H4X diffusion pump

Abstract

The Contra-Flow leak detection technique consists of letting the tracer gas (helium) diffuse upstream through the high vacuum pump toward the mass spectrometer sensor. The test object can remain at relatively high total pressure, as high as the tolerable forepressure of the high vacuum pump. Because this environment is not in the molecular flow regime, the relationship of detection sensitivity to pump performance parameters is sufficiently different from those of conventional leak detectors to warrant an investigation. Comparative tests were made with both types of detectors over a wide range of test pressures using water vapour, air, nitrogen and argon to simulate various test object pressure conditions. The results reveal that at test pressures above approximately one pascal the Contra-Flow method provides at least ten times higher sensitivity. Exposure of the test object to mechanical pump oil vapour can be prevented by a continuous stream of a helium-free gas or by the use of zeolite traps. The latter do not absorb helium significantly but produce a great effect on the residual gas composition.