So, I was thinking about how siphons worked today, and no matter how high or low the tube goes, as long as the end is lower than the start, water will flow. So, if you took a hose, ran it up 50 feet, and then back down to the ground, it would seem like water would flow through that.

However, when I performed some calculations, it looked like after going up for ~33 feet (equivalent of 1 atmosphere when you go down), the hose would be at a complete vacuum pressure.

This would mean that the water would boil off, like in space. Alternatively, when you go up more than 33 feet, would you be at negative pressure relative to vacuum? This intuitively and mathematically does not make sense.

So, how can we resolve this discrepancy?

  • $\begingroup$ Speaking in terms of absolute pressure, there is no negative pressure (below a hard vacuum). And yes ~ 33 ft is your limitation (1 atm). Just like the inverted barometer tube with the evacuated space above the mercury column, you will start to see an evacuated space above the water. Will the siphon still work? - yes. Our atmosphere just limits your working pressure to one atmosphere of pressure! $\endgroup$
    – docscience
    Aug 13 '15 at 0:19
  • $\begingroup$ ... and by the way the evacuated space is not really evacuated - not a hard vaccum. In either case (mercury or water) you will have gaseous particles at the vapor pressure. $\endgroup$
    – docscience
    Aug 13 '15 at 0:22
  • $\begingroup$ Exactly. This is an important principle in designing pumping systems. If you put a pump at the top of the system and try to pump up a distance over about 10 m (what are these "ft" you two are talking about?) then you just create a near vacuum at the top of the pipe and no water gets pumped. So, instead you put an immersion pump at the bottom of the pipe and push the water up. The pressure inside the pipe just above the pump is higher than the pressure outside the pipe. $\endgroup$ Aug 13 '15 at 0:39
  • $\begingroup$ @gleedadswell, all joking aside, So, if you took a tube that was, say, 100 feet long, filled it with water, put both ends into a bucket, and lowered it, you would effectively have gases/steam on the top, and water at the bottom. $\endgroup$ Aug 13 '15 at 0:44
  • $\begingroup$ @docscience, you say that the siphon will still work, even if there is an evacuated space. What happens to the water as it passes through that space? Does it boil off on one side, and recondense on the other? $\endgroup$ Aug 13 '15 at 0:46

Yeah, not really... You CAN have negative absolute pressure, just not in ideal gases. Other materials have cohesion and can sustain a metastable state under negative absolute pressures.

See for instance the 2016 PLOS One paper where they show a water siphon working at 15m: Negative Pressures and the First Water Siphon Taller than 10.33 Meters or http://dx.doi.org/10.1371/journal.pone.0153055

Or the 1902 short article on Science (unfortunatelly behind a paywall) where they show a Hg siphon working at 70cm (local atmosferic pressure 61cmHg): ON THE SIPHON http://dx.doi.org/10.1126/science.15.369.152

  • $\begingroup$ You may wish to include more of the information from the link in your answer in case the link goes dead. $\endgroup$ Feb 21 '17 at 0:00
  • 1
    $\begingroup$ Thanks, I have included a short description os the experiments and put the DOI links to the papers, which are supposed to have long term support. $\endgroup$ Feb 22 '17 at 13:26

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