Permittivity-Based Water Content Calibration Measurement in Wood-Based Cultural Heritage: A Preliminary Study.

Livio D’Alvia,Andrea Cataldo,Zaccaria Del Prete,Emanuele Piuzzi

doi:10.3390/s22062148

Livio D’Alvia, Andrea Cataldo + Show 2 more

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https://doi.org/10.3390/s22062148

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Abstract

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. 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 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 R2 > 0.990 in the 1–12 GHz frequency range. 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 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 water 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, may be used directly as a non-invasive sensor for on-site measurements.

Highlights

Water content is undoubtedly one of the most critical parameters in artworks, historical objects, and buildings conservation [1,2], being the promoter of changes in the size and shape, and accelerating the deterioration rate due to chemical reactions and biological deterioration sources
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
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 [3,4]

Summary

Introduction

Water content is undoubtedly one of the most critical parameters in artworks, historical objects, and buildings conservation [1,2], being the promoter of changes in the size and shape, and accelerating the deterioration rate due to chemical reactions and biological deterioration sources. 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). If gravimetric methods are the most accurate procedures to evaluate water content in the objects, they are destructive and require taking samples of the object under investigation. For these reasons, the curators’ attention in monitoring historical objects through non-invasive and ad-hoc instrumentations, devices, and methodologies has considerably increased in the last ten years. A brief description of the common techniques and instruments is proposed below [8]

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