## 3 Materials and Methods
### Sensors
#### 3.2.1 Water Parameters  
For water quality monitoring in fish farms, the physic-chemical parameters monitored by our system are temperature, conductivity, turbidity, and the presence of an oil layer in the surface of the water. As it is explained above, to create a low-cost system, the price of the sensors is a limiting factor. For this reason, we create our own sensors based on electronic components with low-cost. First, we describe the employed sensor for temperature monitoring. There are several options for temperature sensing. The most employed ones for water monitoring are the RTD and the thermistors. The reasons why these are the most used sensors are their high working range, adequate accuracy, and low price. In our case, a thermistor type negative temperature coefficient (NTC) was employed. The NTC presents greater resistance when temperature decreases. The other advantage of the NTC is the linear relationship between resistance and temperature. The employed NTC is the NTCLE413E2103F520L from Vishay [36]. The price of this sensor is 0.96 \(\upmu\)C. The working range of the sensors according to the datasheet is \(-40\)\({}^{\circ}\)C to \(105\)\({}^{\circ}\)C. Our working range will be from \(5\)\({}^{\circ}\)C to \(35\)\({}^{\circ}\)C. The employed NTC can be seen in Figure 6.  
Following, the conductivity sensor is presented. As for temperature sensing, different options can be used. In this case, and considering that one of the main requirements is to avoid the contact between the sensing element ant the water, we propose the use of inductive sensor. This sensor is composed of two copper coils. One of the coils is powered by a sine wave. The generated electromagnetic field change depending on the conductivity of the water. The secondary coil, depending on the magnetic field, induces an electric voltage. The employed sensor was described by [PERSON] et al. [37; 38]. The coils were done with copper wire with 0.4 mm of diameter. The coil diameter was 25 mm. The powered coil has 40 spires and the induced coil 80 spires. The induced voltage increases when the conductivity of the water increases. In a previous test performed with these coils a PVC tube was used [37; 38]. Nevertheless, in this case, a crystal cylinder will be used. The working range of the sensor goes from 0 mS/cm to up to 80 mS/cm. In the marine fish farms, the expected salinity goes for 0 mS/cm to 29 and to 38 mg/L [39], which corresponds to a range from 44.9 to 57 mS/cm. The conductivity sensor included in our system can be seen in Figure 6.  
The turbidity sensor is described below. There are two main methods to measure the turbidity, the acoustic and the optical method. In this case, the optical method is selected. The employed sensor is described in [40]. It is composed of an infrared (IR) light emitting diode (LED) and an IR  
Figure 6: Employed components for sensor to monitor the water quality. photodetector. The employed IR LED is the TSHG6200 from Vishay [40], the peak wavelength of this LED is 850 nm. Moreover, the employed IR photodetector is the BPW83 from Vishay [41]. Its range of spectral bandwidth goes from 790 to 1050 nm and its wavelength of peak sensitivity is 950 nm. The IR photodetector offers a fast response, the resistance of the photodetector decreases when the turbidity increases. The relationship between turbidity and resistance is linear in the range from 0 to 250 mg/L. If we intend to include higher values, the relationship changes to other type of relation. However, the expected values of turbidity in fish farms are low. Only in adverse conditions the turbidity values can increase. The turbidity sensor can be seen in Figure 6.  
The last sensor that monitors the water quality is the oil sensor. This sensor must be able to detect the appearance of an oil layer in the surface of the water. This sensor is based on the model proposed by [PERSON] et al. [42]. A white LED is employed to illuminate the water surface and IR photodiode is used to measure the light emission.