Adjustable Optical Path Trace Gas Detection System Based on Non-Dispersive Infrared Spectroscopy
Objective At present,the infrared trace gas detection system has problems such as large size,low detection accuracy,and high minimum detection limit. As a result,certain limitations exist in the application scenarios of factories,mining areas and households which may cause serious life safety problems and huge economic losses. Most of the research on infrared trace gas detection starts from the volume fraction detection model,but it has difficult in research and little influence on the final model building results. Meanwhile,some scholars start from the cavity of the gas absorption cell to study the influence of the absorption cell on the detection of infrared trace gas,but most of the studies are based on increasing the optical path to improve the detection accuracy. However,generally the longer optical path requires larger volume of the absorption cell,which is not convenient for multi-scene utilization,and few scholars design the gas absorption cell by analyzing the optimal optical path of gas. Therefore,we employ an adjustable optical path trace gas detection system based on non-dispersive infrared (NDIR) spectroscopy with sound accuracy and low detection limits. This system can obtain the optimal detection path for different gases,and the different automatic matching paths can achieve high-precision and low-detection limit detection of gas data.Methods Automatic adjustment is mainly achieved for different optical paths. First,according to the concept of optical path product,the optical path for detecting the 1×10-6 volume fraction of SF6,CH4,CO,and CO2 gases is obtained by a simple pass-through gas absorption cell (Table 1). Then,based on the NDIR detection principle,the adjustable optical path gas absorption cell is designed by adopting the White cell structure. According to the structure of White cell,it is found that the relative position of the secondary lenses has a great influence on the optical path,and then the adjustment of the optical path is realized by adjusting the relative position of the secondary lenses,as shown in Fig. 4. Then,an automatic optical path adjustment device is designed (Fig. 5). The optical path automatic adjustment program is designed,with the flow chart shown in Fig. 6. Finally,the optical path under different optical paths is simulated to verify the accuracy of the automatic optical path adjustment,as shown in Fig. 7. Next,experiments are carried out based on the automatic adjustment of optical paths,the building of gas volume fraction model,the stability of detection data,and the minimum detection limit to verify the adjustment accuracy,high accuracy and low detection limit of the adjustable optical path trace gas detection system.Results and Discussions The designed adjustable optical path gas absorption cell has sound accuracy and repeatability for optical path adjustment,and the relative error of automatic optical path adjustment is controlled within 1％,as shown in Table 3. The maximum RMES coefficient and the minimum R2 coefficient of the gas volume fraction model are 17.463 and 0.9964,which has sound linearity (Fig. 10). The results show that the data detected under the optimal optical path has a significant improvement,and the relative error of the gas data measured under the action of the adjustable optical path gas absorption cell and the gas volume fraction model is controlled within 8％ (Table 4). Additionally,the lowest detection limit under the optimal optical path is reduced to 0.547 times compared with the lowest detection limit of a single optical path,as shown in Table 5. This indicates that the optimal optical path detection has better detection sensitivity and higher detection light intensity to adapt to more detector types than single optical path detection.Conclusions An adjustable optical path trace gas detection system is designed and manufactured. The system realizes automatic adjustment of the optical path of 1.6‒16 m,and the relative error of the optical path is±1％. The gas volume fraction computation model is built by adopting the least square method. The results show that the maximum detection error of SF6,CH4,CO,and CO2 gases in the experiment cannot be higher than 8％ under the optimal optical path,the lowest detection limits are 1.361×10-6,0.487×10-6,0.420×10-6,and 0.769×10-6,respectively,and the highest detection limit is reduced to 54.7％ compared with the minimum detection limit of a single optical path. The designed detection system can adapt to the gas species with the optimal optical path range of 1.6‒16 m under the White cell structure,and the detection accuracy of all gases is measured at the optimal optical path. Additionally,since the optical path length of the design is not the maximum value of the optical path,the light intensity loss is relatively small,which can adapt to the light intensity requirements of various NDIR sensors and lead to sound adaptability.
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