## 4 Discussion  
Results obtained indicate that pH values recorded vary to a great extent depending on the particular location of exhibition halls and showcases as well as on meteorological seasons. Related to the influence of sunlight, environmental humidity, and temperature in sensing results, sunlight, in this case, does not affect the sensor response since they were installed indoors and were always illuminated by LED systems. It was widely demon  
Figure 7: pH average results obtained during the evaluation carried out on November 2019. Numbers at the bottom of the X axis are the sensor numbers. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sensor nr.** & **Feb-2019** & **May-2019** & **Jul-2019** & **Nov-2019** & **Average pH** \\ \hline
1a & 6.4 & 6.5 & - & - & 6.5 \\
1b & - & - & 6.7 & 6.7 & 6.7 \\
2a & 6.6 & 6.9 & & & 6.8 \\
2b & - & - & 6.8 & 7.0 & 6.9 \\
3a & 6.8 & 6.8 & & & 6.8 \\
3b & - & - & 7.1 & 6.9 & 7.0 \\
3a & 6.8 & 6.9 & 6.8 & 7.0 & 6.9 \\
5a & 6.6 & 6.8 & - & - & 6.7 \\
5b & - & - & 6.7 & 6.8 & 6.8 \\
6a & 6.7 & 6.8 & - & - & 6.8 \\
6b & - & - & 6.8 & 7.1 & 7.0 \\
7a & 6.9 & 6.7 & & & 6.8 \\
7b & - & - & 6.0 & 5.9 & 6.0 \\
8a & 7.0 & 6.9 & - & - & 7.0 \\
8b & - & - & 6.9 & 7.0 & 7.0 \\
9a & 6.5 & 6.7 & - & - & 6.6 \\
9b & - & - & 6.4 & 6.2 & 6.3 \\
10b & - & - & 7.0 & 7.0 & 7.0 \\
5.3 & 5.3 & 3.9 & 6.8 & 5.3 \\
12 & 6.1 & 5.9 & 5.2 & 6.9 & 6.0 \\ \hline \hline \end{tabular} Orange = Inside showcase. Blue = Hall free environment. Red = Outdoor. - Not measured.  
\end{table} 
Table 3: pH average results obtained during the four evaluations. strated in former research [19] that environmental humidity affects the sensor response (obviously, pH depends on the amount of H\({}_{3}\)O\({}^{+}\) and OH\({}^{-}\)), while temperature affects results as far as it influences relative humidity. Temperature and relative humidity are simultaneously measured and controlled during pH sensor sensitization. The patented portable device that measures the sensors' optical responses [25] includes both relative humidity and temperature sensors. Either the calibration curves or the specific software for the corresponding results management take into account humidity and temperature effects. Such effects were widely investigated in [27; 28; 29] and revised in [19]. Concerning sensors two, four and six installed in the so-called Biodiversity hall (main greatest hall and the first one after the museum entrance), the positions of sensors two and six were changed during the third and fourth evaluation. Two other sensors were installed in two smaller halls (number eight in the National Parks hall and number ten in the Mediterranean hall); these were somehow protected from the urban air coming from the outdoors. Sensors two, four and six of the Biodiversity hall recorded an average pH between 6.7 and 6.9 (slightly acidic). Since some sensors were not stabilized after the evaluation period, the exposition time was then extended. This could be due to the particular environmental conditions of the Biodiversity hall, which is directly exposed to the urban air that enters through the very big main door oriented to Paseo de la Castellana, a big avenue with heavy road traffic. No control of such ventilation was provided during the sensor sensitization. Moreover, intake of potential polluted air could be intensified by the high number of visitors accessing the museum by this main door, which is the regular entrance for museum personnel also. However, sensor numbers eight and ten installed in other exhibition halls far from the main entrance recorded pH average values near neutrality (6.9-7.0). Therefore, the connection between urban air, probably polluted, and the slightly acid recordings at the Biodiversity hall seems to be clear. As far as the five sensors installed in showcases (numbers one, three, five, seven and nine) are concerned, all their positions were changed during the third and fourth evaluations.