What is pressure?

Pressure is the force that is perpendicular to the surface divided by the area it is effecting. So pressure equals force per area, or p = F / A.

There are a large number of different pressure units used around the world which may cause confusion. The engineering unit for pressure, according to SI system, is Pascal (Pa), being a force of one Newton per one square meter area, 1 Pa = 1 N / m2. Since Pascal is a very small unit, it is most often used with coefficients, such as hecto, kilo and mega.

Other units of pressure, such as pounds per square inch (Ibf/in2) and bar, are also in common use. Pressure is sometimes expressed in grams-force or kilograms-force per square centimeter (g/cm2 or kg/cm2) and the like without properly identifying the force units. Since a system under pressure has the potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to energy density and may be expressed in units such as joules per cubic meter (J/m3, which is equal to Pa).

Pressure types

Several different pressure types exist including gauge, absolute, vacuum, differential and barometric. The main difference of these pressure types is the reference point against which the measured pressure is being compared. Pressure gauges also are available for all of these pressure types. Also, compound gauges are available, including a combined scale for both positive gauge pressure and vacuum (negative gauge) pressure.