Water Vapor Concentration Detection System Based On QEPAS Technology

Water vapor concentration detection technology is an important issue in the field of gas sensing.In the aspects of natural gas transportation,power transmission,food safety,industrial smelting and precision machinery maintenance,the concentration of water vapor is very demanding and needs strict control So there is an urgent need for in-depth study of water vapor concentration detection technology.Quartz-enhanced photoacoustic spectroscopy(QEPAS)is a new method of trace gas detection in recent years.Compared with traditional photoacoustic cell methods and other traditional detection methods,the volume,it has a certain advantage in cost,sensitivity and anti-interference.In particular,it is not affected by the micro-water vapor contained in the laser,collimator,overcoming the inherent underwater problem of absorption spectroscopy.In this paper,based on the study of QEPAS-based water vapor concentration detection system,the relevant principle is expounded,the function of the system is realized,the optical path and signal detection circuit are optimized,and by frequency lock method,the detection process problems and contradictions are solved and ultimately a low concentration of water vapor accurate calibration can be reached.The QEPAS technical principle relates to the absorption theory of infrared spectroscopy gas and the theory of multiple harmonics due to absorption.In this paper,the principle of QEPAS technique is described and deduced by the infrared absorption of gas molecules,the absorption law of gas molecules,the linear function and the wavelength modulation.Then,the piezoelectric characteristics of quartz tuning fork are described in detail.According to the principle of QEPAS and the related circuit knowledge,this paper introduces the function,composition and design idea of each part of the system,and establishes the QEPAS water vapor concentration detection system.After the program and system parameters are set up,the concentration of water vapor is detected in the atmosphere,the system function is initial realized.In order to improve the detection accuracy of the system,the amplitude of the core part of the system,which affects the signal-to-noise ratio(SNR)of the system,is optimized,and the SNR ratio is improved by one time.The characteristics of the resonant tube and the tuning fork are studied in detail,and the structure and size of the resonant tube are determined.It is found that the center frequency of the tuning fork change is small and the bandwidth change is obvious.A new method for the determination of QEPAS water vapor concentration was established.Aiming at the theoretical analysis and experimental verification of the laser driving signal scanning method and the frequency lock method,it is proved that the frequency lock method has a great advantage in the photoacoustic spectrum system.The SNR of the second harmonic signal is levied by a quantum leap in the air and reaches 500 by applying frequency lock method to QEPAS system.The detection limit of the QEPAS water vapor concentration detection system is 200ppm,the accuracy can reach 6ppm at some concentration,and the stability of the system is explored.Finally,the stability of the system and the long-term stability of the quartz tuning fork are studied.