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紫外荧光法是通过检测气体分子吸收紫外光所发射的荧光强度来确定其质量浓度。当激发光处于SO2分子吸光度最强的波段时,分子吸收紫外光能量并受到激发,被激发的分子回到基态时会因气体浓度不同发射波长和强度不同的荧光,同时透过SO2的激发光即透射光的强度也会减弱。根据比尔-朗伯定律,SO2荧光强度、激发光强度和透射光强度的关系如下:
$ \begin{array}{c} I_{\mathrm{f}}=I_{0}-I_{\mathrm{t}} \end{array} $
(1) $ I_{\mathrm{t}}=I_{0} \exp (-\alpha c L) $
(2) 式中, I0为激发光光强, If为被SO2吸收的紫外光强度, It为被SO2吸收后的紫外光的强度, α为SO2分子对紫外光的吸收系数, L为光程, c为SO2质量浓度。
而探测器探测到的荧光强度Ip为:
$ I_{\mathrm{p}}=\beta\left[I_{0}-I_{0} \exp (\alpha c L)\right] $
(3) 式中, β为探测器的接收系数。当外部条件确定后是一定值。
将(3)式在零点进行泰勒展开,可得:
$ \begin{array}{c} I_{\mathrm{p}}=I_{0}\left[\alpha c L-\frac{(-\alpha c L)^{2}}{2 !}-\frac{(-\alpha c L)^{2}}{3 !}-\right. \\ \left.\cdots-\frac{(-\alpha c L)^{2}}{n !}-\cdots\right] \end{array} $
(4) 大气中的SO2含量较低,属于低浓度的检测,在这种情况下,αcL的值很小,(4)式可化简为:
$ I_{\mathrm{p}}=I_{0} \alpha c L=K c $
(5) 当系统确定后,α,L和I0均为定值,所以K也为一个常数,此时探测器接收到的荧光强度与SO2气体的质量浓度成正比。
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在上面仿真结果的基础上,分别根据传统激发光光路和新设计的激发光光路搭建气室,除激发光光路外,两个气室其它的结构均保持一致,其信号采集与处理系统也完全一致。实验系统的原理图如图 8所示。在该实验系统的基础上进行两组实验,两组实验只做气室上的更换,第1组使用传统激发光光路的气室,第2组使用本文中新设计的激发光光路的气室。根据HJ 654-2013《环境空气气态污染物(SO2, NO2, O3, CO)连续自动监测系统技术要求及检测方法》所要求的方式分别针对两种结构的示值误差与精密度进行对比实验。
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根据HJ 654-2013《环境空气气态污染物(SO2, NO2, O3, CO)连续自动监测系统技术要求及检测方法》设备运行稳定并校准后,通入50%量程即250μg/L标准气体,度数稳定后院记录质量浓度值,再通入零气重新校准,重复3次,计算示值误差。应用两种激发光光路结构的实验数据如表 1所示。
Table 1. Measurement value and indication error of experiment using two kinds of structural domains
serial number mass concentration/(μg·L-1) indication error 1 2 3 traditional structure 254.2 247.3 253.6 0.34% full scale new structure 248.6 252.9 251.3 0.18% full scale 可见使用本文中新设计的激发光光路结构较使用传统结构的示值误差显著缩小。根据国家标准,示值误差由下式计算:
$ L_{\mathrm{e}}=\frac{\overline{c_{\mathrm{d}}}-c_{\mathrm{s}}}{R} \times 100 \% $
(6) 式中, Le为分析仪的示值误差,cs为标准气体质量浓度的标称值(μg/L),cd为待测分析仪3次测量的平均值(μg/L),R为分析仪满量程值(μg/L)。
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根据HJ 654-2013《环境空气气态污染物(SO2, NO2, O3, CO)连续自动监测系统技术要求及检测方法》,待分析仪器稳定后,分别通入20%量程(即100μg/L)和80%量程(即400μg/L)标准气体,度数稳定后记录相应显示的质量浓度值,重复6次,计算使用结构仪器的精密度。表 2和表 3分别为100μg/L下和400μg/L下两种结构所测得的数据及量程精密度。
Table 2. Measurement value and precision of two structures at 100μg/L
serial number mass concentration/(μg·L-1) precision/(μg·L-1) 1 2 3 4 5 6 traditional structure 99.0 98.1 97.3 98.9 99.3 100.1 1.13 new structure 99.2 98.9 99.3 100.2 99.6 98.7 0.53 Table 3. Measurement value and precision of two structures at 400μg/L
serial number mass concentration/(μg·L-1) precision/(μg·L-1) 1 2 3 4 5 6 traditional structure 397.3 401.6 401.2 398.3 397.1 402.0 2.26 new structure 398.6 400.8 399.3 399.5 398.5 401.0 1.1 由表 2和表 3可知,在100μg/L下和400μg/L下,新设计的激发光结构所测出的量程精密度均高于传统结构。其中,量程精密度由下式求得:
$ P=\sqrt{\frac{\sum\limits_{i=1}^{n}\left(x_{i}-\bar{x}\right)^{2}}{n-1}} $
