A multi-point fibre optic sensor array for methane detection ("OMEGA") has been developed and tested under semiquantitative field conditions. The new system employed wavelength modulation spectroscopy using a DFB laser source scanned across the Q6 methane line at 1.665•m. A branched fibre network connected the single source to up to 64 sensor heads. Controlled releases of natural gas were provided for test purposes within an array of four optical sensors and four pellistor reference sensors. An automated system delivered standard gases to each sensor, to enable routine calibration checks to be carried out. Agreement between the conventional and optical systems was excellent in the range 0-100% LEL (lower explosion limit). The optical system offers a simple, intrinsically safe design with a low cost of ownership per sensor head.
The planar pellistor is a thin film microsensor used for the detection of inflammable gases.' In its simplest it consists of a metallic meander structure that serves both as a heater for the catalyst and as a temperature sensor.2 For low power operation thermal insulation between the sensor and the substrate is vital and is achieved by a thin silicon nitride membrane. The optimisation of such a structure in terms of the sensor's dimensions is discussed.
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