference in the falling column, but in a working siphon, the weight of the falling water is greater than the weight of the rising water, so the water in the falling column continues to fall.)

A working siphon can have a larger uptube than the downtube, such that the eight of the water in the uptube exceeds the weight of the water in the downtube, so the weight of the falling water is NOT always greater than the rising water. — Preceding unsigned comment added by 124.177.113.13 (talk) 03:19, 20 December 2016 (UTC)

It seems pretty clear that the core of our difference is the idea that the piston needs to actually pull on the fluid at the top to lower its pressure. First, consider a two liter soda bottle filled with only air at one atmosphere pressure and tightly capped. Take that bottle up into space and slit the side, and the air will push its way out and its pressure will drop. There is no need to pull on the air to get its pressure to drop. The pressure in the air will drop all by itself when the air is no longer being squeezed by a bottle or ambient atmospheric pressure. The same thing applies if the bottle is filled with oil at one atmosphere pressure and the bottle is opened in space, the pressure of the oil will drop when neither the container or ambient pressure is squeezing on it, without needing to pull on the oil. And of course if you start by filling the two liter bottle with air at 1/2 atmosphere pressure, and slitting it in space, the result will be the same, no pulling required. It is also important to remember that vacuum can't pull the gas out. Vacuum is nothing, it can't pull or push or exert any force at all on the gas. You probably know these things, but I think I have to state it to get you in the right frame of mind for what follows.
Now consider a syringe with the needle capped tightly. Say the cylinder has air in it at one atmosphere pressure and the plunger is at about the middle of the cylinder. If you pull the plunger almost all the way out, the pressure inside will drop to roughly a half atmosphere. But note that you will not be actually pulling on the air in the syringe at all, you will be pulling against the ambient atmospheric pressure air that pushes on the back of the plunger. And far from pulling on the half atmosphere pressure air left in the syringe, that air inside will actually be pushing out on the plunger, helping you fight the one atmosphere pressure air outside, much like it pushes its way out in the two liter bottle example above, and much like the gases push their way out of a pellet gun, pushing the pellet(the piston) along in front. Do you disagree with this paragraph? Since this paragraph is particularly fundamental to this issue, it is especially important to let me know if you disagree with it.
Now consider a flying droplet siphon, but in a static condition where the tubes on each side are submerged in reservoirs with equal surface heights. Assume we admitted or removed enough air to the top chamber so that the tubes on both sides are filled with liquid almost up to the bottom of the chamber. Now imagine that at the top of the liquid on the right side (what will become the exit side) there are two metal plates in the tube with practically frictionless seals to the walls, and that the plates are separated by a small gap, where there is only pure vacuum and some springs. Assume the weight and volume of the plates, springs, and vacuum add up to a density equal to the liquid. In this static situation we can easily calculate the hydrostatic pressure just above the top plate and just below the bottom plate. Although those pressures will both be negative relative to atmospheric pressure, they will both be positive relative to pure vacuum, right? So for example if the liquids in the siphon are about one meter above the surfaces of the reservoirs, then the air pressure in the chamber would be about 0.9 atmospheres higher than pure vacuum. So consider the effect of this positive 0.9 atm gas pressure in the chamber compared to the zero pressure of the pure vacuum between the plates. Since the top plate will have positive pressure above and zero pressure below it, it will be experiencing a downward force from the air above it in the chamber. Similarly, the liquid below the bottom plate will have a positive pressure relative to vacuum, say roughly 0.91 atm, underneath it, and vacuum above it. So the pressure of the liquid in the "piston" will be pushing up on the bottom plate. With air pressure pushing down on the top plate and the liquid pressure in the piston pushing up on the bottom plate, the spring between the plates will be compressed, instead of stretched, and there will be no pulling by the top of the piston through the plates, right? Even if we let the exit reservoir on the right spring a leak, and its level falls, and the siphon starts flowing very slowly, the liquid in the "piston" will still be positive relative to vacuum, and pushing up on the bottom plate as the bottom plate moves down, and the air in the chamber starts to expand. And the air in the chamber won't get pulled down, the air in the chamber will continue to push down on the top plate, and far from pulling the air in the chamber down, the top plate will continue to be pushing up on the air in the chamber, somewhat resisting its expansion. Mindbuilder (talk) 05:14, 18 September 2016 (UTC) and again at 23:32, 18 September 2016 (UTC) and again at 20:40, 21 September 2016 (UTC)
I probably made my point already, but I just thought of such a perfect example of a gas expanding itself and lowering its pressure without being pulled on, that I had to add it. The example is the water rocket. You might point out that a water rocket made with a two liter bottle may have a few atmospheres of pressure it it, but that is just a difference in degree to the siphon, not a difference in the principle of a pressurized gas expanding and lowering its own pressure without being pulled on. The water rocket would still work the same, though not nearly as good, if the air inside the water rocket was only pressurized to 0.9atm above pure vacuum and the ambient pressure outside was lower than the pressure inside the bottle. Mindbuilder (talk) 06:30, 24 September 2016 (UTC)

