[benjiboy]

OP is talking about at least 2 stops of error. While each meter might have differences in calibration and behavior, I doubt they should be THAT far off if calibration hasn't drifted. Besides, OP is talking about a meter he acquired for free and I assume he doesn't know the history of previous usage. Anything is possible here. Since we are talking about a measuring instrument, my first step will be to veify its reading with known good standard. (or at least another meter) Then, I would proceed with becoming familier with the unit.

By the way, I have more than two dozen watches. While none of them are accurate to seconds to each other, all of them are within few minutes to each other. To me, that's good enough to be usable for daily life.

I agree with your point if he has a meter of known accuracy and a grey card it's a pretty fair indication of the meters accuracy, at least if it's in the ball park, what I was trying to say was that comparing meter readings is a very quick way to drive yourself crazy