The pressure value measured by the pressure gauge reflects the pressure level, 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 the atmospheric pressure, the measured pressure is called surface pressure. Absolute pressure should be equal to surface pressure plus atmospheric pressure:
Absolute pressure = surface pressure atmospheric pressure
When the actual pressure is lower than the atmospheric pressure, the measured pressure is called vacuum degree, which is also called negative pressure. Absolute pressure equals atmospheric pressure to reduce vacuum degree:
Absolute pressure = atmospheric pressure-vacuum degree
Because the atmospheric pressure is approximately equal to 0.1MPa, when the pressure is relatively high, the surface pressure plus this value is approximately equal to the absolute pressure. For example, if the surface pressure of the lower tower is 0.48MPa, the absolute pressure is 0.48MPa 0.1MPa = 0.58MPa.
