Extension of the range of a constant-conductance flowmeter by a flow divider

Abstract

In this paper, measurement technology of very low gas flows with a constant-conductance method is introduced. The pressure of inlet gas is adjusted in the range from 10−1 to 100 Pa, and then the gas is passed through a small tube with a conductance of 10−9 m3 s−1 for Ar at 296 K in the molecular flow regime. By this method, the measurement range of very low gas flows is from 10−10 to 10−7 Pa m3 s−1, and the combined relative uncertainty is 0.94%. The gas flow generated by the constant-conductance method gas flowmeter is injected into a separated-flow chamber with two small tubes. The conductance values of the two small tubes are 1.703 × 10−6 and 3.196 × 10−4 m3 s−1 for Ar at 296 K in the molecular flow regime, respectively. The dilution ratio for the gas flow is designed to be 187.7:1. By this method, the measurement range of very low gas flows is from 10−12 to 10−9 Pa m3 s−1, and the combined relative uncertainty is 1.2%. The vacuum leaks with a leak rate of 10−10 Pa m3 s−1 are calibrated by using the above measurement technology, and the uncertainty of nonlinearity of a quadrupole mass spectrometer is avoided. The feasibility of using a non-evaporable getter pump in a constant-conductance method gas flowmeter for extending the lower limit is also discussed.