I would ask that the editors consider adding a tutorial along the lines of the pdf here https://sites.google.com/site/jwdooleysite/siphon 2601:985:301:E486:3C23:DE63:BFBC:9564 (talk) 11:36, 7 May 2017 (UTC)jwdooley@aol.com

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Self-starting 'M' siphon

I did a quick search in the article for this type of self-starting siphon, but I couldn't find anything, maybe I was using the wrong search words? If this is useful, great, otherwise please delete it.

https://www.youtube.com/watch?v=4SEv_GxAo70 • Sbmeirow • Talk • 12:06, 27 August 2018 (UTC)

That's not a self-starting siphon though. It's sometimes called a "plunge" siphon or a "kicker" siphon. You start it by hand, by physically moving the pipe up and down (some have a valve to help). The M-shaped kicker siphon works because it fills the M, then the weight of the outlet pipe tips the siphon over, raising the first hill enough to make the siphon start.
There are some self-starting siphons, such as the "Tantalus". These work by being a fixed siphon and the upper fluid level rising until the start. The simplest is just a small air leak at the top of the siphon to avoid an air lock. Note though that these don't work unless the level rises above the top of the siphon, which isn't always possible (the siphon needs to be inside the reservoir, not going over the lip of it). Andy Dingley (talk) 12:53, 27 August 2018 (UTC)

Air chamber in Figure 2 is below atmospheric pressure

An instructive question is this: What *causes* the air pressure in the chamber to be significantly below atmospheric pressure?

2601:985:100:994:D949:FBF9:66EF:2CB (talk) 13:47, 1 March 2019 (UTC)john dooley,jwdooley@aol.com2601:985:100:994:D949:FBF9:66EF:2CB (talk) 13:47, 1 March 2019 (UTC)

Gravity pulling down on the liquids in the columns cancels some of the atmospheric pressure that is pushing the liquid up and confining the air in the chamber, allowing what would otherwise be one atmosphere pressurized air in the chamber, to expand a little, resulting in its pressure drop. Mindbuilder (talk) 00:52, 2 March 2019 (UTC)

video siphon demonstration

Thanks to the creators for continually improving this article. I ask that you consider including a reference to this youtube demonstration on forces in the siphon: https://www.youtube.com/watch?v=zUANlmxw_OM 174.54.215.191 (talk) 14:49, 24 July 2020 (UTC)John Dooley, jwdooley@aol.com

At 8min10sec in your video you state "The two water pistons are under tension..." Actually the "pistons" are in compression, being pushed down on from above by the air pressure in the bubble, which though reduced, is still higher than pure vacuum pressure. And the pistons are also being pushed up, ultimately by atmospheric pressure from below. If there is a tension in the piston, then that would imply something near the top of the piston exerts an upward force on the top of the piston. A pressurized gas, even a reduced pressure one, pushes outward on the surfaces that surround it, but it doesn't pull. A lot of people seem to have a hard time giving up the mental model of suction as a pull force from a reduced pressure fluid, when actually suction is not a pull, but rather a push from the opposite direction. Mindbuilder (talk) 02:44, 27 July 2020 (UTC)

Am I correct that Mindbuilder asserts that compression causes the pressure to be at a minimum at the top of the siphon? 2601:985:104:4470:E5D4:FC26:2B56:1300 (talk) 11:22, 23 November 2020 (UTC)john dooley, jwdooley@aol.com

No that's not what I'm saying. Atmospheric pressure pushes the liquid up into the top of the siphon, compressing any air up there if there is any to be compressed. Gravity pulling down on the liquid counteracts some of the atmospheric pressure from pushing up to the top as hard, thus allowing the pressure in the air bubble at the top to resist compression to not entirely as much as full atmospheric pressure. Mindbuilder (talk) 14:31, 29 November 2020 (UTC)

