5.2.1 Design of a helium leak detector
Helium counterflow leak detectors are designed in accordance with the schematic diagram in Figure 5.2. A mass spectrometer MS is mounted on the intake flange of a turbomolecular pump. A backing pump Sv evacuates the turbomolecular pump via valve V2.
A workpiece 1 is evacuated via the test gas connection with valve V1 open. Valves V2 and V1 are connected in such a manner that the required backing vacuum pressure of the turbopump always takes priority over evacuation of the workpiece. Once the workpiece is evacuated, it can be connected to the backing vacuum, or to a tap on the turbomolecular pump via valve V3. Depending upon the pressure range in question, helium is now sprayed onto the workpiece from the outside and together with the ambient air penetrates into the workpiece through leaks. The helium present in the residual gas flows counter to the pumping direction through valves V1 and V2 and through the turbopump to mass spectrometer MS, where it is detected. The differing compression ratios of the turbopump for helium and air, which differ by multiple powers of ten, are utilized in this regard. While the high compression ratio of the turbopump keeps air away from the mass spectrometer, the helium arrives there at a relatively high partial pressure. The turbopump thus acts as a selective amplifier for helium.
This is why a mass spectrometer enables helium to be detected in the workpiece even at pressures of less than 1 mbar. Several powers of ten of the helium partial pressure, and thus a leakage rate range of between 10 and 10-8 mbar l / s, can be covered by means of various taps on the turbopump (V3), as well as by operating it at different speeds that exponentially influence the compression ratio. A pressure of 10-3 mbar must be attained in the workpiece for the highest sensitivity stage of the leak detector (inlet via V3).
Because of the upstream turbopump, the mass spectrometer always operates at an extremely low total pressure, and is thus well protected against contamination and failure.
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