Efficiency Tests of Hydraulic Turbines

Hello everybody:

As a matter of fact, for hydropower plants, the efficiency of its turbines is measured at the start of operation and, eventually, at regular intervals.

Through the time, two basic types of turbine testing, the absolute and the relative, have been customary.

Even when I understand the principles and differences between them both, I would like to know your comments regarding with:

The absolute methods are more complex, expensive and difficult and are generally done once, to ensure the contract guarantees have been met.

The relative efficiency testing (Index Testing) is considerably easier, determines the relative performance of the entire turbine-generator unit and is a complete on-site system evaluation.

For a particular case, turbines of around 8 MW each, under what premises it is chosen one test or the other?

Can it be used, in a reliable basis, the relative method as a substitute of the absolute method?

Your comments are welcome. Thanks.