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Subject: Re: trying to siphone water from a reservoir

Date: Fri Oct 27 11:09:00 2000
Posted by Andrew Wilson
Position: Faculty, Invention Theory, Wilson & Co.


The reason your siphon failed is probably because the
length of the pipe is to short on the exhaust end.

The length should be long enough to outweigh the water being drawn out, up to the apex of the levy (the highest point)

Make your pipe longer on the drain side, and it should work fine.

Andrew Wilson


Admin note:

John Lindsay adds the following alternative answer:

The water flows into the intake end of a siphon because it is PUSHED in by the weight of the atmosphere pushing down on the surface of the reservoir.

When the pipe is initially filled with water and the ends of the pipe are opened under water, the pipe empties downward on both sides, creating a low-pressure area at the top bend of the pipe. The difference in pressure between the top of the pipe and the atmospheric pressure pushing down on the reservoir is what moves the water up the pipe and, one hopes, over the ridge. BUT, this pressure difference is limited by the pressure of the atmosphere. In the best case, the pressure at the top of the pipe might be zero. In which case the pressure difference will be equal to the atmospheric pressure, or about 14.7 PSI (1.03 Kg/sqCm). But the column of water being pushed up the pipe has weight, so that limited pressure can't push the water higher than a certain height, which is about 34 feet (10.3 meters). Our farmer's ridge is 14 meters higher than his reservoir level, so no can do. The water will rise 10 meter in the pipe and then just sit there, 4 meters short of the top of the ridge.

The only solution to the problem of moving the water over the ridge is a pump to provide sufficient mechanically-supplied pressure to push the water up 14 meters, about 22.5 PSI (1.58 Kg/Cm) The pump needs to be located well below 10.0 meters above the reservoir. It's inlet has the same limitation as the siphon's.

John Lindsay, United AirLines Flight Simulator Engineering


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