RE: DL50 props and RPM
My Hyperion E-meter is as accurate as my electricity board.
It shows 3000 rpm in the house, with the ceiling lamp on, indicating 50 Hz alternating currency frequency.
I assume that the electricity board has got their stuff right, so the rev. counter is OK as well.
If the rev. counter had shown more or less than 3000, then it would have been out of calibration.
(Obviously 3600 rpm for those of you with 60 Hz mains supply.)
Easy check.
Magne