ABSTRACT
The present chapter should be considered as an introduction to humidity measurement by optical and fiber-optic sensors (FOS). Here one can find information about the types of optical humidity sensors and approaches used for these sensors classification. The principle of operation of such sensors is based on the effect of water vapor on the physical properties of humidity-sensitive materials. These materials are also analyzed in this chapter. The main attention is paid to polymers, which are most often used in the development of optical and FOS. The mechanisms of swelling in superabsorbent polymers are also considered. It is shown that other materials such as some salts and complexes of transition metals; fluorescent and phosphorescent materials such as rhodamine; porous metal oxides such as Co3O4, ITO, TiO2, ZnO, BaTiO3, Al2O3, SnO2, silica; and carbon-based materials such as carbon nanotubes and graphene are also acceptable for humidity sensors design. As in the polymers, the change of optical properties of materials mentioned earlier is because of the adsorption and condensation of water vapor on the solid surface. As the air and water have different refractive indexes, the appearance of water on the surface and then filling the pores with water will be accompanied by a change in the effective refractive index of this structure. This means that the water adsorption and the water capillary condensation in the film deposited on the surface of the fiber can influence the interaction between the light guided in the fiber and environment. The amount of water, adsorbed by humidity-sensitive materials, depends on their porosity. Therefore, humidity-sensitive materials are typically highly porous. It has been shown that adsorption processes of water vapor taking place in porous media usually is described by an adsorption isotherm, which can have many types in dependence on the properties of the materials used.
