Errors in Pressure Measurement and Impurity Generation in Hydrogen using Bayard-Alpert Ionization Gauges

Abstract

A Bayard–Alpert ion gauge, with thoria-coated electron emitters, was used to measure hydrogen pressures using emission currents as high as 10 mA without production of serious amounts of impurities. The use of high emission currents does not result in a high pumping speed and has the advantage of reducing measurement errors. Such errors result from the adsorption of carbon monoxide on the grid of the ion gauge and occur at carbon monoxide pressures as low as 0.3% of the hydrogen pressure. The total desorption probability of carbon monoxide is approximately 2 × 10−5 particles/electron and approximately half of the gas is desorbed as ions. At high emission currents the adsorption of carbon monoxide on the grid is kept at a minimum thus reducing the spurious current to the ion collector. For example, in hydrogen at 3.5 × 10−9 Torr, in the presence of 1 to 2% carbon monoxide, the ion-gauge indication of pressure is ∼10% high for operation at 10-mA electron emission, while at 100 μA the indicated pressure increases to ten times the true pressure.