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Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review 360

Posted by timothy
from the suck-it-oed dept.
knwny (2940129) writes "Peeved by the widespread misconception that siphons work because of atmospheric pressure, physics lecturer Dr. Stephen Hughes, [in 2010] wrote a mail to the prestigious Oxford English Dictionary(OED) pointing out the error. To back his claim, Dr.Hughes tested a siphon inside a hypobaric chamber to check if changes in atmospheric pressure had any effect on the siphon and demonstrated that gravity and not atmospheric pressure was the driving principle. [This week, the] paper detailing his experiment was published in Nature. The OED spokesperson responded saying that his suggestions would be taken into account during the next rewrite."
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Siphons Work Due To Gravity, Not Atmospheric Pressure: Now With Peer Review

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  • corrected link (Score:5, Informative)

    by Anonymous Coward on Friday April 25, 2014 @12:13PM (#46841685)

    This is the corrected link to the letter: http://www.theguardian.com/science/blog/2010/may/10/dictionary-definition-siphon-wrong

    • Re:corrected link (Score:5, Informative)

      by michelcolman (1208008) on Friday April 25, 2014 @03:31PM (#46843719)

      And if you look at that link, you'll see it's a letter from 2010. Which is yesterday by Slashdot definitions, of course.

      By the way, the letter was nicely debunked here:

      http://www.theregister.co.uk/2... [theregister.co.uk]

      Gravity is responsible for lowering the pressure in the outbound leg of the siphon, but you still need atmospheric pressure at the inlet to push the water over the top of the siphon. You can't siphon up more than about 10 m under normal atmospheric pressure.

      So in a way, it's not really wrong to say that atmospheric pressure is pushing the water over the top. Just like atmospheric pressure pushes liquid into a drinking straw as well. Would you say that a drinking straw has nothing to do with atmospheric pressure?

      Of course the exact value of atmospheric pressure doesn't matter much as long as it's enough, the flow rate will only depend on the difference in height between the two water levels, but without enough atomospheric pressure the siphon stops working. Which was clearly shown in the experiment described in Nature as well. In fact that experiment dispoves rather than proves his point.

  • Gravity! (Score:5, Funny)

    by JustOK (667959) on Friday April 25, 2014 @12:14PM (#46841697) Journal
    You win again, gravity!
  • by cpotoso (606303) on Friday April 25, 2014 @12:18PM (#46841753) Journal
    The 2012 Impact Factor for Scientific Reports is 2.927. For comparison, that of Nature is 38.597. Still impressive, but please lets be precise.
  • Does it suck or blow?

  • by Anonymous Coward on Friday April 25, 2014 @12:21PM (#46841795)

    Seriously? If atmospheric pressure had any influence, it would do the opposite: The pressure at the lower end of the tube is higher than at the other end, so the fluid would flow upward. Obviously this doesn't happen.

    • I think that aerostatic pressure differences are not the point here.
    • by mark-t (151149)

      If the pressure difference is great enough, then yes... it will flow upward. Have you ever used a straw? That's pulling liquid upward entirely through difference in air-pressure

      However, there is still a maximum height that can achieved even if the higher end were in complete vacuum, and for something like water, that distance is not even 35 feet (it's even less for heavier liquids, like mercury). The difference in atmospheric pressure with so little difference in altitude is not gong to be sufficient

      • by BitZtream (692029)

        That's pulling liquid upward entirely through difference in air-pressure

        Wrong. Thats atmospheric pressure PUSHING the liquid up through the straw due to the lower pressure in the part you're 'sucking' on.

        The reason you can't 'suck' water up past 30 feet or so is because the water column becomes equal to atmospheric pressure, so even when the upper end is in a 'vacuum', the atmospheric pressure is in equilibrium with it and it stops flowing upwards.

        You can not pull water up, only push.

