The Differential Optical Absorption-line Measurement Of Harmful Gases

With the accelerated process of industrialization, environmental issues have been the focus of the people's attention. One of the environmental issues, air pollution causes more and more impact on the people's health, so the detection of environmental pollution gas becomes essential. There are several methods of gas detection, the traditional methods have the drawbacks, such as complicated operation, heavy equipment, some hazards, not real-time measurement. This thesis uses the optical absorption principle to do the gas concentration measurement design for three common harmful gases carbon monoxide, methane and formaldehyde, aiming to design a safe, convenient, real-time monitoring and detecting small regional atmospheric gas meter.The law of optical absorption principle can be described by Lambert-Beer law. In order to apply the law for the gas detection, the analysis of the gases'optical absorption properties is necessary. This thesis gives wave band analysis for the three gases carbon monoxide, methane and formaldehyde in the band of absorption spectra, and the comparative analysis for each other. Three gas absorption peaks for the gases in the mid-infrared are taken as their measurement waves. Direct use of Lambert-Beer law in gas concentration detection has many difficulties due to the existence of some kinds of errors. In this thesis, the differential optical absorption measurement method is used to design the structure of gas concentration measurement. Differential optical absorption measurement method has several specific forms. This thesis analyzes several important ways, and adopts a improved dual-wavelength single-beam method which is provided in this thesis. The LED together with the use of optical filter is selected to obtain dual-wavelength light sources. Dual-wavelength single-beam method on one hand detects the intensity of the light absorption at one of the gas absorption peaks, which can be used to calculate gas concentration, on the other hand implements the same measure at the reference wavelength near the absorption peak wavelength intending to use the quotient of the detected light intensity at the gas absorption peak wavelength and the detected light intensity at the gas reference wavelength to eliminate the errors caused by light intensity fluctuation of light source and affections of other gases and smoke particles. According to the use of three types of gas measurement wavelength, this thesis selects the LED light source, detector and filter, and uses ZEMAX optical design software to do the corresponding optical design such as transmitter and receiver. The loss rate of light path is calculated in this thesis. The detected light signal must be processed initially, which involve the weak signal amplification, filtering and the digital signal converting, so amplifier, filter, peak detection circuit etc are designed in this thesis.