Pressure is the amount of force acting per unit area. As to the units of pressure measurements, PSI ( pounds per square inch), bar and other units of measure such as kg/cm2, inH2O, mmHg, Pa are common to see.
There are also different types of pressure to consider. The type of pressure refers to the zero reference point of a measurement. For example: pressure falls into several types according to the zero reference point, involving:
Gauge pressure:The pressure gauge is referenced against atmospheric pressure.
Absolute pressure:The pressure gauge is referenced against a perfect vacuum. It is equal to gauge pressure plus atmospheric pressure.
Differential pressure:The pressure gauge contains two process connections to measure the difference between two pressures.