Study On The Photoacoustic Spectroscopy In Trace Gas Concentration Measurement

The trace gas detection has great significance in air quality monitoring,industrial production process control,clinical medical diagnosis and so on.The photoacoustic spectroscopy measure the trace gas concentration by detecting the amplitude of acoustic wave generated throught photoacoustic effect.The photoacoustic spectroscopy has the characteristics of high detection sensitivity,large dynamic range and real-time online detection.It has a broad application prospect in the field of online trace gases detection.In this paper,for the purpose of study the application of photoacoustic spectroscopy in trace gas detection and evaluating the demand of trace gas detection,the photoacoustic spectroscopy experimental system based on wave length modulation and second harmonic detection method was built.And an experimenta was conducted to measure the concentration of acetylene（C₂H₂）gas.The main research contents and results are as follows:Firstly,this parper studied the principle of the first order resonant photoacoustic cell,then built the transmission line equivalent model of the first order longitudinal resonant photoacoustic cell and finsihed the model simulation.Based on the simulation results,the mechanical structure and the demension of photoacoustic cell were determined,then designed and made a one-order longitudinal resonant photoacoustic cell.Secondly,In order to obtain the resonant frequency of the photoacoustic cell quickly and accurately,a resonant frequency measurement method based on resonance acoustic spectroscopy was proposed in this paper.It was proved by experiments that the method can quickly and accurately measure the resonant frequency of the resonant photoacoustic cell.Thirdly,the photoacoustic spectroscopy experimental system was built.In consideration of the photoacoustic signal is very weakly and susceptible to noise interference,the preamplifier and digital lock-in amplifier was designed and realized.Fourthly,the C₂H₂ is detected based on the photoacoustic spectroscopy detection system.First,the absorption spectrum at 1531.59nm is chosen as the target absorption spectrum.The specific location of the target absorption peak and the optimal modulation parameters of the experiment are determined by experiments.Then,studied the relationship between the photoacoustic signal and the concentration of C₂H₂ gas and the DFB laser output power.By detecting the 5 kinds of known concentrations acetylene gases,the C₂H₂ gas concentration is calibrated,which provided the datum for the actual measurement.Through the continuous measurement of the 25ppm C₂H₂ gas for an hour,found that the detection accuracy is 4.1×10^-7,at the same time,the experimental results proved that the system has good stability.Finally,measured the 5ppm C₂H₂ gas,the results showed the the limit sensitivity of the system is1.34ppm.