Theory For Gas Sensing Technology Based On Acoustic Spectral Peak Location

Gas sensing technology has wide promising applications in industry process control, environment pollution monitoring and petrochemical industry, etc.. The ultrasonic based gas sensing technology becomes one of the most potential approaches for its attractive features of on-line measurement, rapid response, simple hardware and excellent long-term stability. To design a intelligent ultrasonic sensor for detecting gases qualitatively and quantitatively, the theoretical measurement model must be established firstly.This research begins with the development of the ultrasonic based gas sensing technology. The problems in the existing measurement theory are also introduced in detail. Then, the research focuses on three aspects as follows:Firstly, the traditional ultrasonic based gas sensing technology monitors target gases by observing the whole acoustic relaxation absorption spectra, but the acoustic relaxation absorption spectra cannot be obtained easily in practical applications. To solve this problem, acoustic relaxation absorption spectral peak location is employed in this research to detect gases. Since acoustic relaxation absorption spectral peak can be synthesized by the sound speed and the acoustic absorption coefficient at two measured frequencies, this research proposed a gas detection method based on acoustic absorption spectral peak location. This method combines the ambient temperature and the gas composition together to construct effective relaxation area for gases. The target gas can be detected by locating the measured spectral peak in the effective relaxation area.Secondly, when there are two strong relaxational gases in a mixture together, the decoupling model cannot predict the acoustic absorption spectral peak value accurately. Therefore, the effective relaxation area predicted by the decoupling model is wrong. This research expands the decoupling model with coupling the single relaxation times as a whole relaxation time. The spectral peak location can be calculated with this while relaxation time rather than the first relaxation time. The experimental results indicate that the proposed coupling relaxation time model matches the experimental data better than the decoupling model.Thirdly, the coupling relaxation time model is applied in the spectral peak based gas sensing method. This research proposed how to construct the effective relaxation area by the coupling relaxation time model. The experimental results in CO2-N2 and CH4-N2 indicate that the proposed coupling relaxation time model can replace the decoupling model to construct effective relaxation area. Meanwhile, for the gas mixtures with two strong relaxational gases, such as CO2-CH4, the predicted spectral peak location matches the two frequency measurement method better than that predicted by the decoupling model. The detection of CO composition in the air and the low quality natural gas detection are proposed to validate the proposed spectral peak location based gas sensing method.