Fig. 1. Commercial 32 kHz quartz tuning fork. (a) Physical diagram; (b) model diagram

Fig. 2. Diagram of resonant tube structure

Fig. 3. The first 6 mode shapes of quartz tuning fork

Fig. 4. Normalized displacement of tuning fork arms under different conditions. (a) Different frequencies; (b) different H

Fig. 5. Effect of gap S on tuning fork arm.(a) Normalized displacement of tuning fork arms at different S; (b) schematic diagram of gap between quartz tuning fork and resonance tube

Fig. 6. Influence of resonant tubes with different inner diameters and lengths on normalized displacement of tuning fork arms under symmetric structure

Fig. 7. Normalized displacement of tuning fork arms under different sizes of resonant tubes with asymmetric structure

Fig. 8. Principle block diagram of QEPAS system

Fig. 9. Experimental results of each control group. (a) Second harmonic signal; (b) transmission spot distribution diagrams

Fig. 10. Methane detection experiment. (a) Long term measurement in methane gas with volume fraction of 5×10^-3; (b) Allan deviation