Calibration-Free Wavelength Modulation Spectroscopy for Gas Properties Measuring Basedon 2nd and 4th Harmonics
WANG Yi-hong, ZHOU Bin*, ZHAO Rong, WANG Bu-bin
Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China
Abstract：Wavelength modulation spectroscopy (WMS) based on harmonic detection, a modality of the tunable diode laser absorption spectroscopy (TDLAS), has been widely employed for gas properties measurement and combustion diagnosis, given its advantages of contactless, rapid, high sensitivity and accuracy. In recent years, to expand the application scope of the WMS method and reduce the calibration error of spectral parameters, the research on the calibration-free strategy of the WMS method has gradually become a hotspot. Traditional calibration-free WMS method generally needs to simulate the absorption spectrum according to the spectral database combined with the laser modulation parameters, which puts forward high requirements for the prior spectral parameters and hardware parameters. A rapid and accurate calibration-free wavelength modulation spectroscopy (WMS) method for gas parameter measurement based on 2nd and 4th order harmonics is proposed. Compared with the traditional calibration-free WMS method, the proposed method has the following characteristics and advantages. First, the proposed method, analytically deduced from a much more accurate Voigt function model, enables speedy measurement down to milliseconds and general suitability for various degrees of line-shape broadening. Second, the proposed method only needs the algebraic calculation of the central peak height parameters of the 2nd and 4th harmonics to obtain the key spectral parameters, such as absorption line broadening and integrated absorption area, to realize the measurement of gas parameters such as concentration and temperature.Third,the proposed method does not need square iterative fitting calculation and high-order harmonic calculation, which reduces the requirements of hardware system. Fourth, instead of acquiring the entirely scanned absorption line-shape, the proposed methodonly requires extracting the peak values of the harmonics. This characteristic significantly benefits gas diagnosis at elevated pressure and/or temperature. Fifth, the proposed method only needs to use the spectral parameters such as absorption line intensity and low state energy in the spectral database. However, it does not need to use the prior parameters such as the self-broadening coefficient, temperature index and collision broadening coefficient of other components, which reduces the dependence on the spectral database. In order to verify the feasibility of the proposed method, a WMS measurement system was built in the laboratory environment. The absorption line of the CH4 molecule near 6 046.95 cm-1 was selected, and the 2nd and 4th harmonic peaks were used to measure the mole fraction of CH4 at room temperature. Experimental results show that under the absorption optical path length of 20 cm, the relative error of CH4mole fraction measurement is 1.19%, and the detection limit of the system is 4.28×10-6.
