The principle of He enrichment in a counterflow leak detector with a turbomolecular pump with two inlets

Abstract

The pumping time required to render helium leak detectors operational can be greatly reduced by increasing the He concentration in the gas flow entering the detector. Since this gas flow is composed chiefly of water vapor, a cold trap with liquid nitrogen is a very effective method for concentration. Because the procurement and handling of liquid nitrogen can be difficult, the counterflow principle was proposed and introduced as an alternative many years ago. The tester takes advantage of the fact that the compression ratio of diffusion pumps and turbomolecular pumps for heavy gases is far greater than for light gases. Hence by allowing gas to enter on the forevacuum side it is possible to greatly concentrate light gases such as helium on the high-vacuum side. The sensitivity is then no longer based on the pumping speed of the high-vacuum pump (as with customary leak detectors) but instead on the product obtained by multiplying the compression ratio for He by the pumping speed of the backing pump. As far back as 1967, Becker suggested placing a second high-vacuum pump in the inlet to avoid the relatively long response time arising due to the low pumping speed of the backing pump. Because of the expense entailed, this improvement has not yet been implemented. The problem can be solved at little cost by employing a turbomolecular pump featuring two opposing rotors on a single axle. In this manner one obtains high sensitivity with high pumping speed and no hydrocarbons at the inlet.