The X-Cell Optical Tachometer indicates accurate rotor RPM while flying. If you can see the rotor, you can measure the RPM, during hovering or 3D aerobatics. Convenient "One-Hand" operation with variable rate rocker switch to increase/decrease rpm in slow mode (10 rpm per click of rocker switch) or fast mode (100 rpm per every second the rocker switch is depressed). Features: - RPM range 500 to 3000 win +/- 1 rpm resolution - Measures any rotating blades, from models to full size helicopters - Largest viewing window for easy tracking of the model in fast flight - Start-up feature automatically begins at 1500 rpm - Automatic battery conservation mode - Automatic "Stand-By" mode after 3 minutes