Vacuum Technology Book, Volume II

4.1.3 Ultimate pressure and base pressure

Ultimate pressure $p_e$ is the lowest pressure that is asymptotically approached by the pressure of a blank-flanged vacuum pump under defined conditions without gas inlet. If a pump is operated at ultimate pressure, the usable pumping speed will be zero, as only its own backflow losses will be displaced. Ultimate pressure is a theoretical value. Today, base pressure is specified instead of ultimate pressure. The conditions for achieving base pressure are specified in standard ISO 21360-1:2012. As the base pressure must be attained within a specified period of time, it is usually higher than the ultimate pressure.