Miniature mid-infrared photoacoustic gas sensor for detecting dissolved carbon dioxide in seawater

A miniature mid-infrared photoacoustic (PA) gas sensor with a chamber volume of approximately 73 mu L for detection of dissolved gases in seawater was presented. This sensor offers several advantages: small size, high sensitivity, low cost, and low power consumption. The strong absorption band of carbon dioxide (CO2) near 4.26 mu m was targeted by utilizing a mid-infrared microelectromechanical system (MEMS) thermal light source and a narrow bandwidth optical filter. The thermal light source was electrically modulated, which simplified the sensor structure. Double-pass absorption was implemented to enhance the PA signal, which was detected by a small and low-cost MEMS electric microphone. The gas entered the sensor through a small hole in a diffusion manner without the need for valves or an air pump. This design feature contributes to a compact structure with dimensions of only 2.43 cm x 1.25 cm x 1.8 cm. The analysis results showed that the minimum detection limit (MDL) of CO2 reached 0.72 ppm with an averaging time of 100 s. A polydimethylsiloxane (PDMS) membrane was used as the water-gas separation unit. The performance of the designed PA sensor for detecting dissolved gases in seawater has been verified. When the flow rate of the water pump was 600 mL/min, the time for water-gas equilibrium was only 3.5 min. This can be attributed to the small size of the gas chamber and the rapid water exchange.