So how does a siphon really work?

[james ozzie]

A local physicist in Australia claims to have corrected a very old error in the Oxford Dictionary which apparently states a siphon works on atmospheric pressure. Not so, he says, gravity is what powers a siphon. This raised some disagreements, so maybe you smart Pruners can offer some thoughts?

My own thought is imagine running a siphon in a large chamber and steadily evacuating it (to remove the atmosphere). At some point the water will vapour lock in the siphon, will it not? Is there a "perfect fluid" that could be used instead? Intuitively, a siphon would not work in weightless environment as there is no difference between the upper and lower reservoir (QED?)

So my money says that it needs gravity to move the fluid and pressure is needed to keep it fluid. Any offers?

[brooksjg]

imagine running a siphon in a large chamber and steadily evacuating it
Yup - the water will boil and confuse your results!

Can't see why anyone thought syphons were anything to do with air pressure....

I think this thread needs to stop now before Perpetual Motion machines make an appearance....:):):)

[Old Smokey]

This is not really one for Tech Log, but Gravity is the answer, and ambient atmospheric pressure required to keep the liquid in a liquid state.

In the weightless conditions of space, but in a pressurised capsule, siphoning will not work (the fluid would also become "free floating" bubbles).

Try siphoning a liquid from one container to another in normal atmospheric pressure. Works fine when the first container is held above the other lower vessel. Now raise the lower vessel above the first, and the fluid will transfer in the opposite direction.

Really, this one is for the "Questions" forum.

Regards,

Old Smokey

[rudderrudderrat]

Hi,

With a pressure difference of just one atmosphere, you can pump a vertical column of water to about 35 feet, or a column of mercury to 29.9 inches.

Gravity is definitely the motive force in the syphon, but the maximum working differential pressure of the "pump" (from surface of higher container to top of syphon tube) is limited to one atmosphere.

[Dan Winterland]

Forget the theory. It sucks!

[StallBoy]

Definately GRAVITY:ok:, now would anyone like to hear about my perpetual motion machine :eek:that runs on air pressure, gravity and flying ducks...:)

[Dont Hang Up]

Sorry but the good old Oxford Dictionary has it correct.

A siphon will not work in a vacuum. Yes gravity provides the motive force, but the lift requires ambient pressure to push the liquid upwards (against gravity).

No ambient pressure - no lift - no siphon.

Rudderrudderrat is quite correct with a maximum lift (for water) around 35 feet at sea level. At FL350 it would be less than 10 feet. And in a vaccum it would be zero.

If you can't get past "the water would boil" issue then think of doing it with mercury instead. But then the lift is less by a factor of about 10 (but still zero in a vacuum).

[Checkboard]

My own thought is imagine running a siphon in a large chamber and steadily evacuating it (to remove the atmosphere). At some point the water will vapour lock in the siphon, will it not?

It depends upon the liquid. For a Mercury siphon operating under these conditions - the mercury will vaporise from the exposed surface, but the mercury within the siphon tube will have sufficient pressure from the liquid itself to avoid boiling.

Apart from that, the siphon works just like a slinky (http://mattogno.com/MattognoGirls/wp-content/uploads/2008/11/slinky2.jpg), or if you were to link together a chain of paper clips and place the chain in a paper cup. Take up one end of the chain and lay it over the edge of the cup, pull the chain so that the free end hangs down below the cup - once you have enough weight hanging down, the force of gravity on the hanging chain will pull the rest of the paper clips out of the cup.

[Dont Hang Up]

Apart from that, the siphon works just like a slinky (http://mattogno.com/MattognoGirls/wp-content/uploads/2008/11/slinky2.jpg), or if you were to link together a chain of paper clips and place the chain in a paper cup. Take up one end of the chain and lay it over the edge of the cup, pull the chain so that the free end hangs down below the cup - once you have enough weight hanging down, the force of gravity on the hanging chain will pull the rest of the paper clips out of the cup.


Not really. The slinky is a solid so each part can exert a force on the adjacent parts. Not so for a liquid. There is no (significant) tensile strength with which to "pull" more water through.

Gravity will tend to create a void at the highest point of the siphon tube. Ambient pressure (provided the lift is not too great) will push liquid uphill to fill this void.

Without ambient pressure the siphon tube will simply empty itself.

Yes gravity is the motive force but that is the 'no brainer' part of the equation (we already know that the liquid will pour out if we make a hole in the bottom of the container). The clever bit of the siphon is that the liquid starts its journey going uphill. A phenomenon that owes itself entirely to atmospheric pressure as the Oxford Dictionary so correctly states.

