A simplified formula for the analysis of binary gas containing a low concentration of a heavy vapor in a lighter carrier

The most general formula relating the sound velocity, c_mix in a binary mixture of non-ideal gases A and B to their relative abundances is a complicated function incorporating the gas temperature and pressure and various thermodynamic properties-including the critical parameters of the constituents. In the analysis of mixtures with a low (0.1-1%) concentration of a heavy vapor whose molecular weight differs from that of the carrier by a factor that is large in comparison with the ratio of their isentropic coefficients γ_A, γ _B, the relation between the concentrations (X_A, X _B) and c_mix is considerably simplified. The approximations underlying the derivation of the simplified binary gas equation are examined and appear justified for practical situations exemplified by (a) a relatively heavy hydrocarbon vapor such as toluene (M=92) at <1000 ppm in air of known moisture content, and (b) Tri-Methyl Indium (TMI: MW=160) at <1000 ppm in a hydrogen carrier. Starting with an ultrasonic gas flowmeter, if one adds a binary gas mixture analysis capability, one obtains possibilities for an ultrasonic mass flowmeter