Synthesizing Primary Molecular Relaxation Processes in Excitable Gases Using a Two-Frequency Reconstructive Algorithm

Abstract

Identifying molecular relaxation processes in excitable gases remains challenging. An algorithm that reconstructs the primary relaxation processes is presented. Based on measurements of acoustic attenuation and sound speed at two frequencies, it synthesizes the entire frequency dependence of the complex effective specific heat of the gas, which is the macroscopic "footprint" of relaxation effects. The algorithm is based on the fact that for a simple relaxation process, such as occurs in many polyatomic gases at temperatures around 300 K, the effective specific heat traces a semicircle in the complex plane as a function of frequency. Knowing the high-frequency or instantaneous value of the specific heat provides the capability to not only sense the presence, but also infer the nature and, for mixtures of unlike-symmetry molecules, the concentration of foreign molecules leaking in a host gas.

Relaxation Time Coupling Method for Ultrasonic Sensor Design

“…1, and the other circles are also experimental data. With these five pairs of data, we synthesize them to a measured spectral peak by the algorithm proposed in [5]. The synthesized results are shown as Plus in Fig.…”

“…Then with the calculation results, the spectral peaks are predicted. Finally, we compare the predicted spectral peak value with the measurements data by the synthesizing algorithm [5]. The prediction results and relative errors are given in Table. 2.…”

“…Based on the decoupling model, Zhang et al suggested that there will be as many relaxation times as there are vibrational modes available to a gas, and the relaxation time of the primary relaxation process determines the spectral peak location in most cases [6]. However, we think that the "primary relaxation time" in [6] τ is the whole relaxation time of the whole relaxation process [5].…”

“…Based on this method, a gas detection sensor with two pairs of ultrasonic transducers are suggested to practical applications [4]. In a simple relaxation process, the acoustic relaxation absorption spectral peak location is determined by the relaxation time [5,6]. Therefore, calculating the relaxation time in excitable gases is a key issue to apply the spectral peak location based method in gas detection.…”

A prototype acoustic gas sensor based on attenuation

“…That is where measurements of the acoustic attenuation come into play. As shown in our previous work, 12 precise measurements of sound speed and attenuation can lead to ͑quasi͒ quantitative gas analysis via the identification of the molecular symmetry properties, if not of the molecules themselves. In the future, this could lead to the development of rugged "smart" acoustic sensors capable of quantitatively determining gas composition in various environments and processes.…”