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光声光谱气体检测技术是通过探测气体吸收光能产生的声波大小进而获得待测气体体积分数的技术。在光声池中充满待测气体,处于基态的气体分子对特定波长的光能选择性吸收并跃迁到激发态,通过无辐射弛豫的方式释放热能,导致光声池中的气体膨胀,产生声波。当入射光被周期性信号调制时,光声池中将产生同频声波[25]。根据Lambert-Beer定律,声波的大小和光声池中待测气体的体积分数呈线性。通过微音器检测光声信号Y可以反解待测气体体积分数,检测到的光声信号可以表示为:
$ Y=s P F \varphi \alpha(\nu) $
(1) 式中,s为微音器的灵敏度; P为入射光功率; F为光声池池常数,和光声池的结构和材料有关; φ为待测气体的体积分数; α(ν)为该气体对波长为ν的光的吸收系数。
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为了验证组合激光光源的双组分微量气体检测系统的性能指标,搭建一套性能测试系统,系统结构示意图如图 6所示。利用两个质量流量控制器(七星华创,CS200)将气体体积分数为1000×10-6的CH4和500×10-6的C2H2混合气体和高纯氮气以特定比例混合。分别配置出CH4/C2H2体积分数为1000×10-6/500×10-6,500×10-6/200×10-6,250×10-6/100×10-6,100×10-6/50×10-6,50×10-6/10×10-6的混合气,将不同体积分数的混合气按照由小到大顺序依次通入光声池,通过DFB激光器的波长扫描对CH4和C2H2气体吸收线附近的2f-WMS信号进行探测。检测过程中通过控制关闭光声池以避免环境噪音和气体流动对检测过程造成的干扰。
Figure 6. Schematic diagram of the performance test system structure of the dual laser photoacoustic spectroscopy gas detection system
通过设置两个激光器的偏置电流的扫描范围,使检测CH4气体的DFB激光器输出电流从102.0mA~109.0mA, 该电流范围覆盖CH4气体的吸收谱线1650.90nm,检测C2H2气体的DFB激光器输出电流从98.0mA~114.0mA,该电流范围覆盖C2H2气体的吸收谱线1531.58nm。两个激光器的工作温度稳定在25℃,锁相放大器的积分时间设置为1s。利用三级小波分解技术对光谱信号进行降噪处理[24],得到的不同体积分数的CH4和C2H2气体的光声2f-WMS信号分别如图 7a和图 7b所示。其中2f-WMS信号峰值处对应的波长即为DFB激光器的中心波长,此时检测CH4气体的DFB激光器偏置电流为105.5mA,检测C2H2气体的激光器偏置电流为105.6mA。为了验证光声信号与气体体积分数的线性关系和系统检测范围,选取由低到高的5种不同体积分数CH4气体/C2H2气体进行检测,通过寻峰算法选取2f-WMS信号的峰值作为光声信号检测值进行线性拟合[20],结果如图 8a和图 8b所示。由图 8可以看出,CH4和C2H2气体分别在体积分数为0~1000×10-6和0~500×10-6的范围内具有良好的线性响应,每10-6体积分数的检测响应度分别为16.1831μV和5.8969μV。该系统检测范围超过DL/T1498《变电设备在线监测装置技术规范》[26]中规定的变压器油中溶解CH4气体和C2H2气体的最高体积分数(分别为600×10-6和200×10-6),满足现场检测需求。
Figure 7. a—second harmonic signal of different volume fraction of CH4 b—second harmonic signal of different volume fraction of C2H2
Figure 8. Linearity fitting of measured values of photoacoustic signals at different volume fraction
为了进一步测试系统的检测下限,根据DL/T 1498《变电设备在线监测装置技术规范》测量误差技术指标,选取体积分数为3.00×10-6/0.50×10-6的CH4/C2H2混合气体进行精度和重复性检测,重复测试5次,测试结果如表 1所示。对体积分数为3.00×10-6的CH4气体进行5次检测的平均值为3.00×10-6,显示的最高值为3.30×10-6,最低值为2.70×10-6,最大绝对误差为0.30×10-6。对体积分数为0.50×10-6的C2H2气体进行5次检测的平均值为0.64×10-6,显示的最高值为0.70×10-6,最低值为0.60×10-6,最大绝对误差为0.20×10-6。因此,该实验结果确定了系统对CH4和C2H2的检测下限分别达到3.00×10-6和0.50×10-6。
Table 1. Low concentration repeatability and error test
φ(CH4) φ(C2H2) volume fraction of standard gas 3.00×10-6 0.50×10-6 test volume fraction 1 3.10×10-6 0.60×10-6 test volume fraction 2 2.70×10-6 0.60×10-6 test volume fraction 3 2.70×10-6 0.70×10-6 test volume fraction 4 3.20×10-6 0.70×10-6 test volume fraction 5 3.30×10-6 0.60×10-6 average concentration 3.00×10-6 0.64×10-6 maximum absolute error 0.30×10-6 0.20×10-6
