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 (Pa). A pressure of 1 Pa is small; it approximately equals the pressure exerted by a dollar bill resting flat on a table. Everyday pressures are often stated in kilopascals (1 kPa = 1000 Pa).

The SI unit for pressure is the Pascal (Pa), equal to one Newton per square meter (N/m2 or kg·m−1·s−2).Non-SI measures such as pounds per square inch and bars are used in some parts of the world, primarily in the United States of America. Pressure is sometimes expressed in grams-force/cm2 or as kg/cm2 and the like without properly identifying the force units. But using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as units of force is expressly forbidden in SI.



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.

1 Patm equivalent to:

1.01325 Bar

14.69595 psi

101 kN/m2

101.325 Kpa

760 mmHg

10 mH20

At sea level, the gases and liquids that make up the atmosphere, exerts a pressure of approximately 14.69psi or1.03 Kg/cm2. To distinguish this, two specific pressure units are PSIG and PSIA. PSIG or pounds per square inch gauge is referred to atmospheric pressure. PSIA or pounds per square inch absolute is referred to absolute zero. Barometer is standard instrument to measure atmospheric pressure.

Atmospheric pressure at sea level is also called barometric pressure.

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


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


Vacuum pressure is the pressure less than atmospheric pressure. Vacuum is often measured in inches of water (inH2O) or mercury (inHg). A vacuum is a lack of air fluid. 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.


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

Absolute pressure = gauge pressure + atmospheric 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 is defined as the pressure exerted by fluids at rest. Static pressure is independent of kinetic energy of fluid.


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.


Common units of pressure are Pascal (N/m2), Bar, Kgf/cm2, PSI (lbf/in2), Torr (mmHg) and mmH2O. Pressure less than 1 PSI are normally considered as low pressure and low pressure are usually calibrated in inches of water (1PSI = 27.7 inH2O).


Factors affecting liquid pressure are

· Height of the liquid

· Density of the liquid

· Specific gravity of the liquid

· Pressure exerted on the top of the liquid


Specific gravity is a reference number that compares the weight of specific volume of a liquid to the same volume of a liquid to the same volume of water at 60 C and a specific volume of gas at specific temperature to the same volume of air at same temperature. Water and air are assigned specific gravity constant 1.

Specific gravity of some liquids

Liquid Specific gravity

Ethyl Alcohol - 0.7939

Kerosene - 0.8200

Benzene - 0.8794

Water - 1.0000

Ethylene glycol - 1.1155 -

Glycerin - 1.2600

Mercury - 13.5700

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