Australian Scientists Discover That You Can Build A Siphon Way Taller Than We Thought

You Can Build a Siphon Way Taller Than We Thought

If you've ever siphoned petrol from a tank, you'll know that the height between the source and the destination of the liquid is important. But scientists have shown that the height limit of a siphon is much bigger than we thought. The maximum height of a siphon is usually thought to be defined by the atmospheric pressure of wherever you happen to be siphoning. In a paper published in Scientific Reports, researchers from the Queensland University of Technology explain why that's the case:

This limit arises because the pressure in a siphon above the upper reservoir level is below the ambient pressure, and when the height of a siphon approaches 10 meters [10m], the pressure at the crown of the siphon falls below the vapour pressure of water causing water to boil breaking the column.

And when the column breaks, your siphon stops working. Typically, at sea level, that happens when the siphon reaches 10m in height. But what the Australian team realised was that the "boiling" they refer to — perhaps less confusingly referred to as cavitation, because it's more the formation of bubbles because of lower pressure rather than anything to do with heating — is a result of excess gas in the water inside a siphon. As the pressure changes, dissolved gas forms into bubbles and then disaster strikes.

So they carried out a set of experiments using degassed water instead of the usual stuff that comes out of your tap. They left water under a vacuum for more than three weeks, forcing large quantities of gas out of it — then they tested out how that affected the ability to create a taller siphon.

Their results show that it's possible to create a siphon that is 15m tall which operates quite happily at seal level. That's because, according to the researchers, the degassing of the water prevents the cavitation from occurring: Without gas in the liquid, which can form bubbles as the pressure changes, the water's essentially "stronger" and much harder to tear apart.

The researchers speculate that their small experiment suggests that the limit can be pushed further still — at least to 91m and maybe even beyond. "If tensions as high as the transient tension of several 100 bar can be maintained at the apex of a siphon," they write, "then in principle a siphon should work up to a height of several kilometres. However, it would be challenging to verify this experimentally, requiring a helicopter or UAV."

Sounds like a challenge.

[Scientific Reports]

Image by Morphart Creation/Shutterstock


    "requiring a helicopter or UAV"
    Sounds like someones making a funding pitch...

      Yeah, not sure why you can't just use a balloon.

    I'm sure I've siphoned using a 20 metre garden hose before. I was draining a pond and laid the hose down a hill.

      They're talking about how high the hose can reach above the source, not how far along the ground. I'm not sure why you'd need a UAV or helicopter to test their theories though. I'd have thought they could just run a pipe over a 91m high mountain.

        I think they mean for the "height of several kilometres". A mountain could work in that case too but you'd need a looooooong hose :)

    "Their results show that it’s possible to create a siphon that is 15m tall which operates quite happily at seal level."

    seal level? :-)

      Don't tell me you've never watched a Steven Seagal movie?

        Boom tish

      *brakes into song singing kiss from a rose by seal*

    And the most important bit is the last sentence of the abstract:

    This experiment provides conclusive evidence that siphons operate through gravity and molecular cohesion.

    Some people still think syphons (this is the proper Australian spelling, not sure why QUT is using American spelling) work via air pressure.

    An extension on Stephen Hughes work from prior to 2010, where he got the Oxford Dictionary to correct their definition of a Siphon (it's a latin derived word, the english spelling with a 'y' appears to be a linguistic anomaly).

    I think one problem in people's thinking of these simple tube structures, is that in order to start a static siphon (with no control valves at the inlet) one must "suck" the water up to the top of the tube and over the hump (a foot valve obviates this necessity).

    Standard Atmospheric Pressure is 101.3 kPa (1013 hPa) at mean sea level, (14.696 Lbf/in^2)

    One cubic centimetre of pure water = ~ 1g
    10 (1000cm) metre column of water (1cm in sectional area) ~= 1000g = 1kg

    1013 hpa = 1013 grams per square cm. = 1033.22 cm of water (10.33m)

    The clear connection between atmospheric pressure and a siphon, is that you cannot SUCK the water more than 10.33m up and over the hump (to get it started), as one has difficulty generating less than 0Pa of pressure in order to "pull" the water up the 'incline".

    Once the flow is established, under normal conditions dissolved gases present the initial limit to the minimum pressure which will maintain a flow state (once the flow path between liquid levels is broken by vapour, the siphoning effect will stop).
    The gases were dissolved into the liquid at atmospheric pressure, they will leave the water at reduced pressures (relating to gas pressure and surface tension in water), and their vapour pressure will be a lot higher than that of the water itself.

    The Vapour pressure of water at 20deg.C is 2.3 kPa = 23 hPa, so one would think that it would boil / cavitate / vaporise at this pressure (0.023bar ~= 10.09mH20) if it was totally gas free, though if cooler than 20deg C, the rate of vaporisation will decrease, as long as the water is flowing, low quantity of vaporisation could be expected.

    It is interesting that their cohesion experiments have lead to a much higher theoretical siphon height. Please note that it is unlikely to get large quantities of degassed water which one would have a need to siphon more over heights greater than 10 m, I guess a pump to get the liquid up the incline will be needed.
    (I remember S.Hughes saying back in 2010, that the "tensile strength" of the substance to be siphoned is the limiting factor, showing that a chain or other flexible strong object will readily "siphon" with very great "siphoning heights" (h) if the initial conditions are satisfied.)

    We have much to learn from trees, which appear to "siphon" water to heights well above 15m every day. (Search for articles indicating that stomal pressure is minus several atmospheres of pressure, a feature not thought theoretically or physically possible, like temperature pressure scales only go to zero.)

    Atmospheric pressure does not appear to be a specific limiting factor to a functioning siphon, though it may well be a limiting factor to getting a siphon to operate (without the correct tools).

    Last edited 30/01/16 4:26 pm

      Thanks for this great explanation - it's simultaneously comprehensive and clear.

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