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

Posted by timothy on 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."

6 of 360 comments (clear)

  1. corrected link by Anonymous Coward · · Score: 5, Informative

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

    1. Re:corrected link by michelcolman · · Score: 5, Informative

      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...

      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.

  2. Gravity! by JustOK · · Score: 5, Funny

    You win again, gravity!

    --
    rewriting history since 2109
  3. Re:Actually it's both. by TechyImmigrant · · Score: 5, Funny

    >A straw with a hole in it cannot siphon.

    A straw has two holes in it.

    A straw with only one hole can't siphon.

    --
    I should use this sig to advertise my book ISBN-13 : 978-1501515132.
  4. Plot twist: by LordLimecat · · Score: 5, Informative

    Atmospheric pressure is actually due to gravity.

  5. Re:Actually it's both. by michelcolman · · Score: 5, Informative

    Nope. Strange how many people get this wrong, it's really not that complicated.

    The water doesn't work like a chain, the cohesion of water is only just enough to hold a drop of water together, certainly not enough to pull a whole column of water along through a siphon. The motion is caused by gravity BUT atmospheric pressure is needed as well (as shown in the actual experiment that was referenced in the Slashdot summary and described in more detail in Nature). Here's how a siphon acually works:

    Suppose you have a source reservoir and a destination water reservoir, with the water level of the destination lower than that of the source. The reservoirs are connected by a tube that goes from the source reservoir up to an apex above both water levels and then down into the destination reservoir. The tube is filled with water (you have to start the siphon somehow by filling it with water before it can work).

    Now, if you would calculate the pressure at the apex starting from the inlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the source reservoir level. On the other hand, if you calculate the pressure at the apex starting from the outlet, it should be equal to atmospheric pressure MINUS the water pressure from the difference in height between the apex and the destination water level. If the destination water level is lower, the latter value for the pressure at the apex is lower than the former. Of course there can only be one pressure at the apex, which will be in between these two pressures. It is lower than what you would expect when calculating from the inlet, and higher than what you would expect when calculating from the outlet, so the pressure gradient will suck water in from the inlet and push it out of the outlet.

    But note the two times I wrote "MINUS" in bold capital letters. You can't go below zero pressure. When the atmospheric pressure is too low to push the water from the source reservoir up to the apex, the siphon breaks up.

    That's exactly what happened in the experiment described in Nature. They tested it with a 1.5 meter siphon in a pressure chamber. The water in the siphon broke up when they reduced pressure to below 0.18 atmosphere, which makes perfect sense because at that point the pressure at the apex would start to approach zero. The siphon actually turned into a double barometer with vacuum (or a bit of water vapour, actually) in between.

    So yes, the motion is caused by gravity but you DO need atmospheric pressure or it simply won't work. In fact, if you look at it a certain way, it's not even wrong to say that atmospheric pressure is pushing the water up to the apex and therefore making the siphon work.