Research On Gas Detection Technology Of Photoacoustic Spectroscopy Based On Ultrasound Frequency

In the power system,the role of the transformer is indispensable,and its healthy and stable working state is a necessary condition for the normal operation of the power grid.After summing up experience,people have found that H2,CO,C2H2,C2H4 and other gases will be dissolved in the oil when the transformer fails,and the composition and concentration of these gases are closely related to the cause of the transformer failure.Photoacoustic spectroscopy gas detection technology has the advantages of high sensitivity,low detection limit,good long-term stability,and supports simultaneous detection of multicomponent gases.It is regarded as an ideal method for online gas detection in oil.This thesis is based on the principle of photoacoustic spectroscopy gas detection.Aiming at the problem that the resonant frequency of the currently applied photoacoustic spectroscopy gas detection system overlaps with the background noise frequency and the volume of the photoacoustic cell is large,a resonant frequency is designed.Compared with the ordinary photoacoustic cell system,the miniature photoacoustic cell in the 30.1k Hz ultrasonic frequency band has the advantages of small size,low co-frequency noise,simple structure,and low cost.It provides a miniaturization and productization for photoacoustic spectroscopy gas detection technology.The main contents of this thesis are as follows:(1)According to the gas absorption spectrum theory and the generation and detection principle of photoacoustic effect,the different working modes of resonance and nonresonance are compared,and the factors that determine the strength of the photoacoustic signal are analyzed,which is the structure of the photoacoustic cell.Optimization and simulation design determine the theoretical direction.The principle of wavelength modulation and second harmonic detection technology is analyzed,and the signal detection scheme is determined.(2)Based on the principle of photoacoustic spectroscopy detection technology,the infrared light source and laser light source and their respective modulation methods were compared and analyzed,and the microphone and its frequency response were selected and analyzed,and then the structure and size of the photoacoustic pool were analyzed using matlab and COMSOL software carried out a simulation design,analyzed the principle of cross-correlation detection and phase-locked amplification,and finally determined a resonant photoacoustic cell with a tunable DFB laser as a light source,a miniature MEMS microphone as an acoustic sensor,and a resonance frequency around 29 k Hz in the ultrasonic frequency band.,Combined with phase-locked amplification to extract the second harmonic signal gas detection system program.(3)Through the construction and test of the experimental system,the actual resonance frequency of the photoacoustic cell was measured at 30.1k Hz,the quality factor was 16.45,and the appropriate modulation depth for acetylene gas detection was determined to be138 m V,and then the 50ppm-1500 ppm concentration of acetylene gas was measured.The second harmonic detection and analysis showed that the final acetylene gas detection limit of the system was 5.4ppm,and through noise comparison analysis,it was determined that the background noise level of the system was reduced by about 32.4% compared to the low resonance frequency of the photoacoustic pool of 1900 Hz,thus effectively reducing the background noise interference of the same frequency.(4)According to the experimental results,the shortcomings of the experimental system are analyzed,and the improvement plan including the optimization of the photoacoustic cell structure and improving the integration of the system is proposed,which points out the direction for the further improvement of the performance and practicality of the system.