[Piltdown Man]

Atmospheric pressure plays no part in the performance of syphons save for the premise that both surfaces are exposed to the same atmospheric system, be it high pressure, low pressure or even a vacuum. Gravity and gravity only makes a syphon. Once you change the pressures over the surface, the action becomes that of a pump.

PM

[Dont Hang Up]

According to Wikipedia...

The maximum height of the [siphon] crest is limited by atmospheric pressure, the density of the liquid, and its vapour pressure

[tocamak]

Surely it must all be down to gravity; after all where does the atmospheric pressure come from if not the effect of the earths gravity exerting its force on the air molecules (and of course vice versa).

[Jimmy Macintosh]

I'm with Don't Hang Up on this one.

Take it to an extreme, you have a vessel of water on one side of a wall that is 36' tall. On the other side you have a receiving vessel that is 10' lower than the first vessel.

This will not siphon. Once you've managed to suck* the water to crest the wall the water in the tube will drain to both sides again.

Lower the wall to 20', it will siphon, as atmospheric pressure is pushing the water up over the wall allowing gravity to pull the fluid through the rest of its journey.

Dependent on the fluid and the pressure depends on how high one can make the 'wall'

Both are required in my opinion.

(Good point that gravity is why there is atmospheric pressure in the first place though...which does make it purely based on gravity breaking it down to first principles)

*for sake of brevity, suck was used but if you wish...once you've managed to reduce the pressure on one side of the tube sufficiently to have atmospheric pressure on the other side force the water through the tube...(LM, I don't recall that level of pedantry when you taught me for the ATPL's :))

[Pugilistic Animus]

Watched a classic film where Marilyn Chambers was explaining to the poor male motorist the action of of siphon but the poor girl got distracted before she could finish:ouch:



:}:}:}

[Lightning Mate]

managed to suck the water

Er....sorry, but there is no such thing as suction.

Do you mean a depression?

(unless of course you are talking about an entirely different subject.......)

[Pugilistic Animus]

Quite simply:


w= ∫ρgAhdh} h2,h1


:)

[barit1]

Dont Hang Up and (shudder) wikipedia have it right. Think mercury barometer. Think water barometer (13.6 times as high).

But the flow is driven by Δh. So both gravity and atmospheric pressure are required.

[Agaricus bisporus]

This gets my nomination for;

"Aviation's Most Pointless and Irrelevant Thread of the Year"

(see the natural sucessor to this thread on Jet Blast.)

Well, you asked for it!!!

[Pugilistic Animus]

just for those who like accuracy, I forgot to add an 'h',as a result, I wrote the pressure integral but I meant the work integral... as work, is a path function...why ''work function' the work of lifting a fluid to a height 'h'? because the question was "How does a siphon really 'work'" :}

get it:}

[barstow]

james ozzie, there are a plethora of boards out there devoted solely to physics.

Why you feel the need to start pilots/ground staff/etc. arguing physics issues is way beyond me.

Hence Agaricus bisporus' apt reply, and one of my maxims:

First step if you ever thinking about entering in an argument with a physicist: make sure you're a physicist... :cool:

And another: Wikipedia is no substitute for a 4 year degree.

[Pugilistic Animus]

P =F/A and F=mg,....the mass of the column of liquid is ρ*Volume and
Volume = Ah

so, F =mg =ρAgh then pressure at the base of the column = F/A =ρAgh/A = ρgh

:8

[cwatters]

The Physicist was Stephen Hughes. One of his papers is here...

http://eprints.qut.edu.au/31098/8/31098a.pdf

Some of it was a total surprise to me. Here is an extract...

Another seeming ubiquitous misconception is that the maximum height of a siphon is dependent on atmospheric pressure. The maximum height of a water siphon actually depends on the tensile strength of water – i.e. the maximum weight that hydrogen bonds are able to support.

[Mr Optimistic]

you realise that without an atmosphere you wouldn't have liquid water and without gravity you wouldn't have pressure (because there would be no atmosphere ).

[james ozzie]

Oh dear oh dear, I seem to be guilty of the most unthinkable discourtesy to our serious readers! My humblest apologies to Barstow and the clever boy who does the Big Red Letters!

Yes, as was pointed out in the beginning, this could have been posted elsewhere. But isn't it surprising that some are able to find the time and inclination to flame me for it? Perhaps they can ask a moderator to delete it if it is so offensive?

But I am heartened by the more cerebral posters out there who have made thoughtful contributions.

And Barstow, perhaps you could add this to your collection of maxims:

"University degrees are only important to those who don't have one"

(Yeah, yeah, been there, done that, 4 years too...)

[Smudger]

My brain hurts... goodnight all... and thank you...

[PappyJ]

Fluid moving via Siphon = gravity. Fluid moving via Vacuum = PRESSURE. Basic physics.