Fig. 1. Schematic of different types of electrical hydrogen sensors^[4]. (a) Electrolytic sensor (electrical current type); (b) MOS sensor; (c) catalytic combustible sensor; (d) thermal conductivity sensor

Fig. 2. Performance of FBG sensor based on hydrogen absorption and exothermic properties. (a) Hydrogen response of FBG coated with Pt-loaded WO₃^[20]; (b) improvement of sensing performance by fabrication optimization based on metal-organic framework^[21]

Fig. 3. FBG-based hydrogen sensing system with optic-heating assistance^[24]. (a) Overall configuration; (b) schematic of sensing probe

Fig. 4. Optical sensing chip of hydrogen detection. (a) Based on surface plasmonic-catalytic effect^[32]; (b) based on MZI effect^[33]

Fig. 5. Internal layouts of spherical detector

Fig. 6. Buried pipeline leakage test^[53]. (a) Schematic of pipeline and distribution of monitor points; (b) temperature change curves at monitor point #1‒3, #5, and #9; temperature change curves of cable (c) before and (d) after leakage

Fig. 7. Fiber helical wrapping structure and detection results in frequency domain^[63]. (a) Schematic illustration of reference region (Zone 0) and monitored region (Zones 1‒3); (b) helical wrapping structure, where Ac stands for accelerometer; (c) detail photo of one of pipe segments; (d) overall averaged DAS signal spectra from Zone 2 under different pressures; (e) averaged signal spectra from accelerometer placed close to the leak (center) and the edge close to flange (end)

Fig. 8. Classification of hydrogen energy storage and transportation equipment detection technology