OIL BACKSTREAMING IN TURBOMOLECULAR AND OIL DIFFUSION PUMPS

State of the art turbomolecular pumps have oil vapor backstreaming rates too low to measure by classical methods (AVS method, metal-foil gravimetry, quartz piezoelectric method, or drop microvolumetry). The authors analyzed the rarefied gases in the chamber; however, great care must be taken to prevent the probe from reacting with the residual atmosphere in the chamber. Experiments clearly demonstrate that oil backstreaming is primarily a function of time: any oil vapor present in the system will eventually find its way into the chamber. Backstreaming was measured in a number of gas bearing and ball bearing turbomolecular pumps as well as in up to date diffusion pumps with minimal backstreaming. A gas bearing turbomolecular pump exhausting to atmosphere through a dynamic seal is the only type completely free of oil vapor backstreaming, regardless of operating time. Today's oil diffusion pumps have backstreaming rates comparable to those of turbomolecular pumps backed by oil-sealed forepumps.