back to article Siphon Wars: Pressurist weighs into Gravitite boffin

We've just had a missive from a US reader regarding that most pressing scientific question of the moment: Just how do siphons work? Those of you with a scientific bent will recall the recent Oxford English Dictionary outrage, in which one Dr Stephen Hughes of Queensland University of Technology laid bare a 99-year-old gaffe in …


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  1. SuperTim


    It needs air pressure and gravity. it is gravity that causes the liquid columns to have weight. They then create a pressure differential which the air pressure can work on pushing liquid through the pipe.

    It wont work in a vacuum and it wont work in zero gravity. Which is why there are are no syphons in space!

    1. Anonymous Coward

      Reassuring news

      "there are are no syphons in space!"

      That's probably why the Space Shuttle can pass over Liverpool without losing any fuel.

    2. Intractable Potsherd

      Is it true that there are no siphons in space?

      I'm serious - I really don't know. Has this experiment been done in reduced gravity? Surely this is the best way to prove the point - air pressure but no/very low gravity. It would be easy to video and post for all to see.

      1. Richard 120


        How do you get rid of gravity? The only way I know is to get far enough away from the object exerting the gravitational pull, so do it in space then? Easy to do that, get a ticket to the ISS, oh hold on, what's happened to the other part of the experiment, the air pressure. I think we might lose that too.

  2. Chemist

    Re : siphon

    I've already posted this but here go again :

    Gravity alone drives the thermodynamics. The energy comes from the difference in height of the reservoirs multiplied by the mass of water moved.

    The mechanism requires that the 'hump' over which the liquid has to climb is such that the liquid will not form a vapour bubble at the highest point - this point will depend on the atmospheric pressure. With a high-boiling liquid and a low hump the 'outside' pressure could be quite low. Practically this would require the entire app. to be a partial vacuum chamber

    In a normal environment, of course, atmospheric pressure is related to gravity.

    1. wheel

      True, but not vapour point

      But the point is that atmospheric pressure could not be zero, even with a theoretical liquid with an infinite boiling point. The static pressure at the top of the siphon tube (the 'hump', as you call it) has to be less than the atmospheric pressure at the surface of the reservoir, or the liquid cannot flow. This is not to do with vapour point, but pressure gradient: the liquid will always flow from a region of higher pressure to lower pressure.

      If there is no atmospheric pressure, the liquid cannot flow up the tube; the static pressure at the top of the tube can never be less than zero.

      In other news, the liquid only flows through the siphon because it is denser, and therefore heavier, than the surrounding atmosphere. But if it wasn't, it would float away...

      As an aside, surely this isn't difficult to understand? I'm a musician, for heaven's sake...

      1. Chemist

        Re : True, but not vapour point

        You may have noticed that I never mention zero atmospheric pressure. The vapour pressure is important however - the 'weight' ( actually pressure = weight/area ) of liquid in both legs is resisted by the outside pressure until a point is reached with a hump that's too high (~10m with water at normal atmospheric pressure) when the liquid pressure cannot be balanced by atm. pressure and a vapour bubble forms at the point of lowest pressure - the top of the hump. This is exactly the same as a barometer - if you tried to siphon mercury ( don't suck !) the hump would only work up to ~1m at normal atm. pressure.

        As I mention above the system would work with a high-boiling liquid and a low 'hump' at very low atmospheric pressures

        1. wheel
          Thumb Up

          I wasn't disagreeing

          I was agreeing.

          I was merely taking the argument one step further by considering a hypothetical situation with a zero atmospheric pressure. It wouldn't then matter how high your vapour point was; a vacuum would still form at the top of the hump, at the same level as the liquid in the reservoir.

          Sorry if it sounded like criticism; you clearly understand the problem, but I'm amazed at how many people do not. Good call with the barometer analogy; I was thinking the same myself, but couldn't remember what the damn thing was called!

          Taking things further again (and again not criticising), I think the gravitational force is also relevant as to how high your hump could be. On the moon, you would need far less atmospheric pressure to overcome the 'weight' of the liquid in the hump than on earth...

      2. Claus P. Nielsen

        More pedantery

        "the liquid will always flow from a region of higher pressure to lower pressure"

        This is wrong.

        A siphon transport a liquid from a reservoir A to another reservoir B as long as the surface of the liquid in A is elevated above the surface of the liquid in B.

        Under normal conditions, this higher elevation means that the atmospheric pressure at the surface of the reservoir A is lower than the atmospheric pressure at the surface of reservoir B (Since there is a slightly higher column of air above reservoir B than above reservoir A).

        So the liquid runs from low atmospheric pressure to higher atmospheric pressure.

        Which is exactly why stating that the siphon is driven by atmospheric pressure is misleading.

        1. wheel

          Not just atmospheric pressure

          Pressure isn't just caused by the atmosphere; it's also caused by the column of liquid. I do not say that gravity doesn't cause the siphon effect, but that the siphon effect could not happen in a vacuum, as the pressure of the liquid alone would not be enough to move the liquid over the 'hump'. A vacuum bubble would appear at the same level in the tube as at the surface of the reservoir.

          The only reason that the liquid and atmosphere cause pressure at all is gravity. Gravity also makes liquid flow downhill; this is not a factor of pressure but rather the transfer of gravitational potential to kinetic energy. When the liquid is flowing uphill – as in the part of the siphon before the 'hump' – its gravitational potential is increasing: gravity is working against the flow of the liquid in this section.

