Fig. 1. In HITRAN database, the simulated C₂H₂ absorption lines near 1542 nm. (a) The absorption lines intensity of C₂H₂, CO₂ and H₂O; (b) The simulated spectrum of the selected lines and the peaks fitting

Fig. 2. Spectral line intensity at 6489.074 cm^-1 and 6490.020 cm^-1 and its ratio change with temperature

Fig. 3. Sectional view of gas pool body, heating furnace, and partial flange view. (a) Sectional view of the gas pool body;(b) Fiber heating furnace (half piece); (c) Window outer flange

Fig. 4. Schematic diagram of experimental system

Fig. 5. Direct absorption spectral signals of C₂H₂ gas at 900 K

Fig. 6. Comparison of measured C₂H₂ concentration at various pressure at 900 K with the sample reference concentration

Fig. 7. Comparison of spectral detection of measured C₂H₂ concentration with sample reference concentration at different temperature

Fig. 8. Direct absorption spectral signals of 1% C₂H₂ gas at different temperatures and the pressure of 4.04 × 10⁵ Pa

Fig. 9. Comparison between the values of two-wire temperature measurement and the mean values of the three thermocouple readings