This argument is the same, whether or not the tube exists. I will conclude with two versions of the same statement: 1) Without tension there is no siphon. 2) You can't siphon sand. 2601:985:104:4470:29AC:69ED:45FF:A14C (talk) jwdooley@aol.com — Preceding undated comment added 11:09, 3 January 2021 (UTC)

Bell Siphon

Why is the bell siphon not mentioned in this page. I don't feel confident to edit the page. Below some starters. https://gogreenaquaponics.com/blogs/news/what-is-bell-siphon-and-why-we-use-it-in-aquaponics https://www.youtube.com/watch?v=s_brue6URFg

I can see a couple of reasons why the bell siphon doesn't qualify for mention in this article. Firstly, the information at Go Green Aquaponics is conspicuously lacking a diagram; it tries to explain what a bell siphon is and how it works, using nothing but words. There is no diagram so if the aim is to explain, it fails. Secondly, the Go Green Aquaponics information is unashamedly an advertisement. Wikipedia requires that any information likely to be challenged should be able to be verified using a reliable published source. See WP:VERIFY. The Go Green Aquaponics information is not a reliable published source. Dolphin (t) 13:12, 19 September 2021 (UTC)
I have now watched the You Tube video you identified. It shows how one practitioner builds what he calls a "bell siphon" but it doesn't explain how it works or what principles it displays. Consequently I would have to say the You Tube video doesn't qualify as a reliable published source to support the concept of a bell siphon. Wikipedia is not a How-To Guide or a Manual. Please have a look at WP:NOT. Dolphin (t) 13:31, 19 September 2021 (UTC)

A Masterpiece

This article is a masterpiece of clarity. I'm old as the hills and never have I read such a well constructed explanation. I think a cigar is in order.Longinus876 (talk) 08:11, 22 April 2022 (UTC)

Evidence for the "chain model."

If the beaker on the right in the first figure is raised until the water levels are the same in each beaker, the flow stops, but the tube remains filled with water. Without the tensile strength of water (the "chaininess") the water would simply fall out of the tube into one of the beakers.....jwdooley@aol.com 2601:985:100:80D0:F99C:6BCC:9BF7:39C8 (talk) 07:13, 9 January 2023 (UTC)

