401-405 SERIES - 25 -
6.2. Appendix 2 - Gas Conversion Factors
The gas correction factors (GCF’s) presented in this manual were obtained by one of four methods.
The following table summarizes the different methods for determining GCF’s and will help identify for
which gases the highest degree of accuracy may be achieved when applying a correction factor.
1. Empirically determined
2. Calculated from virial coefficients of other investigator’s empirical data
3. From NIST tables
4. Calculated from specific heat data at 0° C at 1 atmosphere
The most accurate method is by direct measurement. Gases that are easily handled with safety such as
inert gases, gases common in the atmosphere or gases that are otherwise innocuous can be run through
a standard flow meter and the GCF determined empirically.
Many gases that have been investigated sufficiently by other researchers, can have their molar specific
heat (C’ p) calculated. The gas correction factor is then calculated using the following ratio:
GCF = C ’apN2
C’apGasX
GCF’s calculated in this manner have been found to agree with the empirically determined GCF’s
within a few tenths of a percent.
The National Institute of Standards[LH1] and Technology (NIST) maintains tables of thermodynamic
properties of certain fluids. Using these tables, one may look up the necessary thermophysical property
and calculate the GCF with the same degree of accuracy as going directly to the referenced investigator.
Lastly, for rare, expensive gases or gases requiring special handling due to safety concerns, one may look
up specific heat properties in a variety of texts on the subject. Usually, data found in this manner applies
only in the ideal gas case. This method yields GCF’s for ideal gases but as the complexity of the gas
increases, its behavior departs from that of an ideal gas. Hence the inaccuracy of the GCF increases.
Hastings Instruments will continue to search for better estimations of the GCF’s of the difficult gases
and will regularly update the list. Most Hastings flow meters and controllers are calibrated using
nitrogen. The correction factors published by Hastings are meant to be applied to these instruments. To
apply the GCF’s, simply multiply the gas flow reading times the GCF for the process gas in use.
Example:
Calculate the actual flow of argon passing through a nitrogen-calibrated meter that reads 20 sccm,
multiply the reading times the GCF for argon.
20.000 x 1.3978 = 27.956
Conversely, to determine what reading to set a nitrogen-calibrated meter in order to get a desired flow
rate of a process gas other than nitrogen, you divide the desired rate by the GCF. For example, to get a
desired flow of 20 sccm of argon flowing through the meter, divide 20 sccm by 1.3978
20.000 / 1.3978 = 14.308
That is, you ` (adjust the gas flow) to read 14.308 sccm.
