Pressure and types of pressure


Pressure is force per unit area applied in a direction perpendicular to the surface of an object. Pressure is measured in any unit of force divided by any unit of area. The SI unit of pressure is (the Newton per square meter) which is called the Pascal ¶.

For liquids, the formula may be written:



Atmospheric pressure is the pressure measure on the surface in the atmosphere. However, the difficulty is that it changes with altitude and humidity. Thus, atmospheric pressure may differ from one area or place to another.Atmospheric pressure at sea level is also called barometric pressure.

Standard atmospheric pressure at sea level = 760mmHg = 29.91inHg = 14.696 PSI.

GAUGE PRESSURE Gauge pressure is the pressure above atmospheric pressure. Hence, the zero of the gauge pressure scale depends on the atmospheric pressure at that point. Sometimes it is called internal pressure.

P gauge = P abs – P atm

ABSOLUTE PRESSURE Absolute pressure is the pressure measured with respect to zero pressure (vacuum).

Absolute pressure = gauge pressure + atmospheric pressure.

VACUUM PRESSURE Vacuum pressure is the pressure less than atmospheric pressure. Vacuum is often measured in inches of water (inH2O) or mercury (inHg).The vacuum scale extend between the absolute zero reference point and atmospheric pressure, thus it is not a positive pressure. It is treated as a sucking force or negative force.

DIFFERENTIAL PRESSURE Differential pressure is the difference in pressure measurement taken at two related points. It is calculating by subtracting the lower port pressure reading from the higher port pressure reading. E.g. Between unknown pressure and a local atmospheric pressure.

STATIC PRESSURE Static pressure is defined as the pressure exerted by fluids at rest. Static pressure is independent of kinetic energy of fluid.

DYNAMIC PRESSURE Dynamic pressure is the pressure above static pressure that results from the transformation of fluid kinetic energy into potential energy. Dynamic pressure is the pressure above static pressure caused by the movement of fluid.

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