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With regard to the high accuracy optic-fiber sensor for monitoring methane concentration, the
choice of light source depends on methane peak values. Besides, the environment of mine should be
considered, that is to say other gas should be considered, such as vapor, CO and CO2 etc, without absorbent
spectrum in the decided wavelength. It has been reported that vapor, CO and CO2 have no obvious absorption
in 0.85μm, 1.3μm and 1.66μm area, CH4 has no obvious absorption in 0.85μm area. So diode laser with
1.3μm or 1.66μm peak wavelength is chosen as the optic-fiber sensor's light source for detecting methane
concentration. On the basis of the principle of optic absorption varied with methane concentration at its
characteristic absorbent wavelength, the advantage of optic-fiber sensor technology and the circumstance
characteristic of the coal mine. An optic-fiber sensor system is presented for monitoring methane
concentration. Space Division Multiple Access Technology (SDMAT) and long optical path absorbent pool
technology are combined in the study. Considering the circumstance characteristic of the coal mine, the
optic-fiber network sensors for detecting methane concentration from mix gas of vapor, CO, CH4 and CO2 are
used. It introduces the principle of an optic-fiber sensor system for monitoring methane concentration in coal
mine. It contains the structure block diagram of monitoring system, the system is mainly made up of diode
laser for monitoring methane concentration, Y-shaped photo-coupler with coupled rate 50:50, optical switch
1×2, gas absorbent cell, the computer data process and control system and photoelectric transformer. In this
study, in order to decrease to the influence of the dark-current of photodiode, intensity in light sources and
temperature drifts of processing circuit on the system accuracy in measurement, a beam of light is broken
down into two beams in the coupler of Y-shaped coupler, the one acts as the reference optical path, the other is
known as the sensing optical path. The experimental result shows that diode laser with 1654.141nm in
wavelength is taken as the optic source for detecting methane concentration, the detective limit of the sensor
is below 4.274mg/m3 when the optical path of absorbent pool is 20 centimeters, and the prevision and stability
could satisfy practical application. The whole instrument can also reach on-line measurement with multiple
points on different spot.
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