Absolute and gauge pressure
What is the difference between PSIA and PSIG?
PSI stands for spounds per square inch, it is a unit for measuring pressure.
A means absolute. Absolute pressure is the pressure relative to zero or absolute vacuum.
G means gauge, that is what pressure gauge measure. Gauge pressure is the pressure relative to the ambient atmospheric pressure.
- Absolute pressure is always greater that Gauge pressure.
- Absolute pressure indicates the total pressure, whereas Gauge pressure indicates the pressure relative to the atmospheric pressure at the location.
- Absolute pressure does not change with altitude but Gauge pressure changes with change in altitude.
- Absolute pressure can never be negative but PSIG can be negative. Vacuum pressures in Gauge pressure below atmospheric pressures are negative
PSIG = PSIA – 1 Atompheric pressure In absence of data at the exact location the atmospheric pressure of sea level (14.7 PSIA) can be used.
Does -A and -G applied only to PSI?
No, -A and -G suffixes used with other units also: KPa(g), Bar(g), Atm(g).
What if there is no suffix in Unit?
If it is just PSI, BAR, kPa, etc - it usually means absolute pressure.