(7) 式中, P为量程精密度(μg/L),xi为第i次的测量值(μg/L),x是n次测量的平均值(μg/L),n为观测次数(n≥6)。
紫外荧光法SO2监测仪激发光光路的设计仿真
Design and simulation of excitation light path of SO2 monitor by ultraviolet fluorescence
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摘要: 为了解决国内传统的SO2检测仪存在气室的荧光检测区域荧光强度低、进而导致仪器监测精度低的问题,设计了新的激发光光路结构。在该光路结构中,点光源通过平凸透镜准直,利用一窄缝消除竖直方向上的远轴光线,通过一双凸透镜汇聚到气室荧光采集区域的中心,并利用光阑减小杂散光的干扰。采用射线追踪算法作为严格矢量分析的工具,对设计的光路进行仿真分析。结果表明,优化后的光路使得激发光的能量损失降到了10%,且弥散斑也大为减小;用该光路与传统结构的光路进行实验对比,其示值误差由0.34%满量程变为0.18%满量程,质量浓度为100μg/L时的精密度由1.13μg/L变为0.53μg/L,质量浓度为400μg/L时的精密度由2.26μg/L变为1.1μg/L,两项指标均得到了提高。所设计的激发光光路结构能够有效解决传统光路的不足之处。
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关键词:
- 光学设计 /
- SO2检测 /
- COMSOL Multiphysics软件 /
- 紫外荧光法
Abstract: The traditional SO2 detector in China has the problem of low fluorescence intensity in the fluorescence detection area of the gas chamber, which leads to the low monitoring accuracy of the instrument. In order to solve this problem, a new excited optical path structure was designed. In the light path structure, the point light source was collimated by a plane convex lens, and a narrow slit was used to eliminate the far-axis light in the vertical direction. The point light source was converged to the center of the gas chamber fluorescence collection area by a double convex lens, and the interference of stray light was reduced by the diaphragm. The ray tracing algorithm was used as a tool of strict vector analysis to simulate the designed optical path. The results show that the optimized optical path can reduce the energy loss of excitation light to 10%, and the dispersion spot is also greatly reduced. The experimental results show that the indication error of the optical path is changed from 0.34% full scale to 0.18% full scale. The precision of 100μg/L changed from 1.13μg/L to 0.53μg/L. The precision at 400μg/L changed from 2.26μg/L to 1.1μg/L. The two indexes have been improved, which can effectively solve the shortcomings of the traditional optical path. -
Table 1. Measurement value and indication error of experiment using two kinds of structural domains
serial number mass concentration/(μg·L-1) indication error 1 2 3 traditional structure 254.2 247.3 253.6 0.34% full scale new structure 248.6 252.9 251.3 0.18% full scale Table 2. Measurement value and precision of two structures at 100μg/L
serial number mass concentration/(μg·L-1) precision/(μg·L-1) 1 2 3 4 5 6 traditional structure 99.0 98.1 97.3 98.9 99.3 100.1 1.13 new structure 99.2 98.9 99.3 100.2 99.6 98.7 0.53 Table 3. Measurement value and precision of two structures at 400μg/L
serial number mass concentration/(μg·L-1) precision/(μg·L-1) 1 2 3 4 5 6 traditional structure 397.3 401.6 401.2 398.3 397.1 402.0 2.26 new structure 398.6 400.8 399.3 399.5 398.5 401.0 1.1 -