Poor Man gets his candies (6X weston cells)

[zhtoor]

-

[tggzzz]

Don't do it, any more than you would with any other type of battery/cell.

If you want to know the average voltage, measure each voltage on its own and calculate the average.

Apart from any other considerations, that will enable you to discount the effects of an individual failing cell.

I'm sure you already know that you must ensure there is zero current drain, and that you must be careful with the mercury and cadmium compounds that have leaked.

[tggzzz]

http://conradhoffman.com/stdcell.htm

Easily found using google.

[HighVoltage]

We had a long discussion about this already in the thread by Dr.Frank:

1000: My (hi)story of the Weston cell, of the Volt, and of being a volt-nuts
https://www.eevblog.com/forum/metrology/1000-my-(hi)story-of-the-weston-cell-of-the-volt-and-of-being-a-volt-nuts/

Your cells seem all ok.
Make sure to set your voltmeter to High-Z impedance.
A typical 10 MOhm load is too high for these cells.

[HighVoltage]

these cells are 1-tube design, i haven't seen anything like it in the literature. (might have missed a lot)

regards.

Can you take one of the cans apart and show us the inside?
I have also not seen a one tube Weston Cell before.
 

[Gyro]

these cells are 1-tube design, i haven't seen anything like it in the literature. (might have missed a lot)

regards.

Can you take one of the cans apart and show us the inside?
I have also not seen a one tube Weston Cell before.

The Muirhead K-375-C is a single tube design (saturated)...

https://www.eevblog.com/forum/metrology/muirhead-std-and-ref-cells-all-catalogue-info-uploaded-!-(ayce)/

[HighVoltage]

The Muirhead K-375-C is a single tube design (saturated)...

Thank you, I had not realized that!

[MisterDiodes]

Read your reply #2 above.

This is a big issue with these cells - do NOT draw current during a measure; you want that discharge rate as slow as possible - and watch the temperature.  Now you remembered why you wanted that KVD and null meter too....

[tggzzz]

There are many things you could do, but that doesn't mean you should do them.

In this case you should work out what benefit you expect from your idea, and whether there are better ways of achieving those benefits.

I strongly suggest you learn standard metrology processes and procedures, and why they are that way.

[VintageNut]

Read your reply #2 above.

This is a big issue with these cells - do NOT draw current during a measure; you want that discharge rate as slow as possible - and watch the temperature.  Now you remembered why you wanted that KVD and null meter too....

thanks MisterDiodes,

my mouth still waters at the mention of KVD+Nullmeter combo! 
i came across the following researching weston cells.

please comment.

As to the quote of your article....
If the cells were in parallel, they would not be for "comparisons" it would only be one comparison.

My guess is that the cells were connected to a switching multiplixer to take intercomparison measurements.

The primary lab that I visit once a year to have my 731Bs calibrated has a bank of references that are connected to a switch multiplixer. There are a couple of open spots in the wiring of the multiplexer for references to be measured.

The bank of references are compared to each other once every week. This data is saved over many years and used when calibrating an unknown reference.

[MisterDiodes]

Read. Your. Reply. Number. 2. 

Whatever moldy tomb of info your getting that from doesn't mention how long that arrangement lasted and at what discharge rate, either.  The parallel connection discharges some cells faster and leaves you with a mess of haz-mat gloop basic equivalent one cell (for a short while) that's going completely fail when any one of the group fails.  You lose all the advantages of having multiple cells to average and compare - and be able to spot the bad cells and remove those from the group as required.

I just realized I repeated your reply #2 but it will sink in at some point.

By the way, watch the bias current of your DMM also.  When everyone is telling you "draw zero current" from these cells, we aren't kidding.  This is a very big issue with these cells.

Null meter time, or really verify your meter's input Z and bias current.

[VintageNut]

Read your reply #2 above.

This is a big issue with these cells - do NOT draw current during a measure; you want that discharge rate as slow as possible - and watch the temperature.  Now you remembered why you wanted that KVD and null meter too....

thanks MisterDiodes,

my mouth still waters at the mention of KVD+Nullmeter combo! 
i came across the following researching weston cells.

please comment.

As to the quote of your article....
If the cells were in parallel, they would not be for "comparisons" it would only be one comparison.

My guess is that the cells were connected to a switching multiplixer to take intercomparison measurements.

The primary lab that I visit once a year to have my 731Bs calibrated has a bank of references that are connected to a switch multiplixer. There are a couple of open spots in the wiring of the multiplexer for references to be measured.

The bank of references are compared to each other once every week. This data is saved over many years and used when calibrating an unknown reference.

the lab that you visit, does it have weston cells on a switch matrix?

No. There more than 10 total of a mixture of 732B, 732A, 7001, .....  They are all very well aged and the 732Bs are calibrated at Fluke every year.

Nobody would do serious work today with Weston cells today as far as I know. The temperature control needed is a serious bath and control equipment.

