#1000: My (hi)story of the Weston cell, of the Volt, and of being a volt-nuts

[saturnin]

I have found another nice document about the drift of the Weston Cell (see attachment)
"THE EVALUATION OF WESTON CELL RELIABILITY ON THE BASIS OF THE ELECTROMOTIVE FORCE DRIFT"
This one is from 2013

The article seems very suspicious to me. Or is it only my impression? What do they want to present actually? They measured EMF of the Weston cell by Voltcraft 4095 (from Germany, of course ), which seems to be an ordinary handheld multimeter (https://commons.wikimedia.org/wiki/File:Digitalmultimeter.jpg) with ~1% accuracy and 10M input resistance, which imho they didn't even take into account??

Also, some statements in the article are quite misleading, e.g.:

Apart from Weston cells the technological evolution has brought on the market other devices, such as the Zener diodes [10,11] or Josephson Junction Array [12], but the equipment that measures and tunes up their reference voltage values has also to be calibrated in a standardised laboratory using the Weston cells.

Do they want to say that if you want to measure output of JJA (i.e. an intrinsic voltage standard), you would calibrate your voltmeter using Weston cell? How many cal labs use Weston cells for calibration today? 

[HighVoltage]

Hello HighVoltage,

I assume, that the 34461A does not induce any current spikes from the AZ chopping, otherwise you probably wouldn't have measured 131GOhm.
What about the room temperature over these 30min? Did you monitor the cells temperature?
A standard cell has about -40µV/°C, so a temperature rise of 1/3°C simply explains your observation.

Frank

This is  a good point with the room temperature.
The thermometer has shown constantly 24°C. So I don't think it has changed by 1/3°C but I can not be sure.
I will take some more measurements and insert a modern probe and record the temperature with another instrument.

[lowimpedance]

Hello HighVoltage,

I assume, that the 34461A does not induce any current spikes from the AZ chopping, otherwise you probably wouldn't have measured 131GOhm.
What about the room temperature over these 30min? Did you monitor the cells temperature?
A standard cell has about -40µV/°C, so a temperature rise of 1/3°C simply explains your observation.

Frank

This is  a good point with the room temperature.
The thermometer has shown constantly 24°C. So I don't think it has changed by 1/3°C but I can not be sure.
I will take some more measurements and insert a modern probe and record the temperature with another instrument.

 When monitoring the Muirhead cell I recovered from the bin here at work on a 3458a (HiZ) the drift noted was dominated by small temperature changes in my lab., as the air con is controlled during the day but put into a "sleep" mode overnight, the change in room temp. is easily seen on the cell voltage. Note the cell was connected to the DMM continuously and also my lab does not contain any 'work' standards  thus does not require 24H air con.
 Of course to see the long term drift months of logging would be needed, unfortunately someone else needed the 3458 ........bummer  .

@Saturnin , Weston cells went the way of the Dodo well before my time here at 'the lab'  .
 However I was surprised to find the old one tossed in the bin. Of course I coundn't leave it homeless  .

[HighVoltage]

I got a second Weston Cell.
The owner did not know, what he had and it was cheap, almost free.

When it arrived, I measured 1.001 V with my 34470A in High-Z mode
So, it was kind of dead on arrival.

Well, in one of these many papers that I read on Weston Cells, it said to
keep it in an oven at 60 Degree C for a few hours and it may revive. That is
what I did with this dead cell yesterday.
After it came out of the oven, I let it cool down and then hooked up to the 34470A again.

To my total surprise, it did come back to where it is suppose to be.
The sharp dip of falling again was this morning, when I came in to the lab and it got warmer
Will be interesting to see, if this one will finally settle to a fixed value.

Also, interesting, it had a sticker on the cell, reading:
1.017700
1/1/91
Fluke 883A

So, it seems like this cell was already low in 1991, when it was correctly measured with a Fluke 883A differential meter.
Or may be some cells only had 1.0177V instead of 1.018V?

I did not expect Weston Cells to be so interesting. Thank you Dr.Frank for starting this subject.

[Kleinstein]

The relatively large temperature dependence suggests this is a saturated cell. Thus the voltage should be higher, not just 1.0177 V.

[HighVoltage]

Yes, that is what I thought.
Now it is above 1.018V and I will just keep the 34470A hooked up for a few days to see the trend.

Also, this Weston Cell came with a rather need looking Certificate / Advertisement.
May be someone here likes to keep a copy as well.

[Dr. Frank]

It's a Standard Cell, that is a saturated cell, definitely.
Unsaturated cells are usually called Reference Cell.

Even the value from 1991 is too low, 1,0177V, so it was defective or at least of very bad quality at that point.

Frank

[johansen]

this is mine. it was mounted upside down in a fluke 801, for who knows how many years.

I measured it in 2010 at 1.0190 volts, at that time it only worked if tilted about 30 degrees from vertical.

