Ppb-Level Miniaturized Flow Gas Sensor Based on Photoacoustic Spectroscopy With Advanced 3-D Printed T-Type Resonant Cell

We present a highly sensitive, miniaturized flow gas sensor with advanced T-type photoacoustic cell (TPAC) for dynamic trace gas detection. The tube effect inside the TPAC was innovatively discovered and analyzed, which would affect the acoustic excitation mode, leading to a first-order cylindrical resonance mode with three operating frequencies. A TPAC with a rounded transition cross section was proposed, as well as its acoustic and fluid properties were investigated. The geometry of the transition section and the tube was further optimized to reduce the flow noise and enhance the signal. The TPAC was 3-D printed with a volume of 5.7 mL. The wavelength modulation spectroscopy and second-harmonic (WMS-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2{f}$ </tex-math></inline-formula>) technique was applied in the gas sensor. At a flow rate of 50 mL/min of acetylene, the minimum detection limit (MDL) reached 64.5 ppb with 1-s integration time, corresponding to a normalized noise equivalent absorption (NNEA) coefficient of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.28\times 10^{-{8}}$ </tex-math></inline-formula> cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{1}}\cdot \text {W}\cdot \text {Hz}^{-\text {1/2}}$ </tex-math></inline-formula>. With the relatively large flow rate up to 500 mL/min, the signal-to-noise ratio (SNR) stayed above 60 dB and the MDL was measured to be 277.7 ppb. This work allows new perspectives for the applications of photoacoustic spectroscopy.