## 6 Experimental Results
### Case Study: Periodic Surface Flow Monitoring
#### 6.4.1 Overall Energy Consumption  
Table 1 reports the energy drained by the three devices, at full and half resolution and with color and monochrome images, for each of the four phases outlined in Figure 10 during a 15-min measurement cycle. The table includes the overall energy, in bold, and in the rightmost column, the length of the idle time.  
From the table, we can immediately notice that the figures of merit that emerged with the analysis focused on the single OTV phase also apply to a 15-min cycle measurement. The Raspberry Pi 3B+ is the least energy-efficient device, followed by model 3B, while the clear winner in all configurations is model 4, with a significant advantage compared to other devices. Specifically, compared to 3B and 3B+ in the same configuration, it always allows saving, respectively, more than 200 J and 350 J for each measurement cycle. For all devices, we can also notice that the energy consumed for monochrome image extraction from the 20-s video is halved compared to the color case, and once again, model 4 is the most efficient platform, even if by a small margin in this case. By looking at the idle time reported in the rightmost column of the table, we can notice, not surprisingly, that model 4 has the longest time in this phase. Its power management system is much more efficient than other devices, draining less than 1/3 currently. This fact, coupled with the energy figures that emerged in the OTV phase and the energy required for video extraction, always enables maximum energy efficiency deploying an out-of-the-box Raspberry Pi 4 system despite a slightly higher consumption for image acquisition. It is worth noting that a better energy handling of the idle phase would be possible with additional external circuitry for models 3B. However, it would add additional complexity/cost and hardly compensate for the default efficiency of model 4.  
We recall that, when targeting the maximum energy efficiency with Raspberry Pi devices, it is crucial to disable via software commands USB ports when not strictly necessary since they drain a significant amount of energy. In our setup, the only phase requiring active USB ports is the acquisition phase, carried out with a color USB camera in our experiments, lasting for construction of the first 20 s of each measurement cycle.  
Figure 11, shows the instantaneous power consumption measured with a Raspberry 4 in the four examined configurations for the first 200 out of 900 s of a single measurement  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Res/Type/Model** & **Acquisition [J]** & **Extraction [J]** & **OTV [J]** & **Idle [J]** & **Overall Energy [J]** & **Idle Time [s]** \\ \hline F/Color/3B & 81 & 103 & 392 & 265 & 841 & 680 \\ F/Mono/3B & 81 & 103 & 180 & 293 & 657 & 753 \\ H/Color/3B & 81 & 50 & 307 & 293 & 730 & 750 \\ H/Mono/3B & 81 & 50 & 136 & 314 & 580 & 805 \\ \hline F/Color/3B+ & 92 & 110 & 439 & 360 & 1000 & 710 \\ F/Mono/3B+ & 92 & 110 & 200 & 392 & 793 & 773 \\ H/Color/3B+ & 92 & 57 & 342 & 389 & 879 & 766 \\ H/Mono/3B+ & 92 & 57 & 148 & 413 & 710 & 814 \\ \hline F/Color/4 & 101 & 84 & 343 & 87 & 615 & 775 \\ F/Mono/4 & 101 & 84 & 163 & 91 & 440 & 816 \\ H/Color/4 & 101 & 42 & 230 & 92 & 465 & 818 \\ H/Mono/4 & 101 & 42 & 105 & 94 & 343 & 844 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Energy consumed by the three devices, configured to take advantage of SIMD and Multicore processing capabilities, during each phase of a 15-min measurement cycle. Additionally, the rightmost column reports the seconds of idle time.  
Figure 10: The four phases composing a measurement cycle.  
cycle. We can perceive how deploying monochrome images at half resolution is the optimal choice regarding energy efficiency.