Weston cells are very highly temperature sensitive compared to a modern compact voltage standard such as a Fluke 732B.

[MisterDiodes]

...no matter, we all know how to do it better now.

Again - when you connect all 6 in parallel, it takes on cell to fail to bring the whole group down - and now you're drinking from the cup of sorrow.  You're also the proud owner of 6 dead cells when you could have had 5 remaining good cells - if you had only listened to your nutter-brothers trying to help you.

In 1929 you could buy more cells too...and that was before we knew what haz-mat meant.

One lower-risk way:
You can also measure each cell individually, average them, and then pick -one- cell that you will use for measures.  On that one cell you use it's value offset from the average.  Only use that cell to measure (measure + offset to relate that back to average), but check that cell  against the newest recalculated average value occasionally.  Or do the same technique using all the cells round-robbin for measures so each cell gets about the same amount of energy withdrawn at each measure.

There are other ways as well but in Metrology it's all about checking across as many -separate- references as possible, with as many different techniques as practical, and looking for bogus measures and eliminating those from the measuring system.

You're going to keep your measures relatively quick, and not very often (compared to a Zener Diode Vref that will run all the time).  Remember that you only have a limited amount of energy in each cell.  You're going to keep your measure current at zero, or at least as low as possible, say a few hundred fA or less during a measure.  The more you discharge those cells the faster you'll be out of good Vrefs, so plan accordingly.

You might look at some of the various older app notes etc. that describe very low input current / low noise buffers etc. for these cells.  HINT: Fet or tube buffer followers might be more help than op-amps here but you'll have to research that on you're own.  Everyone here will have a passionate opinion on a good buffer circuit I'm sure.  Even with a buffer amp you'll keep your measures relatively short, and keep the cells disconnected and at a stable temp  as much as possible.

The main thing you'll probably learn quickly is what an absolute pain in the ass Weston cells are to use and maintain - but Have Fun!

[Edwin G. Pettis]

Naw, CK5886 tubes aren't that hard to find, I've got them.  They are made for low voltage operation and input current is on the order of 25fA.

If you want to shell out the bucks, you can get a Weston cell measured to .04 PPM but that comes with restrictions.  They are interesting devices, probably fun to play with but they have been obsolete for quite a few years, solid-state Vrefs are much more robust, less temperature sensitive and possibly even more stable with time.  Plus, in many places, because of their chemistry, they have been classified hazardous, harder to buy and even harder to get rid of 'legally'.

Properly cared for, properly used and kept under constant temperature, they are very good voltage references and also low noise to boot, but today, it just isn't worth the hassle of keeping and handling Weston cells when much better technology is available.  But have fun experimenting with them, just be careful of how you handle them.

[MisterDiodes]

That transistor in the op-amp note is replaced by just disconnecting standard cell when you turn off the amp.  Maybe think about a set of switch contacts on your On-Off switch as a replacement for that transistor switch.  I wouldn't leave the cell connected to any complete circuit while not in use.

Remember that's a slightly higher current design, looks like it's pulling some 10's of pA from the cell. which (I think) the Eppley's were a little more forgiving than what you have, maybe.  Somebody might have more info on that.  I might think about a lower input current design on the amp...   You can probably use just a unity gain buffer, you don't need the complexity of the resistor-feedback gain necessarily.

Edwin suggested a tube and that's really the bee-knees for low current.  Makes your project even more fun!

There is another standard cell buffer sample circuit floating around either at Linear / AD or TI/National.  I think it was around a pA current on the cell but I might be wrong on that.  But have a look around.  The older app notes will have other ideas - if they are still online.  But you should be able to find an appropriate op-amp if you want to do it that way.  Use the parametric search tables.

I'll have to let you consider those thoughts for now.

[VintageNut]

I have exactly one good Weston cell. It is the kind that degrades normally without any help of drawing current.

The way I measure it is a >10G DMM to measure the different of the cell vs. a 731B divided down via a KVD.

I did this every month or two for over a year and made a spreadsheet.

The best way to use the Weston cell is also with a KVD.

If I recall correctly, the 731B has a procedure to calibrate it vs a Weston cell of known voltage. That was back in the day when Weston cells were not obsolete.

Since I have had my 731Bs calibrated, I have lost interest in the Weston cell as it requires regular calibrations to know what the voltage is as it degrades.

[MisterDiodes]

You can get a price quote at NIST to measure your cells against their JVS down to around no less than .04ppm uncertainty, runs around ~$4200 last time I knew as a special quote (might be more or less today), but you pay extra fee on top of that for hazmat shipping and handling, and it is sent in a thermal isolated box, last I knew.  I think you have to have something like an insurance bond in place in case one of the cells break and they have to clean the lab, I know at one time that was the case.  Just getting the cells to and from NIST is not easy nor cheap, and you do that every year or 6 months or so to find out the unique discharge rate of each of your cells.

