PSIG and PSIA
G in PSIG is stands for Gauge and A is stands for Absolute.
PSIG is the pressure read from a gauge which reads the difference between the pressure in the pipe and the pressure of the atmosphere.
PSIA is the total pressure or absolute pressure including the pressure of the atmosphere.
PSIG is lower than PSIA
PSIG + 1 atm = PSIA
PSIA – 1 atm = PSIG
atm is stands for atmosphere
1 atm is approximately 14.7 psi, so we use the value to convert the PSIG to PSIA or vice versa, or we use the actual atmosphere value measurement for your place if you have it.
Atmospheric pressure at sea level is 14.7 PSIA = 0 PSIG
Resumed from onlineconversion.com