# Monitoring and Calibrating Building Materials Drying Kinetics with Capacitance Sensors
## 1 Introduction
Several capacitance sensors have been tested for use with different building materials [17; 18; 19; 20; 21]. These capacitance sensors use different electronic designs, resulting in various application techniques and calibration approaches. The dielectric properties are influenced by several different factors, such as material intrinsic factors (mineralogical chemical composition, microstructure, grain size, etc. ), but also extrinsic factors, such as environmental conditions. Due to the complexity and variability of these different parameters, it is evident that each material requires individual calibration in order to obtain reliable moisture measurements. The calibration of capacitance sensors for in situ measurements is even more challenging for cultural heritage materials [12]. Capacitance methods remain a pioneer among various indirect methods due to their high operation speed and good accuracy, automation of measurements and data processing [22]. However, despite their advancements, capacitance-based methods for determining moisture content have some disadvantages compared to other methods These issues are related to the need for calibration in order to determine the gravimetric to capacitance correlation. Additionally, sensor placement within building materials may be inconsistent across different installations, introducing potential measurement errors. Furthermore, after events where building materials may be saturated, the material must reach a steady state to reliably obtain a consistent measurement. Despite the wide variety of techniques available for calculating moisture content, measuring and monitoring moisture using a minimally destructive method, in real time, remains a challenge for both historical and modern buildings [23; 24]. Although advanced imaging technologies and computational methods are widely used in building investigations, the determination of moisture content is still in need of further elaboration [25]. Additionally, climatic change, along with its accompanied phenomena, such as sea level rise, extreme weather conditions, floods, etc., constitutes a challenge for conventional methods. Consequently, only permanent monitoring of moisture content can evaluate the risk of decay or damage and support decision-making for the preventive conservation of structures [26]. The main objective of this study was to investigate the monitoring of materials' moisture content measured simultaneously by direct and indirect methods. For this reason, different building materials comprised of main structural elements were selected (such as bricks and stones), but also different kinds of mortars (cement, lime, and hydraulic lime-based), most frequently used in historic and modern buildings. More specifically, the materials' moisture content was measured in the laboratory using both the gravimetric method and the application of embedded capacitance sensors. The main task of the investigation was to evaluate the capacitance sensor measurements in relation to the gravimetric results during a dynamic drying test experiment. A calibration model was developed which correlates a material's actual moisture content to capacitance values for the entire range of drying, from completely saturated to dry state, in constant environmental conditions.