Fig. 1. Simulated absorption spectra of CO and interference gas in infrared band

Fig. 2. Schematic diagram of infrared CO sensor system.

Fig. 3. Infrared emission spectra of the used light source

Fig. 4. Structure and simulation of the absorption cell

Fig. 5. Photo of the digital orthogonal lock-in amplifier and test result

Fig. 6. Calibration of the sensor system

Fig. 7. Dynamic response time test of the sensor system

Fig. 8. System stability measurement results on volume fraction 0×10^-6 CO gas sample and Allan deviation curve

Fig. 9. Long-term monitoring result on the CO sample with a volume fraction of 0×10^-6 and Allan standard deviation curve after Kalman filter

Fig. 10. Cotton，paper and wood smouldering experiments and CO concentration measurement results