Pressure measurement is a very common requirement for most industrial process control
systems and many different types of pressure-sensing and pressure-measurement
systems are available. However, before considering these in detail, it is important to
explain some terms used in pressure measurement and to define the difference between
absolute pressure, gauge pressure and differential pressure.
Absolute pressure: This is the difference between the pressure of the fluid and the
absolute zero of pressure.
Gauge pressure: This describes the difference between the pressure of a fluid
and atmospheric pressure. Absolute and gauge pressure are therefore related by the
expression:
Absolute pressure D Gauge pressure C Atmospheric pressure
Thus, gauge pressure varies as the atmospheric pressure changes and is therefore not
a fixed quantity.