Experience with a pulsed gas flow technique for calibrating vacuum gauges

Abstract

A method for calibrating ion gauges and mass spectometers is described in which the calibrating concentration of gas molecules is established by injecting a measured flow of gas into a chamber of known volume for a measured time, while the chamber is isolated from its pumping system. THe switchable gas flow system which has been developed provides gas flow rates down to 3 × 10 14 molecules sec −1 , and allows the duration of the gas inflow pulse to be as short as about half a second As the volume of our calibration chamber is about 60 litres it is practical to calibrate down to about 3 × 10 9 molecules cm −3 . An attraction of this calibration technique is the ability to calibrate directly to low gas concentrations without using either a very low gas flow rate or alternatively a very large system as is required by the more usual orifice limited continuous flow calibration technique. Experimental results are presented from calibration studies with hydrogen on IOG-13 and ETI ion gauges MS10 and PPG mass spectrometers. This work includes studies of gauge pumping and contamination production. A few calibration results are also presented for deuterium and argon.