Dielectrometry measurements of moisture dynamics in oil-impregnated pressboard

The dielectric spectrum of pressboard, (i.e. its complex permittivity as a function of frequency), is related to its moisture content and to its temperature. Consequently it is possible to use dielectrometry measurements to measure a pressboard sample´s moisture content. The complex permittivity of pressboard is measured in two different ways. The first method places the sample between two parallel metal plates, in effect creating a lossy lumped capacitor, whose complex impedance can easily be measured. The complex permittivity thus calculated is effectively averaged across the sample´s bulk. The other method involves interdigitated electrodes, applied only to one surface of the sample. Since the electric field´s depth of penetration is dependent on the spatial wavelength of the electrodes, different sensors can obtain information from different depths in the material, making it possible to measure spatial distributions of the dielectric properties. This can allow the study of the diffusion of water in pressboard