Be aware that under any AC light (flourescent or incandescent) you can get mixed up readings. especially with flourescent. That is because the light is pulsing on/off 120 times per second. point the tachometer at a flourescent light and it should read 3600 when set to 2-blade prop. When I'm working with the tach indoors (like with an electric motor), I use a car headlight to provide the light through the prop.