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.

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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

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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