Standard Pressure Units and Conventions

To communicate the amount of pressure requires the use of an agreed on scale and convention. Just as the conventions and units for temperature are referred to as Fahrenheit, Centigrade or Kelvin; industry uses PSID, PSIA, PSISG and PSIVG notation for pressure. PSI (Pounds per Square Inch) is the pressure unit while the last letter(s) refers to the convention used to measure the pressure. You would use PA( Pascels) units instead of PSI in the metric system.

By nature, pressure is the difference between two points or ports. Pressure between two ports (or pipe connections) is differential pressure. A designation of 10 PSID means 10 Pound per Square Inch Differential.