That cost is for -each- cell by the way, not for a set of cells.  There might be some discounted price for a cell set.

I know Boeing Cal lab used to do this service also, but they no longer accept hazmat cells for testing.  That will be very common at most labs these days, and that's why nobody except you is trying to use them today.   That's another big reason these are such a P.I.T.A.

Yes I was going to suggest the technique shown at LT app note 9 (same as your 1052) - in this use the cells are stacked in series which will give you a higher unknown voltage (and also means you can't tell which cell is bad), and inherently averaged into one unknown cell voltage:

http://cds.linear.com/docs/en/application-note/an09f.pdf

The trouble with 1052 (and similar chopper IC styles)  is the very high current spikes so you want to isolate the amp from the cells via RC filter at least. You want to bypass the amp input to give those current spikes a place to go someplace besides the standard cells - the makers of zero-drift chopper amps always leave that little part out.   Those short switching spikes (at chopper freq) can be up to several hundred times higher than what you see listed on the datasheet for the low freq current noise. 

Pay attention to your op-amp offset current AND bias current, no matter what.

The problem with something like LT1097, LT1008 etc. is the offset voltage and drift those introduce (that's why those example circuits include gain added with a variable pot - now input Voffset is not important).  The trouble with IC amps in general is you tend to trade off low input current for higher voltage noise.  You can get various electrometer IC op-amps but look at the voltage noise and offset.

That's exactly why Edwin and I were suggesting something more along the lines of a Tube or Fet type buffer as a discrete design.

Really:
In your case I'd just stick to using a DMM with 10G input impedance, and I would at least try to find out which way around your input bias current runs on that meter so that at least you're not trying to "charge" those cells.  They are not batteries.

Remember: You are trying hard to not discharge nor charge these cells. At. All.  These devices work best in an age where mirrored galvos, KVD's and null detectors meters were the norm. Normally you'd put a safety limiting resistor in series with your cell while it's connected to your bridge balance measure with your glavo or null meter, and then as you get thing close to in balance on the KVD, then you remove the safety resistor and do the final adjustments on the KVD to bring the bridge into balance.  Then you record the measure and remove the cell from the circuit.  The idea is to keep the current flow through the cell as close to nothing as possible.

Why not just use a few LM399's?  For what you have to work with you'll still have an Vref voltage source, steady but unknown.  Or for what you have to work with for measuring, why not just use an LT6655? You don't have much budget to work with, and at least something like a '6655 will get you in the ballpark of a Vref at a close known value.  Or use several of those in parallel.  That might be a better starting out place for your lab's local definition of a "Volt" reference.

If you just need an unknown but quiet voltage source - say for instance you need to do a a ratiometric measure - then a NiCd battery or sealed lead acid battery can work as a very quiet voltage source.  Rechargeable and relatively cheap too!

For a step up:

As VinatgeNut points out - you can find 731b's around for relatively cheap, and they WORK GREAT for what you're doing - you don't need a 732 at your level.  731b's will replace every function of a standard cell with ease, and much more reliable and easier to use than anything you're doing with a standard wet cell.  You can even get them cal'd at most any lab for around $100 or so.  Easy Peasy Cheesy.

[Gyro]

I don't think anyone has mentioned the technique of comparing cells connected back to back.

Obviously it doesn't give you the absolute reference voltage, but it does allow you to compare the voltage deviations of the cells more frequently, with minimum current draw and maximum resolution. You can then share the burden of measuring the absolute reference voltage across all of the cells.

You could even have a 'sacrificial' cell, which is used for providing absolute voltage measurement, and which is only referenced to the other cells by back to back comparison.

[MisterDiodes]

...Yes that can be done to compare cells.  The only problem is if you only have a DMM to use for measure (with bias current) then you're always trying to charge one cell and discharge the other.  Discharge is bad enough, but trying to charge a cell can really throw off it's discharge rate - which we hope will be at least kinda sorta constant over time.  At least when the cell is newer during its usable period.  As the cells age the real discharge rate gets a bit wobbly anyway, so it might not even matter in this case.

[fcb]

I've used a silicone conformal coating to seal epoxy packaged references and the edges of the (AN82) style PCB relief, but I do an elevated temperature run first for a few days.

[fcb]

If you can't get hold of MG (what we use) silicones in your country, then you can forget getting parylene.

I've had some success mounting epoxy voltage references in little glass bottles, but the seal is awkward and you have to really use solid state potentiometers or a DAC to perform adjustments. It does yield a stable humidty and isolation from air-pressure changes (if you get the seal right!).  There are some cheap 42ml jars on amazon.

Might be cheaper to use references that are already sealed though.

[Kleinstein]

For the humidity problem I would consider having a sealed metal container for the whole circuit, not just the reference part itself. With sufficient size and some silica or similar buffer one could get away with epoxy feed through.

