Research Of Gas Concentration Detection System Based On TDLAS

Gas concentration detection is of great importance in environment monitoring,industrial production,food safety,medical assistance,and daily life,and it has always been focused by domestic and foreign scientific research workers and social workers.With the development and application of gas concentration detection technology,spectrometry has been popular with the advantages of anti-interference ability,high sensitivity,non-contact and real-time on-line measurement.Generally,the gas detection sensitivity can be increased by extending the effective optical path length?EOPL?in the field of gas concentration measurement by spectrometry.In this paper,the theoretical,simulated and experimental research of diffuse cubic cavity,which is a new technique extending EOPL,were studied.Based on the tunable diode laser absorption spectroscopy?TDLAS?,we studied concentration measurement of oxygen with diffuse cubic cavity as the gas cell.Meanwhile,the measurement of carbon dioxide was studied based on tunable multimode diode laser absorption spectroscopy?TMDLAS?.The TDLAS experimental system of this paper combined the wavelength-modulation spectroscopy?WMS?which effectively improved the signal-to-noise ratio of the experimental system by modulating the signal to a high-frequency band.First of all,the study and experiment of oxygen concentration measurement were studied.We setted up the TDLAS experimental system and the optimal modulation parameters were found through experimental analysis.According to Beer-Lambert law,we defined the optical parameter?OP?,and the relationship between EOPL and OP with the air as the gas to be tested was derived.And EOPL of diffuse cubic cavities with different side length were calibrated.According to the analysis and study of the light propagation law in the diffuse cubic cavity,the theoretical approximate model of light propagation in the diffuse cubic cavity was established based on the finite element method.We calculated the simulated value of single reflection average optical path(L_ave)by MATLAB.Based on the radiation theory,we developed the theoretical formula of EOPL,port fraction,diffuse reflectivity and L_aveve of the diffuse cubic cavity.By changing the opening size of top cover,the experimental values of L_aveve were obtained by the formula fitting of origin.The simulated values were compared and analyzed with the theoretical value and the experimental value,which proved the accuracy and stability of the finite element method simulation.In addition,we sealed diffuse cubic cavity,and prepared different concentrations of oxygen using flow meters.The OP of oxygen with different concentration were given by the calibration curce between the oxygen concentration and OP.We analyzed the stability,sensitivity,signal-to-noise ratio,and detection limit of the TDLAS experimental system using a sealed cubic cavity as a gas cell.Due to the limitations of single-mode diode lasers,in this paper,we used multi-mode diode lasers near 1570nm to measure carbon dioxide based on TDLAS.A TMDLAS concentration measurement experiment system was established.By controlling the flow rate of carbon dioxide in and out of the gas cell,absorption signals of carbon dioxide at different pressures were obtained,we also studied the relationship between pressure and absorption signal intensity.