[volt-nuts] How long can standard cells last?
m1k3k1 at hotmail.com
Fri Nov 30 07:33:18 UTC 2012
Well from that site the tempco is about 5 microvolts per degree, and from the bottom of the page:
The un-saturated cell should have an e.m.f. of 1.0190 volts @ 20°C when new. When the voltage drops to 1.0183 volts, the
cell should be discarded, as it is no longer stable. Most reputable labs will not certify a cell below this value.
The NBS recorded data on approximately 600 un-saturated cells. About 5% showed an increase of e.m.f. with time, the average
change being 28 µV/year. The remaining 95% decrease in e.m.f., at an average of 85 µV/year. Of that group, nearly half
changed by more than 50 µV/year, and one fourth by more than 100 µV/year. That data suggests that certification at
yearly intervals is required to insure .01% accuracy. The expected life of un-saturated cells is 7-14 years.
So my guess would be that it is near the end of its life, but measure and see.
> Date: Thu, 29 Nov 2012 23:15:19 -0800
> From: wb6bnq at cox.net
> To: volt-nuts at febo.com
> Subject: Re: [volt-nuts] How long can standard cells last?
> Hi Ed,
> The type of cell you have is an UNsaturated type. These types are portable. The
> saturated type of cells are non-portable and very temperature sensitive requiring
> the use of a constant temperature enclousure.
> Here is a web site that describes these devices that is quite good:
> ed breya wrote:
> > I just junked out a very beat up old Fluke 803 differential
> > voltmeter, and found deep within, an old-school Cd/Hg standard cell.
> > It was well protected in an aluminum box, and wrapped in foam and
> > foil. It looks brand-new, and still measures around 1.018... V. I'd
> > like to keep this one as another reference point if it's still good.
> > I assume that it just wasn't used much, or that the Fluke circuits
> > were very good at not loading it down.
> > I'm sure it is the original unit installed in the instrument - marked
> > 5/12/1960. It is a Muirhead D-845-C. There's no test voltage tag or
> > any other info but a serial number.
> > So, I'm wondering if a 52 year old standard cell can still be OK, and
> > if anyone knows the specs on these, or where to find the info. I
> > don't know if it's possible, but I'd like to find what the official
> > voltage was supposed to be to a few more digits resolution. I think
> > various types and brands each had slightly different nominal voltages
> > around that determined by the basic chemistry. I remember in the old
> > days, every one I saw included a sticker with the 25 deg C exact
> > voltage measured as accurately as possible back then against the NBS.
> > I'd like to especially know if this is a saturated or unsaturated cell type.
> > Ed
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