Talk:Siphon/Archive 5

Removing the Siphon coffee section.
I am wondering why why there is a section trying to make the point that the vacuum pump method of making coffee is a 'siphon'. It is not. In fact the redirect, called 'siphon coffe', goes straight to 'vacuum coffee'. I would think it is confusing to readers and misdirectes them. One does not point to a coffe brewer and say it is a siphon. It it were so, one could point to any vacuum pump and say it is a siphon. It makes no sense. Comments? Tobermory conferre 02:31, 17 March 2014 (UTC)


 * I agree that the siphon coffee section should be removed or maybe put lower down in the article with all the other items that are called siphons just because they involve liquid flowing through tubes (but don't share the operation of that special device that makes water flow up hill powered by gravity). The fact that liquid will flow up hill under pressure, as in the siphon coffee maker, probably surprises almost no one. I suspect that section was prominently added by a seller of such devices. Mindbuilder (talk) 07:57, 17 March 2014 (UTC)


 * Thanks Mindbuilder, reading through the article puts more doubt in my mind. Having had a look around it seems as there has been some sort of competition between those who assert that atmospheric pressure is the driver of the siphon and those who say it is not. It is odd how mistaken beliefs can exist long after they have been disproved in the literature. I recall a few years ago there was a comparision made between the Encyclopedia Britannica and Wikipedia, so I went to the EB description of siphon:


 * '... usually in the form of a tube bent to form two legs of unequal length, for conveying liquid over the edge of a vessel and delivering it at a lower level. Siphons may be of any size. The action depends upon the influence of gravity (not, as sometimes thought, on the difference in atmospheric pressure; a siphon will work in a vacuum) and upon the cohesive forces that prevent the columns of liquid in the legs of the siphon from breaking under their own weight .'


 * Compare that to this article which tortuously describes the action of a siphon wrongly and includes references to virtually any device that pumps liquid through a tube and calling it a siphon! However, I suspect (from looking back through the page history) that any attempt to edit this article will immediately attract the 'no, it's atmospheric pressure' people to change back anything that differs from what they feel comfortably wrong about. The presently mangled article is I think a result of that sort of process. What are your thoughts? Tobermory  conferre 03:26, 23 March 2014 (UTC)


 * In following this siphon article it has amazed me how desperately people search for any crazy explanation to avoid having to admit their beliefs may have been mistaken. When a new way of looking at things casts doubt on your previous understanding, you should be very willing to change your previous understanding. Unfortunately we seem to be masters at convincing ourselves that we're not actually wrong, thus saving ourselves from the pain of admitting error. I try to take pride in recognizing my mistakes and realizing the truth whenever I can overcome the confirmation bias we all have. Luckily I don't have to worry about that very much because everything I believe seems to actually be true :)


 * If the writer of that EB article thinks typical siphons at normal atmospheric pressures "depend ... upon the cohesive forces..." in the liquid, I'd like to hear his explanation for the demonstration in figure four of this Wikipedia article, where the air bubble at top separates the two halves of the siphon, yet it still works. It may help for him to think first about what makes the liquid rise in a barometer or drinking straw. Of course siphons in vacuum do rely on the cohesion, but then they also collapse if there is a bubble, while siphons at normal atmospheric pressure don't. These two different behaviors under two different circumstances of ambient pressure, require two different explanations.


 * I was waiting for you to move that siphon coffee section further down the article, but I'll probably get around to it after a while if you don't. Mindbuilder (talk) 21:35, 23 March 2014 (UTC)
 * I would suggest deleting it, while it is called a siphon it is just a pressure pump... in the same way that a fire extinguisher can be called a siphon! Tobermory conferre 07:10, 24 March 2014 (UTC)

Re Comment ".....Of course siphons in vacuum do rely on the cohesion, but then they also collapse if there is a bubble, while siphons at normal atmospheric pressure don't. These two different behaviors under two different circumstances of ambient pressure, require two different explanations."

It is the same behavior. There is a focus and discussion of gravity, atmospheric pressure and cohesive forces. One key issue not often discussed is that the liquid moves because of the pressure gradient between the liquid inlet and liquid outlet. Atmospheric pressure or the lack of doesn't change the pressure gradient. If there is a bubble, then that can affect the pressure gradient, and it is important that if the bubble is on the downstream leg, then it's vertical height needs to be less than the height difference between the inlet and the outlet. In a vacuum, it is still the pressure gradient that is causing the liquid to move. Cohesive forces is a furphy. The catch with a bubble is that once the pressure drops below the vapor pressure of the liquid it will turn to gas. And the problem that creates for the siphon is the pressure gradient becomes zero, i.e. the vertical height of the liquid on the upside is equal to the vertical height on the downside and flow stops. Water droplets clearly have an attraction to other water droplets. Slowly overfill a glass of water and it will fill above the glass top, however only by 3 to 4 mm. With a dripping tap, the maximum water droplet size is 3 to 4 mm. There is two clear practical examples that water can only support 3 to 4mm of it's own weight. Putting the water into a pipe should not change the cohesive force or tensile strength. Clearly something else is at play. Everyone focuses on gravity and atmospheric pressure, and the idea that a liquid must be either pushed or pulled through the pipe. A pump can not push water into an existing pressured pipeline unless it can firstly create a pressure gradient, that is it's outlet pressure must be greater than the pressure in the pipeline. Fill two buckets with water and sit them on the ground with a tube out of the top of one and into the other. Increasing or decreasing atmospheric pressure or gravity will not get the liquid to move. But raise or lower one of the buckets, seal the top of one bucket and pump air in or remove air, or add a pump into the pipeline and the water can be moved. What do all these methods have in common: they create a pressure gradient." — Preceding unsigned comment added by 124.187.98.130 (talk) 06:03, 28 March 2014‎


 * I suppose you could say I wrote imprecisely. You could say the different behaviors of siphons in vacuum and at one bar ambient pressure, with a bubble in them, demands a single complex explanation for two different behaviors rather than a single simple explanation. I just meant that cohesion can't work to explain the siphon at one bar ambient pressure with a bubble and atmospheric pressure can't explain a siphon working in vaccum. The explantion has to be more complicated than either of the simple models.