          The force driving the liquid up into the 'hump' is a static pressure force exerted on the liquid in the bottom of the tube by the sum of the liquid pressure and the atmospheric pressure above it. This force encourages the liquid up the tube. Because of gravity, the weight of the liquid within the first stage of the tube is also exerting a downward pressure force; this force works against the other force. The difference in force which enables the liquid to flow over the hump is caused by atmospheric pressure.

          The liquid is denser than the atmosphere, so exerts a greater static pressure for a given volume (or height, if you will). Once the liquid is over the hump, it does indeed fall due to gravity, rather than the atmospheric pressure. But the flow is maintained because the static pressure at the bottom of the siphon is higher than the static pressure in the receptacle; while the atmospheric pressure may be slightly higher at the lower altitude, the liquid pressure is higher still, as the liquid is denser than air.

          The problem is that, in the absence of any atmospheric pressure, the liquid simply cannot get over the hump. Without atmospheric pressure, there is no force to drive the liquid against the force of gravity. And if you are thinking 'suction'... well, suction is pressure difference, and there can never be less pressure than zero, so it wouldn't exist in a vacuum.

          (*Aside* There are other things going on too, of course; I notice that some have cited surface tension, by which I assume they mean the bonding between the liquid molecules; surface tension only happens at a surface, and there isn't one inside a siphon. But liquid bonding forces are weak compared with pressure forces; that's what makes liquids liquids. Dynamic pressure, too, undoubtedly plays a part, particularly in the downward portion of the tube, but I'll leave discussion of that to fluid dynamicists.)

          So the siphon – that part of the mechanism that gets the liquid over the edge of the reservoir, rather than the part which delivers it into the bucket – is, arguably, driven by atmospheric pressure.

          I accept that you need gravity for a siphon to work. (Though I should point out the obvious here – the whole concept of a siphon is meaningless in zero gravity: which way is up?) But I will strenuously argue that it won't work in a vacuum, which means that atmospheric pressure is an essential element in the process.

          In short, asserting that a siphon isn't driven by atmospheric pressure is more wrong than asserting that it is.

          1. Chemist

            Re : Not just atmospheric pressure

            I think we need to clarify some terms.

            "driven by" - to my mind that means the thermodynamics of the situation and that is the energy derived from potential energy - in this case gravity

            An 'atmospheric' pressure is necessary for the mechanism to work but it doesn't contribute to the overall energy change. There are a number of factors - height of hump, boiling point of liquid, atm. pressure, temperature, width of pipe, end of pipe in or out of bottom reservoir that modify the behaviour of the system but only gravity drives it.

            The limiting height for the hump is that the shorter limb is less than ~10m in the case of water

  3. Richie Hindle

    Thought experiment

    Here's a thought experiment: set a siphon going and then vary the force of gravity. The rate of flow will vary. Now vary the atmospheric pressure. The rate of flow will not vary (provided the pressure remains high enough to prevent a vacuum forming in the pipe).

    A siphon needs atmospheric pressure to prevent a vacuum forming in the pipe, but it's gravity that actually pulls the liquid through.

  4. Jamie Kitson


    Surely if the atmospheric pressure is the same at both ends then it is the gravity that is doing the sucking?

  5. Pete 2 Silver badge

    surface tension (as opposed to scientific tension)

    The crucial thing about siphons is that the liquid being drawn must be continuous - you can't siphon sand. It;s the continuity that holds the key. Once you have used (the lack of) air pressure to get liquid out of the container and over the hump and down to below the level in the container then yes, the gravity on the liquid _below_ the level in the container does draw it down.

    However, if it wasn't for surface tension holding the column of liquid together, all that would happen is that the bit of liquid you've sucked over the top of the siphon would fall down the tube and the rest would fall back into the original container. It the ST which ensures that once gravity has got a grip on the liquid in the tube which is below the level in the container, it will continue to be drawn out of the container and down the siphon.

    p.s. if scientists can't agree on what causes an effect Archimedes observed X thousand years ago, what hope have they got discovering fusion or researching climate change?

    1. Anonymous Coward
      Thumb Up

      I agree

      Even if you fluidise the sand there is no way it will climb a "primed" siphon tube in the promise of an energetic "reward". The "reward" is communicated by the surface tension.

      For the doubters:

      If i cut the end off a syringe, push it into vibrated, fluidised sand, and pull the plunger, does the sand rise up the syringe? - answer no, just a small swirl that is carried on the wind between the grains.

      1. wheel


        What about the air moving between the grains of sand?

        1. The old man from scene 24

          air moving between the grains

          Yeah, that would lead to losses. But with a high enough pressure, you could still get it to work - not as efficiently as water, though.

    2. Anonymous Coward
      Anonymous Coward

      sorry, second reply

      Just looked at the wikipedia which posits an interesting test case - when there is a bubble in the siphon. If the down-leg of the siphon is long enough it will draw out the bubble and run normally. Clearly there is no surface tension across the bubble, hence it is not required.

      That said, i cannot reconcile this with the previous sand argument.

      1. Pete 2 Silver badge

        bubbles do have surface tension

        >Clearly there is no surface tension across the bubble,

        Then what is it that makes the bubble? Surface tension acts on a drop of water to make it spherical (as that's the shape with the smallest area::volume ratio) since the surface tension will try to minimise the size of the droplet. Bubbles are the same shape for exactly the same reason - because they ARE formed by, and therefore have, surface tension at the air-bubble interface.