[HighVoltage]

Here is a 70 hour measurement of a Weston Standard Cell in HighZ mode.
Although I did not measure the temperature at the same time, this seems to be pretty much a change of temperature.
This test also proves, that we can keep a high resistance meter (>1 GOhm Input) hooked up to the Weston-Cell without any problems.
My earlier assessment that a high resistance DMM would have an influence on the Weston-Cell output seems to be wrong.

I will keep this test running for a while with this 34461A meter.

   

[lowimpedance]

Here is a 70 hour measurement of a Weston Standard Cell in HighZ mode.
Although I did not measure the temperature at the same time, this seems to be pretty much a change of temperature.
This test also proves, that we can keep a high resistance meter (>1 GOhm Input) hooked up to the Weston-Cell without any problems.
My earlier assessment that a high resistance DMM would have an influence on the Weston-Cell output seems to be wrong.

I will keep this test running for a while with this 34461A meter.

 

In my test of the cell I found in the bin! (thread posted with graphed data back in August ),   I had a 3458a connected for 2 weeks with no loading issues or notable changes in the cell voltage. All variations were related to temperature changes in the Lab environment.
Long term measurement needed to see cell voltage age drift. Test again next year and then the next......  .

[tggzzz]

The one on the left side in their box, is a Muirhead 845-C from 1961. Hello - that's my own vintage!

Pah! Mere newbies, both the person and the cell - as shown below

I remember using Weston cells at school during physics lessons to calibrate NiFe cells (using a 1m long resistance wire). Anyway, I have vaguely wanted one ever since, a little moreso since I got hold of an HP3468 and have wanted to check whether it is still "accurate".

There's a local school/college closing and flogging off its "stuff", everything from desks and chairs, though bench PSUs 20MHz scopes and banana leads, to lumps of sodium and potassium. So naturally I pay a visit and have an extended rummage, since the very pleasant "salesperson" openly acknowledges she doesn't know what's what and can't test anything, and anything she can't sell she'll have to pay to be taken away and disposed.

What do I come across on a desk but one "Cambridge Instruments Weston Normal Cell" and a dual "Muirhead Standard Cell" with a yellow anchor and MEL stencilled on the side. Clearly they are shortly destined for the tip. Well, given their age they are unlikely to be "good", and showing her the fragile glass means she couldn't sell it on fleabay, so I offer her £5, and take them away.

Not expecting very much, I measure the voltage, and find the CI Normal Cell is 1.01865V and the Muirhead Standard Cells are both 1.01868V at 17C. OK; not broken, but it means I'll have to find out a bit more about standard cells. Memo to self: resist becoming a voltnut.

On pulling the Muirhead apart and comparing them with the "Muirhead Journal" article posted elsewhere, it appears that:

• they are crystal locked saturated cells
• 1.01859V @ 20C implies a 0.1N solution
• serial number is almost illegible, but it 1702
• date is almost illegible, but is 19-1-49 !

And there was me thinking that a 68 year old standard cell would be worse than useless

I suppose I'll have to see if I can find a 6/7/8 digit DMM, and learn about changes in the definition of the volt     ....    Must. Not. Become. A. Voltnut.

[Conrad Hoffman]

That's amazing! In general, portable cells are un-saturated and survive inversion. IMO, nearly all are useless now, as manufacture has been illegal for decades. They may appear to be in reasonably good condition, but monitoring for a long period of time will show instability not related to temperature. No doubt some are still good, but I had about 6-8 Eppleys and all became useless as standards many years ago. Saturated cells last pretty much forever if made and treated well. They typically aren't designed to be inverted, and/or will take a very long time to recover from it. I recycled all my cells some years back as hazardous waste, as some contain quite a lot of mercury and cadmium. Technically you can't sell them on eBay due to the mercury content, but I see them quite often.

[MisterDiodes]

What a beautiful find, tggzzz... You were certainly inb the right place at the right time! Thanks for sharing!

[HighVoltage]

I found a brand new, originally sealed Muirhead Standard Cell and bought it for cheap.
Only problem, it had no housing.
May be this one was a replacement cell to be inserted in an existing housing?

Manufacturer: Muirhead & Co. Ltd. Beckenham
Model:            D-402-C/P
Date:             18.1.71
Number:         37500
Voltage:         1.01850 V

Of course, this one was shaken a lot during the transportation but the voltage seems very stable.
Since it did not come in a nice traditional housing, I used a modern Bopla housing, that fitted perfectly.
Interestingly, the German dual pipe holder fitted perfectly as well, for distance and diameter, what a coincidence!

What is left, is to make a nice label for the housing and add a PT100 for temperature measurements.

I find it kind of amazing, how stable such an old cell still is after 46 Years!

[beanflying]

So today I received a box of mainly RF adapters Caps and an assortment of old junk I got on evilbay. The two matching boxes inside were curious so I opened them up.  Inside two brand new in plastic 1961 and 1967 Weston cells. They were not shown in the original listing pictures or description...