 * As for the pressure gradient, a pressure gradient is an effect not a cause. The gradient is a description of the physical state of the system, but the gradient description doesn't cause the system to be in the state the system is in, nor does the gradient description cause the system or the system's elements to move or change state. Although to simplify things we may often talk of the gradient as though it does cause movement. The cause of the pressure gradient is gravitational and intermolecular forces. The question is how do the gravitational and intermolecular forces cause the pressure gradient. In tricky situations like this siphon debate, we have to go deeper than the pressure gradient simplification and get down to the four forces of nature. Though I think we can simplify by ignoring the strong and weak nuclear forces and simplify the electromagnetic forces to just the attraction and repulsion between molecules that touch.


 * Atmospheric pressure does change the pressure gradient in combination with a bubble when comparing a siphon in vacuum to a siphon at one bar ambient pressure. If cohesive forces are a furphy then how does the Z-tube work, and why doesn't the liquid in it vaporize as the pressure goes negative? I think part of the explanation is that the adhesion of the liquid to the tube walls constrains its movement compared to a droplet or the liquid at the top of a glass. Another part of the explanation may be that the molecules of a liquid at a given temperature don't all have the same kinetic energy, so the vapor pressure may depend not on the average speed of the molecues, but on the fastest molecues that are able to escape the liquid surface even though the average molecules can't. When submerged in a Z-tube, a few fast molecues might not be able to push open a vapor bubble, even at negative pressure, before having their energy absorbed by surrounding molecules. This may be related to why liquids can evaporate at temperatures far lower than they will boil. Indeed, since the Z-tube is clean and the liquid is undisturbed if vibration is low, the conditions in the Z-tube may be more like pure water in a clean cup in a microwave that won't boil until well above 100C. Mindbuilder (talk) 19:06, 28 March 2014 (UTC)

Re my comment "cohesive forces are a furphy", that should read that "cohesive forces that enable water to have a tensile strength of 10 metres or more is a furphy". An Argument has been put forward that the maximum height of a siphon is due to tensile strength. That is a load of rubbish. Atmospheric pressure in a standard water siphon does two things. One, it ensures that the water remains as a liquid, rather than turn to vapor. And two, since the pressure drops in a siphon as we head up the upward leg, if the siphon height is such that the pressure drops below the vapor pressure, then the water will turn to vapor. Thus atmospheric pressure sets the maximum height for a standard siphon using tap water.

Re "how does the Z-tube work...", well you get a tube, bend it into a Z shape, fill it with a liquid, put it on a flat surface then start spinning it. Tap water will turn to vapor if it drops below vapor pressure. For water to turn to vapor, it appears it needs a catalyst or certain circumstances, not just the pressure dropping below vapor pressure. For tap water, the air it contains appears to be what makes it easy for it to turn to vapor. Remove this air by boiling the water, and tests have shown that we can take water below vapor pressure without it turning into vapor, however the water is very unstable at this point and the slightest disturbance can turn it to vapor. So that fact that water can be put into a z tube and not turn into vapor is no surprise and is no big deal.

Note that in a standard siphon using tap water, if the height is raised, the siphon will going from operating with all water to the top part being filled with vapor. The same occurs if we raise a tube sealed at one end from a container of water into the air. It's either full of water until it gets above 10 metres and the top part will turn to vapor. We don't get the top part filled with nothing, rather it is either water or vapor. If the water is treated to remove all air in suspension, then the tube can be raised such that the height exceeds 10 metres, or the atmospheric pressure reduced to zero and the tube raised to say 1 metre or more. Again, test show that the water will stay as a liquid, but is very unstable, and the slightest disturbance will have it turn to vapor. At no point, does it appear that the water will simply slide down the tube and the area above it filled with nothing. As the Professor would say, Why is this so?

Well, that is all the forces that cause the pressure gradient coming into play. Any water droplet simply moves due to the droplets next to it being at a lower pressure. Even in unstable circumstances as noted above, this is still going to occur. The 4mm cohesion or tensile strength will have all water droplets being quite friendly to the ones next to it. However, the pressure gradient circumstances are deciding which direction for the liquid to move. — Preceding unsigned comment added by 124.185.8.205 (talk) 05:16, 29 March 2014 (UTC)

Interesting new reference, sure to reignite pressure vs gravity
Via news aggregator, I ran across this recent published (22 of April 2014) article in Nature: http://www.nature.com/srep/2014/140422/srep04741/full/srep04741.html It is in favor of gravity as the main engine in a siphon's function. --Reverend Loki (talk) 20:50, 25 April 2014 (UTC)

Yes, this article is awfully wrong about a lot of things. First, atmospheric pressure and height restriction are based on a vacuum being drawn. A piston on the top vessel overcomes this and a syphon will occur, I believe, above the limit where a vacuum can't overcome the force of the weight. It's why submersible pumps are made and why non-submersibles need to be primed. Second, it seems rather intuitive that Bernoulli's principle would be in play and the pressure/gravity system is simply that water flowing in a tube is less pressure than water not flowing, meaning the only pressure difference is between the fluid flowing in the tube and static upper tank. A sealed top vessel can be drained to a point and create a negative pressure in the top vessel vs. bottom for the entire time (it has to be stopped before the liquid runs out or the vacuum cavitates the flow into the tube letting air back in. --DHeyward (talk) 22:21, 25 April 2014 (UTC)

