Electronics > Metrology

From the bin!-- weston cell +meas. data & lastly Muirhead Cat. data sheet

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Dr. Frank:
Very nice find & assembly.
Anyhow, the standard value given in 'international Volt' has to be adapted to SI-Volt, or to 'absolute Volt', as it was called, then.
(In former times, there existed also the differing NBS - Volt, see the NBS document)

I found in my old documents following relationship for the output of a saturated cell at 20°C:

1.018300 Vint = 1.018646 Vabs (SI)

The first order T.C. of about -39.9 ppm/K  (= -40.6 / 1.0183 ppm/K) matches your measurements.

Your Weston cell is still in very good shape, as it differs about -30ppm from the standard value @ 20°C only.

A negative drift over time is also typical; yours would be -0.5ppm/yr., if it had exactly the nominal value in 1960 (which was probably not the case, anyhow)



--- Quote from: Conrad Hoffman on August 22, 2016, 08:06:16 pm ---I wrote up a page from various sources a while back when I still had standard cells-

IMO, all unsaturated cells are now junk, as they haven't been legal to manufacture in several decades. All my Eppleys eventually went unstable and couldn't be trusted. You'll have to record data for a couple months to be sure. Not so the saturated cells, though they need a temperature controlled oven and should never be inverted. They don't travel well. Ultimately I got rid of those too, as I didn't like having that much mercury around.

The best thing to do with old Eppley and Weston unsaturated cells is take the nice thermally lagged case, send the guts to the hazardous waste collector, and install a nice solid state reference. Then you have something that looks cool and is still useful.


--- End quote ---

Any way to recover the platinum wire?

Conrad Hoffman:
I'm not sure on the wire. The ones I've seen keep it very short, with a splice, and you'd have to break the glass, risking a mercury and cadmium spill. IMO, not worth any possible gain. The thing I found so appealing about standard cells is they're the only "easy" method to create a voltage standard without access to calibrated comparison standards. Even that's not true unless you can purify the materials and happen to be a chemist, but it's the thought that counts! You'll do better with solid state references, rather than unsaturated cells. Maybe not true with saturated cells, but they're really a pain to maintain at perfect temperature and not damage them during use.

While trawling through some old trade catalogues in a filing cabinent I came across some Muirhead catalogues and wouldn't you believe it the cell I found is listed with all the specs.
And looking at the data measured and comparing to the catalogue specs. the cell from the bin is still in excellent shape !. Also the suffix letter indicates this cell was supplied without a housing (just as found in the bin!!).
So attached is the scanned data/sales sheet from the catalogue to complete the story.

Now that you have probably put all the catalogues away again  :D, any chance that there is a page on the Muirhead K-375-C standard cell? I've noticed that there are a few floating around on ebay. They seem to be a smaller, single tube design and I'm wondering how they did it. More curiosity than anything.


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