        In the down leg of a siphon, you have the weight of the water from the bottom of the tube to the bubble weighing down. then you have the bubble fully enclosing the width of the tube (otherwise it wouldn't be stable and would just float up) then you have the rest of the water column. The weight of the water below the bubble will expand the size (actually the length) of the bubble - which still holds together under surface tension, unless the weight of water below it is too great and "breaks" the surface tension - thus fracturing the bubble into smaller ones which then wouldn't fit the width of the tube and so would rise to the top of the siphon. As the weight of water increases the size of the bubble, the bubble's volume increases and it's internal pressure drops BUT the bubble still holds together due to the surface tension acting on it. The bubble still gets drawn down the siphon by gravity on the water below it and the surface tension keeping it as a single bubble. So I stand by my original point that it's the surface tension of the water which makes the siphon work. it acts like the "glue" which holds the water column together as it is drawn down the tube.

      2. Rattus Rattus

        reconciling with sand argument

        The reason the sand will not flow through a siphon is because air can move between the grains, equalising the pressure inside the tube with the pressure outside. (Well, the pressure in the tube does not drop in the first place, since there is already air present between the grains, below the level of the inlet and all through the tube. The air can enter the tube through both ends as the sand slides out.)

        With a liquid, there is no air below the inlet, and the liquid will prevent air from being drawn down into the upper container and from there into the tube. (Until the liquid level drops below the inlet level, of course.)

        Gravity certainly plays a role, but the air pressure is still the key.

        1. The old man from scene 24


          You are correct. In addition to your argument, sand has a much greater friction than water. In fact sand acts more like a solid in many cases than a liquid.

          But one could still get a siphon working with sand if the air pressure were high enough and the side of the tube smooth enough.

  6. David Cotterill

    Defintely Gravity

    In the Aquaponics world, we use siphons all the time. Its what drives our systems.

    The atmospheric pressure does not need to be present. Take two sealed containers, half full of liquid, with a connecting siphon and a vacuum. Now lower one. The siphon will start running. What matters is the pressure on both sides needs to be equal to make the siphon work. However it is NOT the driving force of the siphon. Gravity is. QED.

    1. Anonymous Coward

      not true

      As you evacuate the sealed system, a bubble will appear at the top of the previously full siphon tube, and this bubble will grow until the liquid level in both ends of the tube is the same as the surrounding liquid, minus some small capillary effects.

    2. John Gamble

      Under Pressure

      Well, yes, and that's pressure (the fact that it's not atmospheric pressure is irrelevant).

      I think the argument here is whether the start-up mechanism is relevant or not. I'm actually in agreement with most people here; I think it doesn't matter since the main thrust of the definition is what makes the water flow in the process, not what starts the process.

      So leave atmospheric pressure out of this everyone (including you, people from Colorado Springs)! It just confuses the explanation.

  7. Anonymous Coward
    Anonymous Coward

    my 2 cents

    when the atmosphere pressure on the cylinders is the same, then gravity what determines (i.e. balance) the liquid. Otherwise, the liquid will remain unbalanced once you tilt the cylinders (atmospheric pressure is the same, gravity in a different direction)

  8. password

    Dont just post articles from any idiot

    I can assure you that as long as both ends of a pipe are at similar pressures (the difference of say a half a meter from a car's fuel tank to the ground is approximately no differnce in pressure.

    The "Sucking on the hose" the author reffers to is in order to overcome the force of gravity not to create a pressure differential. It is possible (and indeed prefferable for many of the more unpleasant tasting liquids) to immerse the length of hose in the upper pool stick your thumb over the end of the pipe to hold the column of water in place then lower the end of the pipe and remove your thumb.

    Bingo your siphon will start to flow because you have a solid column of water and one end is in a lower gravitational energy state than the other.

    the act of sucking on the pipe is just a handy way to overcome the initial gravitational potential when it is not possible to immerse the hose.

    Asside from the difficulty of whatever liquid you used evaporating there would be nothing to stop you siphoning in a vacuum.

    Regards BSc Physics

    1. Anonymous Coward
      Anonymous Coward

      Not quite

      In a vacuum, there would be nothing stopping a "vacuum bubble" forming at the top of the hump and the liquid falling off in opposite directions. The siphon most likely wouldn't work.

      > the act of sucking on the pipe is just a handy way to overcome the initial gravitational

      > potential when it is not possible to immerse the hose.

      The sucking method would not work in a vacuum of course.

      And this is the reason why the siphon in a vacuum, has problems.

      The liquid in the longer drop is effectively sucking up the liquid in the shorter end because it there is more of it and therefore heavier.

      But it is still sucking.

      1. ElReg!comments!Pierre

        Yes quite

        Atmospheric pressure is required to hold the column(s) of liquid together and preventing it from collapsing, end of. The driving force is gravity.

    2. Anonymous Coward
      Thumb Up


      So right. The key is potential energy as provided by gravity, nothing else. Besides, siphoning in vacuum would actually be faster if one closed the source reservoir and let gas pressure help gravity, at the cost of some liquid.

      The worst thing about science is that it takes absolutely no thinking to bring the most horrible statements out in the world, but driving them back into the cave is hard work...

  9. Anonymous Coward

    Not a great week for Aussies

    Austrlians have had there physics and cricketing abilities tested and they've been found wanting.

    What next? Maybe a study confirming their beer is horsepiss?

    1. Rattus Rattus

      Depends whether you test export or domestic

      We are famous for exporting Foster's. If you test that you will undoubtedly find it is horse piss, at least on the days the emus hadn't been well. We export it just to get the damn stuff out of our country.

      On the other hand, if you test the stuff Australians actually drink you'll find it's better beer than anywhere in the world except perhaps Belgium and the Czech Republic.