Pressure differences vs pushing up, pulling in practical siphons, and pushing up only to start.
@ DHeyward - Your edit upset a long established consensus, so I reverted most of it till we can discuss it. I'm writing up some more comments now. Mindbuilder (talk) 22:43, 25 April 2014 (UTC)

Differences are mathematical values that don't have a physical effect themselves. The atmospheric pressure exerts an actual physical force to push the liquid molecules up in a typical siphon. The pressure at the top of the siphon exerts less force downward and thus there is a difference, and the upward force wins out, but the difference itself isn't a concept that can cause movement, only the force can. Sometimes we simplify descriptions as if it were the difference that makes things happen, but that simplification would be more obscuring here rather than clarifying here. People need to know that in typical siphons the atmospheric pressure is pushing the liquid up and that the fluid at the top of the siphon is pushing DOWN not pulling up. More to come. Mindbuilder (talk) 22:55, 25 April 2014 (UTC)

"Once the liquid reaches the maximum potential energy and begins to drain" I'm not clear on what that means. Would you care to propose alternative phrasing to clarify? Mindbuilder (talk) 22:59, 25 April 2014 (UTC)

"it pulls the liquid out of the top reservoir to drain in the bottom" This is correct for a siphon in vacuum but it is incorrect in a typical siphon as was being discussed in that paragraph. The fluid at the top of a typical siphon is at positive pressure relative to complete vacuum, and thus while it is being squeezed by this positive pressure, the molecules repel each other and do not pull on each other at all. This is even more obvious when a small or large bubble is allowed to flow over the top of the siphon. If there was any pulling going on, then even a tiny bubble would be happy to expand very large from the pull and break the siphon. Mindbuilder (talk) 23:06, 25 April 2014 (UTC)

Although atmospheric pressure is generally almost identical at the entrance and exit, and flow rate doesn't vary much with minor variations of atmospheric pressure, that doesn't mean flow rate is independent of atmospheric pressure. In fact in the siphon of figure 4, there would be no flow at all without atmospheric pressure. The flow rate "depends" on atmospheric pressure for its very existence. Mindbuilder (talk) 23:38, 25 April 2014 (UTC)


 * While it's true that such bubbles in figure 4 will expand to the same volume as the external static pressure, it is not relevant to whether a syphon will occur. As long as the down flow tube has fluid with enough potential energy to overcome the height of the barrier, the bubble will be moved.  It doesn't matter if the apparatus is at sea level or 30,000 feet (the numbers change but not the principle.  As long as the pressure in the tube maintains a fluid state of water, the forces in play are fluid dynamics governed by Bernoulli.  Altitude and syphon height affect only the ability of the fluid to be a liquid.  excess velocity creates cavitation bubbles.  Low pressures turn it into a gas.  The mechanism of the syphon is that fluid in the tube is moving.  The tube by definition will have a lower pressure than the source fluid simply because it's moving.  The analysis isn't as detailed as atomic or molecular attraction.  It's on the fact that moving fluid is lower pressures than static fluid.  The weight of the fluid is sufficient to support pressure fifference flow, whence the Nature paper that doesn't see a difference as the experiment is elevated.  Only until the fluid vaporizes is their a change in flow.  --DHeyward (talk)


 * If there is any pulling on the bubbles, they will expand without limit. I mean like bigger than the solar system. Maybe earths gravity will pull it back from expanding into interstellar space, but the point is, if gas bubbles stop expanding it can only be because the pulling has stopped and something(usually together with atmospheric pressure) is pushing on them all around. They in turn push back. If there is even one little spot where a gas is not being squeezed together, it will expand without limit out that direction until it meets up with something that can push back and resist its pressure, like the atmosphere. A tiny bubble at the top of the siphon will displace all the liquid out of the siphon if the liquid does not push up on the bubble with the help of atmospheric pressure. The existence of air at the top of a siphon is proof that no pulling is going on up there. If there is any pulling on one side or area of the bubble by the liquid the bubble will simply happily expand towards the pulling substance for as long as the pull lasts, filling an infinite volume in the direction of the pull until the pull stops, and the bubble will transfer none of the pull to the other side of the bubble, but instead will continue to push away on whatever is on the other side of the bubble. Although it will push with less pressure as it expands towards the pull, it will still nevertheless continue to push in all directions, not pull in any. If there is no pull across the top of the siphon, then there is no source of force left to lift the molecules on the up side except atmospheric pressure.


 * Speed of flow is not necessary to lower the siphon pressure to make it work. You can put a valve at the exit or entrance of the siphon and slow down the rate of flow to a drop a minute, and the siphon can still empty out the entire upper bucket, just very slowly. There could be a little residue left over from below the entrance and some back spillage when the water level drops low enough to allow air in to break the siphon. If the upper bucket is replenished, the siphoning can go on indefinitely at a snails pace.