基于组合激光光源的双组分微量气体检测系统
Two-component trace gas detection system based on combined laser light sources
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摘要: CH4和C2H2是变压器发生故障时两种重要的特征气体。为了实现对变压器中溶解的微量CH4和C2H2气体含量检测的需求, 采用激光光声光谱气体检测技术, 通过分析CH4和C2H2气体的近红外吸收谱线, 选取合适的激光光源并确定激光调制参数; 设计并搭建了一套以双激光光源和非共振光声池为核心的光声光谱微量CH4和C2H2气体检测系统, 获得了系统对CH4和C2H2气体检测灵敏度和低含量检测误差。结果表明, CH4和C2H2气体分别在体积分数为0~1000×10-6和0~500×10-6的范围内具有良好的线性响应, 每10-6体积分数的检测响应度分别为5.8969μV和16.1831μV; 在低含量CH4/C2H2混合气体对系统的重复性和精度测试中, CH4气体体积分数为3.00×10-6时的检测最大绝对误差为0.30×10-6, C2H2气体体积分数为0.50×10-6时的检测最大绝对误差为0.20×10-6。此研究结果满足测量误差的技术指标要求, 实现了对微量CH4和C2H2气体的高灵敏度检测。Abstract: Methane and acetylene are the most important characteristic gases when the transformer breaks down. In order to meet the demand for detecting the concentration of the trace CH4 and C2H2 dissolved in the transformer, the laser photoacoustic spectroscopy gas detection technology was adopted. By analyzing the near-infrared absorption spectrum of CH4 and C2H2 gas, the appropriate laser light source was selected and the laser modulation parameters were determined. A set of photoacoustic spectroscopy trace CH4 and C2H2 gas detection system with dual laser light source and a non-resonant photoacoustic cell were designed, and the detection sensitivity and low concentration detection error of the system for CH4 and C2H2 gas was obtained. The result shows that CH4 and C2H2 have good response linearity in the volume fraction range of 0~1000×10-6 and 0~500×10-6, respectively, and the responsivities are 5.8969μV and 16.1831μV per 10-6 volume fraction, respectively. The system is tested for repeatability and accuracy using low-content CH4/C2H2 mixed gas. The maximum absolute error of detection is 0.30×10-6 when the CH4 volume fraction is 3.00×10-6, and the maximum absolute error of detection is 0.20×10-6 when the C2H2 volume fraction of 0.50×10-6. The research results meet the technical index requirements of measurement error and achieve high-sensitivity detection of trace CH4 and C2H2.
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Table 1. Low concentration repeatability and error test
φ(CH4) φ(C2H2) volume fraction of standard gas 3.00×10-6 0.50×10-6 test volume fraction 1 3.10×10-6 0.60×10-6 test volume fraction 2 2.70×10-6 0.60×10-6 test volume fraction 3 2.70×10-6 0.70×10-6 test volume fraction 4 3.20×10-6 0.70×10-6 test volume fraction 5 3.30×10-6 0.60×10-6 average concentration 3.00×10-6 0.64×10-6 maximum absolute error 0.30×10-6 0.20×10-6 -
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