Research On Trace Acetylene Gas Detection System Based On QEPAS Technology

High-sensitivity trace gas detection sensors have a wide range of applications in transformer failure,environmental protection,personal safety protection,etc.Traditional electrochemical methods,gas chromatography and other trace gas detection methods have low detection accuracy and slow response speed.The disadvantage of complicated operation.Quartz-enhanced photoacoustic spectroscopy（QEPAS）technology is a high-precision trace gas detection technology,which has the characteristics of fast response,high sensitivity,and low detection limit.Acetylene（C2H2）is a common gas.The detection of trace acetylene concentration in transformer insulating oil can achieve the purpose of transformer fault warning and diagnosis,thereby preventing unnecessary economic losses and ensuring personal safety.Therefore,this thesis uses the quartz-enhanced photoacoustic spectroscopy system to carry out theoretical,simulation and experimental research on the detection of trace acetylene.The main content of this article is as follows:（1）The basic principle of photoacoustic spectroscopy is introduced,and the analytical formula of photoacoustic signal is given.The basic theory of photoacoustic spectroscopy is applied to the QEPAS system for theoretical analysis,and the parameters that affect the lower detection limit of the photoacoustic spectroscopy system are obtained,and the direction of the optimization design in the QEPAS system is obtained,and corresponding simulations are carried out based on the conclusions of the theoretical analysis.And its experimental work.The quartz tuning fork and its piezoelectric effect are introduced.The laser wavelength modulation technology and the second harmonic are theoretically analyzed.（2）The on-axis QEPAS system was simulated based on COMSOL.The model of the tuning fork and the resonance tube in the shaft system is established,and the resonance frequency of the standard tuning fork is obtained by simulation.The sound pressure distribution inside the axial QEPAS system was explored.The optimal laser incident position,the relative position of the tuning fork and the resonance tube,and the inner diameter length parameters of the resonance tube in the axial QEPAS system were simulated and optimized,and follow-up experimental work was carried out based on the conclusion of the simulation optimization.The simulation results have high guiding value for the design and assembly of the on-axis QEPAS system.（3）The off-axis QEPAS system was simulated based on COMSOL.The model of the tuning fork and resonance tube in the off-axis system is established.The sound pressure distribution inside the off-axis QEPAS system and at the opening of the resonance tube is explored.For the first time based on COMSOL for the best laser incidence position in the off-axis QEPAS system,the relative position of the tuning fork and the resonance tube,the inner diameter length parameter of the resonance tube in the off-axis QEPAS system,and the opening depth and The opening width has been simulated and optimized.The simulation steps and results have high guiding value for the design and assembly of the off-axis QEPAS system.（4）The on-axis QEPAS system was built.Firstly,an absorption line of 1532.83 nm was selected for acetylene gas,and a tunable DFB laser was selected as the light source accordingly,and the best working parameters of the DFB laser were selected through experiments.The resonant frequency and quality factor of the quartz tuning fork in a trace acetylene gas environment were tested.According to the simulation results,the parameter optimization experiment was carried out,and the inner diameter ID=0.4mm,the length Lc=4.7mm resonator tube was selected,which made the photoacoustic signal amplified by nearly 17 times,thus obtaining the lower detection limit of 3.24ppm.Subsequently,in view of the low power of the near-infrared DFB laser used in the traditional QEPAS,the EDFA was used to amplify the power of the light source,and the power-enhanced QEPAS system was built to detect trace acetylene,so that the lower detection limit of the system was reduced to 0.239ppm,and the system’s The normalized noise equivalent absorption coefficient is 1.41×10-7 cm^-1W/Hz^1/2.Finally,the stability of the system for a long time is studied by Allen analysis of variance,and the detection limit of 14.2ppb is obtained under the condition of an integration time of 54 seconds.