Chapter Three: Overview
The Gas Correction Factor (GCF)
35
The Gas Correction Factor (GCF)
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 controller. The GCF is a function of
specific heat, density, and the molecular structure of the gases. Nitrogen is used as the baseline
gas (GCF = 1) since flow controllers are usually calibrated with nitrogen.
Appendix C: Gas Correction Factors, page 65, lists the gas correction factors for some
commonly used pure gases. If the gas you are using is not listed in Appendix C: Gas Correction
Factors, page 65, you must calculate its GCF. The equations for calculating gas correction
factors are listed in How To Calculate the GCF for Pure Gases, page 35, and How To Calculate
the GCF for Gas Mixtures, page 36.
Note
1.
When using the GCF, the accuracy of the flow reading may vary by
±5%, however, the repeatability will remain ±0.2% of FS.
2.
All MKS readouts have Gas Correction Adjustment controls to
provide direct readout.
How To Calculate the GCF for Pure Gases
To calculate the Gas Correction Factor for pure gases, use the following equation:
GCF =
(0.3106) (s)
(d ) (cp )
x
x
x
where:
GCFx = Gas Correction Factor for gas X
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
dx = Standard Density of gas X, in g/l (at 0° C and 760 mm Hg)
cpx = Specific Heat of gas X, in cal/g° C
