For gas mixtures, the calculated mass flow controller (MFC) gas correction factor (GCF) is not simply the weighted average of each component's GCF. Instead, the GCF (relative to nitrogen) is calculated by the following equation (for n gases):
Here, a1 through an are the fractional flows of gas 1 through n; s1 through sn are the molecular structure factors; d1 through dn are the standard densities; and cp1 through cpn are the specific temperatures. Values for s, d, and cp are available for most gases; many are listed in our MFC manuals. The values for a1 through an will depend upon the application. (Remember that a1 through an must add up to 1.0.)
Molecular Structure correction factor(s):
Gas 1 = argon at 150 sccm
Gas 2 = nitrogen at 50 sccm
sccm stands for Standard Cubic Centimeters per Minute. Where "Standard" means referenced to 0 degrees Celsius and 760 Torr.
Take the GCF of the gas that you are flowing and divide that by the GCF of the gas that the MFC was calibrated for. Example: the MFC is calibrated for 100 sccm of Argon (GCF 1.39) and you are flowing CO2 (GCF = 0.7) the resulting effective GCF would be 0.7/1.39 = 0.50. Therefore the 100 sccm Argon instrument will be flowing 50 sccm of CO2 at full scale.
The GCF would be 1.
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