## 3 Results and Discussion  
### Temperature Collection  
The initial temperature gradient in the tank wall (\(\Delta\)T = T\({}_{\text{wall surface}}-\) T\({}_{\text{water}}\)) impacts the measurement and defect detection threshold [8; 9]. Therefore, it is necessary to study the best time frame for conducting the IRT evaluation on an HDPE AST in the field. During different times of the day, one half of the tank is sun-faced and the other half is shade-faced; therefore, the uneven heat received from the sun can induce a non-uniform temperature on the tank, as shown by IRT in Figure 3. The tank wall temperatures recorded over 230 days are shown in Figure 4. The east and west sides were selected for temperature monitoring because of how the tank was positioned: the east side is sun-faced in the morning while the west side is shade-faced and vice versa. Figure 4a shows the temperatures recorded at 10:30 a.m. when the sun is on the east side, and Figure 4b shows the temperature at 4:30 p.m. when the sun is on the west side of the tank. According to the weathering data, in the area where the study has been conducted, the sunrise time varied between 5:54 a.m. to 7:48 a.m. in different months and the sunset time varied from 4:54 p.m. to 8:43 p.m. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c} \hline \hline  & & & **C1** & & **C2** & & **C3** & & & **C4** & & & **C5** & & & **C6** & \\ \hline R1 & 1.6 & N/A\({}^{4}\) & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A \\ R2 & 4.4 & 10.40 & 0.42 & 0.55 & 9.47 & 0.46 & 0.50 & 12.51 & 0.35 & 0.66 & 14.59 & 0.30 & 0.77 & 17.35 & 0.25 & 0.91 & 18.27 & 0.24 & 0.96 \\ R3 & 4.8 & 15.85 & 0.30 & 0.83 & 3.14 & 1.52 & 0.16 & 2.30 & 2.07 & 0.12 & 7.19 & 0.66 & 0.38 & 13.87 & 0.34 & 0.73 & 10.08 & 0.47 & 0.53 \\ R4 & 5.2 & 1.98 & 2.61 & 0.10 & 6.53 & 0.79 & 0.34 & 7.90 & 0.65 & 0.41 & 12.30 & 0.42 & 0.65 & 16.14 & 0.32 & 0.85 & 17.61 & 0.29 & 0.92 \\ R5 & 6.4 & 4.34 & 1.46 & 0.23 & 6.32 & 1.00 & 0.33 & 9.07 & 0.70 & 0.48 & 10.82 & 0.59 & 0.57 & 12.48 & 0.51 & 0.66 & 16.04 & 0.40 & 0.84 \\ R6 & 7.9 & 7.23 & 1.10 & 0.38 & 4.23 & 1.88 & 0.22 & 6.26 & 1.27 & 0.33 & 10.50 & 0.76 & 0.55 & 13.10 & 0.61 & 0.69 & 16.61 & 0.40 & 0.87 \\ R7 & 12.7 & 3.97 & 3.20 & 0.21 & 6.26 & 2.03 & 0.33 & 6.67 & 1.91 & 0.35 & 8.53 & 1.49 & 0.45 & 13.63 & 0.93 & 0.72 & 17.03 & 0.75 & 0.89 \\ \hline \hline \end{tabular}  
* Depth from the external surface (D) (mm); 2 AR: aspect ratio which is equal to the ratio of defect diameter (d) to the defect depth (D) (AR = d/D); 3 D/t: Ratio of the defect depth (D) to the wall thickness (t); 4 Could not obtain reliable measurements of depth due to small defect size.  
\end{table} 
Table 6: Characteristics of the defects shown in Figure 2 created on the internal west side of the tank (wall thickness, t = 19.1 mm = 0.75 in). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c c c c} \hline \hline  & & & **C1** & & & **C2** & & & **C3** & & & **C4** & & & **C5** & & & **C6** \\ \hline R1 & 1.6 & N/A\({}^{4}\) & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A & N/A \\ R2 &Figure 4 shows that the temperature of the tank wall varies from east to west depending on the sun's direction. In the morning, the east side of the tank has a higher temperature, and in the afternoon, the west and east sides of the tank have comparable temperatures. In the afternoon, the temperature of the tank wall on both sides increased from the heat accumulation that had started from sunrise. Figure 5 shows the temperatures on the tank wall on two random cloudy days; the temperatures of both the east and west sides are similar and fall between the water and ambient temperatures. In other words, on cloudy days, there is a negative initial thermal gradient (\(\Delta\)T \(<\) 0) on the tank wall, which, according to the previous studies [8; 9], is not suitable for completing IRT evaluation. This was verified for all of the data gathered, and the same pattern was seen on all cloudy days.