Design of a LED-based sensor for monitoring the lower explosion limit of methane

Here we present the development of a gas sensor system for the monitoring of the lower explosion limit (LEL) of methane. A mid-infrared light emitting diode (LED) with central emission wavelength of 3.4mm is used as light source to probe the n3 rovibrational absorption band of methane. To gauge the light intensity, photoacoustic detectors consisting of a microelectromechanical system (MEMS) microphone and pure methane gas inside a hermetically sealed miniature cell are employed. In order to provide means for compensating fluctuations of the LED intensity, we use two equal detectors at two different optical path lengths from the light source. The sensor has been characterized in our laboratory and the results show a limit of detection of about 2,500ppm at an optical path length of 12mm. No cross-sensitivities to humidity have been detected. The concept can be adapted to other gases with absorption bands in the infrared spectrum.