Re: Weston Cells

On Wed, 14 Jun 2006 11:04:24 -0700, the renowned Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Wed, 14 Jun 2006 10:54:15 -0700, "Mark L. Fergerson"
<mfergerson1@xxxxxxx> wrote:

*** on the langwang wrote:
Does anyone still make/sell Weston (a.k.a. saturated cadmium - I think
!) voltage reference cells & if so, who ? I've recently acquired a
vintage thermocouple potentiometer & restoration depends on being able
to get a replacement cell. Googling produces a lot of theory & history
but no practical supplier solutions.

You can build one but I don't think you can buy them any more because
the chemicals are just too hazardous. What you actually need is a
reference voltage of 1.0183 V, right? Did you know that NIST and CERN
now use Zener circuits (as a secondary standard calibrated to a
Josephson junction) because the latter is more stable than a Weston
cell? You can DIY but zener diodes have tolerance issues.

If I wanted a chemical primary voltage standard I suppose I'd start
stacking coins and paper disks soaked in saltwater.

OTOH do you absolutely need that last .0183 of a volt? If not:

http://www.digikey.com/scripts/dksearch/dksus.dll?Filter


Mark L. Fergerson

If *I* were wanting a voltage reference on the cheap I think I'd use
one of the many bandgap chips that are available, add a heater so that
ambient didn't enter into the picture, then calibrate it against a lab
standard.

...Jim Thompson

The standard cell has a fairly high tempco compared to most decent
modern references (something like 50ppm/K), so I'm not sure an oven is
called for. Natsemi has some with a built-in heater and temp control,
but they are fairly pricey ($60 or something like that). Even the
Thaler ones are gross overkill.

But here's a source (hey, $50 off regular price!) and an incentive to
design your own if you go with one that requires a power source:
http://www.tequipment.net/IETHSVR.html

Maybe an 1.024V LM4140, with an op-amp buffer and trim network, but it
probably will draw too much Iq to be left on all the time on
batteries.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
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