An arrangement in which the test port joins the foreline of the detector's own turbo pump rather than its inlet, so helium reaches the mass spectrometer by diffusing backwards against the turbo. The turbo's compression ratio for helium is low, so helium gets through while heavier gases and contamination are held back. This lets the detector test at inlet pressures of a few mbar and protects the spectrometer when it meets a gross leak.
Thanks to counterflow the detector starts reading at a couple of mbar, so we can spray before the chamber even crosses over.
Also heard as
- counter-flow
- reverse flow
Related terms
Helium leak detector
Mass spectrometer tuned to helium that finds and measures leaks by detecting helium drawn into the system.
Compression ratio
Ratio of a pump's outlet pressure to its inlet pressure for a given gas, measured at zero net flow.
Turbomolecular pump
High vacuum pump whose rotor blades spin at tens of thousands of rpm and bat molecules toward the exhaust.