n cook wrote in message
...
Emilio wrote in message
...
Is there an easy way to calibrate (low voltage) digital voltmeter? Is
there- say a battery that can be usd as a reference?
Thanks!
Emilio
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
http://home.graffiti.net/diverse:graffiti.net/
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
http://home.graffiti.net/diverse:graffiti.net/