MKS 246C, Instruction Manual

Page: 57 / 92

Chapter Three: Overview Gas Correction Factor
47
Gas Correction Factor
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 12, page 77, lists the gas correction factors for some commonly used pure gases. If the gas
you are using is not listed in Table 12, you must calculate its GCF. The equations for calculating
gas correction factors are listed in How To Calculate the GCF for Pure Gases, page 48, and How
To Calculate the GCF for Gas Mixtures, page 49.
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 246 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:
Tx = Reference temperature (° K)
Ts = 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.