User guide

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PN 074-566-P1B

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 %)

2000 2500 3000 35001500 4000

TMLn in H2: Impact of Error in Gamma

True

Gamma

20%

Overestimated

Gamma

10%

Underestimated

Gamma