Mid-infrared ppm-level methane detection device using small-size absorption pool and dual-channel lock-in amplifier

A portable dual-channel digital/analogue hybrid lock-in amplifier (LIA) was developed, and its amplitude detection error is less than 10% when the signal-to-noise ratio is larger than −12 dB. Then, a differential mid-infrared methane detection device was experimentally demonstrated based on a wideband incandescence wire-source and a multi-pass spherical reflector. Detailed detection principle was described, and key modules including the optical part and the electrical part were designed and fabricated. Detailed experiments were carried out to obtain the device's sensing performances. With the absorption length of only ∼4.8 cm, the limit of detection (LoD) is about 100 ppm within the detection range of 0-70 k ppm. As the concentration gets larger than 1 k ppm, the relative detection error falls into the range of −5% ∼ +5%. Based on two seven-hour-measurements on the 2 k and 6 k ppm methane samples, the maximum absolute error is less than 5%. Because of the cost effective incandescence wire-source, the small-size and inexpensive dual-channel LIA, and the small-size absorption pool and reflector, the developed standalone device shows potential applications of methane detection in coal mine production and environmental protection.