You are bidding on a  Heli Tach RC Helicopter Optical Tachometer 3600 x10 RPM Speed Tester this is the fastest Tachometer on the market  allowing you  to meassure RPM on smaller Helicopters the other ones on the market go to   1000-3190 only .An optical RC Helicopter Tachometer is something you need to confirm how fast your rotor is spinning while flying Brand New.It requires a 9 volt battery