The weston cells are not really practical for every day use. More like use them for a monthly or maybe 6 month comparison with you next best reference (e.g. LM399 set). They might be an alternative for a long time stable reference if checked against each other and a few checks again a precision calibrated reference a few years apart.

Shipping those refs is best avoided - more like keep in you lab only, preferably a cool place in a basement where they can stay the next years. Shipping or an earthquake might cause changes. That is one reason the got obsolete.

For a not so frequent use, the loading with a high Z DMM may not be such a big problem. Just check the bias of the DMM before use. Depending on the quality of the DMM / amplifier it would be more like each cell separate, maybe 5 in series to compare to a 7 V reference. Keep no. 6 separate, just inc case something bad happens.

[Conrad Hoffman]

I've never seen a single tube cell and without reading every word here twice, is it certain that these are saturated or unsaturated?

[MisterDiodes]

Be careful blaming an observed drift on humidity - that is an effect that usually doesn't appear for several days or weeks after humidity changes.  Sometimes it happens faster but that effect can have the fastest observed effect in the DMM itself.  Another sneaky effect is if you have a rapid temp change in humid conditions where you get an almost invisible film of condensation on your board that mixes with any contaminates left on the board - so always make sure your boards are 100% clean and flux free, and keep them in clean & dry enclosures.  Distilled water (condensate) is non-conductive, but -any- ion contaminates in the water raise conductivity VERY rapidly.

Most practical: An LM399 + PWW resistors and PWW adjust pot will be far more humidity resistant than anything else you have - including the DMM.   An 8 pin plastic dip amp won't see a real humidity change effect for several days at least, SMT version is much more vulnerable of course.

Coatings don't have to be "electronic spray on" plastic.  Coatings from a old school wax dip (keep away from bugs and it attracts dust, and make sure the board stays cool) can work, glass bottles, canning jars or paint can enclosures filled with CO2 / LN2 or whatever the local welder can get you, house paint can slow down absorption (test first to make sure it is non-conductive enough!!), clear wood varnish, linseed oil, run the circuit under mineral oil, a storage cabinet with a desiccant packet or saturated salt solution stored inside - there's a million ways to control humidity, some are better than others.  Test away!  Don't put anything on your board until you know what it does, and I'd wait 6months to a year to make sure your coating under test is really plastic-safe and non-conductive. 

I would also look at setting up your lab space with better temp / humidity control if your delving into the low ppm world.  Get creative.  For instance a humidifier can be made from a cheap box fan pulling air over or thru moist furnace air filter media, and a dehumidifier can be rigged from an air conditioner and a heater.  There are other ways also.  For what you're doing even if you kept humidity controlled to with some ballpark of 30% change range would probably be fine, and even that would be hard to see on a 3468a.  Generally you want to stay over 50% humidity to avoid static problems - the more you go below that the more every surface becomes a static generator.




 

[Gyro]

I've never seen a single tube cell and without reading every word here twice, is it certain that these are saturated or unsaturated?

I can't speak for the OP's cells directly but the Muirhead k-375-C single tube cell is definitely saturated (so it is certainly possible that the OP's are).

[MisterDiodes]

Yep.  Compared to a Zener Vref, these just aren't very good.  They look cool sitting on a museum shelf though.

The other fun thing these do is if you ever draw over around 100uA (the threshold varies, but usually around that) for any length of time (Maybe you accidentally brushed the test leads together and realize the safety resistor was not in the circuit) you might as well throw out the cell after that.  Even if the cell voltage looks like it almost recovers after that high current event, the discharge rate is now so unstable it's not really useful any more.  Which you might find out about a few months later when you're monitoring your discharge trend lines.

A real Barrel o' Laughs when you realize that happened to you.

You'll see this warning on the various old standard cell manuals - and they aren't kidding.

[Conrad Hoffman]

FWIW, I had a pretty good collection of Eppley un-saturated cells. They just don't last forever and eventually I sent all of them to hazardous waste. I also had 3 good saturated cells in a JRL oven. It served me well for several years, but the temp control was primitive and I didn't like having that much mercury and cadmimum around, so it met the same fate. Nothing wrong with it, except it was a standard cell. Solid state is the only reasonable way to go. My Analogic (Data Precision) 8200 voltage source uses the LM399 and is as good as just about anybody needs. Naturally you can do better as some of the projects here show, but in any case the standard cells have had their day in the sun and are best retired.

[HighVoltage]

Another reason why you definitely don't want to put them in parallel is their different output over temperature.
The temperature dependency of the WestonCells is really significant and not always the same.

Here are two of my Weston Cells measured over night.
The first 6 hours during day/evening with activity in the room
The second 6 hours during the night

Although the output voltage is following the temperature profile, the amplitudes of change between the two cells is very different.

[HighVoltage]

Yes, #1 is special! This one always had this value, since I had it.

I only had those two cells at hand last night, may be I will repeat the test with my other weston cells.

