MKS 649A - The Gas Correction Factor (GCF) for Flow Metering

The Gas Correction Factor (GCF) for Flow Meterin
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ter Three: Overview
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The Gas Correction Factor (GCF) for Flow Metering
A Gas Correction Factor (GCF) is used to indicate the ratio of flow rates of different gases which
will produce the same output voltage from a mass flow meter. The GCF is a function of specific
heat, density, and the molecular structure of the gases. Since flow meters are usually calibrated
with nitrogen, nitrogen is used as the baseline gas (GCF = 1).
Appendix D: Gas Correction
Factors
, page 59, lists the gas correction factors for the most commonly used gases. If the gas
you are using is not listed in the appendix, you must calculate its GCF using the following
equation:
GCF =
(0.3106) (S)
(d ) (Cp )
x
xx
where:
GCF
x
= Gas Correction Factor for gas X
d
x
= Standard Density of gas X, g/l (at 0° C and 760 mmHg)
C
p
x
= Specific Heat of gas X, cal/g° C
0.3106 = (Standard Density of nitrogen) (Specific Heat of nitrogen)
S = Molecular Structure correction factor where
S equals:
1.030 for Monatomic gases
1.000 for Diatomic gases
0.941 for Triatomic gases
0.880 for Polyatomic gases
Note
1. When using the GCF, the accuracy of the flow reading may vary by
±5%, however, the repeatability will remain ±0.2% of F.S.
2. All MKS readouts have Gas Correction Adjustment controls to
provide direct readout.