[Kurt Larsen]

Kurt Larsen  asked the Naked Scientists:
   
Hi Chris,
 
Thanks for putting on a great show.  I catch the podcast weekly.
 
My question is about using a siphon.  I was taught the system works due to both gravity, but also the force of air pushing down on the surface of the liquid in the higher container enabling it to move through into the lower container. 

I wanted to know if this would work on the International Space Station?  Is there enough air pressure to force the liquid through the tube and, if there is, could you conceivably siphon in any direction.
 
Thanks again for the show,
Kurt Larsen.

What do you think?

[MikeS]

I,m no expert but I  don't think a syphon works because of air pressure.  After all, air pressure is lower toward the ground because of gravity. 

If the space station were rotating to produce artificial gravity through centrifugal (or centripetal?) force then yes it would work.  (If you dip one end of a flexible hose in a bucket of water and whirl the other end around it will spray water.)

If the space station were in orbit but not rotating then it would not work.

[graham.d]

Not right Mike. Siphons do indeed work because of air pressure and air pressure is higher at ground level (not lower) as I feel sure you know. Have another beer :-)

A siphon would work fine in the ISS provided it was in a pressurised compartment of the craft.

[MikeS]

Not right Mike. Siphons do indeed work because of air pressure and air pressure is higher at ground level (not lower) as I feel sure you know. Have another beer :-)

A siphon would work fine in the ISS provided it was in a pressurised compartment of the craft.

Hi Graham,
So you noticed my intentional mistake 'ahem' [and that was before a beer]. Yes, of course pressure is higher at ground level due to gravity. (Isn't it amazing how many times you can read what you have written and still not notice a glaring mistake?)

Laboratory tests have shown that a syphon can work in a vacuum due to the tensile strength of the liquid as it is gravity that powers the syphon.  Air pressure simply stops the column of liquid from potentially separating, it has nothing to do with what powers a syphon.

"A syphon is a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir, without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity, and is discharged at a level lower than the surface of the reservoir."
http://en.wikipedia.org/wiki/Siphon

[JP]

Not right Mike. Siphons do indeed work because of air pressure and air pressure is higher at ground level (not lower) as I feel sure you know. Have another beer :-)

So here's a question: assuming you somehow prevent the liquid from boiling away, would a siphon work on the earth, but in a vacuum?

[Geezer]

Not right Mike. Siphons do indeed work because of air pressure and air pressure is higher at ground level (not lower) as I feel sure you know. Have another beer :-)

So here's a question: assuming you somehow prevent the liquid from boiling away, would a siphon work on the earth, but in a vacuum?

Yes, but you wouldn't be able to suck on the end of the hose to get it started.

[JP]

Not right Mike. Siphons do indeed work because of air pressure and air pressure is higher at ground level (not lower) as I feel sure you know. Have another beer :-)

So here's a question: assuming you somehow prevent the liquid from boiling away, would a siphon work on the earth, but in a vacuum?

Yes, but you wouldn't be able to suck on the end of the hose to get it started.

You could blow on the hose, though.

[CliffordK]

I'm going to say no to the siphon in weightlessness.  For one thing, if you had an open tub, you couldn't even guarantee the liquid would stay in the bottom of the tub, or even in the tub at all.

You could use a pressure differential to move liquid.  But, again, if you had a rigid container, you would have troubles forcing the water to stay in the path of the hose intake.

If you had a flexible container, or rigid container with a bladder, you could use pressure, but not gravity to force the water one direction or the other.

[JP]

But to the original question, it's been experimentally tested, and it's gravity, not pressure that makes a siphon work.  Water likes to flow downhill, and water in a tube likes to maintain a continuous flow.  The result is that as water gets pulled out the bottom, fresh water gets drawn in at the top.  Because it's a confusing topic, a lot of experiments have been done in vacuum to check if a siphon still works, and it does:
http://www.mindspring.com/~rwramette/nokes.pdf

This means that it wouldn't work in a weightless environment, unless you pressurized one end of the system.  But in that case, you'd really just have a hose, not a siphon.

[Geezer]

You probably don't even need a vacuum. As long as the air pressures at both ends of the hose are always the same, you'd get an identical result (he said very quickly hoping nobody would notice that he might be bluffing.)

[syhprum]

With virtually no gravity there is no reason for the liquid to move.
Air pressure is needed to cope with the up portion of the siphon but unless the "up" section was more than 10 meters the pressure in the ISS would surfice although the inherent stickiness of water means that sometimes more than 10 m is possible 

[rosy]

The air pressure is essential to allow the water to fill the hose as it goes up on its way out of the upper container, because the air pressure is what's holding the water up against gravity. Otherwise it'd cavitate and the water would find its own level in the container (in as far as it wasn't boiling off, of course..).

The siphon is driven entirely by gravity... the water is in a lower energy position at the bottom of the siphon than at the top, so if it can get out of the upper container then it will, but without the air pressure keeping the hose full it's got no means of escaping.

[rosy]

Sorry, didn't read syphrum's post properly. Could just have said "what syphrum said"...

[graham.d]

I realise my mistake was in not actually understanding what a siphon was! I was thinking of what was called a soda siphon (for diluting a perfectly good scotch). Yes, of course it works by gravity although as Geezer says, sucking the tube to get it going could be tricky with no atmosphere.