## 5 Discussion  
A final balance between the two investigated testing methods is summarized in Table 7. The most remarkable difference between the two methods is the total time necessary to retrieve the final 3D hill-chart of a double-regulated turbine. In terms of efficiency difference, the results presented in Figure 14 remains qualitative since they are highly dependent on the shape of the interpolated surfaces using more or less enough information for an irrefutable definition. However, one may state that the differences are general below \(\pm 1\%\) (excepting the regions without enough measurement information to correctly describe the hill-surface), considered acceptable taking into account the amount of information acquired as well as the saved time. Focusing on the microturbine, the static measurements have been performed for approximately 1000 operating points. Forty man-work hours (one full man-work week) were necessary to calibrate/check the instruments, set the test rig and perform the measurements. Indeed, the classical static point-by-point method requires a considerable time for a complete measurement campaign of the full 3D hill-chart because a consistent number of operating points are necessary. For each operating point, once the operating parameters set, a waiting period of several tens of seconds is systematically required to stabilize the operating point. In the case of dynamic measurements, the total time to calibrate/check the instruments, set the test rig and to perform the measurements is reduced to  
Figure 12: Resulting efficiency 3D hill-charts obtained by classical static point-by-point and by the dynamic measurement methods: (**a**) counter-rotating microturbine—result of static measurements; (**b**) PAT—result of static measurements; (**c**) counter-rotating microturbine—result of dynamic measurements; (**d**) PAT—result of dynamic measurements; (**e**) colour plot scale. Figure 13: Validation of resulting 3D hill-charts obtained by the dynamic measurements method with the results of the classical static point-by-point method: (**a**) comparison for the counter-rotating microturbine; (**b**) comparison for the PAT. approximately eight man-work hours (one man-work day). The main advantage is the shortest time necessary to obtain the full 3D hill-chart. \[\epsilon_{\text{S-D}}=\eta_{\text{Static}}^{\text{s}}-\eta_{\text{Dynamic}}^{ \text{s}}\left[-\right] \tag{10}\]  
In the case of the PAT (a single regulated machine), the static and the dynamic measurements required more or less the same investigation total time. Anyway, the advantage on the \"sliding-gate\" method comes from the fact that the method allowed building a more precise 3D hill-chart since the number of acquired measurement points is several thousand times larger than in the case of the static method. Moreover, the dynamic method allowed measuring the S-shaped characteristic curves, which for the classical method can be done only with a special control of the test rig (see the method presented in [PERSON] [19]) and requests a considerable time. The dynamic measurements allow for fast detection of eventual hydraulic instabilities in the machine from local hill-chart humps or using fast sensors mounted in key locations (e.g., fast wall pressure sensors, accelerometers, etc.). Today, hydraulic turbines are often operated at operating regimes far from the best efficiency point. The dynamic method allows investigating the machine behaviour and identifying the eventual hydraulic instabilities that can occur on the full operating range. Further, complementary extensive experimental investigations can be performed for the problematic operating regions dominated by hydraulic instabilities. To cite an eventual drawback of the method, a second digitizer and a relatively higher acquisition frequency are necessary to acquire synchronized dynamic signals from the sensors.