Re: Multimeter Calibration and Voltage Referance


stefan_trethan
 

I think I did okay, considering that I don't have the volt nuts ;-)

Thanks for the clarification.

ST

On Fri, Mar 27, 2015 at 6:46 PM, Sergey Kubushyn ksi@...
[TekScopes] <TekScopes@...> wrote:
On Fri, 27 Mar 2015, Stefan Trethan stefan_trethan@... [TekScopes] wrote:

Chemical Voltage Standards are er.., Standards. They do NOT require
calibration against same standards because they are based on fundamental
constants so they are primary standards on their own right. They last for at
least tens of years and they were what NIST used as primary standards in not
so distant past until better standards were developed. Unfortunately those
are too big and bothersome to keep in one's garage but good old Weston cells
are more than adequate to calibrate ANY regular instrument including
top-of-the-cream HP 3458A and Datron/Wavetek 1281.

The devil is, as usual in the details. First of all you need SATURATED
Weston cells. _ALL_ those you see on Ebay and wherever else are UNSATURATED
ones so they have their EMF falling with a time so they need periodic checks
against real standards to find their actual EMF. This is the price you pay
for transportability and lower thermal coefficient. These days there is
absolutely no sense for those -- LTZ1000A beats them heads down and way more
convenient. However LTZ1000A voltage source require calibration against
known standard. It is pretty stable when calibrated (after several years of
aging of course) so it holds its voltage very good but ABSOLUTE value is
unknown until calibrated. Even LM399AH is better than old unsaturated Weston
cell and those are available from e.g. DigiKey at less than $10 in single
quantities (LM399AH-ND part number.)

SATURATE Weston cells are standards theirselves. They do NOT need
calibration. Their EMF is based on fundamental constants so they ALWAYS give
the exact (to some degree) ABSOLUTE known voltage that can be used as a
standard to calibrate all other sources against.

However it comes with a cost. First of all they have high temperature
coefficient so you have to keep them in an oil bath with very stable and
precise temperature. Temperature should be either kept at exact spot (that
does not require a very linear and precise thermal sensor but the entire
bath must be calibrated to that spot temperature) or its temperature must be
known with high accuracy if it is not at that spot so necessary correction
can be calculated (that is totally different task from keeping it at exact
spot.)

Then, saturated cells are NOT transportable. They can not be shipped so
there is only one way to get those -- make them in place. It is not terribly
complex or outrageously expensive job but it requires some equipment, tools,
materials, and skills that are usually not available. But if one can do some
relatively basic glassblowing, has adequate chemical lab and chemistry
knowledge it is not a very big deal. All materials can be purchased online,
most of them even not requiring additional purification.

I don't think a chemical voltage standard would be all that useful,
since the voltage does change as they age.
In a cal lab they would be periodically compared wih a superior
standard, and they'd keep track of that change.
So if you got an old dusty standard cell somewhere that change may not be known.
Also temperature and stuff has a big influence.

I think there are some reasonably accurate semiconductor references
for sale on Ebay and elsewhere, in the $40 range.
Those may well be your best option for checking or calibrating low end
multimeters.

ST



On Fri, Mar 27, 2015 at 12:01 PM, Gary Robert Bosworth
grbosworth@... [TekScopes] <TekScopes@...> wrote:
Henryk:

I have seen certified voltage standards at some of the companies where I
worked. They usually had their voltage stamped on the outside to 5-digit
accuracy and traceable to the National Bureau of Standards. Perhaps you
could find one of these cells on eBay.

Gary
On Mar 27, 2015 3:13 AM, "henasau@... [TekScopes]" <
TekScopes@...> wrote:

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Posted by: Stefan Trethan <stefan_trethan@...>
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Posted by: Sergey Kubushyn <ksi@...>
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