[volt-nuts] Old Weston cells

[pa4tim at gmail.com]

Are you sure they are unsaturated ? Because most unsaturated cells last only 10-20 years (AFIK). Saturated cells last much longer but they should be kept at 30 degrees celcius for the correct value. On the other hand, as far as I know they do not survive if they are not kept vertical and most of the time they are not when they are kept outside their oven. 


I have 4 weston cells in a guildline cabinet with oven. I have the cal data for the first 10 years they have been used. After this time the oven was turned off , the lab was closed but all gear  stayed put on that same place over 30 years (the lab was turned into a server room but all the gear just stayed unused ) I made a graph from the data. Took a ruler and extended the lines. After a few months I had a change to measure them with a calibrated 6,5 digit meter and a year later a 7,5 digit meter and the values I found where plus/min a few uV on the lines. But more important the difference between the individual cells was spot on with my graph. So I use them as my house standard (I do not measure them direct. I set my 332 at 10V. A fluke 720 of ESI KV at the value the cell must have. Connect them both to a Fluke 845 and set the 332 so the meter reads zero. Then I check my 731 against the 332 the same way and leave them both connected this way for a few hours. If the meter is still at zero after that time I use the 332 for calibrating a meter.


The only problem is the high Rout of a KV. So I use a buffer made of a LTC1052 and LT1010 after the KV  if the multimeter is a 10 M version. But I still wonder if that is needed. I tested it with some meters but did not notice a difference that was within the meters resolution.


Fred, pa4tim 




Verzonden met Windows



Van: Joseph Gray
Verzonden: ‎dinsdag‎ ‎27‎ ‎augustus‎ ‎2013 ‎06‎:‎26
Aan: Discussion of precise voltage measurement

I suspected that there was nothing to do to rejuvenate these old cells. I
thought I'd ask anyway.

Thanks.

Joe Gray
W5JG



On Mon, Aug 26, 2013 at 9:16 PM, zbigniew169 <zbigniew169 at gmail.com> wrote:

> All my knowledge regarding Weston Cells tells me that is not possible in
> any way :-((
>
>
> 2013/8/27 Joseph Gray <jgray at zianet.com>
>
> > I got my hands on a few very old, unsaturated cells. I figured that they
> > would be no good after all this time. Just out of curiosity, I measured
> > them on my 3457A, which hasn't been sent off for calibration yet.
> >
> > DMM was set to 3 V scale to keep the input impedance high. Room
> temperature
> > = 26 C.
> >
> > Cell #1 = 1.018288
> > Cell #2 = 1.018236
> > Cell #3 = 1.018224
> >
> > I read that when they get below 1.0183 V, they are no good. Is there any
> > (easy) way to rejuvenate these cells?
> >
> > Joe Gray
> > W5JG
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