Calibration of secondary standard ionization gauges

A McLeod gauge, designed, built and maintained by us, serves as the primary vacuum standard of Japan. We measure the sensitivities for nitrogen of the secondary standard ionization gauges (VS–1) with an accuracy of ±3% with the McLeod gauge in the pressure range from 0.01 to 0.3 Pa. The VS–1 gauge is a triode type that shows constant sensitivity from 1×10−4 to 0.3 Pa. 258 VS–1 gauges were calibrated since 1964. The average value of the sensitivities is 0.137 Pa−1. About 85% of the gauges have sensitivities with less than ±10% deviation from the average. The main reason for the spread of sensitivities was the electrode displacement. We discuss the technical problems connected with the use of the VS–1 gauge as a transfer gauge for international comparison of vacuum standards.