The tungsten evaporation limit of hot-cathode ionization gauges

Comparative measurements of pressures in the low 10−12-Torr range with hot-filament total and partial pressure gauges have shown discrepancies which can be partly explained by the influence of tungsten vapor from the hot filament on the total pressure gauge readings (tungsten evaporation limit). The method proposed by Alpert and Buritz, with a slight modification, was used for estimates of the tungsten evaporation limit; i.e., a second filament served as an auxiliary source of evaporated tungsten atoms. Three gauges were tested, one gauge with a buried collector and two modulated Bayard-Alpert gauges. The experiments showed that the tungsten evaporation limits of all three gauges fell in the range (1–2)×10−12 Torr (nitrogen equivalent pressure) with an electron emission of 10 mA. Provided the variation of the gauge sensitivity with the emission is known, it is recommended to operate at reduced emission (e.g., 4 mA), at which level the tungsten limit is substantially lower. Low-work-function emitters, as a...