SEARCH FOR PRESSURE-DEPENDENCE IN THE SENSITIVITY OF SEVERAL COMMON TYPES OF HOT-CATHODE IONIZATION GAUGES FOR TOTAL PRESSURES DOWN TO 10-7 PA

Dependence of sensitivity on pressure in He, N2, and H-2 has been investigated for a group of 16 hot-cathode ionization gauges, representing both extractor and Bayard-Alpert types for total pressure as low as 5 x 10(-8) Pa. Absolute sensitivities were determined using a primary high vacuum standard. An independent method, that of measuring the sensitivity ratio of one guage to another, was also employed. Within a scatter of about +/- 3%, +/- 4%, and +/- 4% respectively, the N2, He, and H-2 sensitivity ratio data showed no clearly discernible pressure dependence down to total pressures as small as 5 x 10(-8) Pa. The absolute sensitivity measurements in He and N2 over the range 10(-7) to 10(-3) Pa in total pressure, also were constant within a scatter of about +/- 4% and +/- 3%, respectively. As a consequence of drift in the background component of the total pressure, all the absolute sensitivity measurements at total pressures below 10(-7) Pa exhibited an apparent pressure dependence not evident in the sensitivity ratio results. In the case of H-2, drift in the pressure persisted at total pressures orders of magnitude larger than the background pressure, and for all the gauges led to an apparent difference of approximately 10% between the H-2 sensitivities at 10(-8) and 10(-5) Pa. Results of further investigation suggest that the apparent pressure dependence in the sensitivities is an artifact produced by the well-known phenomenon of thermal dissociation of H-2 at hot filaments and associated processes of H-2 pumping and production of other species such as CO and C2H4.