Helium can contra flow through the turbo pump back into the analyzer. Therefore, the inlet port of a leak detector can be connected to either the exhaust or an intermediate port of the high vacuum pump (mostly a turbo pump).
By doing so, the leak detector system can start testing at higher test pressures. If the pressure drops to levels that are comparable with the required analyzer pressure, the machine can then switch to a “direct” flow mode for higher sensitivities. In this situation, all molecules will reach the analyzer directly.
The advantage / disadvantages are shown with each flow mode below. We start with the contraflow which has lower sensitivity, a shorter test cycle, slower response and maintain a clean analyser, please see the image below.
Now we have done some of the theory we can move on to methods for helium leak detectors here.