PRESSURE
Pressure Terminology P = ρgh There are three different kinds of pressure reported in the literature, and it is important to know the terminology: 1. Absolute pressure is measured relative to absolute zero on the pressure scale, which is a perfect vacuum. (Absolute pressure can never be negative.) Absolute pressure is indicated by P and is identical to the familiar thermodynamic pressure. 2. Gage pressure (sometimes written as "gauge pressure") is measured relative to the local atmospheric pressure. Gage pressure is thus zero when the pressure is the same as atmospheric pressure. (It is possible to have negative gage pressure.) Gage pressure is indicated by Pg and is related to absolute pressure as follows: Pg = P - Pa, where pa is the local atmospheric pressure. 3. Vacuum pressure is also measured relative to the local atmospheric pressure, but is used when the gage pressure is negative, i.e. when the absolute pressure falls below the loc