Gauge pressure is defined as the pressure measured against the zero atmospheric pressure. It is cited against the ambient air pressure. It is determined by most instruments and gauges.
Gauge Pressure = Absolute Pressure – Atmospheric Pressure
Prel = Pabs – Patm
Units for measurement of gauge pressure are bar(g), kPa(g), and psi(g)
Changes in atmospheric pressure affects gauge pressure. Gauge pressure can be positive or negative.