Fig. 1. Schematic of Beer-lambert law. (a) Gas absorption diagram; (b) signal diagram after gas absorption

Fig. 2. Spectral line intensities of methane carbon isotope at different wavelengths in HITRAN database. (a) ¹³CH₄; (b) ¹²CH₄

Fig. 3. Results of CH₄ spectral simulations at 1658 nm

Fig. 4. Schematic of experimental setup

Fig. 5. Flow chart of control process and fuzzy adaptive PID closed-cycle control algorithm

Fig. 6. Photo of test-field equipment (right photo shows sampling rubber balls)

Fig. 7. Signal spectra of methane isotopic gas samples using proposed system. (a) Direct-absorption signals; (b) second harmonic signals(2f)

Fig. 8. Measurement distribution values of methane concentration and carbon isotopic abundance of coal mine samples. Dotted square line stands for distribution values of methane concentrations at different sampling positions of coal mine; dotted star line stands for distribution values of methane carbon isotope abundances at different sampling positions of coal mine

Table 1. Selected spectral data of methane in 1658.4-1659.6 nm

Table 2. List of sampling positions