## 4 Discussion  
Results shown in Section 3 in overall show an agreement between capacitive sensors, MOF-FP optical sensors, and gravimetric measurements in (two) different types of soil. One of the main differences between capacitive and optical results can be seen in Figure 5. There, it can be noticed differences in the time response to humidity changes between both sensors. It is due to two important factors: devices size and type of measurements. On the one hand, capacitive sensors that are used in this study have dimensions of 14.5 cm \(\times\) 3.3 cm \(\times\) 0.7 cm and they give us volumetric measurements by measuring the dielectric constant of the soil, as shown in Figure 10. On the other hand, MOF-FP based sensors have very reduced dimensions (0.7 mm \(\times\) 125 \(\upmu\)m) without the protective cap. This cap is a cylinder 4 cm long having 0.8 cm of radius. The sensing head measures the humidity that is directly in contact with it. Therefore, it can be concluded that capacitive sensors are field sensors in opposition to point optical sensors. Due to this difference, the optical sensor is very sensitive to point humidity changes very close to the sensing head while capacitive measures all of the humidity comprised around it, being less sensitive to local variations. Table 1 shows the measured errors between optical--capacitive sensors and optical sensor--gravimetric measurements. As previously commented, gravimetric measurements present great variations due to the employed technique and its variability depending on the measuring conditions. Therefore, the errors measured between optical or capacitive sensors and gravimetric in some points (i.e., in Figure 9b) at 63% RH in gravimetric measurements) are higher than expected, as can be seen in the case of the maximum error between optical sensor and gravimetric measurement. Another difference shown in the results from capacitive and optical sensors in Figures 6 and 7 is a measuring error when humidity is close to 0%. This is due to the different nature of the sensors. On the one hand, capacitive sensors measure the dielectric constant of the soil and therefore the content of water in soil: moisture. On the other hand, optical sensors measure refractive index variations that are produced by the presence of soil moisture. Relative humidity can be described, as follows:  
\[Relative\ humidity=\frac{Pw}{Pws}\times 100 \tag{2}\]  
where \(Pw\) is the water vapor pressure and \(Pws\) is the water vapor pressure in saturation and it is a function of temperature [28]. The term humidity usually makes reference to the presence of water in gaseous form, but it is often used to refer to expressions that are related to water vapor characteristics and in the field of measurement, there are various terms associated with such water vapor measurements. In addition, the term moisture is frequently interchanged with humidity even though the actual definition of moisture refers to the water in liquid form that may be present in solid materials [37]. Since humidity is a measure of water in gaseous state present in the environment, water vapor found in a gas mixture behaves in accordance with the gas laws and the amount of pressure it exerts equates to the partial pressure of the water vapor components in the gas mixture, as defined by [PERSON]'s law [37]. To set a reference between capacitive sensors and optical sensors when the soil is saturated, the air in holes inside the soil (and therefore the cavity created by the optical sensor) is saturated too, and as a conclusion, both of the sensors show 100% of humidity and relative humidity, respectively. As can be seen in Figures 8 and 9, both of the sensors work similarly in high humidity scenarios. The difference arises when the soil approaches to 0% humidity. Due to the nature of the optical sensor