The gas correction factor (GCF) is used to indicate the ratio of flow rates of different gases for a given output voltage from a mass flow controller (MFC). The basis gas is nitrogen (N2) which, by convention, has GCFN2 = 1.
To calculate the mass flow of a gas for a MFC that is calibrated for a different gas, take the GCF of the gas being used and divide that by the GCF of the gas that the MFC was calibrated for.
Example:
A MFC is calibrated for Argon (GCFAr = 1.39) and the gas of interest is CO2 (GCFCO2 = 0.70). The resulting effective GCF would be GCFCO2 / GCFAr = 0.70 / 1.39 = 0.50.
For a set point of 100 standard cubic centimeters per minute (sccm), the MFC calibrated for Ar will actually be flowing 100 sccm x 0.5 = 50 sccm of CO2. If the gas were actually Argon, the mass flow would be 100 sccm. If the gas were N2, the gas flow would be,
= 100 sccm x (GCFN2 / GCFAr) = 100 sccm x (1.0 / 1.39) = 72 sccm
Note:
1. When using the GCF, the accuracy of the flow reading may vary by +/-5%.
2. The repeatability remains within +/-0.2% FS.