This is sometimes done with any leak-free hose to siphon gasoline from a motor vehicle's gasoline tank to an external tank. An external pump has to be applied to start the liquid flowing and prime the siphon. The train analogy is demonstrated in a "siphon-chain model" where a long chain on a pulley flows between two beakers.Ī plain tube can be used as a siphon. 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. In this analogy, atmospheric pressure holds the train together. What is not obvious is what holds the train together when the train is a liquid in a tube. So long as the valley is below the plain, the part of the train on the valley side of the hill will be longer than the part on the plain side of the hill, so the portion of the train sliding into the valley can pull the rest of the train up the hill and into the valley. For water at standard pressure, the maximum height is approximately 10 m (33 feet) for mercury it is 76 cm (30 inches).Īn 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. When the pressure exerted by the weight of the height of the column of liquid equals that of atmospheric pressure, a partial vacuum will form at the high point and the siphon effect is ended. 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. At the high point of the siphon, gravity tends to draw the liquid down in both directions, creating a partial vacuum. The maximum height of the intermediate point (the crest) is limited by atmospheric pressure and the density of the liquid. The siphon works because the ultimate drain point is lower than the reservoir and the flow of liquid out the drain point creates a partial vacuum in the tube such that liquid is drawn up out of the reservoir. Once started, a siphon requires no additional energy to keep the liquid flowing up and out of the reservoir. Liquids in vacuum are not in equilibrium and typically boil. In practice, atmospheric pressure is required, to maintain the cohesion of the liquid in the siphon. Furthermore, some (notably Encyclopedia Britannica) argue that theoretically, "a siphon will work in a vacuum". They argue that theoretically, internal molecular cohesion is sufficient to pull the liquid up the intake leg of the siphon to the crest. "The siphons were, apparently, flame-projectors, either hand-pumps or reservoirs worked by mechanical force-pumps".Īmong some physicists there is some dispute as to what causes the siphon to lift liquid from the upper reservoir to the crest of the siphon. "Some apparatus called a 'siphon' (σιφων) was used". It is not clear whether these were actual siphons or merely pumps that used air pressure to project the Greek fire. 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. The siphon was first used as a weapon by the Byzantine Navy, and the most common method of deployment was to emit Greek fire, a formula of burning oil, through a large bronze tube onto enemy ships. Even earlier Egyptian reliefs from 1500 BC depict siphons used to extract liquids from large storage jars. His student, Hero of Alexandria, wrote extensively about siphons in the treatise, Pneumatica. It is probable that Ctesibius was the discoverer of the principle of the siphon. 9 Explanation using Bernoulli's equation.6 Sample building code regulations regarding back siphonage.
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