Would a siphon work in a vacuum?

11 April 2010


Splashing water



Would a siphon work in a vacuum?


Dave Ansell answered this question...

The simple answer to that is, no, it wouldn't work. The general way that siphons work in general is that, essentially, you've got air pressure pushing down on both ends of your pipe. One end is higher than the other, then that can push - push it on both sides, push the water up to the top and then it carries and going.

If there wasn't any air pressure, then you could form a bubble at the top of the tube and that bubble would expand and expand until the two levels of water are same level as that their two tanks.

Okay, so that's the simple answer. The somewhat more complicated answer is that water has something called cohesion. The water molecules do stick together as related to the surface tension. And, therefore, as long as you don't form any bubbles, water can actually be pulled and actually will stay stuck together even at a negative pressure, even at zero pressure, or even as you can pull it apart, which is the reason why trees can actually go higher than about 30 meters which is the air pressure which will push the water up, or 10 meters, which push the water up. And so even with a very, very thin tube and until you get a bubble, it would work for a bit but not for very long.


We now know that siphons do indeed work in a vacuum!

So many physicists and physics teachers got this wrong so fret not. The very well-respected PLOS One gives a good writeup here: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0153055

In summary we have discovered that atmospheric pressure is irrelevant to the operation of a siphon, which actually works based upon molecular cohesion and gravity. Yes a siphon will work in space. However, a low atmosphere would let your fluid boil away, so to see this effect you need to use an ionic liquid, which doesn't boil under low pressure.

In general, with large tubes a syphon can't work in a vacuum. This is because if the pressure on a liquid goes below its vapour pressure it is favorable for it to boil. This means that a bubble forms at the top of the tube. The bubble will grow as gaseous water vapour fills it, until the pressure returns to the vapour pressure of the liquid. If the pressure at the top of the tube is too low, the column of water is unstable.

So if you are trying to syphon in a vacuum you are starting at zero pressure, and then effectively pulling on either side of the top of the syphon, so it is definitely energetically unstable, you are creating a negative pressure in the tube.

A negative pressure in a gas is impossible, because this would mean a gas was pulling inwards on the walls of its container. As a gas is made of small particles bouncing off the sides, during the bounce they can't pull, they can only push. If you reduce the pressure on a gas, it will expand until it meets something to contain it.

However this is not true of solids and also liquids. The molecules stick to each other, so they can support tension. This is obviously true in a piece of string, but it is also true in a bubble-free tube of water. So it can support a tension and the syphon will work.

But as soon as a bubble forms the bubble will expand until the pressure increases above the vapour pressure of the liquid again, so breaking the syphon.

I can't find any mention of VASI technology. And no mention of your work and negative pressures.

I have looked at your linked-in page
Which seems to refer to storing energy by compressing air in a tube by pushing water into the bottom. Storing energy by compressing air has certainly been considered and used. It has a variety of disadvantages:

To store much energy you need very large very high pressure vessels, which are expensive and if they fail very dangerous. The energy density, whilst higher than pumped water storage, isn't that high. When you compress the air it heats up a lot, if you leave the gas hot, its pressure is even higher and so harder to contain. If you let the air cool to room temperature, when it expands again it will get very cold, and not expand to its original size, and so it doesn't release as much energy as you put in.

eg if you compress air to 1/10th of its original size starting at room temperature, it will heat up to over 1000C which is hard to deal with. If you let it cool to room temperature and then let it expand again you will only retrieve about 25% of the energy you put in before the mechanical losses.

It is possible to use compressed air for this purpose, but you have to do clever things involving storing heat separately and then returning it to the gas as it expands, or using waste heat from a gas turbine.

[Please note that my original answer was given live on air, so I may not have explained the concepts outlined here as clearly in the first instance.]

Your answer is misleading. You are ignorant about the effect of negative atmospheric pressure to a positive atm. Anyway I can't blame you. You did not research well about the topic. I've done some research about the negative and positive atms and I've actually came to a conclusion that siphon works in a vacuum perfectly. In fact vacuum plus siphon can create a mechanical force that can generate electricity. I called this VASI Technology.

To anybody reading Ruben's comment, he gives away his quack status in the closing two sentences so please ignore.

Add a comment