Design And Implementation Of Optical Gas Detection System Based On Integrating Sphere

In recent years, as people pay more and more attention to their own living environment and the problems of air pollution, how to reduce the harm which the toxic and harmful gases bring to human has become an important problem which needs to be solved urgently. Therefore, it is necessary to monitor the pollution gas effectively. Spectroscopy gas detection method developed fast and has become an important way in the area of harmful gas detection because of its features of non-contact detection, fast response and so on. The research content of this thesis is to build a gas detection platform based on absorption spectrum technology and use nitrogen dioxide gas to verify the performance of the system.Firstly, the theoretical basis of the quantitative analysis of spectrum absorption gas-- Lambert-Beer’s law was introduced, and the scope of the application of the law was also discussed in this dissertation. On this basis, Lambert-Beer’s was modified appropriately so that the results will be more suitable for the actual situation after considering the Rayleigh scattering and Mie scattering existing in actual environments.Secondly, the author focused on the study of gas chamber sensing unit of the gas detection platform. According to Lambert-Beer’s law, the detection limit of the system can be effectively increased by increasing the sensing length of gas path. So how to design an efficient gas chamber is one of the key technologies in the whole system platform. On the basis of comparing and analyzing the advantages and disadvantages of traditional gas chamber, an integrating sphere was adopted as the absorption gas chamber of this system. After the light enters into the integrating sphere, it bounces back and forth in the inner wall of the integrating sphere evenly for many times with high reflectivity coatings to increase the optical length. Due to the light propagates randomly in the integrating sphere, the sensing length in the integrating sphere cannot be calculated using traditional algorithm. Accordingly, a mathematical model for optical length calculation of the integrating sphere gas chamber was established. And the equivalent optical path length was also derived in this thesis. On the basis of discussing the effect that the opening ratio and the coating reflectivity brings to the equivalent optical path length, the design of the integrating sphere gas chamber was completed.Finally, a gas detection experiment system based on absorption spectroscopy technology was built. The system was used to detect the absorption response of nitrogen dioxide gas and by polynomial fitting of absorption and absorption cross section, the differential absorption and differential absorption cross sections were obtained. Then the least square method was used to calculate the gas concentration. At the end of the thesis, the concentration inversion and the inversion error are also given. Experimental results show that the system has high detection accuracy and stability.