Multi-gas sensing system based on miniaturized cruciform photoacoustic cell

Abstract

Photoacoustic spectroscopy has the advantage of multi-gas sensing with only one sound detector because of its non-selectivity. A photoacoustic cell (PAC) is an important part of the system as it amplifies the intensity of sound pressure. In this paper, we report on the development of a multi-gas sensing system based on a specially designed cruciform photoacoustic cell (CF-PAC), whose structure contains two beam paths and is suitable for multiple laser beams incident from different directions independently. It is especially suitable where beams are not easy to combine due to large wavelength differences. The structure of the cell is carefully designed using the finite-element method (FEM) and the resonant frequency of it is about 5050 Hz and the external dimension is only 67×60×40 mm with the inner volume of about 24 cm3. We also present a contrast experiment between the CF-PAC and the conventional “H-type PAC” with similar parameters and the result shows that the former sacrifices only a small amount of performance, but adds beam path effectively. The sensor is designed to detect methane (CH4), nitrogen dioxide (NO2), and carbon dioxide (CO2), and the minimum detection limit (MDL) with a 5 s integration time are 113 ppb, 2.1 ppm, and 1.6 ppm, respectively. The 1σ normalized noise equivalent absorption coefficients (NNEA) of the three gases are 2.54×10−9 cm−1 W Hz−1/2, 5.70×10−8 cm−1 W Hz−1/2, and 2.31×10−6 cm−1 W Hz−1/2, respectively. Modified formulas based on nonlinear regression will be given to make the detection results more accurate in the multi-component gas environment.