Siphon

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A siphon is a continuous tube that allows liquid to drain from a reservoir through a intermediate point that is higher than the reservoir. Once started, no additional energy is required to keep the liquid flowing up and out of the reservoir. The siphon works because the ultimate drain point is lower than the reservoir and the flow of liquid out the drain point creates a vacuum in the tube such that liquid is drawn up out of the reservoir.

The maximum height of the intermediate point is limited by atmospheric pressure and the density of the liquid. At the high point of the siphon, gravity tends to draw the liquid down in both directions creating a vacuum. Atmospheric pressure on the top surface of the higher reservoir is transmitted through the liquid in the reservoir and up the siphon tube and prevents a vacuum from forming. When the pressure exerted by the weight of the height of the column of liquid equals that of atmospheric pressure, a vacuum will form at the high point and the siphon effect ended. For water at standard pressure, the maximum height is approximately 33 feet; for mercury it is 30 inches.

An analogy to understand siphons is to imagine a long, frictionless train extending from a plain, up a hill and then down the hill into a valley below the plain. So long as part of the train extends into the valley below the plain, it is "intuitively obvious" that the portion of the train sliding into the valley can pull the rest of the train up the hill and into the valley. What is not obvious is what holds the train together when the train is a liquid in a tube. In this analogy, atmospheric pressure holds the train together. Once the force of gravity on the couplings between the cars of the train going up the hill exceeds that of atmospheric pressure, the coupling breaks and the train falls apart.

The speed of flow through the siphon is driven by the height difference between the higher reservoir and the drain point. The height of the intermediate high point is not relevant[1].

Where:

v = velocity

g = gravitational constant (9.8 m/s²)

h = height between the drain point and the upper surface of the higher reservoir

An inverted siphon is not a siphon but a term applied to pipes that must dip below an obstruction to form a "U" shaped flow path. At no point does the siphon effect come into play; an inverted siphon will work fine in the absence of atmospheric pressure. Liquid flowing in one end simply forces liquid up and out the other end. Engineers must ensure that the flow rate in such a channel is fast enough to keep suspended solids from settling. Otherwise, the inverted siphon tends to act as a debris trap. This is especially important in sewage systems which must be routed under rivers or other deep obstructions.

Backsiphon is a plumbing term applied to clean water pipes that connect directly into a reservoir without an air gap. As water is delivered to other areas of the plumbing system at a lower level, the siphon effect will tend to siphon water back out of the reservior. This may result in contaimination of the water in the pipes. Anti-siphon valves[1] are required in such designs. Backsiphon is not to be confused with backflow. Backsiphon is a result of liquids at a lower level drawing water from a higher level. Backflow is driven entirely by pressure in the reservoir itself. Backflow cannot occur through an intermediate high-point. Backsiphon can flow through in intermediate high-point and is thus much more difficult to guard against.

A siphon barometer is the term sometimes applied to the simplest of mercury barometers. A continuous U-shaped tube of the same diameter throughout is sealed on one end and filled with mercury. When placed into the upright position, mercury will flow away from the sealed end, forming a vacuum, until balanced by atmospheric pressure on the other end. The term "siphon" is used because the same principle of atmospheric pressure acting on a fluid is applied. The difference in height of the fluid between the two arms of the U-shaped tube is the same as the maximum intermediate height of a siphon. When used to measure pressures other than atmospheric pressure, a siphon barometer is sometimes called a siphon gauge and not to be confused with a siphon rain gauge. Siphon pressure gauges are rarely used today.

A siphon bottle is a pressurized bottle with a vent an valve. Pressure within the bottle drives the liquid up and out a tube. It is a siphon in the sense that pressure drives the liquid through a tube. A siphon bottle is sometimes called a gasogene[1] or even more rarely, a siphoid.

A siphon cup is the reservoir of paint attached to a spray gun. This is to distinguish it from gravity-fed reservoirs. An archaic use of the term is a cup of oil in which the oil is siphoned out of the cup via a cotton wick or tube to a surface to be lubricated.

A siphon rain gauge is a rain gauge that can record rainfall over an extended period. A siphon is used to automatically empty the gauge. It is often simply called a "siphon gauge" and is not to be confused with a siphon pressure gauge.

A plain tube can be used as a siphon. An external pump has to be applied to start the liquid flowing. This can be a human mouth and lungs. This is sometimes done with any leak-free hose to siphon gasoline from a motor vehicle gasoline tank to an external tank. If the tube is flooded with liquid before part of the tube is raised over the intermediate high point and care is taken to keep the tube flooded while it is being raised, no pump is required. Devices sold as siphons come with a siphon pump to start the siphon process.

It is tolerably certain that Ctesibius was the discoverer of the principle of the siphon.[1]

The siphon was first used as a weapon by the Byzantine Navy, and the most common method of deployment was to emit a formula of burning oil through a large bronze tube onto enemy ships. Usually the mixture would be stored in heated, pressurized barrels and projected through the tube by some sort of pump while the operators were sheltered behind large iron shields.

A siphon is also the organ of some mollusks used to draw water in and out of the animal.