The pressure value measured by the pressure gauge reflects the level of pressure, but it is not the actual pressure. According to the working principle of the pressure gauge, the measured pressure is the difference between the actual pressure (absolute pressure) and the surrounding atmospheric pressure. When the actual pressure is higher than atmospheric pressure, the measured pressure is called gauge pressure. The absolute pressure should be equal to the gauge pressure plus atmospheric pressure:
Absolute pressure = gauge pressure + atmospheric pressure
When the actual pressure is lower than atmospheric pressure, the measured pressure is called vacuum, also called negative pressure. Absolute pressure equals atmospheric pressure minus vacuum:
Absolute pressure = atmospheric pressure - vacuum
Since the atmospheric pressure is approximately equal to 0.1 MPa, when the pressure is high, the gauge pressure plus this value is approximately equal to the absolute pressure.
