DMM calibration

[linw]

I have an 18 yr old Metex M-3620 DMM and would like to tweak/calibrate its voltmeter. With output shorted, it reads 1.21 volts across a LM317 current regulation resistor and it should read 1.25V. On the main board there are four pots (three plastic knurled knob ones and one little flat one) but with no info on which one does what I am loath to fiddle.

Without info is there any way of finding which one adjusts the Voltmeter? Is it feasible to tweak the pots ever so slightly to find the correct pot without messing up the other calibrations?

Telling me to forget it and leave well alone is also an acceptable answer!

Lindsay.

[rc404]

I have an 18 yr old Metex M-3620 DMM and would like to tweak/calibrate its voltmeter. With output shorted, it reads 1.21 volts across a LM317 current regulation resistor and it should read 1.25V. On the main board there are four pots (three plastic knurled knob ones and one little flat one) but with no info on which one does what I am loath to fiddle.

Without info is there any way of finding which one adjusts the Voltmeter? Is it feasible to tweak the pots ever so slightly to find the correct pot without messing up the other calibrations?

Telling me to forget it and leave well alone is also an acceptable answer!

Lindsay.
How good are LM317's as a precision voltage sources? Looking at the National Semiconductor data sheet it lists the voltage range as 1.20 minimum to 1.30 maximum. The LM317A is tighter at 1.225 to 1.270. Both at 25 deg C, though the temperature sensitivity doesn't seem to be great. Are these precise enough to calibrate a DMM?

[linw]

Your question is a good one. My answer is probably not!

Thanks, anyway, for your input. Best I just put the case together and reckon on the accuracy as near enough.

[coro]

Read this thread: http://www.rcgroups.com/forums/showthread.php?t=331878
I would recommend simple 3-terminal reference like AD581 to avoid any troubles.
Ordinary references like TL431C are way too different one against another that You would have troubles to find 4 sufficiently similar within 20 others. And You also can use them to measure temperature, as their output strongly depends on it.

[Discharger]

Accurate voltage references for meter calibration is a subject that has come up before.
If you were to find a reference device how do you know it's accurate? Who checks the checker etc?

[AndyOne]

The best way for a hobbyist to check DVM accuracy is to gather as many other DVMs together as possible and connect them all to the same variable power supply, see which ones match the closest over a range of voltages, any of the others that read different to the majority are most likely the incorrect ones.

There is usually some kind of adjustment inside the meter so it may be possible to adjust it to read the same as the good ones.

Andy.

[linw]

Thanks, everyone. Seems a bit hard to find those reference sources in NZ. I think my Metex is pretty right as it gives sensible readings on batteries. I measured a new 1.55V watch bty and it measured 1.55V. My other meter seemed a bit high and measured 1.56 so I found a small pot inside and tweaked it to the same as the Metex.

I am pretty happy they are close enough now but with close tolerances on batteries we do need to be a bit more concerned with accuracy.