Pressure has two distinct reference points in common usage, Atmospheric Pressure and Gauge Pressure
Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth’s atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point.
Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more atmospheric mass above their location. Similarly, as elevation increases there is less overlying atmospheric mass, so that pressure decreases with increasing elevation. A column of air one square inch in cross-section, measured from sea level to the top of the atmosphere, would weigh approximately 65N.
Most pressure measuring instruments take their zero point from atmospheric pressure. The value indicated being called gauge pressure. Instruments that measure gauge pressure sometimes use the letter G after the unit. e.g. PSIG or kPag
Absolute pressure is pressure measured in relation to a complete vacuum.
Therefore: Absolute pressure = gauge pressure + atmospheric pressure
All Instruments that measure absolute pressure will use the letter A in the unit.
e.g. PSIA or kPaA
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