Abstract
The definitions of “true speed” and the influence of the shape of the test dome on the “measured speed” of diffusion pumps are reviewed. Equations are given for the measured speed to be expected from an “ideal pump” (for which no molecules return to test dome) and for the molecular conductance (as measured with a tubulated gauge) of a pipe directly connected to a pump of the same diameter. Approximate equations are also given for the influence on the reading of a nude ionization gauge of the average gas temperature due to molecules evaporating or scattered from the cold surface of a cryopu mp or from the hot surfaces of a diffusion pump.