  10. DavCrav

    Silly question...

    ...but isn't atmospheric pressure due to gravity in the first place?

  11. Steven Knox
    Paris Hilton

    Weighty Discussion

    From Robert Weaver:

    'Air "weighs" about 14.7 pounds per square inch of area on which it rests, including the surface of a liquid; this pressurizes the liquid to this amount.'

    From Wikipedia:

    "Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth's atmosphere."

    So let's assume that Weaver is correct and siphoning is caused by atmospheric pressure. What causes atmospheric pressure? Weight. What causes weight? Gravity. So what force drives this process, even if Weaver is correct?

    1. DavCrav

      Why do I get downvoted...

      ...and you don't, when we said the same thing? Boo, I question the consistency of people who downvote!

      Also, if your total downvotes exceed your upvotes, do you get shouted at by the Moderatrix or something?

  12. /\/\j17

    A little from column A, a little from column B?

    Isn't it actually a case of both, rather than one or the other?

    Gravity acts on the liquid in the open end of the pipe 'pulling' it out. This creates a lower pressure in the pipe vs. atmospheric pressure acting on the surface of the liquid in the reservoir 'pushing' it in to the pipe.

    Take away atmospheric pressure (seal the reservoir for example) and any liquid extracted via gravity will reduce pressure in the reservoir - and I doubt the syphon will keep going for long. Take gravity away (in space should do) and I doubt the liquid will have any inclination to go anywhere in particular.

    Or how about putting a syphon in one of those pilot training centrifuges? This will create multiple lateral G so the syphon should horizontally if it's just gravity!

    1. ElReg!comments!Pierre

      Only column A

      Sealing the reservoir would stop the flow at some point, but not until the pressure in the reservoir is significantly lower than the atmospheric pressure; that is, when the difference in pressure can counter the driving force, which is gravity.

      Pressure is still required to hold the liquid together, but a watertight tubing is required too. Is the watertight tubing the driving force?

      The siphon would actually work perfectly horizontally in a centrifuge, and you make a very good point. I should have thought of that.

  13. Richard Porter


    Indeed. Sucking is only one way to start a siphon, and immersing the whole pipe and putting your thumb over the end is another. A common type of lavatory cistern works by using a piston to start a siphon. The older type of high level cistern uses a heavy iron bell which is dropped over the down pipe to get the siphon going.

    In any case atmospheric pressure is greater at the lower end of the pipe so it plays no part in maintaining the flow (if anything it acts against the flow).

  14. RichyS

    Pounds? Inches? WTF?

    I'm going to ignore this bloke until he learns to use SI units.

  15. tas


    "operative force in a siphon"

    The keyword is operative. You do not need an atmospheric/gas pressure *differential* to operate, i.e. maintain the siphon. Whereas you do REQUIRE gravity.

  16. Chris Walton


    "Take away atmospheric pressure (seal the reservoir for example) and any liquid extracted via gravity will reduce pressure in the reservoir - and I doubt the syphon will keep going for long. Take gravity away (in space should do) and I doubt the liquid will have any inclination to go anywhere in particular."

    That isn't a fair test as the air pressure would work against the siphoning action, the question is does it drive siphoning.

    I am sure that the siphon won't work in a zero G environment though.

  17. Mike Bell

    Thought Experiment 2

    Imagine a coiled chain on a desk. Give the end a push and it will slip off the desk pulling the rest of the chain after it.

    Imagine the same chain, this time rising over a hump on the edge of the desk (with a little help from you) before it falls to the ground. It will still pull the rest of the chain with it - because of gravity - regardless of atmospheric pressure. If the gravity is too strong, however, or the chain is too weak, it will snap at the top of the hump.

    Now imagine a tube of liquid doing the same thing (perhaps contained in a hosepipe) and perhaps in a vacuum. Would it have enough tensile strength to stay in one piece, or would gravity break it apart at the top of the hump as it pulled it in either direction? That must depend on the mechanical properties of the liquid and the strength of the gravitational field. But if it didn't break apart, such a "siphon" would not require atmospheric pressure to operate. Only gravity.

    1. Rattus Rattus

      Siphons don't work like that

      You need both gravity and external pressure for a siphon. Surface tension is not strong enough to hold any substantial amount of liquid together like that. The pressure inside a siphon is maintained by the fluid being pushed from outside, not by the fluid pulling from inside.

      What you are describing with your chain analogy is capillary action, which will only draw until the mass of fluid is enough for gravity to overcome the strength of the surface tension.

  18. Jeff 11
    Dead Vulture

    Dear me

    You spent the whole article spelling it properly, then linked to "Syphon" on Wikipedia...

  19. Lars Silver badge

    And the winner is


  20. Ian Rogers

    Atmospheric pressure FTW

    It's atmospheric pressure that powers siphoning and obviously so.

    The pressure at the two liquid surfaces are equal, but now imagine travelling "up" the long pipe - the higher you go the lower the pressure will be due to the weight of the liquid below you acting against the surface pressure. When you're level with the end of the short pipe this difference in pressure is what drives the siphoning.

  21. Stevie


    All that is required for a syphon to work is that the column of water be continuous (though a certain bubble tolerance is possible depending on pipe geometry) The pressure can vary until the liquid boils or solidifies with no appreciable effect on the syphon itself.

    You *can* prime a syphon with pressure differentials, but you don't have to.

    I syphon my swimming pool cover (and one year the pool itself) by tossing the hosepipe into the water to be drained, filling the hose until water flows freely *onto* the cover, then disconnecting the hose from the tap and dropping the end on the driveway.

