# Experimental Characterization of Thermal Conductivity with a New Compact Hot-Box Prototype
## 4 Results and Discussion  
In this study, a new experimental cell prototype with a physical volume of 0.602 m\({}^{3}\) was designed to measure the thermal conductivity of a specimen with a fixed area of 25 cm \(\times\) 25 cm and a thickness of less than 10 cm, using the hot-box technique. The prototype was equipped with a thermal regulator to adjust the internal atmosphere of the hot chamber, generating a temperature difference across the specimen with the cold chamber. The atmosphere inside the cold chamber depends on the environmental conditions of the laboratory, which are controlled using a dedicated air-conditioning system. To validate the prototype's operation, an experimental test was conducted with an oak-based specimen with dimensions of 25 cm \(\times\) 25 cm and a thickness of 3.81 cm. The oak sample was selected because it corresponded to a well-known construction material with extensively studied thermal properties. This selection facilitated the comparison of the experimental thermal conductivity value obtained with the new prototype against reference values reported by laboratories and studies in the literature. The experimental process conducted with the prototype is illustrated in Figure 5. The total experiment duration was 1.5 h, corresponding to 5400 recorded readings. It is important to note that experimental measurements were recorded only after the hot chamber reached the target setpoint temperature of 70 \({}^{\circ}\)C established for this study. Figure 6a shows  
Figure 4: Normal distribution of uncertainty for thermal conductivity. the temperature behavior captured by the sensors positioned at the center, south, west, north, and east of the oak specimen throughout the entire 1.5 h test. It was observed that the temperatures exhibited minimal fluctuations and remained close to the setpoint value (70 \({}^{\circ}\)C). The ambient laboratory temperature and the temperatures measured on the cold surface of the specimen are shown in Figure 6b. The laboratory temperature remained approximately constant at 21 \({}^{\circ}\)C, while the temperatures on the cold surface of the specimen displayed smooth lines without disturbances, with a central value close to 38 \({}^{\circ}\)C.  
The measurements obtained from the heat-flux sensors positioned at the center and south coordinates of the oak specimen during the experiment exhibited rapid short-spectrum oscillations (Figure 6c). This behavior was reported in other studies [11,16], suggesting that it could be associated with transient phenomena caused by rapid fluctuations in the temperature differential due to small changes in the internal atmosphere of the hot chamber. Figure 5: Process flow for performing specimen testing. Figure 6: _Cont._  
The thermal conductivity (Equation (1)) obtained from the average values of the main parameters recorded in each reading is shown in Figure 6d. Initially, the trend of the curve continuously decreased until it converged to an approximately constant value (Figure 6d). This behavior was expected according to the ISO-9869-1:2014 standard [19] and was observed in theoretical studies by other authors [11; 20]. The total thermal conductivity was evaluated using the experimental measurements recorded under steady-state conditions during the test, totaling 2700 readings over 45 min. The average values for each reading of the main parameters associated with thermal conductivity are presented in Figure 7a. All average values of the hot and cold surface temperatures of the specimen remained close to constant conditions. The heat flux showed short oscillations with a central value of 134.03 W/m\({}^{2}\).