Pressure Instruments – Units of Calibration

Pressure instruments should read in the following units:
• Above atmospheric: Pounds per square inch gage (psig), inches of water (inches H2O), or inches of mercury (inches Hg)
• Below atmospheric: Pounds per square inch absolute (psia), ounces per square inch, inches H2O vacuum, or inches of mercury vacuum (inches Hg vac). Absolute pressure instruments should be ordered with compensation for barometric pressure changes