| The trace gas methane（CH4）is a serious threat to human life at concentrations above a certain level in the air,so there is a need to accurately measure CH4 concentrations in the ambient atmosphere.Tunable diode laser absorption spectroscopy（TDLAS）technology can select the characteristic absorption lines of gas molecules,which has high detection sensitivity,and combines semiconductor laser with long optical path multi-pass cell（MPC）,making it a commonly used technology for CH4 gas detection.At present,scholars at home and abroad have researched a variety of measurement devices for CH4gas detection based on TDLAS technology,but they all suffer from low utilization of optical MPC mirrors,interference with harmonic signal amplitude caused by interference fringe noise,and low detection accuracy of the system.Therefore,the work on MPC dense spot pattern algorithm design,experimental system construction and experimental research are carried out around the TDLAS technology for CH4 gas detection.The main contents are as follows:Firstly,the research status of TDLAS technology and optical MPC used in gas monitoring system at home and abroad is investigated.Compared with the traditional Herriott type MPC,the long optical path MPC with dense light spot pattern enhances gas absorption.Based on the basic principle and theoretical formula of WMS technology and harmonic detection method,the theoretical relationship between gas concentration to be measured and harmonic signal component is obtained.The relationship between the spot pattern,reflection number,and total optical path of gas absorption for three common optical MPCs is also analyzed.Secondly,the circular spot distribution law of Herriott-type MPC is analyzed,and a new algorithm of spot pattern of dense optical MPC is proposed.To verify the accuracy of the proposed algorithm,a series of numerical simulations calculation of light spots are carried out.At the same time,the optical path simulation model of spherical mirror MPC established by Trace Pro software is compared with the spot pattern produced by the MPC in the experiment,and it is found that the simulation model and the experimental results are in good agreement.The results show that the optical path length calculated by the new algorithm is effectively increased,and a long optical path MPC of 12.92m can be obtained.Then,according to Spectraplot database,the wavelength band of 1653nm semiconductor laser is determined,and the CH4 concentration measurement system is established.The assembly of the experimental system,optical path debugging and air tightness inspection were carried out.And then the data acquisition card is programmed by Lab VIEW software to collect the second harmonic signal,the functions of sampling parameters setting,waveform display,data average processing and custom folder name for data saving are realized.Finally,the experiment of system modulation parameter optimization and concentration calibration is carried out.The experimental results showed that there is a good linear relationship between the peak value of the second harmonic signal demodulated by the lock-in amplifier and the CH4 concentration calibrated by the system.The minimum detection limit of the system for CH4is 2.33ppb at an integration time of20s obtained via Allan deviation analysis.And the results show that the long optical path MPC system achieved high detection sensitivity.CH4 is also measured in the ambient atmosphere,and the results are consistent with typical values of atmospheric CH4concentrations.Combined with the addition of RF noise source and Kalman filtering algorithm,the measurement accuracy of methane is reduced from 190ppb to 64ppb.Compared with the experimental results in the literature,the relative error between the CH4 detection system and the existing mature detection instruments is less than（27）6ppb,which can meet the needs of underground coal mine gas detection.Figure[55]Table[7]Reference[88]... |
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