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Old 31st Mar 2017, 11:15 pm   #1
60 oldjohn
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Default Weston Cell

Another thread prompted me to look at my Weston Cell. It seems to be two cells, there are two + and two - and they are not connected. Each cell measures 1.08 on one meter and 1.09 on another meter, I have no idea of what power is left in it I do not want to flatten it by trying.


Are these rechargeable ? There is a plug in the centre that is very tight so I have left it alone. I think it may have had a label many years ago.


John.


I have a photo but my camera is playing up, sorry.
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Old 1st Apr 2017, 12:14 am   #2
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Default Re: Weston Cell

Weston standard cells were normally used with a Wheatstone or other type of bridge and the circuit was arranged so that they supplied no current when the system was in balance.

They provide a reference voltage.

Ideally they should not supply any current. They are a laboratory instrument.
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Old 1st Apr 2017, 5:11 am   #3
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Default Re: Weston Cell

But of course even with a potentiometer circuit (not strictly a bridge, which can be considered to be a pair of potentiometers), the Weston cell will have to suppy some current (through the galvanometer) when it is not balanced.

[Which reminds me of another argument I had with an idiot teacher at school. He claimed a potentiometer-type voltmeter drew no current from the circuit under test. My reply was that that was true _at balance_ but that you could never know you were exactly at balance, you were limited by the minimum current the galvo could detect, which was more than zero. But anyway..]

I seem to remember the current you should draw from a Weston Cell is microamps at most. I think most analogue meters will draw too much, but a digital meter with a 10MOhm input resistance will draw about 100nA and should be OK to do a test.

And no, they are not rechargeable.

They also contain nasty chemicals, I seem to remember the fundamental process is a mercury-cadmium couple, neither of which are exactly non-toxic.
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Old 1st Apr 2017, 7:33 am   #4
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Default Re: Weston Cell

Weston cells come in two flavours - saturated and non-saturated.

The saturated cell has a highly stable voltage, and were mounted in temperature controlled enclosures in standards labs (like NPL). They stayed put for decades. Before the Josephson Junction voltage standard, saturated Weston cells were used as the fundamental method of disseminating the Volt, and Standards labs worldwide collaborated by intercomparing their banks of cells in agreed measurement protocols. As such they are not portable. As a secondary, calibration lab standard, portable versions were designed. Usually mains powered with a temperature controller, and battery backup so they could be transported to a Standards lab for calibration. They still had to be left for a week to settle down before measurement. All a pretty twitchy process.

The unsaturated cell has a lower temperature coefficient that the saturated cell, and is used without an oven. These were often enclosed inside an instrument as a voltage standard, and can tolerate any orientation.

There is a good background article about them here http://conradhoffman.com/stdcell.htm

All before the time of precision buried Zener references, which is what calibration labs use now. They still send them periodically to NPL etc to have them certified against the Josephson Junction standard.
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Old 1st Apr 2017, 11:50 am   #5
60 oldjohn
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Default Re: Weston Cell

Photos of my Weston Cell, at last.


John.
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Old 1st Apr 2017, 1:44 pm   #6
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Default Re: Weston Cell

Tinsley is still in business, they should be able to provide data, specification, etc. A Google search will find them.

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Old 1st Apr 2017, 2:37 pm   #7
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Default Re: Weston Cell

All you need now is a 10 digit multimeter to test the cell.............
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Old 1st Apr 2017, 3:17 pm   #8
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Default Re: Weston Cell

The PDF in post 4 mentions the use of two cells for some redundancy, that's what you have, if they differ you have a duff one. It also mentions that three are even better, in this case you (almost) know which one is duff.
 
Old 2nd Apr 2017, 11:16 am   #9
60 oldjohn
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Default Re: Weston Cell

I have just tested with my better meter, both cells test the same. I wonder if my meter is 0.00043v out of adjustment, it is a few years since it was calibrated. Or perhaps it is within the 0.025% accuracy the makers claim.


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Old 2nd Apr 2017, 4:58 pm   #10
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Default Re: Weston Cell

Ah, but at what temperature? Weston cells have a temperature coefficient.
 
Old 2nd Apr 2017, 5:13 pm   #11
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Default Re: Weston Cell

If it is an unsaturated cell, the temperature coefficient is

E = E(20C) - 0.000005(T - 20) or about 5ppm per degree C

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Old 2nd Apr 2017, 7:04 pm   #12
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Default Re: Weston Cell

Quote:
Originally Posted by merlinmaxwell View Post
Ah, but at what temperature? Weston cells have a temperature coefficient.


I meant to say 19.2 c 24hrs not sure how many hours it has to be at a temperature before it is stable. Cell is in our house so the temperature varies during the day and night. I only brought Cell indoors last night so maybe not stabilized yet.


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Old 3rd Apr 2017, 10:38 am   #13
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Default Re: Weston Cell

Back in the day, the Cambridge portable potentiometers were used to check all our kiln control instruments. The engineer would arrive with pot in his car, where it may have "overnighted" at possibly sub-zero, then brought into the kiln area where in some cases the ambient temp would be 25 Celsius. Not sure what happened to the "settling down effect", but there was none. Hoped for accuracies of +/- 2 Celsius degrees at 1230 Celsius whilst using Pt/Pt-Rh (13%) thermocouples.
Most of these instruments were simple moving coil meters by Electroflo (Elliot) or Ether, but here were also potentiometric units such as Kent Mk 1, Honeywell-Brown Electronik and others which had their own Weston cadmium reference cells, with some form of automatic checking against a working reference, typically a BIG 1.5v Leclanche cell.
Les.

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Old 3rd Apr 2017, 11:45 am   #14
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Default Re: Weston Cell

I seem to remember mercury cells (like the ones used in cameras) were used as sub-references in such instruments. Although not as good as a Weston cell, they held their voltage pretty well. Perhaps checked against a Weston cell every day or something.

Many older digital voltmeters had an internal Weston cell for calibration. There was a special position on the range switch (often with a warning not to leave the switch there) and a potentiometer to set the meter to the appropriate reading. The Weston cell in this case might be a glass H-tube fixed to the chassis, a plastic box with a couple of turret terminals containing such an H-tube, or in one case I remember an resin-encapsulated thing about 2" long and 3/8" in diameter with 2 wires coming out. No idea what it looked like internally.

On a related point, does anyone make a modern semiconductor reference chip that gives the same voltage as a Weston cell (to within its accuracy), either the nominal voltage at 20 degrees C (no matter what the temperature of the chip is) or one with the same temperature coefficient? It'd be quite useful for use with said DVMs, Cambridge Potentiometers, etc.
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Old 3rd Apr 2017, 4:34 pm   #15
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Default Re: Weston Cell

Quote:
does anyone make a modern semiconductor reference chip that gives the same voltage as a Weston cell
I had a look on Farnell, seems not. 1.024V is the nearest and a 2^10 number. One of those and a precision op amp would be capable of Weston volts quite well.
 
Old 3rd Apr 2017, 8:04 pm   #16
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Default Re: Weston Cell

Yes, mercury cells were used by some. !.36v if memory has not failed again. One of the later (not quite "second rate") instrument service engineers arrived with a Cropico, a poor man's Cambridge, and I think a mercury cell in that. I had one which I sold on just a few years ago. I picked it up in a job lot at an electronics auction about 25-30 years ago.
Les.
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