## 6 Conclusions  
We evaluated the effectiveness of inductive coils as a sensor to measure the concentration of OF used in agriculture. The obtained parameters will be used to monitor the quantity of OF in the irrigation systems to detect abnormal situations or deficiencies.  
In this paper, we showed the performance of three different prototypes with four configurations as a sensor to measure the concentration of OF. First of all, the performance of each prototype was analysed using COMSOL Multiphysics, and the WF of each prototype and configuration is determined after testing the sensor in a laboratory environment. Then the calibration and verification are carried out for each one. Finally, statistical analyses are performed to evaluate the capacity of the prototypes to discriminate among different tested concentrations. After analysing the results, we discussed the two major findings of the presented results. First, the effects of changing the configuration on the WF, on Vout, and on its capability to serve as a sensor are discussed. Then, the observed different tendency between Vout and concentration before and after the PF was analysed and compared with the results of other papers. As a final point, we compared the results of the three prototypes, according to the requirements set at the beginning of the paper, to determine which was the best prototype and configuration. The third prototype with the first configuration was the selected one.  
In future work, we intend to make an in-depth analytical study of the influence of electrics parameters, such as permeability and permittivity, in electromagnetic fields, by applying the green function in quasi-static conditions [42]. Moreover, we are going to study the effect of water temperature. Although in other papers, authors do not find any effect from 1 to 20 \({}^{\circ}\)C in soil measures, we have a different medium and higher temperatures. If an effect of temperate is detected, we will develop calibration curves for different temperatures. Moreover, the use of secondary sensors, such as optical sensor will be proposed, and other types of fertilisers will be tested. Finally, theoretical and electrical models will be made to describe the inductance and an internal resistance, in series with an induced resistance due to the conduction losses in the medium, and an additional inductance due to the permittivity and permeability of the medium.  
Conceptualisation, J.L.; methodology, L.P. and M.B.-C.; field measures and sampling, D.A.B.; data curation, L.P. and D.A.B.; writing--original draft preparation, D.A.B.; writing--review and editing, L.P., J.L. and P.V.M.; supervision, P.V.M. and J.L.; validation, P.V.M. and J.L.; project administration, J.L. All authors have read and agreed to the published version of the manuscript.  
This work is partially funded by the Conselleria de Educacion, Cultura y Deporte with the Subvenciones para la contratacion de personal investigador en fase postdoctoral, grant number APOSTD/2019/04, by the European Union, through the ERANETMED (Euromediterranean Cooperation through ERANET joint activities and beyond) project ERANETMED3-227 SMARTWATIR, and by the European Union with the \"Fondo Europeo Agricola de Desarrollo Rural (FEADER)--Europa invierte en zonas rurales\", the MAPAMA, and Comunidad de Madrid with the IMIDRA, under the mark of the PDR-CM 2014-2020\" project number PDR18-XEROCESPED.  
The authors declare no conflict of interest.