Calibration stability of hot cathode ionization gauges: A discussion of the importance of electron path length and gauge constant

Abstract

The ion current, i+, in an ionization gauge is given by the equation i+=Ki−P where K is a gas dependent gauge constant, i− is the electron current, and P is the pressure. Values of K for nitrogen for gauges designed for use at ultrahigh vacuum and extreme high vacuum range from 10/Torr to 106/Torr. It is important to know whether calibration stability is sacrificed when K, and the electron path length are large. Using a simple model, the electron path length is estimated as a function of the probability, β, that an electron will make another pass through the ionizing region. An equation is obtained for K as a function of β. The fractional change in K, ΔK/K, is zero for those gauges where the electrons make a single pass, but is increasingly larger for higher sensitivity gauges with greater probabilities of multiple passes. As an example, assume that the probability of the next pass changes by 1%, then the change in sensitivity is 1.5% for a B-A with K=25/Torr, and 9% for a gauge with K=102/Torr.