The term self-siphon is used in a number of ways. In practice atmospheric pressure is required. Furthermore, some argue that theoretically a siphon will operate 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 chamber needs to be occasionally primed again with liquid to remove the gas.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 ( ). At the crest the gas can be trapped in a chamber above the crest. Conversely, the outlet leg needs to have a steep slope to allow the bubbles to move against the liquid flow. The flow of the liquid moves bubbles thus the intake leg can have a shallow slope as the flow will push the gas bubbles to the crest. Local high points will trap gas so the intake and outlet legs should have continuous slopes without intermediate high points. The longer the liquid is in the siphon, the more gas is released, so a shorter siphon overall helps. Higher temperature accelerates the release of gas from liquids so maintaining a constant, low temperature helps. The siphon itself will exacerbate the problem because as the liquid is raised through the siphon, the pressure drops, causing dissolved gases within the liquid to be "degassed". The gas tends to accumulate at the crest and if enough accumulates to break the flow of liquid, the siphon stops working. Gas in the liquid is a concern in large siphons ( ). Alternatively the siphon may be primed by a pump at either the intake or outlet. If intakes and outlets are submerged, a vacuum pump may be applied at the crest to prime the siphon. The siphon may be primed by closing the intake and outlets and filling the siphon at the crest. Their size requires control via valves at the intake, outlet and crest of the siphon. Large siphons may be used in municipal waterworks and industry. Devices sold as siphons come with a siphon pump to start the siphon process. 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. 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 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. For water at standard pressure, the maximum height is approximately 33 feet (10 m) for mercury it is 30 inches (76 cm).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 vacuum will form at the high point and the siphon effect 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 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 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.
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