In an accommodation pump gas is transferred from one point to another at the same temperature without the application of any external mechanical or electrical forces to the gas - only temperature gradients are used.@footnote1@ If there is no continuous gas flow a static and unchanging pressure difference develops between the two points, requiring no additional input of energy. Little attention has been paid to the energy transfer, i.e. the number of calories per mole of gas pumped, in this process. A simple model is presented in which this energy transfer is analytically calculated for a single-stage Pyrex accommodation pump,upper temperature room (295 K), lower temperature liquid nitrogen (77.4 K), with gas helium, compression ratio 1.2, and pressure in the free molecular range. Under these conditions the physical adsorption of helium on pump surfaces is quantitatively negligible. The model calculates the energy differences between a true accommodation pump with smooth and rough (leached) surfaces and a non-pump of exactly the same geometrical dimensions, but with all surfaces rough. The energy differences are assigned to accommodation pumping. It is found that an energy in the range of 150 calories per mole, is required, independent of the overall size of the pump, but becoming smaller as the cold volume is decreased relative to the warm volume. The results are extended to multi-stage pumps, which are ponderous to calculate analytically, but which bear a simple relationship to a single-stage pump.
@FootnoteText@
@footnote1@ J.P.Hobson and D.B.Salzman, J. Vac. Sci. Technol. A 18(4), 1758-1765, 2000