        Likewise, those people who taught you that airplanes fly because a vacuum on the top edge of t

      • by danbert8 (1024253)

        Right, basically a siphon can only work if the pressure at the high point is above the vapor pressure of the liquid. If you pump a vacuum, then your water will vaporize which will kill your siphon. Gravity has nothing to do with it aside from being the driving force behind atmospheric pressure in the first place. As long as you have an unbroken liquid in your siphon, atmospheric pressure is what drives it. If "tension" between water bonds were what drove it, it would behave like a chain fountain and you wou

  • I'd be more impressed if the Oxnard English Dictionary accepted the change.
  • No doubt the confusion comes from the fact that raising water with a vacuum pump does require pressure. People learned some centuries ago that atmospheric pressure can't raise water more than about 10 feet. Simple siphons are commonly started with vacuum pumping.

    If the top of the siphon is too high for a vacuum pump, some other method must be used, but the siphon action will work at much greater heights because, as the article points out, the siphon action itself does not depend on pressure. What are the height limits, I wonder? Redwood trees are about as tall as trees can get with the capillary action method they use to raise water. I expect siphons work at much greater heights than that.

    • I believe the limit on height is the pressure at which water turns from liquid to a gas at the ambient temperature. If it were to remain liquid at all pressures, then the water column could be lifted the height where the weight of the water equals the pressure of the atmosphere (which would be roughly 33' at STP).

    • Actually, no. The water will only go up the high leg of the siphon up to the height that the atmospheric pressure can support and no higher unless driven by a pump.

      So let's say you use a pump to start the siphon. While you are pumping, the water flows (but that's not a siphon). Now, shut the pump down. The water on in the high leg will fall back to about 33 feet (the height it would reach in a water barometer). Meanwhile, the water in the lower leg will drain out until it is also at 33 feet.

      It is an interes

      • Actually, no. The water will only go up the high leg of the siphon up to the height that the atmospheric pressure can support and no higher unless driven by a
        That is nonsense.

        The water will go as high at *your* end as high it is at *the other end*. So if I have the other end 1000m above your place (like a pipe coming down from a lake in the mountain), it will either spray out at your end roughly 1000m high, or you can simply attach a pipe and feed it into a roughly 1000m high building ... or other lake. T

    • If the top of the siphon is too high for a vacuum pump, some other method must be used, but the siphon action will work at much greater heights because, as the article points out, the siphon action itself does not depend on pressure. What are the height limits, I wonder? Redwood trees are about as tall as trees can get with the capillary action method they use to raise water. I expect siphons work at much greater heights than that.

      Just because you call your tall u-shaped tube a "siphon" does not mean it will behave differently than the tall u-shaped tube someone else calls a "barometer". Once your siphon hump is more than about 10 *meters* (10.3m or about 34 feet) high, the water falls down on each side of the hump, leaving a vacuum (with some water vapour) at the op. The air pressure sets a limit on the height of both suction pumps and siphons.

  • Plot twist: (Score:5, Informative)

    by LordLimecat (1103839) on Friday April 25, 2014 @12:45PM (#46842071)

    Atmospheric pressure is actually due to gravity.

    • by HaeMaker (221642)
      True, but would a siphon work in orbit where there is artificial atmospheric pressure but microgravity?
      • It does work, with some caveats. I was about to post that it wouldn't, and then remembered the first shuttle payload I worked on : http://istd.gsfc.nasa.gov/cryo... [nasa.gov] which transferred helium between to dewars without pumps in a microgravity enuvronment. It's been a couple of decades, and I wasn't on the principal investigators team (I was carrier support), so I don't remember the details of how the transfer worked.

        • SUPERFLUIDS DON'T COUNT.

          (going back to rocking in the corner, hallucinating about my boat sinking into the ocean of superfluid helium as the stuff crawls up over the gunwales and flows down into the hold...)

    • I know you know this, but just to be sure no one else gets confused: A causes B and A causes C does not imply that B causes C. So for instance falling rocks are also caused by gravity but they don't have anything to do with siphons.

