Miniature diffusive mid-infrared photoacoustic gas sensor for carbon dioxide detection

A low-cost miniature diffusive infrared photoacoustic (PA) sensor (MDIPS) was proposed and designed for the detection of carbon dioxide. To reduce the volume and the power consumption, the air pumps and air valves in the traditional PA sensing system were replaced by a breathable hole and filter membrane. Through finite element analysis, the radius of the optimized membrane was selected between 0.2 μm and 0.5 μm. Rapid gas exchange is achieved while effectively suppressing the loss of PA pressure. In addition, a micro-electro-mechanical (MEMS) mid-infrared thermal light source and a MEMS microphone were integrated into the PA sensor. Through theoretical analysis and experimental verification, the volume of the cooper tube in the PA sensor was designed as only 126 μL. A signal processing algorithm combining lock-in amplification and Kalman filtering was implemented in a microcontroller to extract the weak PA signal from a background of substantial interference. The designed sensor achieved a minimum detection limit (MDL) of 2.6 ppm for CO2. The response time measured by 10 %–90 % step was 136 s.