achrn
Posts: 308
Joined: Wed Feb 13, 2013 1:22 pm

Multimeter calibration

Thu Apr 19, 2018 6:37 pm

I've just discovered that one of my multimeters is out of calibration. It's a bit disappointing because it's a relatively young Brymen BM257s. OK they're not Fluke, but the web seems generally to regards them as decent.

I know it's out, because if I rig up a circuit I expect to be drawing about 430mA, the A range tells me it's 0.43A, but the mA range tells me it's about 300mA. Meanwhile my other multimeter gives me the same reading around 430mA in both the A and mA ranges.

So I'm looking for a DMM calibration service that will deal with one meter. Can anyone recommend one?

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bensimmo
Posts: 2991
Joined: Sun Dec 28, 2014 3:02 pm
Location: East Yorkshire

Re: Multimeter calibration

Thu Apr 19, 2018 6:49 pm

In warranty? (relatively young?) then get them to do it, if not do you have the tools to do it yourself and how accurate ?
(

achrn
Posts: 308
Joined: Wed Feb 13, 2013 1:22 pm

Re: Multimeter calibration

Thu Apr 19, 2018 8:30 pm

Out of warranty, sadly (18 months old). Also, grey import (naughty me, though in my defence when I bought it there was no UK distributor that I could find).

It's one that calibration is by feeding specified signals in. I've found how to put it in calibrate mode by a bit of google and a bit of trial-and-error, and established that the first thing it demands to calibrate mA is a 30mA DC. I could do that by rigging up resistors and my bench PSU, but then I don't know what it will ask for next. While I can produce some reasonably accurate DC currents, since the same dial position also does AC, it will presumably move on to asking me for a true RMS 30mA AC (at I-don't-know-what frequency), which will be more difficult for me to jury-rig (I don't really want to be voltage-divider-ing the mains and I don't have a function generator).

I might give it a go, but if anyone could recommend a suitable outfit that wasn't too expensive, I might let them do it.

drgeoff
Posts: 8354
Joined: Wed Jan 25, 2012 6:39 pm

Re: Multimeter calibration

Thu Apr 19, 2018 8:36 pm

Before you assume the meter is out of calibration put the two in series and compare the current readings.

Almost every digital multimeter that "measures" current actually measures the voltage dropped across a resistance in the meter. Inserting the meter changes the voltage in the circuit under test. In a low voltage circuit that change can be significant and cause the actual current flowing to change markedly and the effect is not necessarily the same on all current ranges.

Burden voltage:

http://www.fluke.com/fluke/uses/comunid ... denvoltage

https://www.eevblog.com/forum/beginners ... ultimeter/

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Douglas6
Posts: 4366
Joined: Sat Mar 16, 2013 5:34 am
Location: Chicago, IL

Re: Multimeter calibration

Thu Apr 19, 2018 10:29 pm

And, of course, make sure the batteries are fresh.

achrn
Posts: 308
Joined: Wed Feb 13, 2013 1:22 pm

Re: Multimeter calibration

Fri Apr 20, 2018 6:25 pm

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).

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