Helium leakage detector

A helium leakage detector including an inlet pipe and a mass spectrometer, the improvement wherein said detector includes a molecular diffusion pump, said spectrometer includes a measuring spectrometric cell, and said cell is disposed in parallel with said molecular diffusion pump, an isolating valve within said inlet pipe upstream of said molecular diffusion pump, said molecular diffusion pump having an inlet and an outlet and whose outlet is short-circuited at the diffusion pump inlet by a short-circuit pipe, said short-circuit pipe connecting to said inlet pipe downstream of said isolating valve and upstream of said diffusion pump inlet, and a constriction being provided between the diffusion pump outlet and inlet within the short-circuit pipe;

• whereby, the necessity of using a vane pump is avoided, allowing a recirculating loop of gas to be maintained within the system for some period of time for measuring purposes.