Scales of pressure measurement

Fluid pressures can be measured with reference to any arbitrary datum. The common datum are

1. Absolute zero pressure.
2. Local atmospheric pressure

When absolute zero (complete vacuum) is used as a datum, the pressure difference is called an absolute pressure, P _abs .

When the pressure difference is measured either above or below local atmospheric pressure, P _local , as a datum, it is called the gauge pressure. Local atmospheric pressure can be measured by mercury barometer.

At sea level, under normal conditions, the atmospheric pressure is approximately 101.043 kPa.

As illustrated in figure( Fig : L -7.2),

                  When P_abs < P_local

                  P _gauge = P _local - P _abs                                     L - 7.12

Note that if the absolute pressure is below the local pressure then the pressure difference is known as vacuum suction pressure.