[HighVoltage]

Hello HighVoltage,

 i can't understand what are the two spikes?

regards.

The spikes have nothing to do with the cell.! Probably not a perfect wire connection or a DECT phone call that came in. This was not really a good scientific setup and I need to repeat it.

May be I will hook up 3 of my good cells tonight to 3 different 34461A volt meters. (I have never done that before.) Usually I just take a quick measurement once a month and enter them in to the spread sheet.

[HighVoltage]

If I want to have absolute values correct in comparison, I usually use one meter only.

But in this case I just wanted to see the different change rates over time and temperature and then 3 meters should be ok. The great thing about 3 x 34461A meters: I have 3 graphs captured from the screens without any PC or software involved.

[fcb]

how about locating the weston cell in a gallium triple point cell (primary reference for 29.7646°C)
and then turning this assembly into a voltage reference standard - something like a composite
temperature / voltage standard. note that the temperature 29.7646°C is within the working range
of a weston cell.

lets call this standard a DPS - a dual primary standard for temperature AND voltage. and let us locate
this cell into a measurement equipment like hp 3458a (next generation)

RoHS??

[fcb]

RoHS??

let us forget RoHS for a moment.

regards.

Actually, for all the hassle it caused at the time, I do believe it makes sense. The less lead/cadmium/mercury/etc.. in the environment the better - how can anyone argue against that?

Weston cells contain cadmium AND mercury.

[HighVoltage]

Ok, I did another test over night and here are the pictures of the measurements

3 x Keysight 34461A
Temperature drift is around 1 degree C during this time
All hooked up with the same type of twisted pair low EMF cables

Weston-Cell_1_10h
SPAN: 52uV

Weston-Cell_2_10h
SPAN: 49uV

Weston-Cell_3_10h
SPAN: 223uV

Based on this test, that I had never done before like this, I think the Weston-Cell_3_10h with the larger pk/pk must have a problem.
But this problem has not shown up in my monthly recordings.

In general, these are three good cells and still show a little different behavior and therefore should never be hooked up in parallel.
 
I will keep this test running for another 24 h

[Alex Nikitin]

Weston-Cell_1_10h
SPAN: 52mV

Weston-Cell_2_10h
SPAN: 49mV

Weston-Cell_3_10h
SPAN: 223mV

Microvolts, not mV!

Cheers

Alex

[HighVoltage]

Ooops, what a typo, corrected above.
Thanks, Alex.

[VintageNut]

hello friends

according to the certificates which came with these cells, the cells were manufactured
in february 2004 and when measured at 20 degC were 1.01862 volts.

according to the table provided for temperature correction with these cells, the measured
value (hp 3468a - calibration unkown) was 1.01828 at 26 deg, corrected for 20 deg = 1.01855 volts.
(i need to get a calibrated meter)

this translates into roughly -5 ppm / year drift average over 13 years (this includes a *VERY* rough ride by these cells).

do these numbers make any sense?

comments required from people experienced with weston cells.

best regards.

The cells should not degrade in voltage if the cells were treated properly.

The difference between the original measured voltage and the later measured voltage is more likely adifference between the temperature of the initial measurement and the temperature of the later measurement.

How was the later measurement corrected for temperature? Do you know the temperature coefficient(s)? The temperature dependency may not be linear.

20 degrees Celsius sounds kind of cold for a calibrated measurement.

[HighVoltage]

I only got my first own Weston Cell in 2016, so I have only data for about 1 year.
And during this time I have not seen a drop in voltage.
The changes I have measured are only due to temperature variations.

Out of the Conrad Hoffman document from above, I copied this paragraph:

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.

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

 

 

[tggzzz]

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

My two oldest date from 1949 (68 years old), and are currently (ho ho) 1.018711V and 1.017747V - and 40uV/degreeC

[HighVoltage]

Ok, here is the same test setup again after 48 h

Weston-Cell_1_48h
SPAN: 117uV

Weston-Cell_2_10h
SPAN: 113uV

Weston-Cell_3_10h
SPAN: 444uV

It really seems my Cell 3 has a problem.

[tggzzz]

My oldest cell is from 1971 (46 years old) and still performing well with a voltage above 1.018V

My two oldest date from 1949 (68 years old), and are currently (ho ho) 1.018711V and 1.017747V - and 40uV/degreeC

could you let me know the temperature when you made the measurement on the 68 year old cell,
and what was the cell marked / certificate voltage / temperature of measurement on the cells when it was marked 68 yrs ago?

regards.

Room temp was around 16C, and it was measured with an uncalibrated Solartron 7081, best guess reading ~20-30ppm high.
https://entertaininghacks.wordpress.com/2016/12/28/a-weston-standard-cell-an-introduction-to-voltnuttery/#more-852

[HighVoltage]

... and now i understand the importance of 1.018v output on reference standards.

regards.

