Standard cell valuation and transit query.

[buggies]

First off - I have looked in ebay completed listings to try to find a suitable asking price - so now asking a couple of questions here.

I have two 1979 Muirhead standard cells type D-845 in original sealed packaging. They have been stored in vertical position. I have unsealed the boxes and put DVM probes through the polythene to check the cells are still above 1.018 volts (they are). I no longer have access to any DVMs with sufficient digits to warrant my keeping the cells.
Two questions
- would they survive the postal service? They would have been fitted within bench and semi-portable DVMs originally.
- and any opinions on what they are worth - I am not looking for bids!

[TonyDuell]

A very similar cell (perhaps even this type) was used in some 1960's Solartron DVMs. I have such a meter with the original service manual and there is nothing on the insturment or in the manual about keeping it the right way up. In fact the instrument has feet on the back to store it vertically.

BUT I suspect, due to the highly toxic chemicals inside the glass H-tube (IIRC both mercury and cadmium compounds are present), that such cells are on the Post Office (and most other courriers) 'banned list'.

[buggies]

You are spot on - the semi-portable I referred to was th LM1420 and the cells were in use in the 902, 1010, 1440 and 1480 to my knowledge.
I had not considered the cell contents - that might be a show stopper these days.

[G4_Pete]

Hi,
I would be careful considering posting these as I believe these are Weston standard cells containing mercury and compounds of mercury. Therefore will be considered as toxic.
The industrial precautions for a mercury spill are extensive so a spill in a post office does not bear thinking about!!

ps. I see my post crossed other whilst typing

Pete

[TonyDuell]

And the instrument I have is an LM1420.2 (The '.2' meaning it has the BCD output socket rather than solder pins on the back). And yes I do have the DTU (Data Transfer Unit) to connect to it, but that is getting way off-topic.

Wikipedia confirms that the contents of the cell are pretty toxic. They're quite safe if unbroken of course, but I suspect sending them through the post is a big no-no.

[karesz*]

Hi buggies,
I ordered two similar cells in sommer online for €60. After receiving your post, best is to wait for such hours, and please DO MEASURING VERY HIGOHMish_10Mohm is absolute minimum, an old style voltmeter (i.e. AVOs can destroy it for ever )!! Some weeks later I become on a HAM-fest a third one for only a tenth of that price, also for ca €6_ it was in hes pocket over head staying, after some days its OK Weston cells has a drift of about -1.24ppm/ year and -40µV/°C_ also they are for best resultats to isolate from laboratory environment too or even ovenize it...
Dr. Frank wrote: "They may survive short loading with nA, even a few µA for short time, but this means, it takes time for the cell to recover, but maybe the change is irreversible."
You can find from Dr. Frank a very competent topic over standarsd cells on EEVblog and the links are very interesting too...
Best regards, Karl

[high_vacuum_house]

I have an identical one in a vintage Marconi comparator voltmeter which I think the voltage is a bit low measuring with a high resistance DMM. The comparator readings are out and this looks like the main cause if the internal voltage comparator standard has started to go flat.

Is there an electronic or solid state version of such reference cells these days with ROHS etc.. as I would like to get this comparator voltmeter working properly.

Christopher Capener

[karesz*]

Christopher,
as I know; for similar accuracy, is nothing in integrated version to find, but dont worry, i.e. on eBay are some times such versions of the good old ones to find...
Alternatively I would by an AD584 pcb and apply hes output voltage with an good OP and super precisions resistors to the "ideal 1,018V or so"_better as nothing.
In my linked topik (or in some linked article in it) was written; if a cell has very wrong voltage/is full out: you can back it for some hours(!) and it will regenerated, but please read for exact temp & back-time...
rgds, Karl

[MotorBikeLes]

Approx 65 years ago a factory I worked at purchased some Honeywell-Brown potentiometric temperature controllers. I came on the scene there just 50 years ago when I repurposed one for use with new kilns. I had their engineer to service it, and had a chat to him. I remember it used 6SN7 valves, which he said they now replaced with adapters and 12A*7 (ECC8*), I forget which one. By then they also had available a solid state chopper for the DC amp, as well as a solid state "Zener diode controlled" reference cell to replace the Weston Cadmium standard cell.
So, YES, they were available, and I guess should be simple to replicate with today's technology.
Les.

