Digital Voltmeter Calibration

Try and find someone with a "Weston Standard Cell" voltage reference. My one is decades old with calibration certificate stating 1.01863V at 20 deg C and as far as I can tell is still much the same voltage now to the last digit accuracy. I cross-compared via DVM with a much more recent one. Never draw any current to speak of and don't knock them and they seem incorruptible. Failing that find a 0.01 per cent resistor

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I've heard that lithium batteries make reasonable calibration sources. You'd be best off finding an accurate multimeter to compare to though, then the calibration source is not critical.

James Sweet wrote in news:%tBAf.1237$jO3.145@trnddc07:

I'd use one of those integrated circuit voltage references;some of them have excellent accuracy,and are trimmed to precise,KNOWN voltages.

You could also buy an older Fluke differential voltmeter;they can source an accurate voltage,that you can dial up the near full-scale voltage for checking each DMM range.

--
Jim Yanik
jyanik
at
kua.net

No battery in the consumer market is precise enough. In order to properly calibrate any voltmeter, you need a true calibration standard such as the Fluke 5700 series.

Regrettably, such equipment usually carries a five-figure price tag. There are older units -- DC and AC references and such -- that you can often find on Greed-bay or at electronic surplus places, but keep in mind that you may need multiple units for DC, AC, and Ohms.

Happy hunting.

--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR, 
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

"Emilio" bravely wrote to "All" (21 Jan 06 15:11:44) --- on the heady topic of "Digital Voltmeter Calibration"

Em> From: "Emilio" Em> Xref: core-easynews sci.electronics.repair:356047

Em> Is there an easy way to calibrate (low voltage) digital voltmeter? Is Em> there- say a battery that can be usd as a reference?

Em> Thanks! Em> Emilio

You can use a band-gap temperature compensated voltage reference IC. They are pretty inexpensive and unlike a battery will not discharge. They come in various useful precisions like 1% and better to 0.1%. The more precision the more expensive they are. A very common IC is the TL431 with 1% tolerance and it can be trimmed still better.

However, it is always better to calibrate a voltmeter on the highest voltage range. Then any error is divided into the other lower ranges. By comparison if it is calibrated on a low range the error gets multiplied into the higher ranges.

A*s*i*m*o*v

... If all else fails, hurl it across the room a few times!

For anyone with access to a Weston cell but not access to a 5 or 6 digit DVM. I think this is how I cross-calibrated. I've dug out the docs and mine was calibrated at manufacture as

1.01866V at 20 deg C and -40 ppm/ deg C , 14 March 1979. estimate of uncertainty 10 ppm

Perhaps 5 years ago I did the following with my cell and someone else's secondary standard cell. My DVM has a 300mV range for its 4 digits, or 200mV will do the same. With a NiCad in good condition in mid discharge and left for some hours to reach room temperature is a nominal 1.2V. What the actual voltage is does not matter as long as it is stable. Assume for convenience here 1.2V. Only use with DVMs ( high input impedence) . Then commoning negatives of Weston cell and NiCad, measured the difference so came in the 200mV range. Which brings it into the first 6 of 1.01866 then ratioing of the flashing digits gives an estimate for the fifth digit. So reading of 181.4 mV With DVM 2/3 time reading 181.4 and 1/3 time reading 181.3 so implying Weston Cell voltage of 1.01863 at 22 degrees C. The other cell / DVM test was about 1/3 to 2/3 the other way round on last digit agreeing with that cell's yearly calibration value.

At the same time I checked a small standard cell salvaged from a bit of kit and it too was many years old but almost the same voltage, only last digit different again.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on