Yes, that was a long transition from Weston Cells to DC standards.
This is also one of the reason why Null-Meters have been so important for so many years.

Even my old Fluke 731B has a 1.018 and a 1.019 output plus a delta knob to bring it
to zero with a Weston Cell on a Null meter.

I think the next generation Fluke 732C will be the first one without the 1.018V output.

[tggzzz]

i wonder why these wonderful beasts were retired? the drift rates are better than 732a/b !
and predictable (most of the time; that is, -ve drift).

The problem is saturated Weston cells are far more fragile (physically and electrically) and unpredictable than modern equivalents. My other two are in spec, but nowhere near as good.

[VintageNut]

This week I am taking some measurements on a Fluke 731B while the temperature in my house will vary from 65 degrees F to 80 degrees F. It is fall and there will be some cold mornings and warm afternoons. It is a good time to find out the temperature sensitivity of the 731B.

From the cold side of the week so far, the number is about 0.74 ppm / degree C.

If your Weston cell varies 40uV / deg C, then it is 50X more temperature sensitive than a Fluke 731B.

In addition, your 1.018xxx cell requires a KVD to use for just about any voltage calibration purpose.

And, you cannot draw current from the Weston cell. The 731B can be used with calibrated resistors to create a well known current for calibrating a picoammeter or an electrometer.

My Eppley cell was manufactured May of 1975.

There is a sticker that states that the cell was 1.01918 on May of 1985.

It has degraded about 1.2 uV/ month since 1985.

So, if you want to do sub-ppm work, you have to calibrate the cell often.

It is fun to play with this Weston cell, but not useful for calibration work.

[Conrad Hoffman]

When I wrote my page on the things, I tried to be very specific as to what applied to saturated vs. un-saturated cells. I seemed to have a lot more info on the un-saturated type. The reason is that properly constructed saturated cells don't seem to have any inherent degradation that limits their life. It's the users that damage them. I'm pretty sure they can go more than 100 years if the materials were pure, the oven temperature is low and people stay the heck away from the terminals.

Un-saturated cells, OTOH, have a finite life. Certainly all Eppley un-saturated cells are useless by now. I've no idea who might have produced them more recently, but odds are that all unsaturated cells should be retired. They may well produce almost 1.018 volts, but the value won't be stable over time, even if the temperature is. That's why a minimum voltage is required for certification. IMO, some of the cases were really pretty, especially the Weston models, and can make a nice case for a solid state reference. Just install a DC jack in the rear and run it off a wall wart. The cases are thermally lagged with aluminum and even copper on the older ones. You could ovenize it if you wanted.

(BTW, my page was decent when I wrote it, but the discussion here and pages people have done since, really add to the total information.)

[VintageNut]

This week I am taking some measurements on a Fluke 731B while the temperature in my house will vary from 65 degrees F to 80 degrees F. It is fall and there will be some cold mornings and warm afternoons. It is a good time to find out the temperature sensitivity of the 731B.

From the cold side of the week so far, the number is about 0.74 ppm / degree C.

If your Weston cell varies 40uV / deg C, then it is 50X more temperature sensitive than a Fluke 731B.

In addition, your 1.018xxx cell requires a KVD to use for just about any voltage calibration purpose.

And, you cannot draw current from the Weston cell. The 731B can be used with calibrated resistors to create a well known current for calibrating a picoammeter or an electrometer.

My Eppley cell was manufactured May of 1975.

There is a sticker that states that the cell was 1.01918 on May of 1985.

It has degraded about 1.2 uV/ month since 1985.

So, if you want to do sub-ppm work, you have to calibrate the cell often.

It is fun to play with this Weston cell, but not useful for calibration work.

i agree with most of what you said, but consider these:-

1. long term *stability* (not tempco) of around -1 ppm/year, stabilities in 100's of ppb/year (!) are reported.
(Instrumentation Reference Book by Walt Boyes)

2. low noise to the extent that they *can* be calibrated down to 40 ppb (!) against a JJA.
(edwin g. pettis, misterdiodes)

3. very well defined tempco curve.
refer to an equation which came with the cells in one of my earlier posts.

4. yes the tempco is *horrible*, -40uv / degC near 20 degC (but reaches zero at around 3.1 degC)
so a cooled weston cell at around 3.1 degC will have zero tempco and could make a *very* decent standard.

5. un-saturated variants (your Eppley cell) have a *lot* lower tempco, but they have a *horrible* drift of around -85uV / year.

6. last but not the least. look at the *simplicity*

other than above, i agree with most of what you say, it is *definitely* fun to look at a wonderful piece of work
from our past and wonder what have *we* been doing. if we throw the technology available today at this cell
(or some variant) i am almost sure that the next generation of measurement technology can benefit.

regards.

It may be picky semantics, but your *horrible drift * is just predictable degradation in my opinion. Being able to measure this degradation gives me confidence in my 731B and my KVD and my ability to measure small ppm drift over time.

