The sum of atmospheric and gauge pressures, it is the total force per unit area exerted by a fluid and the pressure above a perfect vacuum (zero pressure) in vacuum systems. U.S. units for absolute pressure are pounds per square inch absolute (psia).
Pressure measured with respect to total vacuum abbreviated as "psia" or "bar." Absolute pressure is equal to the sum of a pressure gauge reading and atmospheric pressure (14.69 psia or 1 bar at sea level).
Air at standard conditions (70 degrees F air at sea level with a barometric pressure of 29.92 in. Hg.) exerts a pressure of 14.696 psi. This is the pressure in a system when the pressure gauge reads zero. So the absolute pressure of a system is the gauge pressure in pounds per square inch added to the atmospheric pressure of 14.696 psi (use 14.7 psi in environmental system work) and the symbol is "psia".