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本文对波长调制光谱（WMS）技术进行了改进，并以其为基础测量了高吸收度的甲烷气体。WMS常被用于气体浓度测量，其依赖于二次谐波幅值与气体浓度之间的线性关系，但是传统的WMS技术只适用于气体吸收度远小于1的情况，这是因为在传统WMS理论的推导中，需要对朗伯比尔定律进行一阶近似，而一阶近似仅在低吸收度下成立，所以在高吸收度下二次谐波与气体浓度的线性关系不成立。在本文的改进方案中，不需要对朗伯比尔定律做任何近似处理。将激光分为测量光与参考光两路，测量光被待测气体充分吸收后由光电探测器收集光强信号，参考光的光强信号不被吸收直接由另一个光电探测器直接探测，两个光电探测器的输出信号经模数转换后传输至上位机，上位机对两路信号均先取自然对数，然后根据参考信号确定二次谐波的解调相位，这样解调出来的二次谐波信号即使在高吸收度下也与气体的浓度保持线性关系。本文介绍了传统WMS理论与改进后的WMS理论，并分别测量了一系列浓度梯度的甲烷气体，对比了传统WMS和改进WMS的实验结果，证实了在高吸收度下，传统WMS理论中的线性不再成立，但改进的WMS仍能保证二次谐波与甲烷浓度之间的线性关系，验证了改进方案的优势；最后通过艾伦标准差分析，得到该甲烷测量系统在平均时间103.6s时稳定性达到最优，对应的艾伦标准差为26.62ppbv。In this paper, the wavelength modulation spectroscopy (WMS) technique is modified and used for measuring methane with large absorbance. WMS has been frequently used for gas measurement and relies on the linear relationship between the second harmonic amplitude and the gas volume concentration however, the conventional WMS technique is only applicable for gases whose absorbance is much smaller than 1, which is because of the first-order approximation to Lambert-Beer's law in the derivation of the traditional WMS theory, but the first-order approximation holds only at low absorbance, hence the linear relationship between the second harmonic and the gas concentration does not hold at large absorbance. In the modified WMS in this paper, there is no need to make any approximation to Lambert-Beer's law. The measurement light is absorbed by the gas to be measured and then collected by the photodetector, and the reference light is directly detected by another photodetector without being absorbed, and the output signals of the two photodetectors are transmitted to the computer after analog-to-digital conversion. In this way, the demodulated second harmonic signal remains linear with the gas concentration even at large absorbance. In this paper, the traditional WMS theory and the modified WMS theory are introduced, and a series of methane with concentration gradients are measured separately to compare the experimental results of the traditional WMS and the modified WMS. It is confirmed that the linearity in the traditional WMS theory no longer holds under large absorbance, but the improved WMS can still guarantee the linear relationship between the second harmonic and the methane concentration, which verifies the advantages of the modified scheme. Finally, through Allen's standard deviation analysis, we obtained that the stability of this methane measurement system reaches the optimum at the average time of 103.6s, and the corresponding Allen's standard deviation is 26.62 ppbv.
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Keywords:
- Wavelength Modulated Spectroscopy /
- Methane Measurement /
- Infrared Spectroscopy /
- Large Absorbance
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