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Absolute and gauge pressure

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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.


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