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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
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2 Gas Correction Factors
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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