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:
------------------------------------ Posted by: Stefan Trethan <stefan_trethan@...> ------------------------------------
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