NOTE 6: I believe just about all the units on the market work in the same fashion, that is they precisely measure the time it takes for the shaft to make a fixed number of vibrations. (I heard there was a unit that measured the number of oscillations that occur in a fixed length of time. If this is true I can't imagine an accuracy of better than 5 or 10 cpm.) My first Club Scout measured the time to complete six oscillations and displays this value of time. A chart is then provided to convert the time to cpm. This requires a simple division. My subsequent Club Scouts II, III, IV, Jr. Club Scout and most other analyzers use a microprocessor to actually perform this division to convert the time to cpm and display the value. I assume most analyzers use a crystal controlled clock to time the oscillations. These clocks are typically accurate to parts per million. Given a specific input oscillation I would assume all analyzers would come up with virtually the same number. I've checked a few analyzers and this was pretty much the case. I did find one that was off a bit but it was the exception.
There are three means I know of the sense the shafts vibrations. One is a reflective sensor approach in which an infrared light beam is bounced off of the shaft and its reflection is picked up by a phototransistor. The Junior Club Scout uses this approach, as do a number of other units I know of. It works very well but it can be bothered by outside light sources especially fluorescent lights and doesn't pick up reflections off of a dark colored shaft very well. Usually a piece of reflective tape is wrapped around the shaft. A second approach is to mount a light source and phototransistor pointed at one another. The vibrating shaft passes between the two and interrupts the beam sending a pulse each time to the timing circuitry. The Club Scout II, III and IV use this technique. It has several advantages. You don't need white tape on a black shaft, it is not as susceptible to outside light sources, and it handles wobblers a bit easier. A third approach is to have a vibration pickup in the clamping fixture. This seems like a reasonable approach but I would think it would increase the cost unnecessarily. Phototransistors and diodes are cheap, simple and reliable.