## 4 Results and Evaluation
### Assessing Soil-Specific Measurements  
The impact of the node platform being presented on the quality of the soil data gathered depends on the analog or digital nature of the soil instruments, and in both cases, specific attention was given to minimize the impact. Specifically, for digital output cases (i.e., for the TEROS 12 and 5 TE units) the system transfers exactly the data that are intercepted and digitalized by the soil instrument internal logic. Consequently, provided that no data losses or error reports occurred due to the instrument, the supporting platform gathers exactly the originally measured quantities. In other words, when data are received, they should be in accordance with the manufacturer's specifications. For the analog instrument data cases (i.e., for the TEROS 10, 10 HS, and ML2 units), one can verify that no distortion to the original values has occurred either by the power supply subsystem or by the digitization process utilizing the analog-to-digital converter (ADC) of the 32u4 microcontroller. The flawless behavior of the system is verified by the testing accuracy using a very precise voltage reference unit (i.e., the Precision LM4040 Voltage Reference Breakout by Adafruit) and by the fact that the measurements provided by the analog instruments compared with the ones provided by their digital counterparts are close to a satisfactory extent with respect to potential deviations implied by their intrinsic characteristics documented by their manufacturers and the research community. In Figure 9a,b, the measured \(\theta\) values are plotted against \(\theta_{\mathrm{m}}\) for the loam sample in order to investigate the performance of TEROS 10 and TEROS 12 sensors in predicting soil water content values. The porous material of loam was packed in a bucket to achieve a constant bulk density. A loam sample oven-dried at 105 \({}^{\circ}\)C was then mixed with the test solution to reach an electrical conductivity (EC\({}_{\mathrm{w}}\)) level of 10 dS/m for the TEROS 10 and 6 dS/m for the TEROS 12. The two sensors were placed vertically in the middle of the sample, and measurements were taken in laboratory conditions at constant temperature. For the same value of \(\theta_{\mathrm{m}}\), the TEROS 10 provided readings that were almost in agreement with the actual ones. The TEROS 12 had a similar behavior other than a slightly underestimation from \(\theta\) = 0.15 to \(\theta\) = 0.25 cm\({}^{3}\)/cm\({}^{3}\). The obtained values showed that both sensors gave reliable results for \(\theta\), which were in agreement with the actual values \(\theta_{\mathrm{m}}\), and the trend line is strongly linear for the two studied cases. The impact of increasing salinity level of the aqueous solution on dielectric permittivity is shown in Figure 9c along with the obtained values from the TEROS 10 and 12. The data from both sensors were much affected by the increasing EC\({}_{\mathrm{w}}\) of the solution and did not remained constant [22; 38]. In Figure 9d are shown the \(\varepsilon_{\mathrm{a}}\) values obtained by the TEROS 10, TEROS 12, and 10 HS sensors versus the \(\theta_{\mathrm{m}}\) values of the loam sample with EC\({}_{\mathrm{w}}\) = 0.28 dS/m. The soil moisture level readings of the three sensors were close except that the TEROS 12 underestimated \(\varepsilon_{\mathrm{a}}\) at saturation point \(\theta_{\mathrm{m}}\) = 0.40 cm\({}^{3}\)/cm\({}^{3}\), which is also a behavioral characteristic documented by recent research outcomes [25]. Figure 10a illustrates the time evolution of the originally in situ measured quantities by the proposed system (i.e., the instrument output) for the TEROS 10, TEROS 12, 5 TE, and ML2 instruments. All results shown cover the same three-day period, which included an intense dry sandy loam soil wetting event on the first day.