Permittivity-based water content calibration measurement in wood-based cultural heritage: a preliminary study

In this work, the dielectric permittivity of four kinds of wood (Fir, Poplar, Oak, Beech Tree), used in Italian Artworks and structures, was characterized at different humidity levels. Measurements were carried out using three different probes connected to a bench vector network analyzer: a standard WR90 waveguide, a WR430 X-band waveguide and an open-ended coaxial probe. In particular, we have investigated the relaxation model for the four wood species, showing how a log-fit model presents in the 1-12 GHz frequency range a determination coefficient R2>0.990 through the open-ended. This result has proved helpful to fill the frequency gap between the measurements obtained at different moisture contents with the two waveguide probes showing an R2>0.93. Furthermore, correlating the log-fit vertical shift with the water content, it was possible to find a calibration curve with a linear characteristic. These experimental results will be helpful for on-site non-invasive moisture monitoring of wooden artworks or structures. Moreover, the final results show how the open-ended coaxial probe, with a measurement deviation lower than 7% from the waveguide measurements, be used directly as a non-invasive sensor for on-site measurements.