Research On Opticcal Fiber Methane Measurement System Based On Spectrum Absorption

With the rapid development of industry and agriculture, safety and environment protection of mine, oil field and electric power system is of great importance more and more. Thousands upon thousands of large-scale power factories and mine exhaust a great deal of flammable, explosive and poisonous gas in the air, and the living space and safety of human's are seriously menaced, so rapid and real-time measurement of the gas is very important for safety production of mine and environment protection. In this paper, the main component of mine gas methane is used as a research sample. An optical fiber gas measurement system is set up based on spectrum absorption method. The system has the advantages of high sensitivity, fast responding speed, freedom from inflammation and explosion, large dynamic range and freedom from poisoning, and the detecting segment can be placed in the flammable, explosive, poisonous places that people can't come in and instruments can't be placed in. Thus the concentration of methane can be remote real-time detected, and the system will give an alarm to people when it is dangerous, so the explosive accident caused by gas in mine can be avoided. After analyzing the theory of near-infrared selective absorption of gas, we describe the academic base of the measurement of the gas by absorption theory. The absorption spectra of methane are confirmed and the spectra characteristics which is fit for low-loss optical fiber to transmit a long way is also discussed. Distributed feedback laser diode (DFB LD) is used as a light source and light source modulation harmonic measurement is presented in this paper. The ratio of the fundamental and second harmonic signal can be used for eliminating the interference owing to light power changing. The mathematical model of gas concentration harmonic measurement is built up. Near-infrared light intensity stability technique of the light source, techniques of DFB LD's frequency locked to the gas' absorption line and the measured signal and reference signal's acquiring and ratio processing techniques are discussed in detail. Light path, gas cell and circuits of thesystem are designed and the system components including lock-in amplifier, weak signal measurement circuit and microcomputer processing circuit are also shown.Experimental research of the optical fiber gas sensor for methane has been completed and the feasibility is enhanced. The results of experiments are shown in the end.