Tunable diode laser absorption spectroscopy, a commonly used gas concentration detection technology, has the advantages of non-contact real-time measurement, high sensitivity, and strong selectivity. It includes direct absorption spectroscopy and wavelength modulation spectroscopy. Compared to direct absorption spectroscopy, wavelength modulation spectroscopy technology has a strong anti-interference ability, higher sensitivity, and lower detection limit; it has been widely used in environmental monitoring, industrial gas detection, combustion diagnosis, and other fields. However, real-time wide-range detection of gas concentration has increasingly become a necessity. For example, the volume fraction of methane in coal mines and petrochemical pipelines varies from 0% to 100%, and the water vapor in air fluctuates significantly. Therefore, there is an urgent need for a new method for wide-range detection of gas concentration in petrochemical pipelines, coal mines, and other fields.
To meet the requirements of wide-range detection of gas concentration in many fields, this study utilizes the high sensitivity characteristics of wavelength modulation spectroscopy, examines the nonlinear characteristics of wavelength modulation spectrum (WMS-NL), and then achieves high sensitivity and wide range detection of gas concentration using the wavelength modulation method. According to the principle of laser absorption spectroscopy, the Taylor expansion of the absorption term is analyzed. Specifically, linear approximation and cubic polynomial approximation of the Taylor expansion are adopted at low concentration (low absorbance) and high concentration (high absorbance), respectively. Moreover, methane (CH4) is taken as an example to verify the feasibility of this method in the wide-range detection of gas concentration. Additionally, combined with the three parameters of absorption line intensity, effective optical length, and gas concentration, the specific details of the method are described based on the calculation of the absorbance of CH4.
Based on experimental verification, this method can achieve the detection of CH4 volume fraction in the range of four orders of magnitude (1.5×10-6-10000×10-6). The volume fractions below and above 1000×10-6 (the corresponding integrated absorbance is below and above 0.0236) are detected separately, and there is a good linear correlation between the inverted concentration and the standard concentration. The correlation coefficients in both the low and high concentration ranges are 0.999. In addition, combined with this method, the error, detection limit, and stability of the CH4 detection system are analyzed. In the range where the volume fraction exceeds 1000×10-6, the maximum relative measurement error is 0.93% and the absolute error is 92.1×10-6. Similarly, in the range where the volume fraction is lower than 1000×10-6, the maximum relative measurement error is 4.00% and the absolute error is -34.2×10-6. In addition, CH4 with a volume fraction of 5000×10-6 is measured for a period of time, and then the Gaussian distribution of the inverted concentration is counted. Its half width at half maximum is 15.9×10-6, and the stability of this method is well demonstrated under high concentrations.
The proposed method overcomes the limitation that conventional wavelength modulation spectroscopy can only measure low concentrations, provides a new idea for wide-range detection of gas concentration, and can considerably expand the application ranges of wavelength modulation spectroscopy.