My Eppley cell is no where near 85uV/yr degradation. If the cell is useless below 1.018 V, then my cell may still be useful 30 years from now. It was manufactured 42 years ago. Not bad!

My point is that very old solid state standards are more useful than a Weston cell and less costly. What you will spend on a temperature regulated oven is less than the cost of an ancient but more useful zener reference.

In the end, enjoy what you like.

cheers

[Edwin G. Pettis]

While the NIST will calibrate a Weston Cell to .04 PPM uncertainty, the cost is prohibitively expensive and the restrictions are just as bad, you can't ship a cell in for this calibration, it must be hand carried, if you want to know all of the requirements just check with the NIST.  Don't forget this cell is full of chemicals considered hazardous so the shipping restrictions are going to be very expensive.  Maintaining one of these cells to anywhere near that .04 PPM uncertainty is going to be expensive and as Conrad mentioned there are a lot of difficulties in doing that.  It wasn't directly mentioned anywhere that I could see on the NIST website but I rather doubt that .04 PPM uncertainty lasts for very long.  You can probably e-mail the NIST and ask them about the requirements and restrictions.

Compared to today's solid-state Vref standards, the cells are very obsolete and fragile with many more restrictions for use.  As I mentioned before, probably the only advantage those old cells had over today's Vref is a fairly low noise level but that is hardly a reason to use them now.  Batteries can supply a lower noise level if it is really needed and they can be measured to a very high accuracy for short term use.

Overall, a LM399 or LTZ Vref will be cheaper in the long run and much easier to maintain, the Weston cell is an interesting piece of technology but it has seen its day, you might be able to maintain your set of cells as a secondary standard but forget using them as a primary, they just don't make the grade now.

By the way, you can't make any definitive statements about measurements made with uncalibrated equipment of unknown accuracy and drift, those are nothing more than approximate at best.

[MisterDiodes]

2. low noise to the extent that they *can* be calibrated down to 40 ppb (!) against a JJA.
(edwin g. pettis, misterdiodes)

4. yes the tempco is *horrible*, -40uv / degC near 20 degC (but reaches zero at around 3.1 degC)
so a cooled weston cell at around 3.1 degC will have zero tempco and could make a *very* decent standard.

5. un-saturated variants (your Eppley cell) have a *lot* lower tempco, but they have a *horrible* drift of around -85uV / year.

6. last but not the least. look at the *simplicity*

Yes it is fun to learn from the older technology, but the real take away you should be focusing on:  Realize how much more practical, useful and safe a Zener Diode Vref is in comparison.  There is absolutely no reason to go to the hassle and risk of a wet cell - especially when you have LM399's / LTZ's available...  In that sense we do not throw out old technology.  You use the knowledge of old technology like this: You be VERY thankful every day you don't have to go back to the "Bad Old Days of Wet Cells" and that's why you treasure a known, stabilized Zener Vref that is so much better than any wet cell.  Especially when you're the one paying the shipping and cal lab invoices.

"Simplicity"?  The cell is simple in construction (depending on where it can be assembled legally), but far from it for actually using it, let alone transport legally - at least in more developed countries with strict hazmat rules.  Really you'll want a KVD and Null Meter to go with those cells - if you want to maximize lifetime and lowest current drawn at each measure.

Don't take that .04ppm possible measure uncertainty (at NIST) of a standard cell too far out of context - that comes at an -extremely- high cost and hazmat risk, at least in developed countries.     Even if you can find a lab willing to take one in for measure these days.  What is possible is not the same thing as what is practical.

If you're not calibrating them often (at high expense) then they aren't going to be much use as being a traceable absolute value in your lab - just another unknown voltage source with a (hopefully) predictable discharge rate.

Looking at it a profitable business expense / risk:  There is really not much use for any wet standard cell these days...More for entertainment value if anything.  Of course it's fun to see something decades old still function at some level - but a Zener Diode is easier, safer, more forgiving and much more profitable to own from a business standpoint.

Have fun!

[Conrad Hoffman]

My understanding of the bad old days was that you hand carried your saturated standard cells to NBS. My Julie oven had fastenings and a shoulder strap- remember, you can't invert the cells, so you can't ship them at all. The recovery time from inversion is near to forever, and it can permanently damage some types of cells. the Julie oven had a strict warning on top of the box against inversion. You'd get a bus ticket for you and a seat for the cell oven. You'd also have to carry two small lead-acid cells to run the 24 volt oven. You'd make the trip, taking several days if necessary, then drop off the cells and let NBS do their thing. When done, you'd repeat the procedure to get them home. Some weeks would have to be allowed before trusting the cells after any travel, nor would NBS do anything until they settled down. I think people must have had too much time on their hands back then. 

[HighVoltage]

I still had the instruments hooked up to one of my Weston Cells.
Here is a good graph to show the huge dependence between temperature and output voltage.

