INFICON Composer Elite Gas Concentration Monitor - Manual (Page 59)

4 - 3
PN
07
4-
56
6-
P1
B
Composer Elite Operating Manual
4.1.4 What To Do when the Specific Heat Ratio
for a Gas is Unknown
Quantitative accuracy for an acoustic measurement technique partially depends on
accurate knowledge of the Specific Heat Ratio,
 , of the individual gas species.
While this ratio is known for many pure common gases, little information is
available on many of the complex Precursor molecules. The information currently
available is given in
Table B-1 on page B-1 . Even without exact knowledge of the
Specific Heat Ratio, useful information and reliable operation can still be obtained.
The Specific Heat Ratio of gases is a parameter that does not have a large range,
it is confined to a range of about 1.1 to 1.7 for almost every known gas.
Consequently, using the wrong Specific Heat Ratio will not make a significant error
in most cases. It is also possible to make intelligent guesses about unknown gases
using the value of the Specific Heat Ratio for a similar gas. It is almost always more
accurate to overestimate the Specific Heat Ratio than to underestimate by the
same amount; see
Appendix B and Figure 4-1 . Consider also that in production,
accuracy is not as important as the reproducibility. As long as the Specific Heat
Ratio is not changed, Composer Elite will produce similar results, day after day.
NOTE: Long term use of Composer Elite with some of the materials listed in
Appendix B may not be possible.
Figure 4-1 When the specific heat ratio for a gas is unknown it is generally
better to overestimate the specific heat ratio
Frequency (Hz)
Concentration (mole %)
5
4
3
0
2
1
2000 2500 3000 3500 1500 4000
TMLn in H2: Impact of Error in Gamma
True
Gamma
20%
Overestimated
Gamma
10%
Underestimated
Gamma