Design of a portable optical sensor for methane gas detection

A detailed investigation has been carried out on the design of a low-cost portable optical sensor for methane detection with a sensitivity of ∼1% of the Lower Explosive Level (LEL) for methane (500 ppm) and able to operate in harsh environments with temperature variation between −20 and 50 °C. The sensor design is based on the use of near-IR LEDs operating around the overtone absorption lines of methane at 1660 nm using a stainless steel tube to direct the light through the gas to the detectors. Various configurations of source/detector layout have been examined to provide appropriate reference and signal paths in order to achieve reliable methane detection at LEL levels in the presence of temperature variation. An optimum design has been identified using two detectors with appropriate optical filtering and with temperature stabilisation of the source and detectors. Based on this design, a prototype instrument has been demonstrated with an ultimate sensitivity of 0.2% LEL methane (100 ppm).