# Comparison of Measurement Possibilities by Non-Invasive Reflectometric Sensors and Invasive Probes
## 4 Discussion
### Discussion of the Calibration Results and Uncertainty Calculations  
In the case of dry samples and a moisture content below 0.05 cm\({}^{3}\)/cm\({}^{3}\), the apparent permittivity determined by means of the surface sensors ranged from 3 to 4. This was a consequence of the value of the apparent permittivity of the solid phase of the tested  
Figure 5: Uncertainty dependence on material moisture: invasive, S1 and S2 sensor, 400 and 600 c.c. samples. material and the apparent permittivity of polyoxymethylene of 3.8 [-] [43]. In the higher moisture ranges, the TDR surface sensor readings showed higher moisture values than the conventional invasive probe. This was also confirmed by the statistical characteristics of the model used, mainly, the coefficient of determination (R\({}^{2}\)) equal to 0.986 and 0.996 for the S1 and S2 TDR surface sensors and to 0.988 for the invasive probe. The RSE values were equal to 1.925 and 3.42 for the non-invasive TDR sensors and to 3.183 for the invasive probe. Similar observations were made in the case of the RMSE. It was equal to 2.70- and 1.52 for the TDR surface sensors and to 2.52 for the invasive probe. The RSE and RSME values were lower for the non-invasive S2 probe, while R\({}^{2}\) was the highest for this probe among the tested sensors [42; 49]. Due to the fact that in the measurements of building materials and partitions the most frequently used quantity is the mass moisture (in %), calibration models were developed for these units. Unfortunately, in the case of the TDR technique, most of the available literature expresses a material's moisture content as volumetric moisture; therefore, for the purposes of literature discussion, the obtained values were converted to cm\({}^{3}\)/cm\({}^{3}\) according to the formula presented in [11; 42]. Tables 7 and 8 show that the RMSE values obtained for all probe models ranged from 1.521% mass to 2.7% mass (0.0091 to 0.0162 cm\({}^{3}\)/cm\({}^{3}\)) for the cellular concrete 400 and from 1.579% mass to 1.929% mass (0.0063 to 0.0077 cm\({}^{3}\)/cm\({}^{3}\)) for the aerated concrete 600. The lowest values of this coefficient, 1.521% mass (0.0091 cm\({}^{3}\)/cm\({}^{3}\)), for the concrete 400 were recorded when measured with the non-invasive S2 sensor. In the case of the aerated concrete 600, the lowest value of this coefficient was achieved in the case of measurement with an invasive probe and was 1.579% mass (0.0063 cm\({}^{3}\)/cm\({}^{3}\)). In turn, the maximum values of the RMSE coefficient for the cellular concrete 400 were obtained with the non-invasive S1 sensor (1.58% mass, 0.0162 cm\({}^{3}\)/cm\({}^{3}\)) and for the cellular concrete 600, again with the same sensor (1.93% mass, 0.0077 cm\({}^{3}\)/cm\({}^{3}\)). According to the data presented by [49], the application of [PERSON] et al. [49] in relation to selected soil centers resulted in uncertainties expressed as RMSE in the range of 0.01-0.066 cm\({}^{3}\)/cm\({}^{3}\). In the case of the model proposed by [PERSON] et al. [50], the RMSE ranged from 0.008 to 0.037 cm\({}^{3}\)/cm\({}^{3}\). The RMSE for the popular calibration formula proposed by [PERSON] et al. (1996) was set at 0.03 cm\({}^{3}\)/cm\({}^{3}\). It should be borne in mind that in most of the cited literature sources, the proposed models were universal. For this reason, the quality of the fit to the measured data was lower. The obtained RMSE values were comparable and in many cases lower than the values determined by the team of [PERSON] et al. for regression models developed individually for each material (0.008-0.034 cm\({}^{3}\)/cm\({}^{3}\)) [51]. When analyzing the characteristics of the influence of moisture on the measurement uncertainty (Figure 5), it can also be noticed that the measurement uncertainties for the sensors S1 and S2 were smaller compared to those obtained for classic invasive probes.