Pressure Measurements Below 10−10 Torr with Bayard-Alpert and Magnetron Gauges

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Abstract
Below 1×10−10 Torr, pressure measurement by means of Bayard‐Alpert type ion gauges is complicated by the existence of a current associated with soft ``x‐rays.'' The precise measurement of a true ion current requires precise determination of the so‐called x‐ray limit of each gauge. The present work describes the use of a Bayard‐Alpert gauge containing a modulating electrode (after the design of Redhead) to measure pressures below 10−10 Torr. Magnetron‐type gauges, which do not have x‐ray limitations, were then compared directly with the Bayard‐Alpert gauges. In addition, gauge comparisons were carried out in a dynamic system involving pressure ratios across a capillary. Both series of experiments showed that the output current of the magnetron gauge was a power function (exponent 1.6) below approximately 2×10−10 Torr (N2). In the latter work it was possible to make measurements to below 1×10−12 Torr N2 equivalent.

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