I too thought it was rather "vague" of him to not explain that. I use both types of tach and have suffered no injuries. I'm not sure, but,I suspect the fact that you need to set the photo sensitive type under the rotor to get a good reading might be what he is referring to. However, most know that you don't try to hold it there. Just set it up under the rotor, step away and run it up. I can see the readout from mine from a safe distance. If you absolutley have to use the memory function of a tach then by all means use the optical type. With those though, you still need to be at a close distance to use them, you just don't have to put it under the rotor so light creates the shadow that the photo sensitive types need to react. I just write the reading down the old fashioned way. He may also be referring to the accuracy of the two types. Perhaps not knowing to within 2 or 3 rpms worries him. But I haven't experienced a lack of accuracy with the photo sensitive types that was enough to worry me or cause any catastrophies.

If you really want to "research" optical tachs go to http://search.yahoo.com/bin/search?p...l%20tachometer. Plan on spending some time on that list as ther are "considerably more than two models to check out.