Gas Detection System Based On CO₂ Laser And Quartz-enhanced Photoacoustic Spectroscopy Technology

SF₆ has been widely used in high voltage power system and switch gears,the detection of trace gases in SF₆ plays an important role in monitoring atmospheric environment and maintaining power system.At present,photoacoustic spectroscopy technology is widely used in the detection of trace gases,which has the advantages of fast response and high accuracy.A conventional photoacoustic spectroscopy gas detection system is used to detect SF₆,and the detection sensitivity of this system is0.09 ppm,however,this system is easily affected by external noise.In order to overcome this weakness,quartz enhanced photoacoustic spectroscopy(QEPAS)has been proposed by domestic and foreign researchers in recent years.In this thesis,CO₂laser is used as excitation source,and a gas detection system of SF₆ based on quartz enhanced photoacoustic spectroscopy is designed.In this paper,the basic principle of photoacoustic spectroscopy,including the infrared absorption theory of gas molecules and the generation of photoacoustic signals,are briefly described firstly.The characteristics of QEPAS and conventional photoacoustic spectroscopy are compared and analyzed.The fundamental of tuning fork as a sound accumulation device is analyzed from the mechanical structure and piezoelectric effect of tuning fork.The off-beam assembly structure of the miniature acoustic resonance tube and tuning fork is designed,and the size of the acoustic resonance tube is optimized.Besides,different kinds of gases may have overlapping absorption peaks in the same spectral range,therefore,the absorption spectrum of SF₆is compared with that of standard air.Finally,947.867 cm^-1 is selected as the detection spectral line of SF₆.In this research,an experimental system is designed and built to study the parameters of the system.The resonance frequency of the system is 32770 Hz.The influence of temperature,air pressure and buffer gas on the photoacoustic signal is studied,which provides a reference for the calibration of the system parameters.And then,the power characteristics of the system are studied,and the laser power of the system is determined to be 2 W.For the problem of noise caused by the increase of laser power,electrical modulation cancellation method is adopted to effectively reduce the system noise to 1.2V.Then,the concentration of SF₆ is calibrated by experiments and least square method at normal atmospheric pressure.According to the calibration results,A normalized noise equivalent absorption coefficient(1)of 2.17×10^-5cm^-1·W/Hz^1/2 is obtained,the detection sensitivity(1)is 15.4 ppb,and the stable response time of the system is 40s.