Functional principle of a tracer gas leak detector

A modern helium and/or hydrogen tracer gas leak detector is working on the principle of mass separation by different deflection radius of a charged (ionized) particle with different masses in a magnetic field. Therefore, a so called sector field mass spectrometer is used to separate the helium (or hydrogen) atoms from other gases.

Spectrometer cell

In order to achieve the right working conditions for the sector field mass spectrometer, a vacuum system with a backing and turbomolecular pump is needed. This pumping group is used to achieve the working pressure for the mass spectrometer but the turbopump also protects the analyzer cell (mass spectrometer). For this, the leak detector has different modes with different inlets at the turbopump. While the gross leak mode is attached to the bottom part of the turbopump, the high sense mode is connected to the top of the pump. This ensures that the analyzer is protected from a huge helium flow in case of a large leak but still has the needed sensitivity to also detect small leaks.
This so called counter-flow principle was developed by Pfeiffer Vacuum in the 1960s and it shows why the turbopump is so important for a helium (or hydrogen) leak detector.
 

Vacuum scheme