[karesz*]

Les,
yes, Temp Compensated Zener does exist_patented in 1971_but there are for i.e. 6,2-6,4V, in my opinion, not so precise and you must it apply/correct as wrote for AD584 too...
Alternatively has Nationals LM113 so an Zener too, but has 1,2V output- with onchip electronics. More commerce, but you can select 1N756/757 (and even some others)too, of course all Zener versions have an nominal value in procent range_not even a standarad cell...
I think the best similar ultra precise reference element is LinearTechs (yet ADI) LTZ1000A , delivers ~7,2V-super precise, BUT hes nominal value is between 6.9--7.5V _ thats why I wrote; I dont know similar precise integrated device as a standard cell!
rgds, Karl

[buggies]

Having looked at the Royal Mail prohibitions and restrictions I see that mercury is prohibited as a corrosive along with caustic soda etc. I doubt it could escape from the strong casing containing the glassware.
I also note that asbestos is prohibited so perhaps a few AC/DC radios are included in that?

[buggies]

Originally Posted by TonyDuell

And the instrument I have is an LM1420.2 (The '.2' meaning it has the BCD output socket rather than solder pins on the back). And yes I do have the DTU (Data Transfer Unit) to connect to it, but that is getting way off-topic.

Is it ok to send you a PM re DTU - I have some old bits.

[TonyDuell]

Sure, send me a PM.

I have a couple of DTUs, one better spec'd than the other. The better one has 3 input relay cards (along with the scanner board to drive them), controller, clock, DVM interface, manual entry unit and parallel output board (officially for a Facit 4070 punch). I have the service manual too.

[Craig Sawyers]

There are two versions of the Weston cell. The unsaturated cell was used in instruments, could be inverted, and had minimal temperature coefficient. However the voltage, although precise, was not precise enough for standards work.

In standards work, saturated cells are used. These have precise and long term stable voltage, but are very temperature sensitive, and must never be shocked or inverted. They were used to define the volt until 1990 when the Josephson Junction voltage standards were developed. I think the standards labs still keep their Weston Cells in their temperature stabilised oil baths as back-up insurance.

Anyhow, they contain not only mercury, but also cadmium.

Craig

[TonyDuell]

Thanks for that explanation. I suspect that any Weston cell we are likely to come across will be the 'unsaturated' type, certainly all the ones I've seen (and the instruments that contain them) do not have warnings about never turning them over, etc.

The Josephson Junction was known as a voltage 'standard' before 1990. I remember doing an experiment on one at university in 1987. It was explained that while the experiment we were doing was to determine a value for the combination of certain fundamental constants (h/e) the device was more practically useful as a voltage standard since said constants were accurately known from other experiments. However it may not have been used as the definition of the volt before 1990.

A quick explanation as far as I can remember it. If you have a Josephson Junction in an RF (a couple of GHz is practical) field and consider the voltage across it as a function of the current through it then the voltage goes in discrete steps (the I-V curve looks like a staircase on the 'scope). The size of those steps depends only on the RF frequency (and frequency (effectively time) is something that is relatively easy to measure accurately) and some fundamental physical constants (I think Planck's Constant and the charge on the electron, but I may be mis-remembering that). Therefore you can calculate what the size of the steps should be and use that to calibrate a voltmeter or whatever.

[Craig Sawyers]

Indeed. Josephson Junctions have been around since they were predicted by Brian Josephson as an student aged 22, in 1962. For which he won the Nobel prize. It is basically a superconducting tunnel junction, and in one variety produces a DC voltage related to the radio frequency field applied.

Since frequency is very well defined in terms of the Ceasium transition, the voltage across a junction is defined to the same accuracy. The hydrogen maser should allow even more accurate determination.

Since 10V is a convenient for a voltage standard, typically a 20,000-odd array of junctions are fabricated in series, and typically 75GHz is used. The junctions have to be at liquid helium temperature.

The current uncertainty in 10V is 0.02ppm at 95% confidence.