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BFL13

Victoria, BC

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Posted: 02/06/20 10:26am Link  |  Quote  |  Print  |  Notify Moderator

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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2oldman

New Mexico

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Posted: 02/06/20 10:30am Link  |  Quote  |  Print  |  Notify Moderator

I guess I'd try a AA cell or maybe the wall socket.

pauldub

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Posted: 02/06/20 10:35am Link  |  Quote  |  Print  |  Notify Moderator

Two obvious options, buy a good meter (like Fluke) or buy a cheap voltage standard. Here's a link about cheap voltage standards.

DrewE

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Posted: 02/06/20 11:07am Link  |  Quote  |  Print  |  Notify Moderator

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.





wopachop

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Posted: 02/06/20 11:07am Link  |  Quote  |  Print  |  Notify Moderator

Keep an eye out for a truck with ladders that are secured with romex.

Ask the dude if he has a fancy meter and compare your reading to his?

The battery idea seems good too. Stack several in series to see how accurate in the 12-14v range? (assuming you are most concerned about accuracy in that range)

Or keep an eye out for kids/adults flying drones or RC planes. They usually have several voltage testers that measure even down to the thousandths and come pre calibrated.

DFord

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Posted: 02/06/20 11:21am Link  |  Quote  |  Print  |  Notify Moderator

Multimeter DMM Calibration and Repair Service

Multimeter Calibration Services

Calibration Services


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BFL13

Victoria, BC

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Posted: 02/06/20 11:25am Link  |  Quote  |  Print  |  Notify Moderator

Thanks, that video was helpful, showing what to expect at various voltages. I checked the new meter against some small batteries, and it comes out fairly well I think.

A 9v batt reads 9.17 and four 1.5s in series reads 6.22 where the batteries are all showing "good" with my battery tester. (Also not calibrated)

My other RV meters are to one decimal place and say 13.8v, while the new meter says 13.84. I don't know when those other meters roll up to 13.9 passing through 13.85.

It seems the new meter might be reading a teeny bit high, but it is accurate enough for anything I use it for, based on all that. The old one said 13.56 instead of 13.8 on the Trimetric.

The part I don't like is how these meters go off calibration slowly so you don't notice it. You can think you have voltage drop when you don't for instance, or not as much as it appears.

You can adjust the voltage reading on the Trimetric in P18 to match what your meter says, but first you need to know if the meter is right. [emoticon]

Have to do a meter check on a regular schedule from now on. Lesson learned.

theoldwizard1

SE MI

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Posted: 02/06/20 02:12pm Link  |  Quote  |  Print  |  Notify Moderator

Absolute accuracy is actually not that important. What is more important is the difference in readings between two different meters. You can eliminate this problem by only using one meter.

This is a pain, but if you store your meter WITHOUT batteries inside, you can at least glance at them before you install so that you know how old they are.

Flute Man

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Posted: 02/06/20 02:23pm Link  |  Quote  |  Print  |  Notify Moderator

If I were you, I would look at the accuracy specs of that particular meter. It may be that it is well within the accuracy of that particular device.


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BFL13

Victoria, BC

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Posted: 02/06/20 03:05pm Link  |  Quote  |  Print  |  Notify Moderator

The owner's manual does give accuracy specs for the various parameters. However, that does not tell you if the base reading is correct. It tells you that whatever the reading is, it could be more or less than that within the accuracy range.

Specs on page 7 (but link is slow today ) I got this one on sale. I would not pay full price for this meter, but is a good deal at $25 IMO.

https://www.canadiantire.ca/content/dam/canadian-tire/manuals/0520726P_EN_FR.pdf

* This post was edited 02/06/20 03:26pm by BFL13 *

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