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The instrument adopts ultraviolet fluorescence method. The basic working principle is as follows: the zinc lamp emits ultraviolet light, which is filtered by a monochromatic filter into a monochromatic light with a wavelength of 214nm, which is focused on the SO 2 reaction chamber. The SO 2 in the sample gas absorbs the ultraviolet light and produces an energy level transition. SO 2 changes from the ground state to the excited state, and the excited state so 2 is unstable. A fluorescence with the central wavelength of 330 nm is emitted instantaneously The concentration of SO2 is proportional to the concentration of SO2, and the optical signal is transformed into electrical signal by photomultiplier tube. In order to eliminate the measurement error caused by the fluctuation of the light path, two monitoring channels are used in the optical path. One is to measure the fluorescence intensity excited by the light path of the sample gas, and the other is to measure the reference light path to monitor the change of the light source of the zinc lamp and automatically compensate for the change of the light source.