Extension Leaflet 311
1950
FARM WATER SYSTEMS
Automatic Air Volume Control
By Paul E. Johnson—Agricultural Engineering
FREQUENT STOPPING and starting of the farm
water pump can be eliminated by using a pressure
tank. The tank cushions the pump discharge and protects the pressure switch from unnecessary pulsations.
This, in turn, compensates for the leaking at the
various water outlets which causes the pump to stop
and start too often.
Water is not compressible; therefore, the useful
capacity of the pressure tank depends upon the proportion of air in the tank. A volume of air is compressed in the upper part of the pressure tank. As
the air expands, it drives water out of the tank with
a  gradual receding pressure.
In pressure tanks, the volume of air governs the
reserve supply of water available from the tank. Automatic air volume control maintains the correct relation
between air volume and quantity of water. Under
pressure the water gradually absorbs the air. The
higher the pressure, the more rapid the absorption.
Therefore, if water is drained from the tank some air
leaves with it. If air is not replenished the system
becomes "water logged".
A water logged system operates as though there were
no pressure tank. A small water leak from an outlet
will cause the motor to start and stop frequently.
Draining the tank completely and refilling it or pumping additional air into it will correct a water logged
system. Some systems have fittings so air may be
pumped into the tank with a tire pump.
Sometimes an air leak in the upper part of the tank
will allow air to escape faster than it comes in. The
plug in the top of the tank and any other fitting should
be air tight. Many air leaks can be located by filling
the tank with air and applying oil or soap suds to
the various fittings. It is possible to get too much air
in the tank so that large bubbles are carried into the
piping systems This will cause noise and spitting at
the faucets. Automatic air volume control does away
with the troubles and should be provided in every
water system to insure satisfactory operation.
For best performance, the tank should carry about
one-third air and two- thirds water at top operating
pressure. Under these conditions about one-fifth of
the total tank volume can be withdrawn between 40
pounds pressure and 20 pounds pressure.
The float and the diaphragm type of air volume
control are commonly used on farm water systems,
Two distinct kinds of float type controls are used. One
of them controls the amount of water admitted to
the tank and is designed for shallow well and deep
well jet pumps (see figure 1). This type of control
can be identified by the copper tube which connects
the control to the pump. The float raises along with
the water level in the tank. This opens the air valve
which allows air to pass into the copper tube and
into the suction side of the pump. The air mixes with
the water and is forced into the tank through the
discharge of the pump.