## 4 Results
### Calibration
Figure 8: Representation of the frequency spectrum of P2 in the four different studied cases. For the third prototype, the results are represented in Figure 9. For this prototype, ranges with Vout higher than 3 V cover between 130 kHz and 150 kHz. In this case, the highest Vout that we registered is the smallest one among the three prototypes, 6.35 V. P3 shows that the voltage peak is located in a higher range of frequency than in other prototypes. Table 4 shows the highest difference of Vout between the minimum and maximum concentration of OF. The maximum difference is found at 140 kHz in three out of four configurations and 130 for the other, as found with P1. Notwithstanding what we detected in P1 and P2, the data of P4 shows the maximum Vout for the less concentrated sample, in three out of four configurations. The maximum difference between higher and lower Vout is detected in the configuration 3, with -1.47 V.  
With regard to configurations 1 to 3, which share the same WF, the difference among the lowest and highest concentrations are 1.29, 1.15, and 1.47 V respectively. Meanwhile, configuration 4, characterised by the lowest WK, has a variation in the measured Vout of -1.09 V, which is lower than the variation for other configurations. If we analyse in detail, the different maximum and minimum values, not only the differences between them, configurations 1 and 3 have a similar pattern. Both configurations have similar values, 7.65 V for configuration 1 and 7.65 V for configuration 3 in the case of the less concentrated sample. Meanwhile, for configuration 2, the Vout is 7.23 V for the same sample, a variation on nearly 0.4 V is found. It has no sense to compare the differences with the results with the fourth configuration since the last configuration is characterised by a behaviour similar to the one found in P1. All of the evaluated prototypes presented a similar range in which their respective Vout is affected by the concentration of tested samples. At these frequencies, their WF, we obtain higher Vout values and higher Vout differences. This range is located between the 90 k kHz and 140 kHz. The next step is to obtain the mathematical model that fits with the data gathered for the calibration at the WF for each prototype and their four configurations. The obtained calibration models for all configurations of P1 are presented in Figure 10. Among the 27 possible models available in Statgraphics, the one that offered the best adjustment, in terms of Coefficient of determination (R2), is the lineal model for all the configurations of P1. This model has the advantage of its low complexity. The values of R2 are indicated in Figure 10, and they have a minimum value of 0.937 for configuration 2. Configuration 4 is the one that presents the best adjustment between  
Figure 9: Representation of the frequency spectrum of P3 in the four different studied cases. the mathematical model and gathered data with R2 of 0.995. With regard to the mathematical models, they are represented in Equations (9) to (12) for each one of the configurations of P1. These equations will be included in the node when the sensor will be part of a WSN to convert the measured electric signal into the value of the sensed parameter. Their confidence and prediction intervals are displayed in Figure 10.  
\[\text{\it Concentration (g/L) = -343,014 + 367,813*Vout(V)} \tag{9}\] \[\text{\it Concentration (g/L) = -245,51 + 385,388*Vout(V)}\] (10) \[\text{\it Concentration (g/L) = -300,911 + 487,033*Vout(V)}\] (11) \[\text{\it Concentration (g/L) = -283,199 + 442,308*Vout(V)} \tag{12}\]  
On the other hand, calibration models for the configurations of P2 are shown in Figure 8:0 The values of R2 are included in Figure 8:1 Configuration 3 presents the lowest R2, with a value of 0.980. On the contrary, the best adjustment between the mathematical model and gathered data is found in configuration 1, with R2 of 0.995. The mathematical models are detailed in Equations (13)-(16) for each one of the configurations of P2.