7/19/2023 0 Comments Minivna tinyA gravimetric water content ~140% represents the Water Saturation Point (WSP). In the study of moisturized wood, it must be taken into account that two particular points characterize the phenomenon of water adsorption: in the range 0% to 25% or Fiber Saturation Point (FSP), the water is transferred into the wood cell wall above this point, the cell walls are fully saturated, and the water passes into cavity cells. In particular, the physical–mechanical properties of wood objects and structures are more affected by water content variations, due to the highly hygroscopic nature of the material, than other materials commonly used in historic buildings (stones, bricks). Water content is undoubtedly one of the most critical parameters in artworks, historical objects, and buildings conservation, being the promoter of changes in the size and shape, and accelerating the deterioration rate due to chemical reactions and biological deterioration sources. Moreover, the final results show how the open-ended coaxial probe, with a measurement deviation lower than 7% from the waveguide measurements, may be used directly as a non-invasive sensor for on-site measurements. These experimental results will be helpful for on-site non-invasive water monitoring of wooden artworks or structures. Furthermore, correlating the log-fit vertical shift with the water content, it was possible to find a calibration curve with a linear characteristic. This result has proven helpful to fill the frequency gap between the measurements obtained at different water contents with the two waveguide probes showing an R 2 > 0.93. In particular, we investigated the dispersion model for the four wood species, showing how a log-fit model of the open-ended data presents a determination coefficient R 2 > 0.990 in the 1–12 GHz frequency range. Measurements were carried out using three different probes connected to a bench vector network analyzer: a standard WR90 X-band waveguide, a WR430 waveguide, and an open-ended coaxial probe. In this work, the dielectric permittivity of four kinds of wood (Fir, Poplar, Oak, and Beech Tree), used in Italian Artworks and structures, was characterized at different humidity levels.
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