TANK PRESSURE GAUGE

369

in tlie chamber to a definite datum level, and this is effected by working
the hand air pump which forces air into the chamber, the excess of
back pressure pushes the liquid back into the tank through a small
orifice at the datum level of the chamber from which air bubbles are
seen emerging in the diagram. This preliminary operation having been
performed the gauge scale will now record the depth of liquid in the tank.
The Principle is illustrated in Figure 10. The downward pressure
of the liquid increases with its depth, or height, and forces its way
through the orifice into the balance chamber. The entrapped air is
consequently forced back through the air pipe line to a cistern of mercury
at the gauge, the mercury being thus pushed back into the bore of the

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TRAPPED AIR IN BALANCE-CHAMBER
COMPRESSED BY HEAP OF LIQUID IN TANK.
Fig. 10.
tube. The length of the column of mercury in the tube corresponds
to the pressure of the liquid and the scale expresses this pressure in
feet and inches.
If water were substituted for mercury in the gauge glass it would
automatically find its own level, but a scale of the same depth dimen-
sions as the tank would be required, the air entrapped in the pipe
merely acting as a cushion and serving no useful purpose. But the
specific gravity of mercury is 13*59 so that it compresses into a column
13*59 times shorter than one of fresh water when subjected to the same
pressure, with a corresponding reduction in the length of the scale.
For example, in Figure 10, the head of water in the tank is 3 feet and