Research Of CO₂ Gas Concentration Detection Basedon QEPAS And The Impact Of H₂O

Carbon dioxide?CO₂?is the most important factor of greenhouse effect.The CO₂ gas concentration detection plays a positive role in monitoring environmental quality,the governance of industrial emissions and prevention of human diseases.Quartz tuning fork enhanced photoacoustic spectroscopy?QEPAS?is a new gas detection technology based on spectroscopy.This technology has become an important method in the field of gas detection because of its advantages of good gas selectivity,fast response time and high measurement sensitivity.Based on the theory of gas infrared absorption spectroscopy,laser wavelength modulation,harmonic detection and characteristics of quartz tuning fork,this paper described the measurement principle of QEPAS technology.Using QEPAS technology to detect the photoacoustic signal,the concentration of CO₂ gas to be measured was obtained by the system inversion.Humidity is an important factor affecting the detection of QEPAS system.Based on the structure and electrode properties of gas molecules,the D-L collision energy transfer model of CO₂-N₂-H₂O gas mixture was established.By discussing the energy transfer and molecular relaxation process in the mixture,the relationship between H₂O concentration and the amplitude of the photoacoustic signal was determined.And the effect of H₂O on the relation time rate and photoacoustic signal of gas molecules was analyzed.Built a CO₂ gas concentration detection system based on QEPAS.According to the absorption characteristics of CO₂ gas based on HITRAN molecular spectral database selected near infrared 1580.0395nm as the absorption line to be measured.Selected the DFB laser as the light source and realized the wavelength modulation of the laser by the laser driver.Established the collimation focusing path to improve the laser utilization.Completed the signal demodulation and filtering through the transimpedance preamplifier and the lock-in amplifier.Proceeded the demodulation signal control and measurement data collection through the electronic control center CEU.Carried out data processing and signal acquisition based on the preparation of LabVIEW procedures.In this paper,the output wavelength of DFB laser was 1580.35nm.The operation temperature and current sweep of the DFB laser were determined by temperature and current tuning.Make the output power of the laser reach the maximum by adjusting the optical path.The performance of the detection system was optimized by determining the optimum H₂O content,the optimum detection pressure and the best modulation depth.Based on the QEPAS technology,different concentrations of CO₂ gas were measured and the concentration of CO₂gas to be measured was calculated by the system inversion.In order to analyze the influence of H₂O on the amplitude of the photoacoustic signal and the signal-to-noise ratio of the system,implemented the gas concentration detection in the optimum H₂O content and without H₂O system by selecting the same gas concentration of CO₂.By comparing the test results of the optimum H₂O content of the system with no H₂O system measurements,it is confirmed that H₂O has the effect of increasing the amplitude of the photoacoustic signal and the system SNR.The detection limit of the system was 496ppm for the CO₂ concentration measured in the gas sample with an H₂O content of 0.8%.