Chapter Five: Theory of Operation
Gas Correction Factor
31
Gas Correction Factor
To interpret the output signal for gases other than nitrogen, use the Gas Correction Factor (GCF).
The GCF is a number used to indicate the ratio of flow rates of different gases which will
produce the same output voltage from a MFC. The GCF is a function of specific heat, density,
and the molecular structure of the gases. Since controllers are usually calibrated with nitrogen,
nitrogen is used as the baseline gas (GCF = 1). The GCF of any other gas is determined by using
the following equation:
GCF =
(0.3106) (S)
(d ) (Cp )
x
x
x
where:
GCFx = Gas Correction Factor for gas X
dx = Standard Density of gas X, g/l (at 0° C and 760 mmHg)
Cpx = Specific Heat of gas X, cal/g ° C
0.3106
= (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
Refer to Appendix B, Gas Correction Factors, page 41, for a list of the Gas Correction Factors
for the most commonly used 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.
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