Causes of unstable and nonreproducible sensitivities in Bayard–Alpert ionization gauges

Abstract

The fundamental ionization gauge equation p=i+/(Si−) can be rigorously derived. However, the values of the two currents i+ and i− can only be inferred from measurements of the ion collector current Ic and electron emission current Ie. Measurements are reported in which some physical parameters upon which i+ and i− are dependent were varied under closely controlled conditions. Experimental evidence is presented to show that the sensitivity S of Bayard–Alpert (BA) gauges varies with changes in the electron emission density pattern along the length of the hot cathode. Because the grid intercepts an appreciable fraction of the electron current on each pass through the grid, small changes in the emission density pattern can cause the fraction intercepted to vary widely. Therefore, the amount of ionizing current i− in the ion collection volume varies causing sizable changes in the rate of ion production and in the gauge sensitivity. It is concluded that these causes of changes in sensitivity in BA gauges are inherent in all ionization gauges in which a change in the emission density pattern can effect the rate of ion production.