 * Do you acknowledge that atmospheric pressure pushes the liquid up a barometer and a drinking straw? Do you acknowledge that the pull of gravity on the water in down tube of figure 4 will leave a reduced pressure zone at the top of the siphon? Do you acknowledge that Gas pressure can push a liquid from a higher pressure zone up to a lower pressure zone? Do you acknowledge that a bubble at the top of a one meter siphon will have a gas pressure of about .9 atmospheres absolute and therefore will be exerting an outward force of about 9N against each and every square centimeter or about 13 pounds against each square inch of surface, solid or liquid, around the bubble? (assuming the siphon is slow moving or static with the two reservoirs at equal height so that pressure changes with velocity are negligible) Do you acknowledge that if the bubble expands ten percent, double, or a hundredfold, that it will still be exerting an outward force against everything around it? — Preceding unsigned comment added by Mindbuilder (talk • contribs) 07:18, 26 April 2014 (UTC)


 * Of course the atmospheric pressure matters in siphons that use atmospheric pressure to prime them.  It's also important in determining max lift height.  The bubble volume in figure 4  will be less than the outside air pressure and determined by the weight of the water and the vaporization pressure of water.  But that picture of an apparatus is not yet a siphon.  Once the siphon starts, the apparatus can be elevated from sea level to 40,000 feet without changing the flow rate at all.  Note also, that the atmospheric pressure at the outlet of the siphon is slightly higher than the pressure on the high side.  Gravity overcomes this.  The only condition is that fluid can't become a gas through either vaporization or cavitation and those conditions are related to the atmospheric pressure.  The atmosphere is what creates static pressure on the high and low tank, the flowing water in the tube is at a lower pressure and it's that pressure difference, created by gravity, that matters.  Lakes that use siphons often have a fill port at the top of the syphon and valves and the bottom.  They only have to fill the pipes, close the fill port and open upper and lower valves.  There is no need for any air pressure differences.  This is also apparent when filling a small surface area container from a large surface area container.  Flow stops when the water reaches the same gravitational potential. --98.165.73.36 (talk) 18:04, 26 April 2014 (UTC)


 * Consider a siphon just over one meter tall with two vertical tubes filled with water except for some air at the top. Lets say there is enough air so that the water only reaches just below the bent part of the tube at the top. That is to say that the interface between the water and air is in the straight vertical section of the tube. For simplicity lets start with the two buckets of water at the same height so that the siphon is static. Now lets focus on one molecule of water, one meter above the surface of the bucket, at the top of the water, in the middle of the tube, on what will become the up side of the siphon. Above the molecue is air at a pressure of .9 atmospheres. Because pressurized air pushes outward, up, down, and all around, that air will be pushing down on our molecule of water in this static siphon, right? Gravity will be pulling down on our molecule. That is two down forces on our molecule, yet the molecule doesn't go down in our static siphon. So there must be some force or forces pushing up. Since there is only gas above the molecule, and gasses cant pull, there is nothing above it to pull it up. There are only four or five forces of nature, depending on how you count. We can ignore the strong and weak nuclear force. Gravity is pulling down, so the only thing left for an up force are electromagnetic forces. The molecues at the entrance and exit can't reach out telepathically and apply forces to our molecule. Our siphon and its contents are non-magnetic and have a neutral electrical charge, so the net electromagnetic forces from other molecules at a distance from our molecule are negligible. The only electromagnetic forces that matter are from the other molecules touching our water molecule. The pressure forces at the exit and entrance of the siphon only affect our molecule by one molecule touching another along the siphon. Since the air above our molecule in the siphon is pushing down, the only thing left to apply an upward force to our molecule is the other water molecules below pushing up. But for every action there is an equal and opposite reaction, so if the molecules below our molecule are pushing up, then our molecule is pushing down on them. Gravity is also pulling down on them, so why don't they go down? Because the molecules below them are also pushing up. And so on down the column of water, the molecules below are pushing up on the molecules above, until we get to the bottom where atmospheric pressure supplies the upward force at the base of this process, just like in a barometer or drinking straw.


 * Now lets say we lower the bucket on the down side of the siphon. What happens to our molecule of water at the top of the water on the up side, as we start to lower the bucket on the down side? Our molecule starts to move up. When an object is at rest and starts to move, there had to be a force or imballance of forces to make it move. The air above our molecule will have slightly lower pressure because we're lowering the down bucket and gravity starts pulling the liquid in the down side down, but the air pressure will still be very nearly .9 atm and so the air will still be pushing down on our molecule, right? Why does our molecule move? Because with slightly less pressure above our molecule, the force from molecules below is now able to overcome gravity and the force above to move our molecule up. Were did that force below ultimately come from? Atmospheric pressure - not pull from liquid cohesion above because there is no liquid above, nor a pulling gas from above.


 * Now lets put the buckets back at equal height and remove the air at the top of the siphon and replace it completely with water. Looking again at our same water molecule at the same one meter above the water bucket surface, our molecule now has water above and below it. By the equation of hydrostatic pressure in a siphon, the water just above our molecule has the same pressure as the air had, .9 atm. Although it is possible for water to pull, the water above our molecule has a pressure of .9atm and thus just like with the air, these water molecules being crowded together will be trying to spring apart and exerting a reaction force outward, upward, downward, and all around. So despite the capabillity of water to pull, our water molecule will instead still be experiencing a downward pressure force from the water molecules above it. Gravity will still be pulling down on our molecule. So since our siphon is static there must still be an upward force on our molecule from below. And as before the root of that force at the bottom is atmospheric pressure.


 * Now lets lower the bucket a little on the down side. This results in pressure being lowered slightly at the top of our siphon and above our water molecule. But the water above our molecule is now only slightly below .9atm and therefore at just below .9atm it is still pressing down on our molecule, right? Gravity is still pulling down, but our molecule starts to go up. Why? Because the molecules of water below our molecule are pushing up slightly more than the molecules above and gravity are pushing down . And why are the molecules below pushing up? Because down at the bottom is atmospheric pressure pushing up just like in a barometer or drinking straw. So even in a siphon completely filled with water the force that pushes molecules up the up side comes from atmospheric pressure, not a pull from the liquid above. Indeed the liquid at the top of a siphon is pushing down on all the liquid below, impeeding the upward flow of liquid, not assisting it.


