Development Of Infrared Methane Concentration Detecting System

In the petroleum drilling and coal bed methane exploration, the flame ionization detector (FID) is used to analyze the hydrocarbon from drilling fluid. The FID exist much weakness such as complex structure, high price, long test periods and hardly continuous measurement. In coal mine, the electrical gas detector and electrochemistry gas detector are used to detect gas (CH4). They exist much weakness such as low sensitivity, low precision and easily being poisoned.To overcome the defect of gas concentration detectors above, using ADμC845 integrated with 24 bit ADC as the core, and using infrared light of 3.31μm wavelength as the excitation light, the thesis designed an infrared methane concentration detecting system based on the gas molecular infrared absorption spectrum. In the design of hardware, the thesis compared and chose the infrared light source, infrared sensor and filter, and proposed a new structure design scheme. The new structure realized the integrated design of the optical devices and gas chamber. Additional, the thesis designed filter and amplifier circuit, signal processing circuit and MCU interface circuit. In the design of software, the thesis proposed a method of data acquisition, and established a calibration scheme to calibrate the gas concentration detector.The system has experimented in the lab and field. The results show that the detecting system can operate stably and display the methane concentration accurately. The sensitivity of the system is 200ppm.The measuring range is 200ppm~100%. The precision is±0.78%FS and response time is less than 3 second. The system has overcome the weakness of the FID, and can replace the FID to detect the methane concentration of the drill fluid.