    If you connect two vessels with a pipe and change the pressure in one of them you don't have a syphon, you have a closed system in which the atmosphere is seeking to equalize pressure throughout. Any liquid in or around the end of the pipe will be forced through because it is in the way. You could force water uphill this way with little problem.

    Syphons only work when draining downhill.

    Because that's what water does when it is pulled by gravity.

    Experiment : Set two beakers such that one is 100 cm above the other. Place a quantity of water in the upper one and arrange a traditional inverted U tube syphon. Once the pipe is primed it works as expected.

    Now do the same experiment in a sealed environment at a reduced pressure, say 1/2 an atmosphere. You won't see any appreciable change in the flow rate.

    The OED is still full of snot. Better burn yours now before it gets you into more trouble.

  22. Lou Gosselin

    I thought reg readers were smarter than this!

    This type of authoritative feedback is shameful...

    "The atmospheric pressure does not need to be present."

    "Asside from the difficulty of whatever liquid you used evaporating there would be nothing to stop you siphoning in a vacuum."

    Despite the confidence of the authors, these quotes are very wrong.

    Gravity accounts for downward force in both legs. It's true that the weight of the liquid in the longer leg is greater than the weight in the shorter leg. However gravity is NOT the force which pushes (pulls?) the liquid up!!!

    Whether the siphon is started by sucking or any another means, gravity merely causes a vacuum to form at the top. If and only if the atmospheric pressure on the shorter leg is stronger than the force of gravity, then the liquid will be pushed up into the vacuum.

    Whether the atmospheric pressure is caused by gravity or some other means is not relevant to the functioning of the siphon.

    If the pressure is completely removed, then there will be a total vacuum both inside and outside the siphon. With no force to oppose gravity (ignoring capillary effects), then the force of gravity will pull the liquid down both legs with no siphon possible.

    I'm baffled as to why more people didn't learn this in high school physics.

    1. Lou Gosselin

      @I thought reg readers were smarter than this!

      Utterly shocked that the correct posts are getting down voted...

      The voice in my head keeps saying "if you explain it again, people will understand. Tell them to consider the sum of force vectors acting on each leg. Start with just the downwards force of gravity, and then with upwards force of atmospheric pressure. Now explain that water does not work like a chain since it has no significant tensile strength..."

      But I'm going to have to be content with the fact that some of us can understand basic physics, and some of us can't.

      I guess it's time for myth busters to come in and solve yet another elementary physics problem.

    2. Frumious Bandersnatch

      I thought reg readers were smarter than this!

      > gravity merely causes a vacuum to form at the top.

      No. Gravity would cause a *partial* vacuum to form. Or, more concretely, it reduces the pressure at the top of the siphon. If the pressure is low enough, the liquid will evaporate--it'll be more *like* a vacuum, since water vapour is less dense than liquid water, but still not a vacuum. Nature abhors those, apparently, and the effect is that water is pulled up from both legs to prevent this. Of course, if the water at the top *does* evaporate, then a new equilibrium is reached. This time, the vapour can expand (either by losing pressure, which a gas can do more easily than a liquid, or by causing more liquid to evaporate) much more than the pull of gravity down the long leg of the siphon can compensate for, and so no more liquid can flow past the hump.

      > If the pressure is completely removed, then there will be a total vacuum both inside and outside the siphon.

      No. There's no total vacuum in the tube for a start (it's why we call it a "vapo(u)r lock". And you've failed to account for the weight of water above the tube in the higher container.

      > I'm baffled as to why more people didn't learn this in high school physics

      It's probably something to do with the "state of education" nowadays. As it always was, probably.

      1. Lou Gosselin

        @Frumious Bandersnatch

        You're right about the partial vacuum, a "total vacuum" doesn't exist even in space. But my point was that the only reason a vacuum doesn't form is because of air pressure.

        "And you've failed to account for the weight of water above the tube in the higher container."

        The force caused by gravity on the water outside the tube cancels out the force on water inside the tube at the same height, therefor there is no net force difference for any of the liquid below water level.

  23. ElReg!comments!Pierre

    Atmospheric pressure and driving force

    The prof is still right. The driving force for the siphon is gravity, because that's what creates the difference in pressure. Of course the siphon won't work without atmospheric pressure, however pressure applies on both ends and is (or can be) canceled out. Actually you can have more atmospheric pressure in the receiving end and the siphon will still work provided the 'step' downwards provides sufficient energy; the driving force is really gravity. Atmospheric pressure is required, but so is the fluid, and the fluid is not the driving force, is it? Some sort of tubing is required too, but that tubing is not the driving force either.

    BTW the '32 feet' argument is true (for water), that is because atmospheric pressure is required to hold the column of fluid together. A bit like how the tube's wall must be watertight. No driving force there pal, sorry.

  24. Sean Nevin


    Two containers, one higher than the other, one filled with water and the other with just a little water. Say a few centimeters. Fill a hose with water and place one end in the higher container, the other in the lower UNDER the water already present.

    The atmospheric pressure will be equal on each end of the hose yet water will flow. Gravity does the work. The professor is correct and the dictionary was wrong. It happens...

  25. This post has been deleted by its author

    1. ElReg!comments!Pierre


      You just demonstrated that the liquid flows uphill in a siphon. Congrats. That should take care of humanity's need for energy: set up a siphon with a reservoir in the attic and the other in the basement; as the atmospheric pressure is higher in the basement the water will flow happily from there to the attic; now set up a tube with a turbine to bring the water back down in the basement, and yay! free leccy!