- Delta Temp:      5.7 °C
- Delta Voltage:  246 uV
- Which comes down to about 43 uV/°C

[MK]

or in other words about the same as a k-type thermocouple...

[Kleinstein]

There seems to be some common drift up on all cells (or the meter drifting down). With the limited way to measure the voltage there is a limited use in sending out a cell for calibration / check. It might need more time to see which cells are actually good. One might as well take the absolute values of those cells as a kind of primary standard (as it once was) - good enough for a 5 digit meter and it is not sure that an external calibration will help much as transport can add quite some errors.

[Kleinstein]

The temperature during transport is only one thing. There are also mechanical forces and possible tilting that can be a problem.

For a portable constant temperature box, I would not use a Pelletier element - they have quite a thermal leakage path and thus don't behave that well when used with low power. Just a heater and good insulation is likely the better way.  One may not need to set the temperature very high as the cell itself has essentially not self heating.

Form the shown data it is hard to tell a difference. It could be the 3rd temperature reading was off a little and this way caused slightly odd temperature readings. There are too little point and to much noise / uncertainty to really tell a difference.

[Conrad Hoffman]

FWIW, the Julie oven I had consisted of a heavy walled aluminum box with heaters or heater wire wrapped on it, and a mercury thermoregulator, inside another aluminum box set up the same way. The outer box was significantly above ambient, and the inner box just slightly above the outer box. A sensor and bridge were included so you could detect temperature fluctuations. Everything was insulated from everything else, and it was enclosed in a wooden case. The problem with saturated cells is that you need a really good thermal control system that can be trusted not to drift over a long time period.

[MisterDiodes]

zhtoor:
I think you need to listen to everyone and maybe think about another way besides wet cells.

I would suggest don't even worry about spending limited resources on getting a wet cell calibrated.  You don't really have any equipment that's going to see any benefit - and it's very very unlikely you're going to successfully get a wet cell to the lab and back without destroying the calibrated measure anyway.  In other words:  The cost to benefit ratio is very, very low for getting a calibrated absolute measure on a wet cell.

The other problem is with wet cells - the more you try to take a measure, the more you're changing the discharge curve.

For your situation getting something like a working 3456a (or similar), get that calibrated to within a few ppm (usually very inexpensive), and you're miles ahead for having a known stable way to measure your wet cells to a reasonably accurate absolute value.  That's probably the most bang for your buck to get some ppm measuring ability.  Those old meters are big and slow - but they can offer really good yearly drift specs (often better than 3458a).  Used, functional 731b's work very very well also and they don't break the bank - those can work well down to very low ppm per year drift also.  Which is still going to outperform and outlast your wet cells if you are using them at all.

We just measured a 731b recently that really wasn't drifting more than a ppm per year, and they don't get too picky if you keep them powered on or not.  A bit noisier than a 732a or battery, but they can be rock-solid stable as a value Vref.

You can still use the wet cells as a fairly quiet voltage source as required - as long as you're willing to discharge.  They will -always- discharge, not matter what you think you're doing to draw no energy...

But I wouldn't use the wet cells to transfer an absolute voltage measure to your location.  There's a ton of better ways to do it that are more realistic for your modest uncertainty of measure capability.

[MisterDiodes]

I think that's still probably a better chance of getting an absolute measure transferred to your lab.  Unless your calibrated  measurement facility was across the street where you could walk your wet cell (in a thermal box) over to get tested - there's just not a realistic good way to handle wet cells getting form point A to point B without disturbing the measure.

Remember:  You're not a calibration lab and you don't have extremely tight measurement uncertainty.  But that doesn't mean you can't run Vref experiments. 

The mark of a GOOD metrologist to is to recognize exactly where your limits of measurement uncertainty are, and keep your measuring results and analysis within your actual real measure capability.

In other words you learn when to stop putting digits to the right of the decimal point, and realize what data is meaningful and what data is just noise.

[MK]

I think that's still probably a better chance of getting an absolute measure transferred to your lab.  Unless your calibrated  measurement facility was across the street where you could walk your wet cell (in a thermal box) over to get tested - there's just not a realistic good way to handle wet cells getting form point A to point B without disturbing the measure.

how about driving 300 Km with the cell in a "thermal box"?

-zia

It would need to be heated, and how smooth are those 300k of roads, the more it is bounced about the longer it takes to settle down each time, it could be a month to get stable again.

[HighVoltage]

When I take one of my weston cells and move them a little bit, they immediately change the value.
I think they are only made to sit still and don't move at all.

Moving them 300 km in a car is a bad idea.

[Edwin G. Pettis]

The 3N164 is quite close to the 2N3609 and should work with out problems.  These MOSFETs come with a shorting ring around the pins, do not remove the ring until the FET is soldered into the circuit and remember, the gate is extremely sensitive to static discharge, the slightest zap will puncture it.