Ionization gauges, sensitivity

However, most ionization gauges are thermionicgaugesihai obtain positive ions through bombardment of the gas molecules with electrons produced thermionically from a heated filament and accelerated by a grid biased 100 to 200 V positive with respect to the filament. The ions are collected by a cathode that is typically about 20 to 50 V negative with respect to the filament, and the ion current in the cathode circuit is measured with a sensitive electrometer. 

RELATIVE SENSITIVITY OF BAYARD ALPERT IONIZATION GAUGES TO VARIOUS GASES... 

Relative sensitivities for different Bayard-Alpert ionization gauges may differ by as much as 15% as a result of differences in applied voltages, electron current, and electrode structure. The table below presents the average of the measurements of 12 experimenters on Bayard-Alpert ionization gauges in various gases. The sensitivity relative to nitrogen is tabulated. 

Table 6,2 Approximate ion gauge (Bayer-Alpert gauge) sensitivities (Sjc) relative to nitrt en, and ionization energies (IE), for some representative gases...

Relative Sensitivity of Bayard-Alpert Ionization Gauges to Various Gases.15-12... 

Di must be multiplied by a factor of 0.22. This factor is due to either/(0), y, or the sensitivity difference of decaborane and nitrogen in the ionization gauge. 

They should be used in conjunction with a TC gauge the ion gauge filament should not be on until the TC gauge indicates that the pressure is below about 10 pm. When not in use, turn off the filament, or at least set the meter to the least sensitive scale. The ionization and collection process actually removes some gas from the vacuum space, and this pumping can improve the vacuum or maintain a vacuum in the presence of a small leak. Much larger versions of this device are made specifically for pumping. 

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