## 3 Results and Discussion
### Field Testing  
The authors followed the guidelines mentioned in Section 3.7 to evaluate two inactive HDPE ASTs. The goal was to estimate the time needed to finish the evaluation of a single tank and to identify the field testing challenges. The information on each tank is summarized in Table 9. The wall thickness of the tanks was measured using a pulse-echo UT probe. The P-wave velocity of HDPE was estimated by measuring the temperature of the surface and the regression data from Ref. [9]. The first tank was empty, while the second tank was full of a brine solution. Both tanks were currently reported by the owner as being in \"good condition\". Since the first tank was empty, the \(\Delta\)T was assumed to be zero, and therefore the heating-cooling cycle of 60-60 was chosen for this tank. For the second tank, the external wall temperature was 30 \({}^{\circ}\)C (86 \({}^{\circ}\)F), and the brine inside the tank had a temperature of 26.1 \({}^{\circ}\)C (79.0 \({}^{\circ}\)F); therefore, the heating-cooling cycle of 40-120 was chosen. For both tanks, the bottom 1.3 m (4.27 ft) all around the tanks were examined.  
Due to the hoses connected to the fittings and outlets, it was impossible to directly heat the fittings and evaluate the fittings in one step. Therefore, the fittings were heated twice: once from the left side and once from the right side. The circumference of the tank was divided into nine segments, as shown in Figure 18, with each segment consisting of four spots. While the first spot was cooling, the second spot was heated for 60 s on the first tank to complete 60-60 cycles. On the second tank, when Spot #1 was in the cooling phase, Spots #2, #3, and #4 were heated for 40 s each to complete 40-120. Each segment had an average size of 1.25 m\({}^{2}\)\(\pm\) 0.22 m\({}^{2}\) (13.5 ft\({}^{2}\)\(\pm\) 2.4 ft\({}^{2}\)).  
Figure 18: Image of two in-service HDPE ASTs with configuration of one segment consisting of four heated spots labeled.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Tank Capacity** & **Tank Age** & **Tank Type** & **Wall Thickness** & **Tank Diameter** & **Empty/Full** \\
**(gal (L))** & **(Years)** & **Tank Type** & **(mm, (in))** & **(m, (ft))** & **Empty/Full** \\ \hline
5000 (18,900) & 12 & Single wall & 19.1 (0.75) & 2.60 (103) & Empty \\
5000 (18,900) & 17 & Single wall & 19.1 (0.75) & 2.60 (103) & Full \\ \hline \end{tabular}
\end{table}
Table 9: Information on the two HDPE ASTs tested in the field.  
For the first tank, each segment required approximately 20 min to complete the IRT investigation. Therefore, an estimated 180 min is needed, but the actual inspection took 375 min to complete due to moving the heating source and other equipment, switching laborers and inspectors, replacing batteries, etc. For the second tank, each segment required approximately 13.3 min to complete the IRT investigation; therefore, an estimated 120 min is needed, but the actual inspection took 285 min to complete only the IRT inspection. Comparing the total inspection time for both tanks shows that completing the IRT on a full tank reduces the time needed for the evaluation by approximately 25%, due to having a positive thermal gradient induced by the liquid inside the tank. Looking at the long practical time required to complete IRT on a single tank shows that it is better to start the evaluation from the east side in the morning and finish it on the west side in the afternoon. One can reduce the inspection time by increasing the number of stacked heaters to cover larger areas and therefore reduce the number of segments required for completing the inspection of an AST.