      1. Frumious Bandersnatch

        You just demonstrated that the liquid flows uphill in a siphon

        Of course it does. I figure you're not entirely serious, what with talk of free energy and such. But of course, this is only a local inversion of entropy. If you take the entire system into account, the overall entropy is increasing. Just like in any thermodynamic system (or hydrodynamic, since it applies to us ugly bags of mostly water too).

  26. Anonymous Coward


    This back-and-forward babbling of so-called intellects has gone on too long!

    For centuries, science has divided and joined around one theory or another. Waging petty little inkwell wars over the shape of a circle or the weight of an angel. A thousand little fatwas issued by tweed-suited, chemical-reeking chalk-dust overlords, to snuff out the career of one infidel intellectual or another!

    This has to stop!

    You are all wrong!

    Circle's are orange-shaped! Angels weigh 375grams at sea level, but less than that in heaven due to decreased gravity and increased levity!

    As for the siphon, the force of curiosity drives water into the siphon. The social dynamics of crowds in queues prevents it from turning around and leaving again, and the fear of tight spaces forces it to forge onwards to the bottom of the siphon.


  27. The BigYin

    Oh, FFS

    Get two reservoirs.

    Place them at different heights.

    Get a tube long enough to connect the reservoirs.

    Flood the tube in the upper reservoir.

    Bung one end.

    Carefully remove the bunged end of the tube (so the free end does not meet air).

    Place bunged end in lower reservoir.

    Remove bung.

    Be awed at the might of GRAVITY! No sucking required.

    Next week: Why an aeroplane on a tread mill does bloody well take off.

  28. Schultz


    The atmospheric pressure is almost the same at the top or bottom of the siphon. Ergo it has nothing to do with the driving force of the siphon (Mike bell put it nice with his thought experiment).

    So why did the Reg publicise poor Mr. Weavers ill reasoning so prominently? Physically challenged in the gravity department?

  29. Bounty


    Can you siphon ice?

  30. Jim 59

    rightish, but hoist by his own petard

    Hmm. There are subtleties here that deserve more care than the superficial treatment doled out by Weaver.

    Obviously you need atmospheric pressure for the sucking bit at the start. But that's a red herring. The siphon could also be started without an atmosphere, by a piston or propeller in the pipe.

    However I think that a siphon would not work in a vacuum. Without outside pressure on the water, voids (ie. bubbles of void, nothing) could freely open up within the water at any point in the pipe. The water in the downward leg could happily flow downwards, leaving the upward water where it is. Between them would be a void. The "downward" water could exert no force on the "upward" water to pull it over the lip.

    With an atmosphere, such voids cannot form because atmospheric pressure tends to close them up. Bubbles of air can form, of course, and can break the siphon if you are careless enough to let the required amount of air in. The siphon depends on the water remaining in one coherent flowing body.

    It might be possible to form a very weak/slow siphon in a vacuum. The self coherence of the water might be enough to stop voids forming and hold the water together if the tube were very narrow. But I think that would be capillary action, not a siphon.

    Which proves how idiotic it is staying up this late on a Tuesday morning to make my overblown points in the folorn hope that somebody somewhere will be interested. Still, I now feel confident that my toilet will not work on the moon, which is bound to save embarrassment sooner or later.

    1. rciafardone

      You forget only 1 thing

      Your reasoning is correct, but you forget that in a vacuum, water has the odd custom of freazzing and boiling at the same time!!! :D.

      Otherwhise it would work as you think.

      1. The First Dave


        No, water does not freeze in a vacuum, it _only_ boils. You are probably one of those idiots that think space is _really_ cold, and that is the only place to find a vacuum.

        What many people seem to miss is that _two_ things happen in a siphon - firstly gravity drags down any liquid that is past the bend. All things being equal, this creates a vacuum at the top. If the 'short' leg is more than about 33 feet high (on Earth) then that is the end of the story - nothing can suck water higher than that. All of this is entirely due to gravity, directly.

        The second part is that atmospheric pressure (which is itself merely a manifestation of Gravity) then acts like a set of old-fashioned balance scales, pushing the water up the short leg to balance out the weight of the column of air above it. If the top reservoir is covered with a bell jar, then the decrease in volume as the water leaves will eventually lower the pressure to such a degree that the syphoning will stop, despite the force of gravity remaining the same, proving definitively that air-pressure is a requirement for the syphon to work.

        Indeed, if you went the other way, and increased the atmospheric pressure of the _lower_ reservoir by enough, then it will come as little surprise that the syphon will 'reverse', pushing water uphill, against the force of gravity.

  31. Frumious Bandersnatch

    prof. from Colorado is wrong

    He says:

    > Air "weighs" about 14.7 pounds per square inch of area on which it rests, including the surface of a liquid; this pressurizes the liquid to this amount.

    Air pressure acting on the liquid in the reservoirs on *both sides* is equal. Actually, it's slightly higher on the lower one. You could even submerge both ends of the tube in the liquid and it would still work, though you will get reduced flow since the outflow tube will now be fighting against water pressure in the lower reservoir.

