AVS 51st International Symposium
    Vacuum Technology Wednesday Sessions
       Session VT-WeA

Paper VT-WeA7
A Study of the Variation of Sensitivity of Hot Cathode Ionization Gauges as a Function of Gas Adsorption

Wednesday, November 17, 2004, 4:00 pm, Room 303D

Session: Vacuum Measurement, Sensors and Control
Presenter: P.J. Abbott, National Institute of Standards and Technology
Authors: P.J. Abbott, National Institute of Standards and Technology
P. Mohan, National Physical Laboratory, India
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Hot cathode ionization gauges of the Bayard - Alpert design are the most widely used gauges for pressure measurement in the high and ultra high vacuum range. The National Institute of Standards and Technology (NIST), has considerable experience in calibrating hot cathode ionization gauges of this type for customers and for its own use. Several different envelope geometries have been encountered for example, nude gauges, gauges with glass envelopes, and the more recent gauges having metal envelopes. The sensitivity of this particular type of metal envelope ion gauge is claimed to be highly repeatable, typically within a few percent over the period of one year. However, we have noticed much larger sensitivity shifts in these gauges during routine operation. We report the results of a systematic study of the change in sensitivity for two glass envelope ionization gauges, six metal envelope ionization gauges, and one extractor gauge over the pressure range 2x10@super -@@super6@ Pa to 7x10@super-@@super3@ Pa. An orifice flow (dynamic expansion) system generated the nitrogen and argon calibration pressures, and a spinning rotor gauge was used as a check standard for pressures above 10@super-@@super3@ Pa. For both argon and nitrogen, we found each gauge's sensitivity to be dependent on the quantity of gas the operating gauge was exposed to prior to calibration, i.e., the product of pressure and time. For argon gas, sensitivity shifts of up to 8 % were observed between an initial calibration and one performed after exposing the gauges to 0.028 Pa of argon for about five hours. For nitrogen, the shifts were as large as 14 %. Interestingly, for argon the sensitivity shifts occur towards the high-pressure end of the calibration range, while for nitrogen the shifts occur at the low pressure range. We believe that ionic pumping of gas and its subsequent release by the walls of the metal envelope is responsible for the observed sensitivity shifts.