## 5 Test Protocol  
### Measurement with the Sensor Not Inserted in the Soil  
The operation of soil water matric potential sensors based on the porous block depends basically on the characteristics of the porous ceramics. Thus, the first test performed was an evaluation of the sensor's response for different values of the volumetric water content in the porous block, by measuring the \(V_{BE}\) of the sensor as a function of the time. To avoid the presence of entrapped air in the pores, the sensor was soaked for twelve hours in water (at atmospheric pressure), followed by an additional one hour under a pressure of approximately 70 kPa, to guarantee a complete saturation of the ceramic [12]. Based on the observed weight change from dry to saturated, the saturated volumetric water content of the porous ceramic was calculated as \(\theta_{v}=46.4\%\). Next, the saturated sensor was placed on a scale (with a 0.1-g resolution) and left in an ambient condition with controlled temperature (\(25\pm 2\,^{\circ}\)C). During the evaporation process, for different values  
Figure 4: (a) Sectional view; (b) photograph of the fabricated sensors. of \(\theta_{v}\), heat pulses were applied, and measurements of \(V_{BE}\) as a function of the time were taken during each pulse. The volumetric water content \(\theta_{v}\) of the porous ceramic is defined as the weight of water absorbed into the soil divided by the weight of the porous ceramic. The value of \(\theta_{v}\) is commonly presented in %. In this prototype, the measured values of \(V_{BE}\) were not converted to temperature inside the microcontroller. The values of \(V_{BE}\) recorded in the SD card were read in a PC, where they were converted to temperature using a spreadsheet that calculated Equations (2) and (3). It is worth noticing that for the operation of the sensor in the field, it is not necessary to calculate the values of temperature. If we measure the initial and final \(V_{BE}\)s of the transistor, using Equation (3), we can write:  
\[V_{BE}(T_{0})=\left(V_{g0}+\eta\frac{kT_{r}}{q}\right)-\lambda T_{0} \tag{6}\]  
and  
\[V_{BE}(T_{f})=\left(V_{g0}+\eta\frac{kT_{r}}{q}\right)-\lambda T_{f} \tag{7}\]  
Thus, subtracting Equation (7) from Equation (6), we obtain:  
\[V_{BE}(T_{0})-V_{BE}(T_{f})=\lambda(T_{f}-T_{0}) \tag{8}\]  
Therefore, the difference \((T_{f}-T_{0})\) is proportional to \(\Delta V_{BE}=V_{BE}(T_{0})-V_{BE}(T_{f})\), and during the calibration in the laboratory, the values of \(\theta_{v}\) should be calibrated as a function of \(\Delta V_{BE}\). This value can be easily calculated inside the microcontroller and used by the automatic irrigation system. The values of the calculated temperatures (as well as the temperature difference between \(t_{0}=1\) s and \(t_{f}=10\) s, as a function of time) are simultaneously plotted in Figure 5, for values of \(\theta_{v}\) between 6% and 46.4%. In Figure 6, we present a plot of the calculated values of \(\Delta T\) (measured at \(t=10\) s) as a function of \(\theta_{v}\). It is important to observe that, for \(\theta_{v}=40\%\), the value of \(\Delta T\) in this sensor (with an energy heat pulse \(E=0.745\) J) was \(\Delta T\approx 3.1\,^{\circ}\)C, while in [13], where the transistor was in direct contact with the soil and the energy heat pulse was 100% higher, the value of \(\Delta T\) was \(\Delta T=4.5\,^{\circ}\)C, which is only 45% higher. However, this does not imply that the sensor with the porous block is more energy efficient. This difference found in \(\Delta T\) occurred simply because we had two different mediums serving as a heat sink for the transistor: the porous ceramic saturated with water and the soil saturated with water. Considering that the ratio of the thermal resistivity between a soil with a high organic content and a quartz sand soil can be as high as 3:1 [22], it is clear that what happens here is that, when saturated with water, the ceramic had a higher thermal resistance than the soil used in the experiment conducted in [13].