Abstract
The paper describes the progress made at the NPL with a study of the McLeod gauge and gas expansion methods for the calibration of high vacuum gauges. It is shown that a precision McLeod gauge is capable of the calibration of Bayard-Alpert ionisation gauges down to pressures approaching 10 −6 torr with an accuracy in the region of 1 or 2 per cent, and results are obtained for the values of the mercury vapour stream effect with seven gases which agree well with a theoretical formulation. The gas expansion apparatus, which has been used to calibrate similar gauges to pressures as low as 10 −7 torr, has given results in close agreement with those obtained with the McLeod gauge, and it is concluded that the expansion technique, combining high metrological accuracy with convenience of operation, has great promise as a practical calibration standard.
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