All you Need to Know about Relative Pressure Formula
Relative pressure is a pressure expressed with atmospheric pressure as a reference, and is one of the methods for expressing pressure. The other type of pressure is based on absolute vacuum, which is called absolute pressure.
Since the pressure measured by most pressure gauges is relative pressure, the relative pressure is also called gauge pressure. The pressure displayed by the pressure gauge is the gauge pressure (relative pressure), which refers to the difference between the real pressure and atmospheric pressure somewhere inside the device.
Take atmospheric pressure P. The pressure port U calculated from the zero point makes relative pressure. That is, the absolute pressure minus the local atmospheric pressure, the relative pressure value is obtained, and is expressed by the letter Pq, (Formula 1)
When the atmospheric pressure Pa is on the free surface of the liquid,
Pa = yh
Knowing from Formula (1), the absolute pressure is equal to the relative pressure plus atmospheric pressure, that is
P = Pq + Pa
In most practical engineering problems, because the atmospheric pressure experienced by the fluids is balanced with each other, the real pressure is relative pressure, so relative pressure is generally used to express static pressure.
The zero pressure indicated on a general pressure gauge is atmospheric pressure. The pressure gauge reading reflects the pressure difference between the fluid pressure and the surrounding air. Therefore, the scale on the pressure gauge is relative pressure.