Conductance modulation method for the measurement of the pumping speed and outgassing rate of pumps in ultrahigh vacuum

Abstract

A new method, named the conductance modulation method (CM method), was developed for the measurement of pumping speeds (intrinsic and net) and outgassing rates in vacuum systems. The details of the formulation and the procedure of the CM method are described. The principle of the method lies in the measurement of the pressure modulation with the change of the conductance between a chamber and a pump. The pumping speed is obtained from the ratio of the modulated pressures and the value of the conductance without measurements of absolute pressures. This feature makes the method quite appropriate for measurements in ultrahigh vacuum. The CM method was applied to the measurement of the gas balance in a vacuum system pumped with a Ti getter pump. The intrinsic and the net speed of the Ti getter pump were measured for H2 in a wide pressure range down to ultimate pressure under various conditions of getter films and the amount of H2 sorbed. The outgassing rates from the wall of the chamber and the pump were simultaneously evaluated. These results confirmed the usefulness of the CM method for the quantitative analysis on the gas balance in vacuum systems.