Gage vs. Absolute vs. Sealed Pressure

The conventional units for defining pressure are PSI, pounds per square inch. However, in all pressure measuring devices, a reference pressure has to be established and the PSI qualified to indicate the reference datum. The suffixes "G", "A", and "S" identify the measured pressure as PSIG - Gage Pressure; PSIA - Absolute Pressure; or PSIS - Sealed Pressure.