 * But the situation is much different with a siphon in vacuum. Lets say earth's atmosphere were to suddenly dissappear so that we had a perfect vacuum at sea level (or to be more realistic you could imagine this on the moon, though with less strong gravity). Now the water in our buckets would tend to boil, but if we chill them to just above freezing the vapor pressure might be less than the surface tension and it might not boil too much. There is a Youtube video showing water in a vacuum almost stops boiling as it gets chilled to near freezing. But even if there is a little boiling in our buckets, there may not be any vaporizing in our siphon tube as has been shown in the Z-tube and in Andrew K Fletcher's 24m siphon. But if it's too much to imagine the water not vaporizing in a vacuum siphon you can imagine this going on with a non vaporizing ionic liquid. Before, the equation of hydrostatic pressure one meter above the bucket surface in our static siphon, showed our molecule at .1atm below the atmospheric pressure at the surface of the bucket (1 - .1 = .9atm) But now that there is a vacuum above the bucket. The pressure at our molecule one meter up will be 0 - .1 or negative .1atm. A negative pressure is a pull. The liquid just above our molecule will be at slightly less than -.1 atmosphere and will be pulling up on our molecule. The molecules just below our molecule will be at a pressure just a little more than -.1atm and therefore will be pulling down on our molecule instead of pushing up as before. If we lower the out bucket, our molecule will rise because it is being pulled up by liquid cohesion and our molecule will pull up the molecules below.


 * As the siphon starts moving the pressures will change from the static ones due to effects related to Bernoulli's principle. But the pressure changes won't be too much in a slow moving siphon. Maybe the liquid in the top of a 1m siphon will only be .8atm instead of .9 or something. But the liquid at top will still be at positive pressure and will still be pushing down on the molecules in the up side, and atmospheric pressure will still be supplying the force to push the molecules up. Of course gravity will still be providing the energy to lower the pressure at the top to enable atmospheric pressure to do its thing. Mindbuilder (talk) 21:19, 26 April 2014 (UTC)


 * If you're not sure if the dynamic effects would change the situation too considerably, then just imagine the surfaces of the two buckets being extremely close in height so that the siphon runs at a speed that is extremely slow, but not quite zero. It might take an hour for a molecule to get from the entrance to the exit, but it is still doing what a siphon does. In such a situation, dynamic pressure changes would be negligible, but atmospheric pressure would still be pushing the liquid up. (powered by gravity pulling down on the other side of course). Mindbuilder (talk) 21:57, 26 April 2014 (UTC)

I think your missing a crucial point. In your 2 bucket at the same level thought experiment, to get a 0.9 atm pressure bubble, you evacuated the air and it's the external 1 atm on the water surface of the bucket. That's the way water starts moving up the tube. Same as when you suck on a straw: the difference between atmospheric pressure and the pressure in the mouth end of the straw determines the force. The surface area of the straw (say in square inches) times delta in pressure between the atmosphere and your mouth (using your .9 Atm, it would be approximately 14 PSI - 12.6 PSI = 1.4 pounds per square inch of force.) You are evacuating air and the pressure difference between the bubble and the open air surface is the force counterbalanced by gravity. The weight of the water in the tube is exactly equal to the difference in the pressure between the atmosphere side and the lower than atmosphere bubble. The height limit of the syphon is reached when the bubble pressure is at the vapor pressure of the liquid (the lowest possible pressure). You seem not to be counting that there is atmospheric pressure on both sides of the siphon when siphon flow is created and there is no bubble. A bubble is not needed and the force on molecules near your bubble example is because water is an incompressible fluid. Once you lower the bucket and actually create syphon flow, the atmospheric pressure on the downside side is slightly higher than on the upper bucket (extremely small in most cases) and the net effect of the atmosphere is slightly negative. Imagine the top bucket at a height above sea-level that is 0.8 atmospheres of sea level, a pipe that rises above that bucket 6 feet and then extends all the way down to sea-level. When the sipon starts, the net atmospherice effect is negative (there is 1 atmoshphere of pressure pushing from the down pipe and only 0.8 pushing on the up pipe. It still works though because the force is gravity driving Bernoulli's principles that a flowing fluid is lower pressure than .  There are many ways to start the siphon flow but consider these two: The first is prefilling the tube with liquid and flow will begin immediately against the atmospheric pressure gradient (high pressure on the low side simple due to altitude).  The second way would be evacuate the tube below the upper pressure of 0.8 atm so that the rise can be created.  The only thing the atmospheric pressure on the high side does is limit the lift potential above that surface but that to is because of the weight of the water in the upward column and the physical PVT properties of water and when it vaporizes.  The water column will separate at it's maximum lift creating your bubble at the vapor pressure of the liquid. That's the limitation that atmospheric pressure places on the siphon (coupled with the physical properties of the liquid but that's not the driving force of the syphon. --DHeyward (talk) 01:29, 27 April 2014 (UTC)


 * I should have mentioned that I realize the atmospheric pressure at the surface of the lower bucket is slightly higher than at the upper bucket. I have been mostly assuming that the atmospheric pressure is equal at the upper and lower surfaces because the difference is negligible. The change in air pressure due to altitude is about 800 times smaller than with water. Thus changes in hydrostatic pressure dominate over changes in air pressure with altitude. I also generally assume that the air pressure is constant within the bubbles inside the siphon although there is a negligible change with altitude in the bubble as well. Of course there is no difference in atmospheric pressure when the surfaces of the buckets are at the same height in a static siphon.


