Fibre optics in palladium-based hydrogen-sensing

Hydrogen sensing at low concentrations in the order of tens to hundreds of parts per million (ppm) over extended periods requires highly sensitive and stable sensor technology. Palladium and its alloys are widely used in hydrogen sensing as they show a high and selective affinity for hydrogen. On exposure to hydrogen, Pd experiences a volumetric expansion, roughly proportional to the hydrogen concentration in the environment. This volumetric expansion can be measured either directly by dimensional monitoring or through monitoring of secondary physical effects. Optical sensor technology is a preferred technique for hydrogen sensing because of its inherent safety for use in a potentially explosive environment, and several optical hydrogen sensors have been reported. However, the majority have been designed to measure hydrogen concentrations of a few per cent in air, whereas the sensors described here are capable of operating in concentrations down to hundreds of ppm. Fibre Bragg gratings (FBGs) and interferometric length monitoring are shown to be able to monitor the dimensions of a palladium sensor element. Hydrogenation not only induces physical strain but also changes the electronic configuration resulting in a change in refractive index which can be monitored using a long-period grating (LPG). The paper discusses the respective characteristics of these different optical measurement techniques using Pd and Pd/Ag alloys as sensor elements for hydrogen concentrations between 100-ppm and 1%.