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www.thinkSRS.com 1 Gas Correction Factors for Bayard-Alpert Ionization Gauges The sensitivity factor, Sg, supplied by gauge manufacturers (usually in Torr-1), is valid only for the gas for which it is specified and the readout of the controller provides a direct pressure reading only for that specific gas. The standard gas, used by the entire industry for gauge specification, is nitrogen and, unless gas correction factors are applied, all readings are considered to be 'nitrogen-equivalent pressures'. In This Application Note Gas Correction Factors 3 Nominal Gas Correction Factors for Common Gases 4 References 5 (408)744-9040 Stanford Research Systems www.thinkSRS.com 2 Gas Correction Factors Stanford Research Systems (408)744-9040 www.thinkSRS.com Gas Correction Factors 3 Gas Correction Factors The sensitivity factor, Sg, supplied by gauge manufacturers (usually in Torr-1), is only valid for the gas for which it is specified and the pressure readout of a BAG controller provides a direct reading only for that specific gas. The standard gas, used by the entire industry for gauge specification, is nitrogen and, unless gas correction factors are applied, all readings are considered to be 'nitrogen-equivalent pressures'. Nominal relative sensitivity factors, Rg, to convert nitrogen-equivalent readings into direct pressure readouts for gases other than nitrogen, are available from most gauge manufacturers and from the public vacuum literature. A summary table and reference list, is included in this application note. For gases where little or no data are available, it has been shown that a reasonable approximation to the relative sensitivity factor, Rg, can be obtained from the ratio of ionization cross sections at 150 eV of electron energy. Several ionization cross section tables are also available in the scientific literature. Once the relative sensitivity factor is known, direct pressure readings are calculated from the straightforward mathematical equation: P = [ Ic / (Sg