Anomalous ion currents in Bayard-Alpert ionization gauges

Abstract

A theory has been developed to account quantitatively for the anomalous ion-current behaviour which leads to errors in pressure measurement in Bayard-Alpert ionization gauges. The equilibrium anomalous ion current has been studied as a function of the gauge electron current and operating pressure. Analysis of the experimental results in the light of the theory yields values for the total atom and ion desorption probability per electron, the ion desorption probability per electron and the sticking coefficient of gas molecules on the grid surface. Though the theory suggests ways in which the pressure measurement error can be minimized, experiment shows that the error can be eliminated by the correct choice of grid material. Thus, for the residual gases normally found in vacuum systems, ionization gauges with either molybdenum or tantalum grids exhibit anomalous ion currents whilst gauges with tungsten or platinum grids do not. A practical form of grid utilizes platinum-clad molybdenum wire.