    The original pedant/boffin had the right idea in trying to explain water as a chain. They're maybe not his exact words, but it makes sense if you think about it like this... A previous poster here (Mike Bell) suggests an experiment where you have a physical chain dangling out of a drawer. Once you have enough of the chain dangling over the edge, it will pull the rest of the chain with it. Water is pretty much exactly like a chain, except that at lower temperatures it loses its chain-like nature and becomes a gas, so the individual molecules don't form chains. Once the pressure drops again, and the water condenses, at re-attaches to neighbouring molecules and you've got your chain links back again. It's slightly more complicated than that because there's an energy gap that needs to be crossed in changing state from liquid to gas

    The prof from Colorado is completely wrong if he thinks (and it would seem that he does) that atmospheric pressure is the operative force. See my example at the start to disprove that (ie, air pressure at the lower reservoir is *greater*, and the siphon works when both ends are submerged). Also, he completely discounts the much greater *water* pressure which operates on the higher reservoir.

    I've rambled on a bit more than I wanted. In summary, though, this is simply a case of hydrostatic equilibrium. You have to take into account all the forces:

    * gravity affects atmospheric pressure on both reservoirs

    * it also affects water pressure at the inlet and outlet (the weight of water above the tube openings)

    * it also affects the water in each leg of the tube

    * in all cases where gravity is in effect, you have to add the weight of all the air/water above; in each reservoir the "weight" of the water above it is simply proportional to the depth (ignoring any compression), while within the tube the higher up you go, the less pressure there is

    * if and only if the pressure at the top of the tube is not so low that a vapour lock forms, then gravity will naturally continue to draw fluid from the higher to the lower reservoir (aka, "water finds its level").

    I'll be heretical here and say that is follows from this that a siphon *can* actually work in a vacuum, provided there is gravity. It would need to be set up so that there was some liquid in both reservoirs to begin with. Depending on the rate of boil-off from the surface of the reservoirs, you would still get a siphoning effect, which would work until there wasn't enough *water pressure* (everyone seems to have completely forgotten about water pressure: it's much more important than atmospheric pressure!) in the upper reservoir to prevent an air lock from forming at the top of the tube, or there is no water pressure at all at the tube's ingress or egress (ie, the tube is just at the water level, which allows all the water on that side to flow down back into the reservoir). You could easily verify this by setting up weighing scales on both the reservoirs, and running the experiment once without the siphon being opened, the other with it open. If the weight of water in the lower reservoir increases or simply decreases at a slower rate (balanced by an increased rate of loss at the higher reservoir) then the experiment would prove that you *can* operate a siphon in a vacuum.

    I guess people don't study applied maths in schools these days.

    1. Jim 59


      A chain can bear tensile force, ie. you can pull on it. In order to pull on water, you must first put it in a sealed pipe and subject both ends to (atmospheric) pressure. Only under those kinds of conditions will it be chain-like, not in a vacuum.

      I agree with Waver that a siphon won't work in a vacuum. Not sure about his 32 foot limit though. The momentum and speed of the water may make that limit a bit higher.

      1. Frumious Bandersnatch


        > A chain can bear tensile force, ie. you can pull on it. In order to pull on water, you must first put it in a sealed pipe and subject both ends to (atmospheric) pressure. Only under those kinds of conditions will it be chain-like, not in a vacuum.

        I'd love to have read a post that could have taken what I'd written and shown, clearly, what was wrong with my reasoning. This was not it. I've explained that water in its liquid state can act like a chain, and also that this falls apart when the density falls low enough to form vapour bubbles. I've also described how water pressure plays a big part in the whole system, and this is completely independent of atmospheric pressure.

        I'll suggest an alternative experiment. This time connect two reservoirs, one higher than the other, with a piece of tube going from the bottom of the higher into the bottom of the lower one (the tube will just hang between them and will have a 'j' shape). Will water flow from the top to the bottom? Obviously it will. Will it work in a vacuum? Yes it will, because gravity is the only operational force. Now what's the difference between this experiment with an upright J tube and a siphon, with an inverted J? The difference is that at the top of the inverted J tube (aka, a siphon), the pressure is less than that in the upright J tube.

        I've already agreed with everyone that in a siphon, if the pressure at the top of the tube isn't enough to prevent the water from evaporating, then an air lock forms and it can't "pull" the water down the long leg (because it loses its chain-like nature, to put it back in metaphorical terms). But as you (and the other down-thumbing commenters) still seem to be missing is that the pressure at the top of the siphon depends not only on atmospheric pressure but on the weight of water above the tube's ingress/egress. The greater the weight of this water on the ingress tube, the higher the pressure on the water in the short leg of the siphon, and thus, the higher the top arc of the siphon can be relative the the ingress of the tube.

        I'll suggest one further thought experiment... between the upright J case (which I'm sure everyone can agree will work, even in a vacuum) and the inverted J case (a siphon, which is in dispute), we've got the case of two reservoirs, both initially of unequal height of water, connected with a horizontal tube. It seems fairly clear that water will indeed "find its level" in this system too, even in a vacuum, so we'll end up with water levels oscillating in both vessels, first with one higher, then the other, until both have boiled off. What the "no siphoning in a vacuum" argument is saying is that even if the tube has the slightest kink so that the highest point in the tube is above the highest point on the ingress/egress tubes, then no flow is possible. I submit that that idea is ridiculous.

        And the reason? Again, it all comes back to *water pressure* and simple hydrostatic equilibrium calculations. So long as there's a sufficient head of water above the ingress/egress points, there will be sufficient pressure within the tube to prevent an airlock forming.

        What say all you naysayers to that?

  32. Jim 59

    and another thing

    Just realised that several people above already expressed my argument and did it more concisely, the bastards.

