Absolute Pressure, Gauge Pressure and Vacuum Pressure
The pressure on a fluid is measured in different system.
Absolute pressure: When pressure is measured with reference to absolute vacuum pressure then it is called absolute pressure. At this pressure, the molecular momentum is zero.
Gauge pressure: This is the pressure measured by of pressure measuring instrument, in which atmospheric pressure is taken as datum (reference). The atmospheric pressure is taken as zero in this system. The instrument used to measure this pressure is known as Pressure gauge. The pressure always used to indicate the pressure above atmospheric pressure.
Vacuum pressure: Measurement of pressure below the atmospheric pressure, in which atmospheric pressure taken as reference. This is also known as negative pressure or suction pressure. The instrument used to measure vacuum pressure is known as Vacuum gauge.
Relation between Absolute pressure and Gauge pressure
Absolute pressure (Pab) = atmospheric pressure (Patm) + Gauge pressure (Pgauge)
Vacuum pressure = atmospheric pressure (Patm) - Absolute pressure (Pab)
👉Standard atmospheric pressure is defined as the mean pressure at sea level at 40° latitude and it is taken as 1201.3Kpa or 1.1013bar or 76cm mercury or 10.3m water