A mid-infrared methane detection device based on dual-channel lock-in amplifier

Abstract

A portable dual-channel digital/analogue hybrid lock-in amplifier (LIA) is developed, and its amplitude detection error is less than 10% when the signal-to-noise ratio (SNR) is larger than −12 dB. Then, a differential mid-infrared methane (CH₄) detection device is experimentally demonstrated based on a wideband incandescence wire-source and a multi-pass spherical reflector. The experiments are carried out to obtain the sensing performance of the device. With the absorption length of only ∼4.8 cm, the limit of detection (LoD) is about 71.43 mg/m³, and the detection range is from 0 mg/m³ to 5.00×10⁴ mg/m³. As the concentration gets larger than 714.30 mg/m³, the relative detection error falls into the range of −5%–+5%. Two seven-hour-measurements are done on the CH₄ samples with concentrations of 1.43×10³ mg/m³ and 4.29×10³ mg/m³, respectively, and the results show that the maximum relative 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 device shows potential applications of CH₄ detection in coal mine production and environmental protection.

Keywords

Maximum Relative Error / Detection Device / Absorption Length / Tunable Diode Laser Absorption Spectroscopy / High Concentration Range