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BFL13's avatar
BFL13
Explorer II
Feb 06, 2020

Multimeter Calibration?

What is the best way to check your voltmeter?

Some years ago I ran into the problem where the meter read voltages too high when its battery was low. That took a while to discover since it happened gradually.

Now I found my meter has been reading low for an unknown time. Kept seeing a difference of about 0.2 volts between Trimetric and other meters but thought it was those other meters that were reading high for some reason like voltage drop that confuses things. Anyway, I finally decided that there was something wrong with the multimeter.

Battery ok, fuse ok, leads ok confirmed by swapping them. No change.

Got a new meter and the manual says to first check its voltage to make sure it is working right. Ha! Check it against what?

It is reading about the same as the Trimetric and the other meters, so they seem to have voted on it, and sort of agree on the actual voltage within a 0.1 range.

ISTR on here there was a way to confirm your meter's voltage using flashlight batteries or whatever it was. However, I also remember those eBay meters that are only accurate at higher voltages and are wonky at low voltages.

So what can an RVer do "trying this at home" without having a calibration lab?

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  • DrewE's avatar
    DrewE
    Explorer II
    Feb 06, 2020
    You need a sufficiently well calibrated/trusted voltage source.

    A normal AA cell is not that precise. Wall voltage varies as well, within limits. Perhaps the best simple method for a quick check is to find someone who has a good, trusted meter and compare readings--preferably someone with one that has been calibrated and certified by a proper calibration lab, if absolute accuracy is required.

    If you want to build a calibrator yourself, you might start with a precision voltage reference chip. Something like this TI LM4040 variant looks promising; that, plus a current limiting resistor plus a voltage source > 10V would give a 10V reference that's within 0.1% or so under any reasonable operating condition, quite sufficient to calibrate or check inexpensive and not overly precise multimeters. This is not at all NIST-level precision.

    There are many precision voltage reference devices available at varying costs, outputs, degrees of precision, etc.

    For many jobs, absolute accuracy is not essential; the actual values of 12.3V or 12.4V might not matter as much as the fact that there's a 0.1V change between them. For other jobs, of course, the actual true value of a reading may be rather critical.

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