Effect of gas composition on vacuum measurement

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Abstract
Accurate pressure measurement with most types of vacuum gauges is critically dependent on a knowledge of the composition of the gas being measured. This paper will discuss the gas dependence of measurement with a number of commonly used types of vacuum gauges. Particular emphasis will be placed on recent work in the author’s laboratory which has shown how the accuracy of capacitance manometers can be affected if the possibility of thermal transpiration is ignored. Methods have been derived and proven for applying corrections when capacitance manometers are used with a wide variety of gases. The dependence of other types of vacuum measuring equipment on gas composition will also be discussed and recent investigations with spinning rotar gauges and ionization gauges will be reviewed. In the paper the potential of small mass spectrometers for quantitative gas analysis will be discussed. The most recent results on the evaluation of these instruments will be given for both small quadrupoles and magnetic sector instruments. The various methods of calibrating these instruments will be discussed with particular emphasis on the requirements of process industries and the monitoring of environmental pollutants.

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