For electric aircraft power systems a tachometer and a wattmeter are indispensable tools when trying to determine your power systems efficiency. This is the FD-T Tachometer. An inexpensive, reasonably accurate way to test and measure your power systems RPM.
One important note to remember is this tachometer cannot be used around fluorescent lighting. Older transformer ballasted fluorescents are not a steady light source, they flash on and off 60 times per second in sync with the 60hz AC that comes into our homes. If you look at the pics above you will see the tachometer registering 360, it is reading the fluorescent lighting in the room that it is in. 60 cycles per sec X 60 sec = 3600 cycles per minute.(Multiply the display on the tach X10 for true RPM)