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

Table 1. Summary of direct detection technology options

Table 2. Summary of indirect detection methods