  • Well, obviously, since it is all due to gravity.
  • Huzzah! If only my high school physics teacher was still alive. We frequently argued this point.

  • Who didn't understand that siphons used gravity to move fluids?
  • Of course it's both forces (pressure and gravity). This is simply a pedantic attack at the way the dictionary defines the process.

    Dictionary definition:

    "A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe."

    This definition is correct as atmospheric pressure differences start the process. However the dictionary doesn't explain that gravity eventually takes over. Dr. Hughes sums up:

    As any petrol thief knows, to get the liquid over the "hump" of the tube you have to suck the other end or, more pedantically, lower the pressure in your lungs to beneath atmospheric pressure by expanding them. Once the liquid has passed the highest point in the tube, the continuous chain of cohesive bonds between the liquid molecules in the tube, and the force of gravity, do the rest.

    • by j-beda (85386)

      Of course it's both forces (pressure and gravity). This is simply a pedantic attack at the way the dictionary defines the process.

      Dictionary definition:

      "A pipe or tube of glass, metal or other material, bent so that one leg is longer than the other, and used for drawing off liquids by means of atmospheric pressure, which forces the liquid up the shorter leg and over the bend in the pipe."

      This definition is correct as atmospheric pressure differences start the process. However the dictionary doesn't explain that gravity eventually takes over. Dr. Hughes sums up:

      As any petrol thief knows, to get the liquid over the "hump" of the tube you have to suck the other end or, more pedantically, lower the pressure in your lungs to beneath atmospheric pressure by expanding them. Once the liquid has passed the highest point in the tube, the continuous chain of cohesive bonds between the liquid molecules in the tube, and the force of gravity, do the rest.

      Gravity is acting as a suction pump, which requires the air pressure to push the water - get rid of the air pressure and you can't "suck" the stuff up in the first place or keep it flowing. The "cohesive bonds between the liquid molecules" are pretty darn weak compared to the forces involved in stealing "petrol" or siphoning most other fluids. Under normal atmospheric pressure you cannot siphon over a hump of about 10m.

  • Since he had to go to some length describing the troubles he had because the low pressure formed bubbles due to cavitation, etc. (remember he could not perform this at zero atmospheric pressure because the water would boil), why use water?

    Why not use a liquid that will not boil in a vacuum, like (I think) mercury? That would very easily prove that atmospheric pressure is not required to make a siphon work (because there's no atmosphere!).

    Take a flexible tube and dunk it in a bucket filled with mercury lett

  • by Toad-san (64810) on Friday April 25, 2014 @01:37PM (#46842639)

    http://www.britannica.com/EBch... [britannica.com]

    "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."

    • by readin (838620)
      Would the cohesive forces hold the weight regardless of the diameter of the tube?
  • None of the physics is new. Just good old newtonian pohysics. So why not write out the equations of motion and see exactly what is happening?

  • by jmv (93421)

    ...a 1.5 m high siphon was set up in a hypobaric chamber to explore siphon behaviour in a low-pressure environment. When the pressure in the chamber was reduced to about 0.18 atmospheres...

    Atmospheric pressure isn't enough, but it's still required. In this experiment, 0.18 atmosphere is just enough for (in theory) a 1.8 meter siphon, had the guy attempted to get it to work at 2 meters, it would have failed because the atmospheric pressure needs to be high enough to hold the column of liquid.

  • The article says: "how could a siphon possibly work by a difference in pressure when atmospheric pressure is the same for the liquid at both ends of the tube?" It does work by a difference in pressure, just not a difference in atmospheric pressure. The liquid falling out of the exit end of the siphon causes a difference in pressure.
  • Gravity pulls one side down; pressure pushes the other side up.

  • it holds the water column together (so gravity can act on it), but to say that it is driven by air pressure makes no sense, because if I am not mistaken (?) air pressure should be greater at lower altitudes (no?), so that it should push it up the hose, which it presumably does to a minor extent.

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