Gas Conversion Factors - Teledyne HASTINGS HFM-300 Instruction Manual

4. Gas Conversion Factors

Gas conversion factors (GCF's) for gasses metered using Hastings Instruments products, can be found
by visiting the Hastings Instruments web site. The web address can be found at the end of this
document. The gas conversion factors (GCF's) provided by Hastings Instruments (HI) fall into five
basic accuracy domains that, to a large extent, are dependent on the method by which they are
found. The following table summarizes the different methods used to determine the GCF's. The
table lists the methods in decreasing order of the degree of accuracy that may be achieved when
applying a conversion factor.
Methods Used to Determine Gas Conversion Factors
1. Determined empirically at Hastings Instruments
2. Calculated From NIST tables
3. Calculated using the virial coefficients of independent investigators' empirical
data using both temperature and pressure as variables.
4. Calculated from virial coefficients using temperature only.
5. Calculated from specific heat data at 0 C and 1 atmosphere
1. The most accurate method is by direct measurement. Gases that can be handled safely, inert
gases, gases common in the atmosphere, etc., can be run through a standard flow meter and the
GCF determined empirically.
2. The National Institute of Standards 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.
3 and 4. Many gases that have been investigated sufficiently by other researchers, can have their
molar specific heat (C'p) calculated. The gas conversion factor is then calculated using the following
ratio.
GCF's calculated in this manner have been found to agree with the empirically determined GCF's
within a few tenths of a percent. Data from investigations that factor in pressure as well as
temperature, usually supply a higher degree of accuracy in their predictions.
5. 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 continually searches for better estimates of the GCF's of the more complex
gases and regularly updates the list.
Most Hastings flow meters and controllers are calibrated using nitrogen. The conversion factors
published by Hastings are meant to be applied to these meters. To apply the GCF's, simply multiply
the gas flow reading and the GCF for the process gas in use. For example, to calculate the actual
flow of argon passing through a nitrogen-calibrated meter that reads 20 sccm, multiply the reading
and the GCF for argon.
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 1.4047.
Manual: 151-032015 300-302 Series
'
C
GCF 
pN 2
'
C
pGasX
20 x 1.4047 = 28.094
20 / 1.4047 = 14.238
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