Douglas6 wrote: ↑
Thu Apr 19, 2018 10:29 pm
And, of course, make sure the batteries are fresh.
Yes - I'm feeling dumb now. Actually, I woke up in the night thinking "batteries!" and having now changed the batteries, the two meters are reading within 5% individually, and within 1 digit in series.
I had considered burden voltage, but disregarded it because both meters supposedly have exactly the same value (though the more expensive just quotes it as the burden figure, and the cheaper meter lists the same value as 'typical') (actually, comparing manuals, the cheaper meter has a better/more detailed specification section, but a rubbish how-to-use-it section). The one that was adrift was way off its spec value - if the reading error was attributed solely to burden it would need to be about 2.5 times what's in the manual to account for the reading I got (manual says 1.7mV/mA, I'd need 4.3mV/mA to explain the reading).
However, having changed batteries, I find my cheaper meter is reading within 1mA of what I calculate I'm expecting allowing for the specified burden voltage (and I'm using nominal value of 1% resistors in the calc, so that's within tolerance), the more expensive meter is reading 5% lower (but that's close enough for me).