Infrared analyzer with MEMS microspectrometer optimized for handheld explosive gas detection

This paper presents a compact handheld gas analyzer, optimized for the multicomponent analysis of hydrocarbon gases, based on a micromachined, tunable Fabry-Pérot filter (μFPF) as a key element. The tuning range from 3.0 μm to 3.7 μm and spectral resolution of 30 to 40 nm (FWHM) cover a region of characteristic absorption of many combustible hydrocarbons and allow an accurate measurement of the concentration of each individual gas. The analyzer functions like a microspectrometer and outperforms the current state of the art for small gas detection handheld devices, using pellistors and NDIR-infrared detectors. An innovative MEMS design with two movable reflectors is insensitive to acceleration forces. Due to the handheld application, especially the vibration response in the frequency range of 10 to 100 Hz was carefully characterized. An uncooled photodiode is integrated as fast detector, together with a transimpedance amplifier and a sapphire lens for light concentration. The photodiode is working in photovoltaic mode without supply voltage. In comparison to a photoresistor this gives the advantages of less power consumption and compatibility to explosion protection regulations. A demonstrator was developed by designing a smart sensor module including complex analog and digital signal processing. Integrating this into a commercial, handheld, explosion-proofed Multi-Gas-Detector model results in a robust instrument, which enables field tests under harsh environmental conditions, typical for fire brigade missions. Spectral analysis of combustible gases (methane, ethane, propane and acetylene) is demonstrated with focus on safety-relevant LEL (lower explosion level) detection.