You can make an easy way to check these cheap $20 laser tachs. On a piece of white matte board or anything fairly stiff draw a circle about 3" diameter then another circle inside the first about 1/4" smaller and another 1/4" smaller, and again. you will end up with bands 1/4" wide. On one band paint one 1/4" black square. On the next two squares 180 degrees apart. The next paint four squares 90 degrees apart. Once you have enough bands cut your circle out. You will need an AC induction motor with a marked known speed to mount this disk on. Now you can test for multiples of that fixed speed. 1x, 2x, 4x......... you can add more concentric circles and more black squares if you need to go higher. I am using a 3600 rpm synchronous motor from an old turntable. This is a good indication of how much your cheap tach is deviating at that known speed. If your motor is large enough you can make your wheel larger and add more bands or make them wider. Easier to keep the laser on the right circle. My cheap tack is consistent not quite 3% fast up to 20k rpm. j
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