The pressure at a point within a fluid mass is designated either as an absolute pressure or a gage pressure:
- Absolute pressure is measured relative to a perfect vacuum
- Gage pressure is measured relative to local atmospheric pressure.
A gage pressure of zero corresponds to pressure that is equal to the local atmospheric pressure.
Absolute pressures are always positive, but gage pressures can be either positive or negative depending on whether the pressure is above atmospheric pressure (positive value) or below atmospheric pressure (negative value).
- A negative gage pressure is also referred to as a suction or vacuum pressure