Improved determination of the gas flow rate for UHV and leak metrology with laser refractometry

Abstract

A system often used for vacuum metrology purposes in order to calibrate vacuum gauges in the UHV region and to calibrate gas leak rates is the throughput system, employing the continuous (or dynamic) expansion method. An important component in such systems is the flowmeter, which has to deliver a pure and well-determined gas flow into the system. To determine the generated gas flow, a number of factors including the pressure inside the flowmeter have to be determined. However, it has turned out that the calibration uncertainty when measuring the pressure in the flowmeter gives a main contribution to the total uncertainty (of typically about 0.1%) for the generated flow, thereby limiting the accuracies of the generated vacuum pressure as well as gas leak rates in UHV metrology. A feasibility study is reported in this paper about the possibility of using laser refractometry to monitor dynamic gas density in situ in the flowmeter, as an alternative and possibly more accurate means of determining the generated gas flow, thereby potentially improving the calibration gas leak rates in the range 10−8–10−4 Pa m3 s−1.