I don't know how much experience you have with stroboscopes John, but the only trap involved is that you can get what looks like a valid reading at a harmonic of the actual speed, depending on what the moving part looks like. The standard way to deal with it is to put a chalk mark, bit of sticky tape, or whatever somewhere on the moving part, then raise the instrument's speed setting until you get two images of that mark, exactly opposite each other. Read the speed, divide by two, set the instrument for that, and if you've got a clear image, you can rely on that being the right speed. When you can only see part of the rotating bit, you may not be able to follow this procedure and you can easily get a wrong answer.