Measured both at 1.0185xx on the 34401a quickly. But now what to do with them long term?

[Macbeth]

Measured both at 1.0185xx on the 34401a quickly. But now what to do with them long term?

Really nice find!

First thing to do is make sure you use 34401A Hi-Z mode when you next measure them. The default 10M ohm is not good enough! 

[Conrad Hoffman]

I've mentioned this before, but the thing to do with the beautiful Weston cell cases (or Eppley) is recycle the cell as hazardous waste, put a DC jack on the back and a nice solid state reference inside the thermally lagged case. Use it as a moderate precision standard- sort of like a Weston cell. 

[beanflying]

I have done a little reading on these so already onto the HiZ input measurement but thanks anyway 

As to doing away with the internals maybe if it was to fail electrically might be a use for the case. I already have a sort of Weston cell in a recent acquisition of a 735A and another Project in a 740B where both should be useful. At this stage most likely place them in my chamber and check the voltages weekly on the 34970a for fun.

Just need to add a Null Voltmeter now to complete a vintage voltnut set 

[tggzzz]

I've mentioned this before, but the thing to do with the beautiful Weston cell cases (or Eppley) is recycle the cell as hazardous waste, put a DC jack on the back and a nice solid state reference inside the thermally lagged case. Use it as a moderate precision standard- sort of like a Weston cell. 

...or use a saturated Weston cell as a slow-acting limited-range thermometer.

[HighVoltage]

I have done a little reading on these so already onto the HiZ input measurement but thanks anyway 

I have a few of these cells by now and keep readings every week and also record the temperature.
My cells are almost all perfectly at a TC of 40uV/°C
Even in HiZ mode it is interesting to watch them change over 24 h or a week.
Especially with a modern meter with a graphing display, like the Keysight 34470A.

As to doing away with the internals maybe if it was to fail electrically might be a use for the case. I already have a sort of Weston cell in a recent acquisition of a 735A and another Project in a 740B where both should be useful. At this stage most likely place them in my chamber and check the voltages weekly on the 34970a for fun.

That 735A looks really nice for its age!

Just need to add a Null Voltmeter now to complete a vintage voltnut set 

Yes, a Null-Meter is the next step.
You are on a good way to become a voltnut.

[blackdog]

Hi,

I measured my D845-D cell today with my newly calibrated HP 3458A,
and I think he is still pretty good.



Furthermore, I have two questions, the first question, how much has the value of this cell dropped?
There have been several changes to the voltage standard since this cell was made, if I am correct 1968.

Second question, would it make sense to build this cell in an oven of about 40 Centigrade?
Making an oven is not a problem for me, as I have already done this for various voltage references.

Kind regards,
Blackdog

[beanflying]

I actually removed my second one from it's original plastic bag today after 50 years 

The certificates on the old one show it measured to 1.0192 V precision while the 'new' one was measured to 1.01927 V. A saturated Weston Cell should read 1.018638 (measured to what standard?). Here in lies part of the problem to what standard at the time were they measured? Most of the accepted national standards at the time were 'consistent' rather than accurate in absolute terms to the 'volt'. I seem to remember reading the US was .3 mV away from the Volt at about that time as were a lot of other national standards not sure about the UK?

Measured today at just on 20 degrees C mine are as follows.

1961  1.018516 or 0.000684 low from new but only 0.000122 from saturated 'standard'.
1967  1.018575 or 0.000695 low from new but only 0.000133 from saturated 'standard'.

There is some math to correct for Temp offsets and the certificates approximate this as 5 micro volts / deg C. Additionally depending on the actual chemistry of the cells there is another minor change in the micro volt range.

Given they are both 50+ years old a likely -2-3 PPM/year isn't to bad 

[HighVoltage]

There is some math to correct for Temp offsets and the certificates approximate this as 5 micro volts / deg C. Additionally depending on the actual chemistry of the cells there is another minor change in the micro volt range.

All my cells have shown around 40 µV/°C in TC
Interesting, how come yours is listed at 5 µV/°C?

[zhtoor]

There is some math to correct for Temp offsets and the certificates approximate this as 5 micro volts / deg C. Additionally depending on the actual chemistry of the cells there is another minor change in the micro volt range.

All my cells have shown around 40 µV/°C in TC
Interesting, how come yours is listed at 5 µV/°C?

beanflying's cells are un-saturated ones, which have a lower tempco than the saturated ones.

-zia

[beanflying]

Sort of Cross posting from the TEA thread (stuff gets lost in that behemoth thread) but as of today I have added two working 1943 vintage Weston Cells to the existing pair. The more recent Cambridge appears to be a dud at this stage. The Advantest reads low by a touch but will recalibrate it when my others get back from Keysight.

Older than my Mother and just younger than my departed Father. Hope my ticker is running that well at 76