# Fother Characterization Using a Novel Bubble Size Measurement Technique
## 3 Results  
### Comparison of Novel and Conventional Bubble Size Measurement Techniques  
Bubble size measurement tests using the two different techniques were carried out in a background solution of medium salinity (with an ionic strength of 0.083 mol/L and a conductivity of 6.85 mS/cm) with the addition of 25 ppm MIBC. A comparison of the two techniques is presented in Table 3.  
As can be seen from the table, PVM provides more details at the micron level, while APBS's field of view is in the millimeter range. Specifically, APBS has a field of view of 68.5 mm by 91 mm, which allows the capture of millimeter-to-centimeter-sized bubbles, as illustrated by Figure 7b. On the contrary, the PVM probe has a much smaller field of view, 0.89 mm \(\times\) 1.3 mm, which allows the capture of only small bubbles in the micro-sized range, as shown in Figure 7a. The smallest bubble size obtained with the APBS probe is 207 um in size, while the PVM probe detects a significant quantity of bubbles as fine as 20 um, as shown in Figure 8.  
One of the possible reasons for the lack of detection of fine bubbles by the APBS probe is that the segmentation algorithm is not optimized for such fine bubbles. The resolution of the image and subsequent image segmentation is too low to tell if this is the case. APBS is a commercial system with its own software developed, so we cannot just replace with a better camera resolution or lens. Another possibility is that the very fine bubbles are not reaching the measurement chamber at all. They are likely being prevented from entering the sampling tube by the downward flow of water during the sampling process. Conversely, the APBS probe picks up several bubbles in the range between 2500 and 3300 um in size, while the largest bubble detected by the PVM probe was 1800 um. This is most likely due to the small field of view of the PVM camera (1.17 mm\({}^{2}\)), which is smaller than the size of the largest measured bubble. The Hough transform algorithm can \begin{table}
\begin{tabular}{c c c} \hline \hline
**Comparison** & **PVM** & **APBS** \\ \hline \multirow{2}{*}{Application field} & Micro-field & mm-cm-field \\  & (a few μm\(-\)1.3 mm) & (200 μm\(-\)a few cm) \\ \hline \multirow{2}{*}{Size} & 0.55 m\(-\)height & 2.00 m\(-\)height \\  & 0.076 m\(-\)diameter & \(\sim\)0.300 m\(-\)diameter \\ \hline Weight & \(<\)1 kg & \(\sim\)10 kg \\ \hline Lab installation & Easy & Difficult \\ \hline Picture taken point & In situ & 1 m above sampling point \\ \hline Time requirements per test & \(\sim\)30 min & \(\sim\)20 min \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the PVM and APBS bubble size measurement techniques. determine the size of a partially visible bubble, but reaches its limit at bubbles larger than the maximum detected size. As a result, the PVM probe gave a slightly smaller bubble size than the APBS probe does. Figure 9 shows the bubble size distributions obtained using the two techniques. Error bars represent 95% confidence intervals obtained from three experimental repeats. As can be seen, there was 8% of the total bubbles captured with the PVM probe that were smaller than 177 \(\upmu\)m and were missing from the APBS curve. Conversely, 0.57% of bubbles captured with the APBS probe were larger than 2 mm and were not detected with the PVM probe. This small proportion of large bubbles affected the Sauter mean bubble diameter calculation. Figure 8: Bubbles captured with PVM probe. Figure 7: Field of view of PVM (**a**) and APBS (**b**) cameras. Table 4 compares the Sauter mean bubble diameter obtained using the two techniques. The use of the PVM probe gave a smaller Sauter mean bubble diameter than APBS (after pressure correction), but the difference was not large (147 \(\upmu\)m difference). A summary of the measurement parameters for the two techniques is presented in Table 5. The first major difference is the number of images analyzed as part of the measurement and the consequent number of bubbles detected during the measurement.