# Monitoring and Calibrating Building Materials Drying Kinetics with Capacitance Sensors
## 2 Materials and Methods
### Measurements/Calibration  
In this work, the different building materials examined are typically found in historic and modern buildings, such as traditional and modern mortars, bricks, and stones. The \begin{table}
\begin{tabular}{c c c} \hline \hline
**Material** & **Code** & **Type** \\ \hline Brick–Italy & \(BRI\) & Fired, handmade \\ Brick–Greece & \(BRM\) & Fired, handmade \\ Limestone & \(SRY\) & Quarry stone \\ Travertine & \(SST\) & Quarry stone \\ Lime & \(ML\) & Hydrated lime putty, river sand, 1:3 per weight \\ Hydraulic lime & \(MHL\) & Natural hydraulic lime 3.5, commercial premixed mortar \\ \multirow{2}{*}{
\begin{tabular}{c} Lime \\ \end{tabular} } & \(MLP\) & Hydrated lime powder with artificial pozzolan, commercial premixed mortar \\  & & premixed mortar \\ Cement & \(MC\) & Cement binder (CEMI/B-M), river sand, 1:3 per weight \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Materials under investigation. Figure 3: The 5-MHz sensing circuit. main aim of the investigation is to calibrate the moisture content as measured by the capacitance sensors based on correlation with the results obtained using the gravimetric method, while drying the samples at constant conditions (T = 21 \(\pm\) 1 \({}^{\circ}\)C and RH (%) = 50 \(\pm\) 5%). The materials' microstructure characteristics were investigated with the aid of a mercury porosimeter (MIP, Pascal 140, 440, Thermo Electron Corporation, Waltham, MA, USA), in a pressure range of 0.01-200 MPa. During the experiment, capacitance changes as the different materials are conditioned to the different specific moisture content levels from 100% saturated state to dry state. Dry building materials are usually electrically insulating resulting in a weak dielectric response to the electric field, with a dielectric constant ranging between 1 and 5. However, their porosity leads to moisture absorption and storage. The high dielectric constant (80) of water results in a higher dielectric constant for the moisture-containing materials relative to their dry state. The overall dielectric constant increases with the amount of moisture. Thus, the dielectric constant can be an indicator of the moisture content in these materials. The moisture content (MC) can be determined by measuring the change of the dielectric constant in relation to moisture content [20; 22]. For each material, three samples underwent two series of drying measurements:  
1. After measuring the mass of dried samples, the samples were immersed in distilled water. Then, their weight was recorded every hour, using a precision balance (RADMAG, PS 10100.R2.M), with an accuracy of 0.01 g. The moisture content \(X\) (kg/kg db) (db: dry base), for each material is estimated as: \[X=\frac{Xsaturation-Xdry}{Xdry}\times 100\] (2)
2. After drying, samples were saturated with immersion in water, and then were left in the same environmental conditions (T = 21 \(\pm\) 1 \({}^{\circ}\)C and RH (%) = 50 \(\pm\) 5%) to dry. This time, the difference between the reference voltage _Vb_ref_ and _Va_ was measured \(V\) (mV/mV db) as: \[V=\frac{Va-Vb\_ref}{Vb\_ref}\] (3) Then, the two dimensionless values were correlated for the different materials. Usually, sensor calibration tests are performed for specific moisture content levels when the material is either completely wet or dry. For example, [PERSON] and [PERSON] (2022) tested eight different types of sensors, including two types of capacitance sensors, for measuring the moisture content of sandstone. The capacitance sensors were calibrated when samples were fully saturated with water and after drying [16]. This gravimetric calibration method considers only discrete moisture thresholds and does not capture the entire range of the drying process. The advantage of the present investigation is that the calibration was performed for the dynamic drying phenomenon, and not only for specific moisture contents. For calibrating the dynamic drying phenomenon, it is necessary to have 'curve shapes', as in Figure 4[28].