Definition - What does Base Pressure mean?
Base Pressure is the standard unit of pressure used to determine the volume of gas. The volume is measured at the operating pressure and is then corrected to the Base Pressure volume. It is also known as base conditions. In order to ensure high levels of accuracy, it is essential to refer to Base Pressure while measuring the volume of a sample of gas or liquid. It is applied to both flow and static measurements.
Petropedia explains Base Pressure
The Base Pressure of a gas reaches an equilibrium value when there are no leaks in the gas chamber, the gas does not deliberately flow into it and the gas is continuously pumped for several days. As the pressure reaches the equilibrium asymptotically and the outgassing rate goes through an exponential decay, the chamber, even for a long time held under vacuum, never reaches a stable pressure. However, the variations in room temperature, vacuum gauge calibration, back streaming, pumping speed, etc., counter the reduction of real pressure which makes the chamber to reach to its steady state. The density of liquids and gases both depends upon the temperature and pressure of the well fluids.