Pressure is the force that occurs for each unit area in the plane, the unit of pressure which is widely used is kg/m2, Newton/m2, lb/inc2, kip/ft2 and so on. Besides that, there are units of pressure that are known and used in steam boiler or power plant such as atmospheric, Bar, psig).
In addition to a wide - range unit used for pressure, the pressure can be also classified based in its definition either for liquid, gas, vapor or steam. There are three types of pressure:
Absolute pressure is the actual pressure of fluid
Pressure gauge is measurement of how much the pressure of fluid compared to the outside air pressure (atm).
Vacuum pressure is equal to the pressure gauge but it is smaller than the atmospheric pressure (atm).
The differences of absolute pressure and pressure gauge can be seen from its unit. Absolute pressure can be expressed as Psi (Pound square inch), then the pressure gauge is expressed in psig (pound / square inch gauge).