PSI = Pounds Per Square Inch / PSIG = Pounds Per Square Inch + Gauge / PSIA = Pounds Per Square Inch + Absolute

PSI is the most popular unit to measure pressure and stands for pounds per square inch. It is an absolute measure, meaning it is compared against a perfect vacuum.

PSIG is pounds per square gauge. It is the pressure measurement that is measured relative to ambient atmospheric pressure. 14.7 psi is the approximate atmospheric pressure at sea level. PSIG is the pressure reading in a pressure gauge which does not include atmospheric pressure. This can be used to calculate the PSIG of certain, fittings, unions or adapters. 

PSIA is pounds per square inch absolute or total pressure. Pressure relative to a full vacuum. 

The Applied Research™ brand adapters, fittings, unions and adapter’s pressure rating are measured in PSIG.