 * I should also have mentioned that in my example of a static siphon I meant to imply that liquid and/or air had been added and/or removed so that after the siphon had stabilized and stopped moving, the water/air bubble interface was one meter above the surface of the buckets. And I meant to start the analysis I was discussing after it had become still. I also assumed that the barometric height of water at sea level is 10 meters even though it is not exactly that. Water barometers reach 10.3 meters, and would go a little higher if there was no water vapor at the top. For simplicity I'm going to continue to assume the barometric height of water is 10 meters.


 * So in my above example of a stabilized still siphon with buckets at equal level and an airbubble/water interface at 1 meter, (1)what object do you think exerts a force on our molecule in the middle of the siphon at the top of the water, to prevent gravity pulling it down? (2)Does the air above our molecule exert a pulling force to counter gravity on our molecule? (3)Does the water molecule below our molecule exert an upward force on our molecule to counter gravity? If the molecule below does, then as you go down the molecules, (4)is it not atmospheric pressure that ultimately holds up our molecule? Now if we lower the reservoir on the down side very slightly, say a half millimeter, so that the siphon starts flowing so slowly that bernoulli's pressure variations are practically nothing, (5)does the air above our molecule start to pull up on our molecule, or is the air above still pushing down? (6)What object exerts the force to move our molecule up? (7)Does the force to lift our molecule not come from the water molecule below it? And (8)does that force not ultimately come from the atmospheric pressure below? If we replace the air above our molecule completely with water, (9)does the water above our molecule not push down on our molecule? And when we set the siphon flowing very slowly, (10)is it not the molecules below our water molecule that apply the upward force to overcome gravity? And (11)is that upward force from the molecule below not ultimately from atmospheric pressure farther down? If you would answer these questions, I think it would allow us to reach an understanding much quicker. I hope you would answer all of them so that I would know what not to explain further with regards to any we agree on. Mindbuilder (talk) 03:52, 27 April 2014 (UTC) (modified) Mindbuilder (talk) 07:42, 27 April 2014 (UTC)


 * Please explain why you think "pressure change in air pressure due to altitude is about 800 times smaller than with water." That doesn't make much sense considering the liquid presence of alpine lakes.   I think you are confusing altitude in the atmosphere with depth in the ocean.  A liter of air at 5500 meters is half the weight of a liter air at sea level.  A liter of water is nearly the same weight at 5500 meters.  At 5500 meters above sea level the air pressure is 500 millibar while the air pressure at sea level is 1000 millibars.  If you place a 1 liter bucket of water at 5500 meters, have the tube raise up to 1 meter above that and then extend down 5500 meters to 6 feet above ground, it will still siphon even though the air pressure is working against it.  Gravity does the work. The fluid that is flowing is lower pressure than the non-flowing fluid in the upper source bucket.


 * For your bubble, lets simplify it. Take a 40 foot length of pipe and seal it at one end (like the straw where you use your finger.  At sea level, submegse the pipe in water so it is completely filled with water with no air bubbles.  Begin pulling it from the water vertically by the sealed end.  At 1 foot, there will be 1 foot of water in the tube.  At 10 feet, the tube is still all water.  At about 32 feet, a gap begins to form between the water and the sealed gap.  It's pressure is set by the water vapor pressure (it's not a vacuum because liquid water will vaporize to create a gaseous bubble).  Any further increase in height of the cap doesn't create an increase in water height.  The pressure of the bubble doesn't change either though its volume does. The water at the interface vaporizes to maintain a constant pressure.  After you reach the maximum 40 foot height of the pipe, the water height is still 32 feet, the bubble is 8 feet long and was the same pressure as it did when it was 1 foot long.  Now, begin lowering the pipe back in the water.  The bubble compresses and water condenses to maintain the same constant pressure.  This the how manometers work.  The difference between atmospheric pressure and the liquids vapor pressure is the force that is applied against it's weight.


 * Back to Figure 4 - if the air bubble starts at atmospheric pressure, gravity pulls on the downward leg causing the volume of the bubble to increase, and thereby decreasing the pressure of the bubble. If the bubble pressure is below the pressure of the upper vessels surface, an amount of water that weighs the same as the pressure difference will enter the upper tube.  The bubble can only drop in pressure and the lowest pressure it can theoretically be is the vapor pressure.  For a siphon to be created in figure 4, the bubble pressure must be reduced  (by volume expansion due to the gravity driven draining in the downward leg) and allow the pressure on surface of the top vessel to overcome lift requirement.   If the bubble started at .9 ATM as you stated, there would be some water in the upward column (the weight equivalent to the 0.1 ATM difference).  The pressure would drop in the bubble as the water drains from the downside (increasing Volume, lower pressure).  The drain flow rate will be faster in the down tube than the flow rate up the tube.  As long as there is sufficient mass and volume in the down tube to reduce the bubble pressure that supports having the upward leg clear it's head space, the siphon will start and the bubble is normally swept away.  The forces are gravity  and the differential pressure force between the atmosphere and the bubble.  --DHeyward (talk) 05:56, 27 April 2014 (UTC)


 * What I mean by 800 times higher pressure change in water than air is that if you take a pressure guage and measure the pressure at sea level in the air and then measure the pressure one meter higher than sea level, again in the air, the difference between the two measurements would be about 800 times smaller than the difference in pressure measurements between one point in a vertical water pipe and another point a meter higher in the pipe. The greater change in pressure with altitude is because water is about 800 times denser than air at sea level. You're right though that this 800 factor only holds near sea level. If the air is half as dense then this ratio would go up to about 1600 times.


