Absolute pressure is the pressure measured above total vacuum or zero absolute, where zero absolute represents a total lack of pressure.
Gauge pressure is the pressure measured above atmospheric or barometric pressure. It represents the positive difference between measured pressure and existing atmospheric pressure.
Most pressure gauges and other pressure-measuring devices indicate a zero reading when the measuring point is exposed to the atmosphere. This point is called zero psig. In fact, most pressure instruments actually measure a difference in pressure.
However, some instruments are designed to produce a reading that is referenced to absolute zero and to indicate a reading near 14.7 psi at sea level when the pressure point is exposed to atmospheric pressure. This reading is generally termed psia. Figure illustrates the relationship between absolute and gauge pressure.
The equation for converting from gauge pressure (Pg) in psig to absolute pressure (Pa) in psia is given by the following:
Pa = Pg + Patm (when Pg > Patm) (1-A)
Pa = Pg - Patm (when Pg < Patm) (1- B) where Patm is atmospheric pressure.
It should be noted that a change in atmospheric pressure will cause a change in gauge pressure. Therefore, a change in barometric pressure will cause a change in the reading of a gauge-pressure-measuring instrument.