The word siphon (from Ancient Greek: σίφων "pipe, tube", also called syphon) is used to refer to a wide variety of devices that involve the flow of liquids through tubes. By definition the word refers specifically to a tube in an inverted 'U' shape, which causes a liquid to flow upward above the surface of a reservoir, with no pump, but powered by the fall of the liquid as it flows down the tube under the pull of gravity, and discharging at a level lower than the surface of the reservoir whence it came.
There are two leading theories about how siphons cause liquid to flow uphill, against gravity, without being pumped, and powered only by gravity. The traditional theory for centuries was that gravity pulling the liquid down on the exit side of the siphon resulted in reduced pressure at the top of the siphon. Then atmospheric pressure was able to push the liquid from the upper reservoir up into the reduced pressure at the top of the siphon, like in a barometer or drinking straw, and then over. However, it has been demonstrated that siphons can operate in a vacuum and to heights exceeding the barometric height of the liquid. Consequently, the cohesion tension theory of siphon operation, where the liquid is pulled over the siphon in a way similar to the chain model, has been advocated. It need not be one theory or the other that is correct, but rather both theories may be correct in different circumstances of ambient pressure. The atmospheric pressure with gravity theory obviously cannot explain siphons in vacuum, where there is no significant atmospheric pressure. But the cohesion tension with gravity theory cannot explain CO2 gas siphons, siphons working despite bubbles, and the flying droplet siphon, where gases do not exert significant pulling forces, and liquids not in contact cannot exert a cohesive tension force.
All known published theories in modern times recognize Bernoulli's equation as a decent approximation to idealized, friction-free siphon operation.
- 1 History
- 2 Theory
- 3 Modern research into the operation of the siphon
- 4 Practical requirements
- 5 Applications and terminology
- 6 Devices that are not true siphons
- 7 In nature
- 8 Explanation using Bernoulli's equation
- 9 Vacuum siphons
- 10 Oxford English Dictionary
- 11 See also
- 12 References
- 13 External links
In the 9th century, the Banu Musa brothers invented a double-concentric siphon, which they described in their Book of Ingenious Devices. The edition edited by Hill includes an analysis of the double-concentric siphon.
Siphons were studied further in the 17th century, in the context of suction pumps (and the recently developed vacuum pumps), particularly with an eye to understanding the maximum height of pumps (and siphons) and the apparent vacuum at the top of early barometers. This was initially explained by Galileo Galilei via the theory of horror vacui ("nature abhors a vacuum"), which dates to Aristotle, and which Galileo restated as resintenza del vacuo, but this was subsequently disproved by later workers, notably Evangelista Torricelli and Blaise Pascal – see barometer: history.
A practical siphon, operating at typical atmospheric pressures and tube heights, works because gravity pulling down on the taller column of liquid leaves reduced pressure at the top of the siphon (formally, hydrostatic pressure when the liquid is not moving). This reduced pressure at the top means gravity pulling down on the shorter column of liquid is not sufficient to keep the liquid stationary against the atmospheric pressure pushing it up into the reduced pressure zone at the top of the siphon. So the liquid flows from the higher pressure area of the upper reservoir, up to the lower pressure zone at the top of the siphon, over the top, and then with the help of gravity and a taller column of liquid, down to the higher pressure zone at the exit.
The chain model is a useful but not completely accurate conceptual model of a siphon. The chain model helps to understand how a siphon can cause liquid to flow uphill, powered only by the downward force of gravity