 * Most of what you wrote in your last reply seems to be about right, so I'm having a hard time seeing where our differences are. It would really help if you would answer the questions I asked before. Do you object to the questions? Yes/No answers are fine. Long answers are fine too. I realize it would be easier for me to see which questions you are answering without you having to copy all the questions if I number them and you put the numbers in your reply, so I'll number them. Mindbuilder (talk) 07:42, 27 April 2014 (UTC)


 * I still am not sure what your saying about water vs. air. A liter bottle of air will way half as much if filled at 5500 meters vs sea level.  A liter bottle of water will weigh about the same if filled at 5500 meters.


 * For your questions: 1) The pressure difference between the air at the surface of the bucket and the air bubble supports the weight of the column. 2) The force of gravity is down, the force of the difference in presure is equivalent and opposite.  The minimum pressure of the bubble would be vapor pressure of water but can be higher since the column is only 1 meter. 3,4) your example is static so the pressure difference exactly balances gravity.  There is no flow.  Note that the pressure of the bubble is determined by the weight of the water.  Consequently, you can't have an arbitrary amount of water with an arbitrary pressure.  5)if you lower the reservoir on the downside while keepin the upper reservoir and top of the loop at the same height, you are adding water to the downside column (by 1 mm in your example.  The additional weight will expand the volume of the bubble so its pressure will drop according to the mass added to the downside tube. The upside also reacts to the new lower bubble pressure by rising.  If you actually lowered the bucket at a constant rate, you would see the bubble get larger and the bubble pressure drop.  The bubble will move to the downside tube because the weight of the additional water moves out of the column faster than the water moves in the upside column. 6) The forces to maintain a column is the pressure difference matching the weight of the water.  The bubble doesn't "push" or "pull", rather the atmosphere and gravity create a bubble pressure by manipulating the volume of the bubble so that it's in static equilibrium.  the bubble compresses and expands but it's lower limit is the vapor pressure of the liquid.  After question 7), it changes from static to dynamic.  It's started by adding mass of water to the down column.  It is no longer a hydrostatic problem.  Flow starts a process whereby the hydraulic pressure is lower in the tube.  This is easier to visualize with lakes at two elevations and no air bubbles (they close off both ends of the tube and fill from the top and then seal the fill port.  The higher altitude lake has less atmospheric pressure than the downside like.  A siphon will still flow from a lower atmospheric pressure bucket to a higher atmospheric one.  The limit of lift is determined by where the weight of the water in the upward column can no longer be supported by the difference in atmospheric pressure and vapor pressure on the upper side.  The column then separates with a water vapor bubble.  It returns to a none flowing static case.  The force that moves the water in a siphon is gravity. Limits of the siphon are atmospheric pressure and properties of the liquid --DHeyward (talk) 10:59, 27 April 2014 (UTC)


 * As for the change in pressure with altitude with water vs air, consider a siphon near sea level where the surface of the upper reservoir is one meter higher in altidude than the surface of the lower reservoir. If you measure the atmospheric pressure at the surface of the lower reservoir and at the surface of the higher reservoir, the difference will be about 12 Pascals. If you have a bucket of water more than a meter deep where the water is not moving, and you measure the pressure near the bottom and you measure the pressure at a point in the water one meter higher in elevation than at the first point, the difference between the two pressure readings in the water will be about 10000 Pascals, or about 800 times greater than the 12 Pascal difference measured in the air. Of course the ratio changes if you are at an altitude where air has less density. If you are at an altitude where air has half the density, then the difference in atmospheric pressure readings with a one meter change in altitude would only be about 6 Pascals. But with the bucket up at that altitude there would be practically no change in the pressure difference with a one meter change in height within the bucket, than when the bucket was down at sea level, because the water doesn't change density with altitude like air does. So then the ratio between the air example at altitude and the water example at altitude would be 10000/6 or about 1666 times. (These calculations are rough because I don't know the exact numbers)


 * So in a siphon at sea level with a one meter change in altitude between the upper and lower reservoirs, there will be a 12 Pascal atmospheric pressure difference impeeding the flow of the siphon. But that one meter difference in altitude between entrance and exit will result in about a 10000 Pascal difference in water pressure to drive the siphon. That 12 Pascal impediment subtracted from the 10000 Pascal driving force is small enough to be ignored in almost all siphons.


 * Your revision at 7:10 that includes the phrase "which causes the bubble to expand" seems to assume that that paragraph is in the context of figure 4 next to it. But I don't think that paragraph was meant to have any relationship to Figure 4 or siphons with bubbles in them. I think its talking about the usual real world case of siphons without bubbles. It is a good insight though to realize that the water in does not exactly equal the water out when there is a bubble in the siphon. I'm not sure though that that particular insight helps clarify things or just overly complicates that section of the article. More comments to come Mindbuilder (talk) 19:04, 27 April 2014 (UTC)


 * In your answer to my question number one and two, You state "The pressure difference between the air at the surface of the bucket and the air bubble supports the weight of the column. 2) The force of gravity is down, the force of the difference in presure is equivalent and opposite."


