Mining Publication: A Second-Generation Remote Optical Methanometer

As the use of deep-cut mining increases, there is expected to be a greater demand for methane (CH₄) measurement devices capable of scanning working faces at depths in excess of 10 m to alert mine personnel of hazardous concentrations of methane and to satisfy regulatory requirements. Remote optical devices offer the most promise to satisfy these objectives while the operator remains safely under supported roof. A second generation remote optical methanometer (ROM) has been designed and is undergoing laboratory evaluation at the National Institute for Occupational Safety and Health. It operates on the principle of the "u₃" fundamental absorption band of CH₄ centered at 3.31 µm in the infrared region off the spectrum. The new instrument differs from the previous prototypes by the replacement of the two-band pass differential absorption technique of measurement with a single-filter gas correlation technique and a refinement of the amplifier and gating circuitry used within the unit. This results in a more accurate isolation of the CH₄ spectral absorption band centered at 3.31 µm. A specially designed 0.4-m-diam variable length gas mixing tunnel was constructed and instrumented with CH₄-sampling capability to calibrate the ROM. Experiments have demonstrated the ability of the detector to respond to various conditions of CH₄ (pct * m) at distances as great as 12 m. Thermal radiation generated within the instrument is the controlling factor in the drift of the output signal and, therefore, the reliability of the instrument. Several approaches to reduce this extraneous radiation interfering with the infrared detector are described, such as installation of a fan in the instrument and thermoelectric coolers on the detector's heat sink. Implementations of the technology will significantly improve the safety of miners.