Wide-range multi-gas detection method based on wavelength modulation spectroscopy and direct absorption spectroscopy

When tunable diode laser absorption spectroscopy (TDLAS) is applied to the field of coal mine and petrochemical industry for gas concentration detection, it often needs to meet the requirements of high precision and wide dynamic range. Therefore, we use the time-division multiplexing method to combine the advantages of direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS) technology to complete a high-precision wide-range calibration-free multi-gas detection system. The driving signal of the laser is designed as a periodic signal, which has linear scanning output signal superimposed with different high-frequency sin-wave modulation. It is used to complete the simultaneous calculation of low and high concentration inversion algorithms by time division multiplexing technology. Moreover, by optimizing the absorbance inflection point of the detected gas, the high-precision and wide-range detection of the gas concentration is realized. Under room temperature and pressure, the concentrations of CH₄, CO and C₂H₂ are detected by experiments, and the absorbance at the optimal inflection point of the two algorithms is determined to be about 0.026 cm⁻¹. The detection range of the system for the concentration of CH₄, CO and C₂H₂ are 0-100%, 0-5000×10⁻⁶ and 0-1000×10⁻⁶, respectively. And also, the minimum concentration detection limits are 2.27×10⁻⁴, 0.21×10⁻⁶, and 1.68×10⁻⁶. The above analysis also shows that the system meets the requirements of wide dynamic range and the accuracy is better than the current coal mine industry standard in the whole range. Moreover, this method can meet the needs of various industrial field applications, and is conducive to promoting the application of laser absorption spectroscopy in industrial processes, security and other fields.