Liquid level measuring devices are classified into two groups: (a) direct method,
and (b) inferred method. An example of the direct method is the dipstick in your
car which measures the height of the oil in the oil pan. An example of the
inferred method is a pressure gauge at the bottom of a tank which measures the
hydrostatic head pressure from the height of the liquid.
IDENTIFY the principle of operation of the following
types of level instrumentation:
Differential pressure (DP)
A very simple means by which liquid level is measured in a vessel is by the gauge glass method
(Figure 1). In the gauge glass method, a transparent tube is attached to the bottom and top (top
connection not needed in a tank open to atmosphere) of the tank that is monitored. The height
of the liquid in the tube will be equal to the height of water in the tank.
Figure 1 Transparent Tube