## 4 Results
### Calibration  
In this subsection, the data gathered for the seeking of the WF and the calibration of the three prototypes is presented. To select the WF, we sought the frequency among the samples, ones in which the difference of Vout, between the most and the less concentrated samples, is maximised. For the first prototype, the highest values of Vout, higher than 3 V, as indicated in the requirements, we registered between 80 kHz and 110 kHz, see Figure 7. In this frame, Figure 7 represents that, in all the configurations, the maximum Vout, 9.65 V, is measured when the PC is fed at 100 kHz. Additionally, Table 4 displayed the maximum difference between the Vout of 0 g/L and 17.5 g/L. The maximum difference is found at the frequency of 90 kHz in three out of four tested configurations, and 100 kHz for the other one. The values of WF for the P1 show a growing tendency, for all of the tested configurations, being the minimum Vout for 0 g/L and the maximum measured voltage for 17.5 g/L. This shows that the different configuration to feed the coil may affect the obtained values, as a low modification of the WF. In the case of configurations 2 to 4, the differences in Vout of calibration tests are almost the same, 0.44, 0.39, and 0.43. Considering that standard deviations for those values are 0.028, we can affirm that no differences are found. Although the differences in Vout between most diluted and most concentrated samples is almost constant, the Vout values for the tested values are different. The results are similar for configurations 2 and 4, and different for configuration 3. For configurations 2 and 4, the mean Vout for the most diluted sample is 6.37 V and 6.40 V, and 6.81 V and 6.83 V for the most concentrated sample. On the other hand, for the third configuration, the values are 6.15 for the most diluted sample, and 6.53 V for the most concentrated sample. These small changes suppose a difference in the Vout of almost 0.2 V between the same samples measured with different configurations. With regards to configuration 1, the difference on the Vout is the highest, 0.53, and the difference between the maximum (9.81) and minimum (9.28) Vout measured with this configuration and the rest are even higher, up to 3 V.  
Concerning the second prototype, Figure 8 shows the gathered data for the calibration at different frequencies. According to the data, we can identify the most potent interaction between the Vout and the sample and with Vout higher than 3 V between the 100 kHz and 140 kHz. In this range, the highest values are located at the peak, 110 kHz in all the analysed configurations. In this frequency, the registered highest Vout is around 9.81 V. The P2 offered higher Vout values when the concentration of OF increased, as with P1. This observation indicates a similar functioning between the two prototypes. Table 4 represents the highest difference of Vout between the lowest concentration sample and the most concentration. Moreover, the measures of the different tests that we did with P2 indicate that the highest voltage difference is found in configuration 4, with 1.84 V.  
Concerning the variability between tested configurations, P2 has high variation in the differences between maximum and minimum concentrations of OF in the WF. While in the first configuration the difference between most diluted and less diluted sample is 1.07 V, in the case of configuration 4, this value reaches the 1.84 V. In the second and third configuration, the difference is 1.41 V. With regards to the Vout values in the samples, configuration 4 is the one with the lowest voltage in the most diluted concentration (7.41 V). On the contrary, configuration 1 reached a Vout of 8.64 V for the same sample. This change on Vout, a higher value for the first configuration, is maintained along with all of the tested samples. So that, a variation of more than 1 V is observed for the same prototype, sample, and WK if the configuration is modified in the lowest.