The operating principle for a MSLD

The equation applies to all gases which are pumped through the piping and thus also for helium. 

In this case the gas quantity per unit of time Is the leak rate being sought the total pressure may not be used, but only the share for helium or the partial pressure for helium. This signal Is delivered by the mass spectrometer when It Is set for atomic number 4 (helium). The value for Is a constant for every series of leak detectors, making It possible to use a 

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