RE: a tach thing, ever heard this before ???
I met up with a guy a few months ago that wouldn't own an optical tach. He's rather hold his tach to the point of his spinner. He was convinced that the optical tachs couldn't be reliable. I observed his technique, and... well ... I think he needs to contact Sylvia Browne to see what he was in his last life, and just how long it lasted. It wasn't too bad up to about 7K ... then it started wabling and vibrating. When it did that, he'd just apply more pressure. After I saw 9K, I couldn't watch anymore. All that kept running through my mind was that joke about a Rednecks famous LAST words. " Hey Y'all ... Watch this"
Optical tachs are pretty sensitive and fairly accurate. I have found that if I put my tape on the spinner and hit it at an angle I don't get as much variation caused by different light sources. Most of the time I use the one we have at work. I'll have to get back to you on the brand, but it's an industrial model with 2 lasers that constantly check against themselves. They have it calibrated twice a year, and so far this year all it has measured is rpms on my engines. I have tried to make it mess up with the light and couldn't. I'll try your method tonight and see what happens. We have some UV lights I can use to duplicate sunlight. The people we bought the tach from sent us a card wanting to know if it ever messes up, what the conditions were and what it did. So far, the other equipment we've bought from them has been nothing but the highest quality (infared heat guns, pyrometers, spectrometers).
I have a Raytech tach that I use here at the house, and in order to get it to read right, it has to be a certain day of the month, the planets have to be alligned just right, and you have to recite an incantation while burning insence.