Measurement of small gas flow rate by the conductance modulation method

Abstract

Conductance modulation (CM) method has been applied to the measurement of small gas flow rate. We have realized the dynamic balancing of gases to simulate the condition of ultimate pressure in ultra-high-vacuum (UHV) and extremely-high-vacuum (XHV). In this condition, we have examined the ability, the accuracy and the detection limit for the measurement by CM method. It has been revealed from the experiments that the method has basic characteristics to measure accurately small gas flow rate in different vacuum conditions. An error of less than 20% is expected when Sig. N × S ∗ C > 1 and the lowest detection limit of the order of 10 −13Pa m 3s −1 is expected when C≒S ∗≒S, where Sig. N : signal-to-noise ratio, S∗: net pumping speed, C: conductance and S: intrinsic pumping speed.