Yes, you can use a light to check calibration on them. Actually any household light will work; it doesn't have to be flourescent. It is due to the cycling of the alternating current at 60Hz, which causes the tach to "see" 3600 rpm's. (Our electric lights turn off and on 3600 times a minute, we just can't see that!)

This tach is bad. After dark, I set up a battery powered search lamp/spot light directly in front of the prop and the tach would still not read properly. It's odd, because it will read household light properly as in above. It will almost read idle rpm correctly if you hold it just a certain way, but goes nuts at any thing higher.

I contacted Glo Bee and they said it must be bad and would have to be sent back.

Bummer, as that's not so easy to do from here in Mexico. Oh well...

Thanks for the input,
AV8TOR