## 3 Materials and Methods
### Electromagnetic Effects on the Coils  
In this section, the process used to perform the simulations prior to testing the sensors in a laboratory environment is described. As previously stated, COMSOL Multiphysics platform was used to model the proposed prototypes and simulate their behaviour in different mediums: air, pure water, seawater, and three different concentrations of OF. The values used to simulate the mediums in which the coils were immersed can be seen in Table 2. The values used to simulate seawater in COMSOL Multiphysics were chosen according to the results found in [34]. As for the values used to simulate the different samples of OF, we used the different conductivity values obtained during the experiments and a relative permittivity close to the one of pure water. In this paper, we will not be showing the simulations of the proposed prototypes in the different mediums. However, we decided to display the magnetic flux density and the electric field of all three prototypes using a 20 g/L of OF sample as a core, and explain the results obtained with COMSOL Multiphysics. Figure 3 shows the magnetic flux density norm and the electric field of the prototypes using a 20 g/L of OF sample. Given the fact that the magnetic flux density highly depends on the permeability of the medium, and the relative permeability values used in the simulations are very similar, COMSOL Multiphysics did not manage to detect changes. The magnetic flux density norm values obtained during the simulations of P1 varied from 11.2 tesla (T) to 5.96 T along the edges of the induced coil, and from 11.2 T to 7.83 T along the centre of the induced coil (see Figure 3a). When the core was filled with air, the electric field of the prototype P1 varied from 148.36 V/m to 39.41 V/m along the edges of the induced coiled, and from 44.66 V/m to 17.91 V/m along the centre of the induced coil (see Figure 3b). In the case of pure water, the electric field varied from 247.28 V/m to 17.95 V/m along the edges of the induced coiled, and from 40.16 V/m to 13.74 V/m along the centre of the induced coil. For seawater, the obtained values of the electric field varied from 243.02 V/m to 87.81 V/m along the edges of the induced coiled, and from 40.33 V/m to 13.24 V/m along the centre of the induced coil. In samples of 2.5 g/L of OF, the electric field varied from 265.67 V/m to 14.11 V/m along the edges of the induced coiled, and from 40.03 V/m to 12.94 V/m along the centre of the induced coil. For the samples of 12.5 g/L of OF, the electric field varied from 267.36 V/m to 15.45 V/m along the edges of the induced coiled, and from 40.32 V/m to 13.14 V/m along the centre of the induced coil. Finally, the obtained values for the samples of 20 g/L of OF, the electric field varied from 268.86 V/m to 17.78 V/m along the edges of the induced coiled, and from 40.51 V/m to 13.35 V/m along the centre of the induced coil. Figure 3c,d display the magnetic flux density norm and the electric field of P2 using a 20 g/l of OF sample. In this case, the magnetic flux density norm values obtained varied from 5.29 T to 1.43 T along the edges of the induced coil, and from 5.47 T to 2.18 T along the centre of the induced coil. For a core filled with air, the electric field varied from 132.67 V/m to 14.07 V/m along the edges of the induced coiled, and from 37.39 V/m to 0.73 V/m along the centre of the induced coil. In pure water, the electric field varied from 191.44 V/m to 11.2 V/m along the edges of the induced coiled, and from 36.52 V/m to 3.23 V/m along the centre of the induced coil. In seawater, the values of the electric field varied from 174.50 V/m to 19.65 V/m along the edges of the induced coiled, and from 36.43 V/m to 3.46 V/m along the centre of the induced coil. In the samples of 2.5 g/L of OF, the electric field varied from 190.72 V/m to 11.56 V/m along the edges of the induced coiled, and from 36.01 V/m to 3.35 V/m along the centre of the induced coil.