Abstract
The conductance modulation method (CM method) is useful for the measurement of pumping speeds and outgassing rates in vacuum systems. This paper describes a formulation for the optimum design of the variable conductance and an evaluation of errors in the CM method. The pressure ratio corresponding to the conductance modulation, P A P B , is expressed as a function of C A C B , C A S p and Q Q P , where C A C B is the ratio between maximum and minimum values of the variable conductance, S P is the intrinsic pumping speed, Q is the throughput of gas evacuated, and Q P is the outgassing rate from the pump. The equation determines experimental conditions for the sensitive measurement of gas balance in the vacuum system. As for the evaluation of errors in the CM method, we have estimated the contributions of errors in the values of the variable conductance and in the pressure measurement. Another factor which may affect the accuracy of the CM method is a variation of the molecular flow in the vacuum chamber during the CM process. A series of Monte Carlo simulations shows that the errors due to the variation of the molecular flow are less than ±3% under practical experimental conditions.
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