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Re: How to calibrate a voltage meter
Fresh batteries, ah, such mythological creatures.
In theory coin cells holde ven tighter tolerances but those can vary as well.
I cheated. I bought some precision voltage reference chips, which are designed to take roughly 8-20V input and put out 10.00v. They may float by a couple of digits in the THIRD decimal place, but with a 4-digit meter that means 10.00 is damn well 10.00, exact to the last digit.
Then I opted for "small" and wired them up to small 12V batteries, in holders, with a power switch and a couple of alligator leads instead of terminal posts. The hardest part of the assembly was obtaining the Altoids mini-tin, because yes, it lives in a small Altoids box. About three bucks cheaper than the smallest "professional" case I could find.
Of course that's just me, I decided to build a proper tool so I can calibrate anything I want for years to come. No, it isn't NTST calibration, but from what I've read when these chips say "10.00" that's a Real Damn Sure 10.00 and I can't ask more than that, especially for maybe $20 worth of parts. I'm sure if there was a market I could job 'em out to China for under $10 each, landed.
Once you have a calibration source of whatever kind you are going to use, you take the back off the meter and look around. You may need to check the manual or ask the maker, but typically there are one or two small screwdriver-adjusted trimpots inside. If there's only one, mark the position it is in, then while the meter is connected to the calibration souce, SLOWLY trim that screw until the voltage matches the source.
If there is more than one trimpot, congratulations, you've got a better meter. Try not to mess with the wrong one. (Which is why you mark it, and check with the maker.)
After calibrating, but the back on the meter again, check calibration again. Sometimes it slips a bit during the final handling and you need to do it again.
Harbor Freight? ROFL, my little HF's were worth what I didn't pay for them, but they are more light test lamps than real meters. Wooboy do they vary.