MKS 247D - Gas Correction Factor (GCF) Calculations

Chapter Three: Overview Gas Correction Factor (GCF)
49
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. Since flow controllers are usually
calibrated with nitrogen, nitrogen is used as the baseline gas (GCF = 1).
Table 17, page 83, lists the gas correction factors for some commonly used pure gases. If the gas
you are using is not listed in Table 17, you must calculate its GCF. The equations for calculating
gas correction factors are listed in
How To Calculate the GCF for Pure Gases
, page 50, and
How
To Calculate the GCF for Gas Mixtures
, page 51.
The equations for calculating the GCF assume that the MFC was calibrated at a reference
temperature of 0° C (~273° K). If you want the 247 unit to read the mass flow as if the MFC
was calibrated at a different reference temperature, adjust the calculated GCF value using the
following equation:
Temperature Corrected
GCF
= GCF x
T
T
x
s
where:
T
x
= Reference temperature (° K)
T
s
= 273.15° K (~ equal to 0° C)
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.