Research On Noise Suppression Method For Flow Sampling Photoacoustic Gas Detection

Gas detection using photoacoustic spectroscopy has the characteristics of high sensitivity,high selectivity,and large dynamic range of detection,especially for the detection of dissolved gases in transformer oil,its limit detection sensitivity reaches ppm and ppb levels,which can real-time monitor the working condition of transformer operation to avoid accidents and major losses caused by transformer failure.In order to monitor the running state of the transformer system online and monitor whether the toxic and harmful gas leaks etc,it is necessary to carry out continuous flow sampling analysis and the flow sampling can achieve the concentration of the gas detection.After data processing,can timely and effectively feedback the operating status of the system,which can effectively protect our lives and property.Then the photoacoustic cell of the system needs to choose the resonance type and needs to have the inlet and outlet ports.The problem is that the noise of the external environment will enter into the photoacoustic cell and affect the signal-to-noise ratio and the minimum detection limit of the system,especially in the frequency range close to the resonance frequency of the photoacoustic cell,which severely limits the limit detection sensitivity of the system.To solve this problem,a muffler is designed in the inlet and outlet of the photoacoustic cell for noise suppression.Considering the simplicity of the structure and the small size of the muffler,the muffler is designed as two Helmholtz mufflers with 8.5mm and 9.5mm nozzles in parallel with the insertion tube length 56 mm,the expansion chamber length 60 mm,the expansion chamber diameter 18 mm and the inlet and outlet diameter 4 mm.The muffler was acoustically analyzed by Acoustic software.Theoretically,the noise reduction reached 80 dB at the resonance frequency of the photoacoustic cell.Through the experiment,compared the experimental measurement value of the muffler transmission loss with the theoretical simulation value,and made an error analysis of the gap between the actual measured value and the theoretical simulation value.The measured mass flow rate is the acoustic loss curve at 10,30,40 SCCM,and the value of acoustic loss is up to25 dB,it shows that the noise suppression effect on the photoacoustic detection system is obvious.Flow measurement of acetylene gas,after calculation,it is found that the sensitivity of limit detection of muffler installed in the inlet and outlet of photoacoustic cell is doubledcompared with that of non-muffler installed.This design provides an effective design reference for noise suppression of photoacoustic spectroscopy systems based on flow measurement.