 * You seem to implicity acknowledge that the atmospheric pressure of the air at the surface of the bucket is contributing an upward force to support the weight of the column, and that from that upward force is subtracted the force from the pressure in the bubble (to get a "difference"), and that the difference in the static siphon must equal the weight of the water column. Is that not the case also in a completely filled and even moving siphon where the liquid in the up side is supported or pushed up by the "difference" between the pressure at the top and bottom? And if the pressure at the top is subtracted in this "difference" calculation then does that not imply that it is a downward force rather than an upward one? If the fluid at the top was providing an upward force then it would be added to the atmospheric pressure below rather than subtracted right? Mindbuilder (talk) 19:41, 27 April 2014 (UTC)


 * You are confusing static and dynamic fluids and creating scenarios that aren't siphons. It is not the case for flowing siphon that the atmospheric pressure difference is driving any flow.  It's gravity.  If anything, atmospheric pressure inhibits dynamic siphon flow despite supporting a static column with an air gap at a lower pressure.  The fluid pressure is less because it is flowing.  This is Bernoulli's principle and can readily be seen with things such as a Venturi pumps and other dynamic fluid flows.  The flowing fluid in the siphon tube is lower fluid pressure than the upper fluid static fluid pressure.  It is completely different than the static pressure to hold the column.  This is why a siphon apparatus can be a static non-moving column until fluid flow is established and the tube fluid pressure drops and flow continues until complete.  Air pressure is not the initiator or driver of the siphon.  Only until enough fluid is moving due to gravity will a siphon be continuous.  This is apparent if you've ever started a syphon by sucking on tube like transferring gas from from a car to a jug on the ground.  The largest atmospheric pressure difference is when the fluid is at the apex.  But the siphon doesn't start when that happens.  Rather, the fluid has to have enough mass in the downside tube to start a gravity flow.  After the apex, the suction applied is less and continues to drop until the downside tube has enough fluid mass to flow due to gravity.  The siphon only continues when gravity sustains the flow.  It's also the reason why the down stream tube is usually open air and not submerged.  The air pressure does not drive siphon, gravity does.    Fluid exiting the downside tube reduces fluid pressure in the tube (not atmospheric pressure).  This is fundamental. --DHeyward (talk) 21:36, 27 April 2014 (UTC)


 * I agree that gravity drives the siphon, but in combination with atmospheric pressure. The big question about siphons is not gravity. Pretty much everyone agrees on its role. You and I agree on its role. The question is why does the liquid go up AGAINST the force of gravity. Gravity being a DOWNWARD force, cannot be directly providing the UPWARD force that makes the liquid go up. A liquid will not go up unless there is an upward force on it, and that force cannot be gravity. What object exerts that upward force on the liquid? Is it the pull of liquid cohesion like in a vacuum siphon, or is it atmospheric pressure? I think you have stated that the upward force is from the "differnece" between the atmospheric pressure at the bottom and the reduced pressure at the top. But a "difference" of forces is not a force. For example, say you have a child and a mother with a shopping cart in the supermarket. Imagine the child wants to stay in the candy isle but the mom wants to move on. So the child pushes on the front of the shopping cart to prevent it from leaving the candy isle. But the mom is much stronger and slowly pushes harder to overcome the child's resistance. The movement of the cart will be determined by the "difference" between the child's force and the mom's force. But if we asked what causes the shopping cart to move, I think almost everybody would say the mom causes the cart to move. Nobody would say "the difference" in force on the cart causes the cart to move. Certainly the child doesn't cause the cart to move forward. The difference will "determine" how fast the cart accelerates, but we don't say "the difference" will cause the cart to move. Or imagine a bucket of water on the floor and someone reaches down, grabs the handle and lifts the bucket. We don't say the "difference" between the force of the arm and the pull of gravity caused the bucket to move up. We say the person caused the bucket to go up. That wouldn't change if a small child grabbed the bucket and tried to pull down but the adult managed to lift it anyway. It still wouldn't be the "difference" between the adult and child that made the bucket go up. It would still be the adult that made the bucket go up.


 * But even if we did say that the difference caused the cart to move,or the bucket to go up, it would only be a simplification combining two forces into one in order to make it easier to think about. But when speaking about this tricky subject, it does not simplify things to combine those two forces into a difference. It is best to talk about them independently and state explicitly which is providing the upward force and which exerts a downward force and only then make it clear what the net differnce is. By keeping the forces separated out of the difference you make it easier to understand the different behavior of vacuum siphons and typical siphons. People need to realize that in vacuum siphons there is a pull at the top but in practical siphons there is not, there is the reverse, a push down at the top.


 * Regardless of whether that reduced pressure at the top is caused by gravity or bernoulli effects or whatever, the fact that the pressure at the top is still positive relative to pure vacuum, and that you have to subtract that pressure at the top from the pressure at the bottom to determine the lifting force on the rising liquid, implies that the pressure at the top is not providing the lifting force to make the liquid go up against gravity. If the liquid at the top is not providing the lifting force to raise the liquid because it's exerting a DOWNWARD pressure, then what is? It cant be gravity because gravity is not an upward force, so it cant make liquid go up directly. What object or objects are applying a DIRECT force to lift the liquid? If it is pressure you realize that pressure is a property of a substance and I'm asking what is the substance and where is the location of that substance that is providing the UPWARD COMPONENT of that pressure difference? Is it not true that atmospheric pressure is the only source of force to make the liquid go up when the top is at a positive pressure relative to pure vacuum?


 * And also lets settle the source of the upward force in siphons operating at an extremely slow speed so we can neglect pressure changes from velocity like in the bernoulli effect. Then once we agree on the simpler case we can move on to faster siphons. Even in siphons flowing at normal speeds the pressure changes from the bernoulli effect aren't that large and don't really change the basic principle that the atmospheric pressure pushes the liquid up into the low pressure zone. Have you calculated the pressure changes from the bernoulli effect in a normal siphon? Mindbuilder (talk) 23:37, 27 April 2014 (UTC)
 * The issue with examining the atmospheric pressure as a separate force is that the atmospheric pressure does no work. You cannot add just the up leg and expect air pressure to push fluid up the tube.  All of the work done within the siphon is the force of gravity through the difference in height between the upper container surface and the outlet.  Zero work is done by the atmosphere --DHeyward (talk) 00:46, 28 April 2014 (UTC)