Study On SF₆ Detection By Use Of Photoacoustic Spectroscopy With Tunable CO₂ Laser

As a gas with excellent insulation property and arc-quenching capacity,SF6 has been widely used in high-voltage electrical equipment,In recent years,with the rapid development of electric power industry,the use of SF6 gas is increasing rapidly,and the leakage of SF6 is becoming more and more serious.The leakage of SF6 not only will threaten the safely operating of power equipment,but also may exacerbate the global warming.Thus,measures and endeavor have been taken to detect the leakage of SF6 by many researchers.A gas sensing system of SF6 based on photoacoustic spectroscopy techniques has been put forward by our lab,and the characteristics and the limits of detection of the system were studied in detail.Firstly in this thesis,the related physical mechanisms of photoacoustic sensing of the system were briefly discussed,and the gas absorption coefficient and the expression of photoacoustic signal were also derived,According to the formula and the HITRAN database,a simulation of gas absorption coefficient was programmed.On the basis of the contrastive analysis of the absorption spectroscopy of SF6 and ambient air,the CO2 laser was chosen as the pumping source and the 951.203 cm-1 and 947.777 cm-1 line was chosen as the detecting line.In order to obtain photoacoustic signal with low loss,the radius of resonant cavity was 3 mm with length of 160 mm,The measured values of chamber constant was 89.74 Pa·m·W-1.Then,We analyzed the noise characteristics of photoacoustic system and got the total noise of the system which was 2.3 m V.Besides,a narrow-band filter with ten times frequency attenuation coefficient of-26 d B and a band-pass filter with one of-20 d B were also used to compress the noise.The detecting capacity of the system was also studied.The intensity of photoacoustic signal was proportional to the power of pumping source,and the optimum signal modulation frequency was 1100 Hz with duty ratio of 40%.The limits of detection(LOD)of 0.06 ppm was achieved using traditional photoacoustic spectroscopy with the laser line at 951.203 cm-1.In order to overcome the interference of the photoacoustic signal of air to the LOD in the atmospheric environment,a sensing method based on dual-wavelength differential photoacoustic spectroscopy was put forward at the final part of this thesis.The experimental result show that under the situayion of the laser with power of 3 watts at 951.203 cm-1 and 947.777 cm-1,by the method of differential photoacoustic spectroscopy detecting,the LOD for SF6 reached to the level of 0.02 ppm.The result show that: the photoacoustic system can detect SF6 in the atmospheric environment and the Difference photoacoustic spectroscopy can effectively reduce the noise and increase the LOD of photoacoustic system,which demonstrate that our research has good practical value.