Locating the inflection point of frequency-dependent velocity dispersion by acoustic relaxation to identify gas mixtures

Measuring adiabatic sound speed is an effective method to characterize gases with different molecular weights because sound speed mainly depends on molecular weight at a given temperature. However, it is still a challenge to apply this method to different gas mixtures with similar or even the same sound speeds. Acoustic relaxation in gases may overcome this challenge because sound speed becomes dispersive due to frequency-dependent heat capacity. Based on our previous work on reconstructing acoustic velocity dispersion with a simple measurement method, in this paper, we propose capturing the inflection point of velocity dispersion to identify gas mixtures. Standard detection areas are constructed using the theoretical location of the inflection point scaled by the acoustic velocity and relaxation frequency with different temperatures for target gases. The captured inflection point is located in the detection areas to obtain gas compositions. Thus, gas mixtures with the same molecular weights, such as 86.9% CO2–13.1% N2, 95% CO2–5% H2 and 95% CO2–5% pH2, can be differentiated using our method from only their acoustic velocities. The results show that the maximum absolute error of the compositions for CO2 can be effectively reduced from 3.8% to 0.2% by our temperature correction function. Therefore, the proposed method can identify gas mixtures qualitatively and quantitatively by only measuring acoustic velocity.