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To measure gauge pressure, atmospheric pressure is subjected to one side of the system
and the pressure to be measured is subjected to the other. The result is that the differen-
tial (gauge pressure) is measured. A tire pressure gauge is a common example of this
type of device.
Recall that as the level of submergence increases (in an incompressible fluid), the
pressure increases linearly. Also, recall that changes in weather cause the absolute
atmospheric pressure to change. In water, the absolute pressure Pa at some level of
depth (d) is given as follows (see Figure 2):
Pa = Patm + kd
where k is simply a constant (i.e.: 2.307 ft of water = 1 PSI)
Figure 2: Pressure Diagram, Detail "A"
INW’s standard gauge submersible pressure devices utilize a vent tube in the cable to
allow the device to reference atmospheric pressure. The resulting gauge pressure
measurement reflects only the depth of submergence. That is, the net pressure on the
diaphragm (Figure 2) is due entirely to the depth of submergence.
