Initial pumping and recovery of ionization gauges

Abstract

Measurements have been made on the pressure decrease in a closed glass system when an ion gauge pumps and also of the recovery of pressure when pumping is stopped after a short period. The pressure decrease may be represented by an equation of the type p−p 1 = C 1 e −a 1 t + C 2 e −a 2 t where p and p 1 are the pressure after any time t and the “final” pressure, respectively. C 1 , C 2 , a 1 ,a 2 are parameters depending upon the nature of the gas and surfaces. For pumping a 1 and a 2 contain terms depending upon the electron current. For recovery a 1 is determined by the sticking time of pumped molecules on the glass and a 2 by a glass characteristic. Average sticking times are found to increase from about 10s to 2000 s as the temperature of the system increases from 22 to 450°C with consequent change in heats of adsorption from 18 to 40 kcal/mole approximately. the increase in sticking times are attributed to changes in residual gas composition.