How to calibrate a volt meter?

I have a Radio Shack digital multimeter, and it seems to me that the DC volts shown for things like batteries may be reading a bit high. What kind of standard measurement could I make that would tell me if the meter is correct? I measured a fresh Panasonic AA alkaline

2-pack and got 1.620 and 1.619. Does that seem to be right?

I have a vague memory that a mercury coin battery had a precise voltage when new, but I don't think they are available anymore. What else could I measure around 1.5VDC that's a known value?

Reply to
George
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1.62 volts is ok for a brand new AA alkaline battery. A brand new 9 Volt alkaline will often red 9.6 volts. Also try you meter on different ranges and see if they agree keeping within the maximum for that range of course.

For a higher voltage measurement check the line voltage on AC volts. I'm not sure where you live so I can't say what it will read, but within a few volts of what it should be.

Also try measuring some resistors. Most resistors are 5% tolerance, see if your meter agrees.

Another resort would be to compare readings from a friends meter. Or bring to a Calibration shop.

Shaun

Reply to
Shaun

Calibration isn't easy, if you are looking for exacting accuracy. I gather from your post that you aren't, though. Just wanting to know if a 1.62V reading means there is a problem with your meter. On that score, it's probably fine. I typically read voltages close to that on fresh alkaline AA batteries. (1.59V and more.) The rating of 1.5V is approximate, in the case of alkalines and if I read one that gave me exactly 1.5V I would suspect that it had been used a little bit before I got it.

Keep in mind that the meter doesn't load down the battery that much and if you use it in some product (like a radio), the actual situation found there will place a load on the battery and it still should provide about 1.5V for the circuit. So you are measuring things in a near-ideal case that is not usually found in actual practice. Some folks will actually place a load on the battery and _then_ measure the voltage, to find out if the battery is relatively fresh or not. A battery that has been used for a while might show a voltage not far from 1.5V, when not loaded at all by the meter, but show something a lot less than that when loaded while measuring.

There are "standard cells" used for calibration. But they must be designed to specifications and they last for a limited time, at a limited temperature range, etc. A Weston Cell comes to mind, though there are others. If you want to build something "kind of okay" and not both accurate and precise to 5 digits, you could look into "voltage reference" parts and drive them appropriately, as they will yield pretty good values for hobby work. They aren't expensive or hard to work with, by and large, and they will let you test various meters very quickly.

Jon

Reply to
Jon Kirwan

Sounds reasonable to me.

Precision voltage reference ICs are available from quite a few manufacturers, e.g.

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Reply to
Gareth

AC lines can't be trusted for cal, that's for sure. I live in the quasi- boonies (i.e. a good way from town, but all my neighbors have 'urban' jobs), and I see voltages on the line from 115 to 130, depending on the time of day and what my neighbors are doing.

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Reply to
Tim Wescott

Just whip out your standard cell - doesn't everybody have one? Mine lives in my millivolt potentiometer...

Lacking that, the serious user would send it to a calibration lab, but the serious user might not start with an RS meter. Not that I have issues with mine, other than its propensity to eat batteries. It's not my high end meter, but then, I won't cry long if it dies in service, and it's a lot easier to carry, plus dirt cheap.

If you have a friend with a "known good" (or actually calibrated) meter you can both measure the same thing and see what you get.

Then you can delve off into silicon voltage references. They may not be precise enough for the calibration lab, but they can more than adequately help sort out if it's the meter or the battery at home. Speaking of which, the battery voltage does not seem overly high; 1.5 volt is "nominal", not precise. Ugh, stupid websites without real links

- watch for wrap.

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Reply to
Ecnerwal

A decent cal lab would cost you more than the price of the RS meter. ;-)

Cheers! Rich

Reply to
Rich Grise

Yah,

The AC lines test was just to see if the meter would roughly measure AC voltages and look for a ball park reading. Where I live in a medium sized city I read 120vac +/- 5 volts. It's not a calibration check, just function / meter check.

BTW: this is not meant to be insulting, but is something that is often overlooked. Is the battery in your meter fresh?

Shaun

Reply to
Shaun

Simply get a precision voltage reference IC, they are only a few dollars, or you can even get free samples from the likes of Maxim easily. More than good enough for your meter calibration.

2.5V is a common reference voltage, but they are available in all- sorts. If your meter is a 2000 count (1999) type then you'll either need a reference under 2V or some precision (0.1%) resistors to drop the voltage below 2V. Forget batteries of any sort.

Dave.

Reply to
David L. Jones

And many will simply refuse to calibrate such a low end meter!

Dave.

Reply to
David L. Jones

I have a battery replacement program in place here: I randomly forget to turn the @#$% meter off, and run the battery down to nothing. Then I replace it.

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Reply to
Tim Wescott

Thanks to everyone for all the responses.

The meter is R/S 22-803, which is an 18-range auto-ranging, auto-polarity, digital multimeter. The ranges are based on

4 - 400mv, 4V, 40V, 400V. I see that I bought it in 1998, and it has just seemed to me that every time I measure voltage, it seems to be a bit higher than I expect - a zener voltage or a voltage regulator, both under load, or whatever. Close, but higher than nominal - never lower.

What brought this up, though, was that I just got a Canon A590IS digital camera, which uses standard AA's. I put in charged NiMH batteries and fooled around with the camera for a few hours, and then the "low battery" indicator came on. So I took the batteries out and measured the voltage, and they were still something like 1.24V. So it didn't seem right that the camera wouldn't still like them.

But of course these measurements weren't under load, so I really don't know how far down they would be if actually powering the camera (particularly the display).

So I guess the meter is GEFGW, and I won't worry about it. But I do need to do something about the batteries and charger. I got a Rayovac charger plus 2 AA's at Wal-Mart for $6, originally for an old HP calculator, and that works fine, but it turns out the charger is just a constant

1.4V, 200ma source, with no smarts, and the batteries are unmarked as to mAH. Oh well. You really do get what you pay for.

Anway, thanks again for the responses.

Reply to
George

Take it for what it's worth, in the lab (years ago), we used fresh carbon cells as the standard. They expected to generate ~ 1.58 VDC into a meter under calibration.

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Reply to
Jamie

Those voltages are normal for new alkaline cells. Leave your meter alone, especially if it has multiple adjustments, unless you have both a service manual and a voltage reference rated for at least 10 times the accuracy of your meter. But leave your meter alone. Voltage reference chips accurate to better than 0.1% can be had for approx. $10. I repeat, leave your meter alone.

Reply to
do_not_spam_me

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