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Calibration-free wavelength modulation spectroscopy for gas concentration measurements under low-absorbance conditions |
Che Lu (车璐), Ding Yan-Jun (丁艳军), Peng Zhi-Min (彭志敏), Li Xiao-Hang (李晓航) |
State Key Laboratory of Power Systems, Department of Thermal Engineering, Tsinghua University, Beijing 100084, China |
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Abstract We derive the expressions of the first and second harmonic signals on the basis of absorption spectral and the lock-in theories, and determine the gas concentration according to the ratio of second and first harmonic signals. It is found that the X and Y components of the harmonic signals are influenced by the phase shift between the detection and reference signal, and the phase shift can be any value in a range from 0 to 2π, which is different from the results obtained previously. Meanwhile, an additional item caused by the residual amplitude modulation will make a great contribution to the second harmonic signal, and may not be neglected under low absorbance conditions. Theoretical analysis indicates that subtracting back-ground signal from the second harmonic signal can remove the influence of this item, and can improve the measurement accuracy of gas concentration. On this basis, we select the transition of CO2 at 6527.64 cm-1 to analyse the approximation errors during the derivation by numerical simulation and then measure the CO2 concentration under low absorbance conditions, with absorbance varying from 1‰ to 6‰.
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Received: 18 April 2012
Revised: 06 May 2012
Accepted manuscript online:
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PACS:
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78.40.-q
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(Absorption and reflection spectra: visible and ultraviolet)
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78.30.-j
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(Infrared and Raman spectra)
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42.68.Ca
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(Spectral absorption by atmospheric gases)
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Fund: Project supported by the National Natural Science Foundation of China (Grant Nos. 51176085 and 51206086). |
Corresponding Authors:
Ding Yan-Jun
E-mail: dyj@mail.tsinghua.edu.cn
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Cite this article:
Che Lu (车璐), Ding Yan-Jun (丁艳军), Peng Zhi-Min (彭志敏), Li Xiao-Hang (李晓航) Calibration-free wavelength modulation spectroscopy for gas concentration measurements under low-absorbance conditions 2012 Chin. Phys. B 21 127803
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