  33. chris huygens

    Another Thought Experiment

    Consider two buckets of water connected by a hose - one higher than the other. Let us grant that atmospheric conditions similar to those on Earth exist: i.e. the atmospheric pressure at the higher bucket is lower than the atmospheric pressure at the lower bucket. Now, let us dispose of the so called "force of gravity" (which is really a general relativistic effect that none of us know enough math to speak about authoritatively anyway. Sorry Newton, but Leibniz invented calculus and you were completely wrong when you dreamed up some magical force that pulls matter together. At least you have a tasty fig bar named after you, though)

    So, now we have two buckets connected with a hose, and one bucket's water has more pressure pushing on it than the other. Which way does the water flow? I would think that it would flow from the area of higher pressure to the area of lower pressure. But wait! The longer column of water weighs more than the shorter one! Err... we supposed there was no "gravity" didn't we, so weight doesn't enter into this experiment.

    I just find it interesting that when gravity is removed from the situation, the siphon flows in the opposite direction.

    1. Disco-Legend-Zeke

      Newton Was Once...

      ...a tasty fig bar.

      Sadly, the bean counters at Kraft reduced the quantity of actual fruit in the product several years ago, yielding a fig-like jelly filling.

      It was once my favorite snack.

    2. Sean Hunter

      A lack of thought experiment

      1)Atmospheric conditions similar to earth cannot exist in the absence of atmosphere and there would be no atmosphere without gravity.

      2)Without gravity there's nothing to keep the water in the bucket.

      1. chris huygens

        Go back to the dark ages where you belong

        Sean Hunter thinks:

        "1)Atmospheric conditions similar to earth cannot exist in the absence of atmosphere and there would be no atmosphere without gravity.

        2)Without gravity there's nothing to keep the water in the bucket."

        Sean Hunter is wrong needs to take a physics class that teaches physics that is not dated. Atmospheric conditions similar to earth can indeed exist without gravity. For example, on an extremely long space ship traveling at a constant acceleration of , oh, I don't know, let's say 10 m/s/s relative to an inertial frame of reference. The air on that ship would be compressed by it's "inertia" in much the same way that the air in our atmosphere is compressed by "gravity", and the "inertia" of the water in the buckets at the end of this ship would keep it there just like "gravity" holds it there on earth.

        Let's all start arguing about whether or not it is "inertia" that is responsible for the phenomenon of the siphon. Or maybe it's bent space-time. Even more pointless of an argument.

        Here's a good one: go up into the Vomit Rocket and see if you can get a siphon going in a state of free-fall. "Gravity" is there doing it's thing, right? Will the siphon work? Does "gravity" just go away when you're in free fall?

        Of course, I know next to nothing about general relativity, but it is quite hilarious to read this back-and-forth about whether a fictitious force is responsible for a siphon or not. I suggest that we next discuss whether or not spontaneous generation is possible in a jar containing broth and phlogisticated air.

        I bet that if there is anyone out there who actually understands the fundamental physical principles of a siphon they are reading this all and thinking exactly what anonymous coward was thinking:


  34. paul 105


    With all of the discussion of a siphon. Tell me how a mile long pipe to the ocean floor can be called a "siphon" as the new media is wont to do.

  35. oopsie


    Unless the 'in' part of the syphon is right at the surface of the liquid (in which case, it ain't going to work for long anyways) surely the liquid above the tube's end point could be considered to be providing atmospheric pressure anyway?

    1. Frumious Bandersnatch


      > surely the liquid above the tube's end point could be considered to be providing atmospheric pressure anyway?

      Well, someone seems to have got this point. Except it's not "atmospheric" pressure. It's "water" pressure, but both are translatable into abstract "forces" so it doesn't really matter if the fluid is air or (liquid) water. See my posts above.

  36. Asiren


    To throw everyone off, surely it's the combination of gravity and SURFACE TENSION?

    Without surface tension, the liquid, whatever it might be, would just break at the top of the siphon and fall down seperately either side.

    It's why sand can't be siphoned, as the tension between each sand particle isn't enough to "pull" it up the pipe.


    1. Chemist

      Re : Actually..

      No it wouldn't necessarily - the column is being held there by air pressure.

      In the case of a siphon with wide tubes AND the long tube not being under the surface of the bottom reservoir THEN air could potentially enter and break the siphon. That could well be related to surface tension.

      There area number of mechanistic reasons for a siphon NOT working but the driving force is just gravity

  37. Lionel Baden

    Fuck It

    It powered by gravity but not possible without air pressure due to it using liquids !!!

    There solved now you can all shut the fuck up.

    yes he is technically right gravity is the driving force pressure is what makes a liqiud.

  38. Thomas 4

    This is awesome

    Watching Gravity fanbois* and Atmospheric Pressure fanbois** duke it out, while a small minority of Surface Tension supporters claim it's all about them at the end of the day.

    Completely unlike anything else on the forums!

    * also known as gravitards

    ** ummm....atmosfanbois? I could come up with.

    1. Frumious Bandersnatch


      Maybe "atmosphreaks"?

  39. pitagora
    Thumb Down

    Check out Bernoulli's Equation

    Guys! Check out Bernoulli's Equation, and look at the terms that it uses to calculate a siphon. Gravity is not a factor! density is, P is pressure, h is height.

    And by the way...siphons work in 0 gravity too :) The gravity however does create air pressure which in term drives the siphon, but it can work in the absence of gravity as well as long as the air pressure exists (i.e. artificially created)

    1. Chemist

      Re : Check out Bernoulli's Equation

      Bernoulli's equation for a siphon includes a term for gravity.

      Siphons do NOT work in zero-g

      Bernoulli's equ. for max. height of siphon is h=P/pg P=external pressure, g= acc. due to gravity, p=fluid density

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