Definition - What does Gauge Pressure mean?
A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is the gauge pressure and can be expressed as:
pg = ps – patm
pg = gauge pressure
ps = system pressure
patm = atmospheric pressure
Gauge pressure is the pressure relative to atmospheric pressure. Therefore, it is positive for pressures above atmospheric pressure, and negative for pressures below it.
Corrosionpedia explains Gauge Pressure
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Absolute pressure is measured with reference to an absolute (and theoretical) vacuum. For example, an ordinary pressure gauge reading of zero does not mean there is no pressure; it simply means there is no pressure in excess of atmospheric pressure.
Gauge Pressure = Total Pressure – 1 atm
The conversion factor that needs to be used in order to convert between gauge pressure and absolute pressure is atmospheric pressure. By adding a pressure of 1 atmosphere to an absolute pressure, the corresponding gauge pressure can be determined.
Gauge pressure is measured with the standard atmospheric pressure as the baseline. Therefore, positive gauge pressures indicate that the pressure is above atmospheric pressure. Conversely, negative pressures indicate that the pressure is below atmospheric pressure.