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

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 C, for gauges designed for use at ultrahigh vacuum and extreme high vacuum range from 10/Torr to 10(6)/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, 8, that an electron will make another pass through the ionizing region. An equation is obtained for K as a function of beta. The fractional change in K, DeltaK/K, is zero for those gauges where the electrons make a single pass, but is increasingly larger for higher sensitivity gauges with C,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= 10(2)/Torr. (C) 2002 American Vacuum Society.