You are assuming that at the surface of the water in each beaker the pressure is zero, and at depth in each beaker the pressure is compressive, and in the tube the pressure is tensile. Your assumption is incorrect; at the surface of any body of water the water pressure is equal to atmospheric pressure. Inside the tube the pressure is less than atmospheric pressure but it is still compressive. The pressure of the atmosphere is considerable and will support a column of water about 33.8 feet (10.3 m) high at which height the water pressure approaches zero. If the length of the column is greater than this critical height the top of the siphon will fill with vapour, not air. The pressure of the water and the vapour never reaches zero, and never becomes tensile. See Atmospheric pressure#Measurement based on the depth of water. Dolphin (t) 21:18, 9 January 2023 (UTC)
I am not assuming anything about the pressure at the surface of the water in each beaker. Your assertion to that effect is incorrect. If the tube has only water in it, the water refuses to fall no matter what the atmospheric pressure is. Your assertion that water "never becomes tensile" is incorrect. Google "tensile strength of water." If the water at the top vaporizes, the rest of the water in the two columns will fall, because the vapor has no tensile strength. 2601:985:100:80D0:85FB:4B8C:89FB:37FD (talk)jwdooley@aol.com 2601:985:100:80D0:85FB:4B8C:89FB:37FD (talk) 19:28, 11 January 2023 (UTC)
As you suggested, I have Googled for "tensile strength of water" and read extensively on the topic. The topic relates to highly specialised fields including very small diameter tubes such as in the xylem of trees; and in certain explosive phenomena. What I read certainly did not support the notion that fluids in a pipe can exist in a state of tension. It would be naive to imagine that the material on this topic supports your suggestion that the water in a siphon is in a state of tension.
Mercury barometers have been in use since the 17th century for the purpose of measuring atmospheric pressure, and particularly for observing the daily variations in atmospheric pressure. The atmospheric pressure is expressed as the height of a column of mercury - typically a height of about 760 mm of mercury. The head space in the glass tube, above the surface of the mercury, is a void filled only by mercury vapor. Atmospheric pressure can also be measured as the height of a column of water but that column will be about 34 feet (10 metres) high so it isn't practical, and in cold climates the water is vulnerable to freezing.
Please have a look at Barometer#History and particularly the account from 1630 about Giovanni Baliani who tried to construct a water siphon greater than 34 feet (10 metres) high, and discovered it wouldn't operate, so he wrote about it to Galileo Galilei. Dolphin (t) 05:55, 12 January 2023 (UTC)
Jwdooley wrote "If the water at the top vaporizes, the rest of the water in the two columns will fall, because the vapor has no tensile strength." That is not typically factually correct. If the siphon is less than the barometric height, as almost all practical siphons are, then atmospheric pressure will push the water up into the water vapor, compress it, and recondense it, "healing" the vapor bubble. Likewise, the easier to demonstrate case of an air bubble just big enough to detach the water on each side of the siphon, and defeat any tensile strength, will also not normally result in the two columns falling. Although the quoted statement IS factually correct in the unusual situation of a water siphon taller than the barometric height of about 10m. In that case, basically the water below 10m will be in compression and the water above 10m will actually be in tension.
@Dolphin - You might be interested to know that water siphons have been demonstrated to heights exceeding the 10m barometric height up to more than 15m. See the very interesting reference 5 (Boatwright, Hughes, Barry) in the main article. Also see the very clever Z-tube. Mindbuilder (talk) 05:18, 15 January 2023 (UTC)
@Mindbuilder. Thanks for your pointer to the article by Boatwright et al. I accept that in a laboratory environment with very careful preparations it is possible to observe a water column higher than the prevailing barometric height. However, I think you and I are in agreement that this information does not validate the “chain model” of the siphon. A chain is mostly (always?) in tension; whereas fluids are mostly (always?) in compression. Dolphin (t) 03:43, 16 January 2023 (UTC)
We agree that water has tensile strength but disagree in its relevance to the siphon.
We agree that water vapor has no tensile strength.
Based on your discussion, we agree that the pressure at the top of the siphon is significantly less than atmospheric pressure.
Now, since the pressure at the top is lower, so also the density of the water at the top is less than the density of water at atmospheric pressure.
The reason for the lower density is that the water on top is stretched. It experiences a tensile strain.
The reason for the tensile strain is that a tensile stress has been added to the compressive pressure from the atmosphere. The tensile stress is caused by the force of gravity pulling down on the water in each tube.
At another time, we will discuss pistons 2601:985:100:80D0:907C:7A44:C064:A089 (talk) 15:39, 16 January 2023 (UTC)
@Dolphin - I can't think of a single practical real world situation where the fluid in a siphon is in tension. However, in those rare circumstances where the siphon is operating in vacuum or above the barometric height, and thus IS in tension, I do think the chain model is sort of valid. It is not a perfect analogy though because you can have a siphon with a fat up tube who's contents weigh much more than in the down tube, and the lighter side can still pull up the liquid in the heavier side. Mindbuilder (talk) 01:34, 18 January 2023 (UTC)
I agree. There are no practical real-world situations where the fluid in a siphon or barometer is in tension. The phenomenon is counter-intuitive and at the frontier of material science. An imperfect analogy or a non-fluid mechanics explanation has no role out there at the frontier. Anyone striving to comprehend the science of fluids in tension is not going to be helped by the so-called chain model. It doesn’t warrant mention in any encyclopaedia. Dolphin (t) 12:04, 18 January 2023 (UTC)
Jwdooly(I think) wrote:
"We agree that water has tensile strength but disagree in its relevance to the siphon."
Yes I agree.
"We agree that water vapor has no tensile strength."
Yes I agree.
"Based on your discussion, we agree that the pressure at the top of the siphon is significantly less than atmospheric pressure."
Yes I agree
"Now, since the pressure at the top is lower, so also the density of the water at the top is less than the density of water at atmospheric pressure."
Yes I agree, but the density reduction of the water is extremely small for such small pressure changes (on the order of 1 part in 10000 volume change over one atmosphere pressure difference), to the point I would call the density change negligibly different from zero for our discussion.
"The reason for the lower density is that the water on top is stretched. It experiences a tensile strain."
"The reason for the tensile strain is that a tensile stress has been added to the compressive pressure from the atmosphere. The tensile stress is caused by the force of gravity pulling down on the water in each tube."
No, the gravity pulling down does reduce the atmospheric pressure, but in a siphon shorter than the barometric height, it doesn't reduce the pressure all the way down to zero or the negative required to put the water in tension. The water under reduced pressure expands to a lower density, not because it is pulled to expand, but because it had been compressed to a smaller volume and the reduction in pressure allows it to spring back and expand itself a little to a lower density. Mindbuilder (talk) 01:34, 18 January 2023 (UTC)
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