The procedures for index measurements and the different approaches are described in the IEC (International Electrotechnical Commission) standard 60041 "field acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump turbines". This standard is worldwide used for complex measurements of hydraulic machines. For measurements in small hydropower plants the IEC standard 62006 "hydraulic machines - acceptance tests of small hydroelectric installations" is appropriate.
Index measurements are considered to be a secondary method. They are usually performed during commissioning and are considered for small hydro plants, as part of the acceptance tests (IEC standard 62006). Index measurements are specially suited for comparative test before and after replacement of runners, modifications within Pelton casings.
Index tests are applicable when only the repeatability of the measurement results is relevant and not the absolute efficiency. For rehabilitation projects guarantees are given for the difference in efficiency before and after. It should be ensured that during the period of the revision the individual measurement instruments or taps for index discharge measurements experience no modification. Index measurements are also suitable to adjust the correct cam between runner and guide vane angles for double regulated hydraulic machines (Kaplan).
Features of index measurements:
- relative efficiency
- used for pumps and turbines
- no limitations with respect to flow rates or head
- measurement of an index flow rate or a pressure difference is needed