## 5 Discussion
### The Best Prototype to be Used and OF Sensor in PA Systems  
These prototypes are the best sensors tested in previous studies [20]. Furthermore, these prototypes differ in the distribution of their layers. With this comparison, we will verify which sensor, with which layer distribution, is the most suitable for monitoring OF. It also serves as a precedent for future research in the development of sensors for fertilizers. After analysing all of the measures in calibration, verification, and statistical analysis, it is time to compare the prototypes and identify the one with the best performance. According to the requirements set in Section 3, we have five different requirements; all of them are analysed in Table 6. The first requirement is a minimum variation of 1 V between the most and less concentrated samples (\(\Delta\)Vout [S1-S9]). According to our data, only the P2 and P3 accomplish this requirement. The second requirement was related to the Vout magnitude, the lower registered Vout (Minimum. Vout) should be at least higher than 3 V. In this case, all of the prototypes fulfil this requisite. The third condition is that Vout must be different in all the samples. To test this requisite, we base on the results of a multiple range test. According to Table 6, this requisite is reached only in three cases, P1 configuration 3, P3 configurations 1 and 4. With regards to the working frequency (WF), all of the prototypes have WFs lower than the required threshold, so that they all are considered as valid. The last requisite refers to the error measured in the verification test, in this case, the AE for the lowest verified concentration (AE [S3]) and the average RE (\(\overline{\text{RE}}\)) must be considered. \begin{table}
\begin{tabular}{l l c c c c c c} \hline \hline  & \multirow{2}{*}{**Conf.**} & \multicolumn{2}{c}{**R1**} & **R2** & **R3** & **R4** & **R5** & **All** \\  & & \(\Delta\)_Vout_ [S1–S9] (V)** & **Min.** _Vout_ (V)** & **N\({}^{*}\) Groups** & **WF (kH)** & **AE [S3] (V)** & **RE (\%)** \\ \hline \multirow{4}{*}{P1} & 1 & 0.53 & 9.28 \({}^{*}\) & 7 & 100 \({}^{*}\) & 0.02 \({}^{*}\) & 0.19 \({}^{*}\) \\  & 2 & 0.44 & 6.37 \({}^{*}\) & 7 & 90 \({}^{*}\) & 0.03 \({}^{*}\) & 0.52 \({}^{*}\) \\  & 3 & 0.39 & 6.15 \({}^{*}\) & 9 \({}^{*}\) & 90 \({}^{*}\) & 0.22 \({}^{*}\) & 3.5 \({}^{*}\) \\  & 4 & 0.43 & 6.40 \({}^{*}\) & 8 & 90 \({}^{*}\) & 0.07 \({}^{*}\) & 1.05 \({}^{*}\) \\  & 1 & 1.41 \({}^{*}\) & 8.64 \({}^{*}\) & 8 & 110 \({}^{*}\) & 0.65 \({}^{*}\) & 7.05 \({}^{*}\) \\  & 2 & 1.41 \({}^{*}\) & 7.95 \({}^{*}\) & 6 & 110 \({}^{*}\) & 0.75 \({}^{*}\) & 8.45 \({}^{*}\) \\  & 3 & 1.41 \({}^{*}\) & 8.40 \({}^{*}\) & 6 & 110 \({}^{*}\) & 0.7 \({}^{*}\) & 7.41 \({}^{*}\) \\  & 4 & 1.84 \({}^{*}\) & 7.41 \({}^{*}\) & 6 & 110 \({}^{*}\) & 0.83 \({}^{*}\) & 9.53 \({}^{*}\) \\  & 1 & 1.29 \({}^{*}\) & 7.65 \({}^{*}\) & 9 \({}^{*}\) & 140 \({}^{*}\) & 0.62 \({}^{*}\) & 8.86 \({}^{*}\) \\  & 2 & 1.15 \({}^{*}\) & 7.23 \({}^{*}\) & 7 & 140 \({}^{*}\) & 0.41 \({}^{*}\) & 6.28 \({}^{*}\) \\  & 3 & 1.47 \({}^{*}\) & 7.65 \({}^{*}\) & 8 & 140 \({}^{*}\) & 0.6 \({}^{*}\) & 8.81 \({}^{*}\) \\  & 4 & 1.09 \({}^{*}\) & 3.79 \({}^{*}\) & 9 \({}^{*}\) & 130 \({}^{*}\) & 0.36 \({}^{*}\) & 8.34 \({}^{*}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Statistical ANOVA test to analyse measured values. The requirements to be met by the prototypes are shown so that they are optimal for measuring OF, where “P” means prototype and “R” requirement. Values with an asterisk indicate that the requirements are met. Analysing the results for each one of the tested prototypes and their four configurations, we only found two cases in which all the requirements are accomplished, P3 configurations 1 and 4. Among both configurations, P3-configuration 1 is characterised by higher \(\Delta\)Vout [S1-S9], higher Min. Vout, and lower AE [S3], although it has a higher WF than configuration 4. In the case of P1 configuration 1, \(\Delta\)Vout [S1-S9] is lower than 1, which does not meet one of the most important requirements.