This one is a beauty! Just attach inline on one of the three brushless motor wires (solder up a male/female connector adapter for convenience) and log motor rpm extremely accurately throughout the motor run.
You need to know how many magnets are in your motor to configure the RDU, but even if you don't know the Emeter can be connected and RDU rpm compared with the Emetertach. Makes it a snap to set it up right, every time... And this unit is NOT plagued by the problems found in some other brands. It's rock solid!