# Capability of Advanced Ultrasonic Inspection Technologies for Hydraulic Turbine Runners
## 2 Experimental Procedure
### Experimental Detection Rate  
The performance of the different ultrasonic inspection techniques tested are compared in Figure 6. Obviously, among all these techniques, conventional UT results have the lowest detection rate. This could be because the fixed-angle beam can miss a flaw more easily than with ultrasounds impinging the flaw from different angles. Although the inspection is carried out to a well-defined standard, the detection rate is highly dependent on the operator. The certified practitioners reported between 7 to 16 flaws out of the 30 embedded in the weld, with an average reporting of 11 flaws. Figure 5: Example of TFM results (Side A, side view). Several difficulties were observed with manual testing; (a) Challenging to report a flaw nearby a larger reflector with all the attention on the large reflector, (b) Manual reporting of flaw position is cumbersome and leads to errors, (c) Low amplitude signal just above the rejection threshold is easily discarded, (d) the report's quality is directly related to the inspector's diligence in taking clear and precise notes. If the report is written several days after the test, the notes' quality is of paramount importance [33]. Manual phased array inspection suffers some of the limitations observed with a fixed-angle beam probe. The detection is done on the fly, and the flaw's information is written during the inspection. If the sample has several flaws, the technician can get confused when measuring and reporting flaws. However, one of the advantages of manual PAUT inspection is the freedom the operator has to explore a suspected flaw by executing various probe motion; pivoting the transducer or orbital movement to reveal the target's volumetric features. The manual phased-array inspection provided the highest maximum amplitude measured and revealed flaws hidden in the trailing edge weld reinforcement. However, even if the manual phased-array found flaws hidden in the geometry, on average, it detected fewer flaws than the linear-encoded phased array. The linear encoded inspection provides repeatable scans, and flaws with low amplitude are detected more readily by comparing the local base noise to check for any localized increase that would reveal a flaw. Close flaws may be separated, and when using the projection views, their location and separation can be measured. Generally, the aperture of 12 and 16 elements produced a reasonably good detection rate, and the small footprint  
Figure 6: Experiment, Different ultrasonic methods. of the 12-element transducer allows it to cover the center of the weld. As the aperture increases, the stand-of distance increases, and the weld's center is no longer covered. This explains why the 32-element tests did not perform better than the 12 or 16 element tests despite having a smaller beam diameter and a more extended working range covering the whole weld path. The TFM method produced a higher detection rate, slightly better than a sectorial scan. One advantage noticed during the test is the operator can readily size the diameter of the flaw. This helped identify small beads from noise since the shape was maintained while probing the area. The TFM could also probe nearer to the surface and provided good coverage of the whole weldment. There is no dead zone compared to a 32 element sectorial scan. In our sample, the material has a low noise level suitable for a TFM inspection, which explains the high detection rate obtained with the study sample. Another advantage of the TFM is flaws could be categorized in apparent size or apparent reflective features. Small ceramic bead produces a specific round-shaped signal area proportional to their size, while larger beads showed either oriented facets or multiple facets surrounding the bead. TFM provided details about the reflective surface's nature, while a sectorial scan did not provide any hints about the flaw's small features. A multi-element probe is, therefore, a better candidate in terms of detection rate.