The Varian 938-41 Porta-Test Helium Leak Detector is based on the Contra-Flow principle - a proven technique that takes advantage of the differences in maximum pressure ratios produced by the diffusion pump for gases of different molecular weights.
This principle is implemented in the 938-41 leak detector by introducing helium (and other inlet gases such as those resulting from a leak in the test piece) into the diffusion pump foreline rather than into the "normal" pump inlet as in conventional leak detectors. Helium, having a much lower pressure ratio than other gases. contained in air, diffuses backwards through the diffusion pump to reach the sensing tube where it is detected as a leak in the normal manner. Although the mechanical pump is also attached to the diffusion pump foreline and removes all inlet gases, including helium, there is no appreciable loss of sensitivity in the 938-41 leak detector.