In Figure 5.12 the right hand diagram shows the schematic for the counterflow
leak detector. The mass spectrometer, the high vacuum system and
also the auxiliary roughing pump system correspond exactly to the configuration
for the direct-flow arrangement. The feed of the gas to be examined
is however connected between the roughing pump and the turbomolecular
pump. Helium which reaches this branch point after the valve is opened will
cause an increase in the helium pressure in the turbomolecular pump and
in the mass spectrometer. The pumping speed Seff inserted in equation
5.4b is the pumping speed for the rotary vane pump at the branch point.
The partial helium pressure established there, reduced by the helium compression
factor for the turbomolecular pump, is measured at the mass spectrometer.
The speed of the turbomolecular pump in the counter-flow leak
detectors is regulated so that pump compression also remains constant.
Equation 5.5b is derived from equation 5.5a: