#1000: My (hi)story of the Weston cell, of the Volt, and of being a volt-nuts  - Page 1
Forum section: electronics / general

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 02, 2016, 10:17:06 am
content: My collection of Weston cells, all salvaged from the dumpster, started in 1982.That's marking my personal emergence as a volt-nuts, as for the first time, I collected Volt artefact fix-points, down to the ppm range uncertainty.These were first measured in 1982, both to 5 1/2 and 6 1/2 digits, about 1.01882(4)V.This was done by a Fluke 8502A, which was calibrated daily or weekly by means of 335D, 720A and an ovenized Standard Cell, which were frequently calibrated against a 2mV Josephson Junction, from a lab being located just across the street.So these values were uncertain to about 10ppm, or better.These unsaturated types, wrapped in tin foil, are from scrapped FLUKE 803B differential VTVMs.They were used to calibrate the internal 500V reference.The first one is from Eppley, now reading 1.01795V, that's a rate of about -25ppm/year.The one on the left side in their box, is a Muirhead 845-C from 1961. Hello - that's my own vintage!It still reads 1.01798V, about -1295ppm from nominal 1.0193V over 55 years.A historical look at the Volt (*):Weston cells defined the Volt until 1948, when the Absolute Volt was defined, and from then on served as mise-en-pratique (representation) for the Volt only.The SI-Volt was defined in 1960, just before all these cells were made.This SI-1960-Volt definition is still valid, up to date, and therefore still uncertain in the SI to about 0.3ppm, currently one of the worst defined units.As there was no precise way at that time to do it better, all main National Standards Institutes had their own Volt, still based on the Weston cells, and the very difficult comparisons between the countries showed differences up to 20ppm.In fact, there was the American Volt, the German Volt, the British Volt, and so on.Between 1968 and 1972, the mise-en-pratique changed to using the single Josephson Junction, and Weston cells were used only as working standards.The Volt at this time was defined by a Josephson constant of KJ(72) = 483594 GHz, so the absolute level of the worldwide Volt was reduced by about -10ppm.As this JJ Volt was still difficult to transfer to 1V or 10V level, obviously there were still differences between the national labs, still on the order of a few ppm.The German PTB claimed to be uncertain to 4*10-8, though.The next time the cells were measured, was on 14th November 1989, another remarkable date in several aspects.At first, the values were now determined to 8 1/2 digits, so that was obviously the first day, when I got a brand new 3458A at university.Secondly, on 1st of January 1990, the SI Volt definition was changed again, exactly the day, when I started my PhD physics thesis.The mise-en-pratique now was defined by the 10V Josephson Junction array, giving about 1000 times better uncertainty worldwide, from 10-6to 10-9level.Coincidentially, the Josephson constant, was increased by about 8ppm, KJ(90) = 483597.9 GHz, bringing the Volt back to nearly the former level.For my 2 month old instrument this meant, that it lost its calibration to SI Volt over night; the American Volt had to be increased by exactly 9.264ppm, so that my instrument was now reading about 9ppm high.https://www.nist.gov/sites/default/files/documents/calibrations/tn1263.pdfThis didn't matter for my work, as I made relative volt measurements, at much lower uncertainty requirements, only.Then I salvaged 3 Muirhead 845-D Reference Cells from about the 1970ties, out of some dead gear.These are also still fine, the one in the middle only decreased about -360ppm over maybe 45 years.The saturated Standard Cell from 1965 also was measured first in November 1989, 1.01857420V @ 22.5°C, which gives U20°C= 1.018672V (SI-90), and 26 years later about U20°C=1.018639V, so that would be a drift of about -1.24ppm/ year.I'm awaiting impatiently July of 2017. This will be the Due Date for the final measurements of the Watt-balances in U.S., Canada and UK, and the German Avogadro experiment.Succesfully determining Plancks constant h within 2*10-8uncertainty for one, and 5*10-8between different experiments, will be the pre-condition for the next new definition of the SI-2018.The definition of the kilogram, the mol and the Kelvin will be changed, and also will finally re-define the SI-Volt.It will then have zero uncertainty by definition of the quantum Josephson Effect, with h and e having zero uncertainty. This will also be identical to its mise-en-pratique, at about 10-10uncertainty level to real world, and within 10-19@ cryogenic world.I think, at that moment, we'll have to re-write Volt-Nuts, in capital letters.Frank(*) taken from:"Volt Metrology: the Josephson Effect and SIS Junctions Arrays", B. Jeannerethttps://home.zhaw.ch/~spma/Scripts/Messtechnik/MT/Unterlagen_Lit&Co/VoltMetrology.pdf

author: EEVblog
author_rank: Unknown
timestamp: October 02, 2016, 10:43:07 am
content: Great 1000th post!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 02, 2016, 12:56:40 pm
content: Good read, even for us non Volt Nuts.I found it interesting that your three salvaged Muirhead cells have such a small serial number range. Obviously the instruments they were salvaged from were purchased in a single lot.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 02, 2016, 01:05:49 pm
content: Your deep understanding of this subject is impressive.What a great collection of Weston cells.That is probably the reason, the Fluke 732 standards have still a 1.018 V output today?

author: Vgkid
author_rank: Super Contributor
timestamp: October 02, 2016, 03:34:07 pm
content: Nice 100th post. It has been a while since I have looked into the historical aspects of the volt.

author: TiN
author_rank: Super Contributor
timestamp: October 02, 2016, 04:05:33 pm
content: Vote for "Volt-Nut" status for FrankGreat read and pleasure to see photos.

author: VintageNut
author_rank: Frequent Contributor
timestamp: October 03, 2016, 11:46:05 am
content: You have very cool toys and you know how to use them!Your collection of voltage cells makes me jealous as well as your 3458A.It is a pleasure to read your well-written and well reasoned posts.

author: Jeff_Birt
author_rank: Regular Contributor
timestamp: October 03, 2016, 12:33:59 pm
content: Dr. Frank,Recently when searching for some manganin wire to rewire the 1K stage of an old Muirhead decade box I found a paper by R. Dzuiba and J.F. Mayo-Wells on the history of the manganin resistance standards used by NIST from 1901 until 1990. This paper mentioned that the Josephson junction array is now used (to calibrate manganin transfer standards). This got me to thinking about where all of these standards come from and how one can be specified without making reference to another, i.e. specifying one unknown by way of other unknowns. Personally I think this would make a fascinating mini-series on Discovery, etc. Thanks for the link to the NIST tn1263, I will attempt to digest it.

author: quarks
author_rank: Frequent Contributor
timestamp: October 04, 2016, 04:00:57 pm
content: Very interesting. Thanks for sharing.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 12, 2016, 07:08:21 pm
content: Dr Frank,After reading your great post, I could not resist and got my first Weston Cell.It was made by Cambridge Instrument Co. Ltd. in EnglandWhen you measure your Weston Cells are you using the multimeter in the 10 MOhm input impedance setting?The load on to the chemical cell most likely will have an influence on the output voltage.But much older multimeters had far less than 10 MOhm resistance, so I wonder what current can a Weston Cell supply,without having a significant voltage drop.I am measuring about1.018, 310Vbut with 10 MOHm at the input of the 34461A I see a constant drop.

author: dr.diesel
author_rank: Super Contributor
timestamp: October 12, 2016, 07:12:55 pm
content: Super Cool, thanks for sharing!

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 12, 2016, 08:02:43 pm
content: Quote from: HighVoltage on October 12, 2016, 07:08:21 pmNice cell, I assume, it's a saturated standard cell.These should not be loaded at all. Therefore, these measurements were always made in differential mode, Null (e.g. Fluke 845A) against an equal comparison voltage, (e.g. Fluke 750A or 720A), giving virtually indefinite high resistance, and that was possible even many decades ago.Nowadays, the low bias of modern bench DMMs is sufficiently low; 10MOhm impedance must not be used.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.Frank

author: tautech
author_rank: Super Contributor
timestamp: October 12, 2016, 08:06:59 pm
content: So just for clarity are DMM's with 10G input impedance OK to measure these Weston cells ?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 12, 2016, 08:19:58 pm
content: Quote from: tautech on October 12, 2016, 08:06:59 pmI don't like at all this characterization ".. 10 GOhm.. impedance.."These instruments are better characterized by their bias current.A low bias current of several ten pA on the 1V or 10V range therefore is allowed for standard cells .PS: These "10 GOhm" are totally misleading. These were somehow theoretically calculated by dividing the voltage range by the bias current, or if you would do it mathematically correct, by calculating the differential dU/dI .This bias current in first order is constant over the input voltage, so unreasonably high resistances might occur, or even negative ones, if the bias current decreases with increasing voltage.Frank

author: HighVoltage
author_rank: Super Contributor
timestamp: October 12, 2016, 08:20:51 pm
content: Well, luckily I had the 10 MOHm multimeter only hooked up shortly, so hopefully the cell was not damaged.At a high-Z setting for the 34461A, the voltage of the cell is very stable and has no drop.So, may be it is OK to have a high resistance multimeter or an Electrometer hooked up in parallel to the Weston Cell?Looking at your pictures, it looks like you have the 3458A hooked up directly to your cells.May be not?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 12, 2016, 08:32:17 pm
content: Quote from: HighVoltage on October 12, 2016, 08:20:51 pm10MOhm create 100nA current, that should not damage the cell if you loaded for only a few seconds.The voltage drop is caused by the about 200..600 Ohm internal resistance of these cells.Of course I used HiZ for the 3458A, when I measured the cells. Its < 20pA bias do not harm the cells at all.

author: Macbeth
author_rank: Super Contributor
timestamp: October 12, 2016, 11:40:34 pm
content: The best #1000 post ever!I have to admit I restrict myself to 6.5 digits in the volt nut stuff. I am tempted further, but then one of my dogs pass by and wags their tail and there is a very noticeable effect immediately visible on my Keithley 2000/2015I quickly realise my lab (not the labrador, the laboratory) is useless for anything in the microvolt level due to unavoidable environmental factors!However I still find all the volt-nuttery very interesting indeed. Please carry on!

author: lowimpedance
author_rank: Super Contributor
timestamp: October 17, 2016, 10:15:21 am
content: Nice to see there is still interest in these obsolete, but not forgotten, metrological artifacts.Attached is a 1958 reprint excerpt from the IRE Transactions On Instrumentation. by George D. Vincent.Hope you find this an interesting read.Had to split it into 2 PDFs to suit the forum file sizes !.ps: I promise this is the last upload on weston cells from me............... maybe.

author: VK5RC
author_rank: Unknown
timestamp: October 17, 2016, 10:40:29 am
content: Thanks for a concise and understandable summary of your professional experience and development of voltage standards.It must have been, and still is I hope, an interesting journey!Rob.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 17, 2016, 12:21:10 pm
content: Many thanks to lowimpedance for that interesting article about the building techniques.I have another one from NBS / NIST, which is complementary.It's too big to upload, so here's the archive address, for a tidy copy:https://archive.org/details/standardcellsthe84hame_0It also depicts the changeful volt-history based on Weston cells.Frank

author: HighVoltage
author_rank: Super Contributor
timestamp: October 18, 2016, 08:28:08 pm
content: 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 2013Also interesting ...I set my 34461A to HighZ input impedance and in this setting the instrument has about 131 GOhm of input resistance (measured).The voltage drop of the Weston Cell can clearly be observed within a 30 min time frame and drops about 13 uV

author: Macbeth
author_rank: Super Contributor
timestamp: October 18, 2016, 08:37:49 pm
content: @HighVoltage - Blimey that seems a heck of a lot for such a huge input impedance. Does the cell recover at all afterwards?

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 18, 2016, 09:04:10 pm
content: Quote from: HighVoltage on October 18, 2016, 08:28:08 pmHello 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

author: HighVoltage
author_rank: Super Contributor
timestamp: October 18, 2016, 09:15:01 pm
content: Quote from: Macbeth on October 18, 2016, 08:37:49 pmYes, the cell actually recovers very nicely after about 1 hour time, the 13 uV are gained back.Since this is my first Weston Cell, I have no idea if this is a normal behavior even for such small currents.I have also hooked up my Keithley 614 Electrometer ( >50 T Ohm input impedance) and spot checked the cell every few hours and did not notice any drop, based on the 614 being constantly hooked up in parallel to the cell.

author: enut11
author_rank: Super Contributor
timestamp: October 18, 2016, 09:36:26 pm
content: Dr Frank, thanks for the wonderful journey back in time wrt to voltage standards. It reminded me of what happened some 40 years ago. I used to spend Sat mornings wandering through our local scrap merchants electronics section. One day I found a Western Cell wired inside a big box. I went back to my car to get my cutters but on returning, it was gone. I was heart broken. I would have destroyed it anyway as at the time my best multimeter was 20kohm/volt!Enut11

author: saturnin
author_rank: Regular Contributor
timestamp: October 18, 2016, 09:43:29 pm
content: Quote from: HighVoltage on October 18, 2016, 08:28:08 pmThe 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 , 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?

author: HighVoltage
author_rank: Super Contributor
timestamp: October 18, 2016, 10:03:30 pm
content: Quote from: Dr. Frank on October 18, 2016, 09:04:10 pmThis 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.

author: lowimpedance
author_rank: Super Contributor
timestamp: October 18, 2016, 11:04:20 pm
content: Quote from: HighVoltage on October 18, 2016, 10:03:30 pmWhen 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' standardsthus 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 21, 2016, 12:14:36 pm
content: I got a second Weston Cell.The owner did not know, what he had and it was cheap, almost free.When it arrived, I measured1.001 Vwith my 34470A in High-Z modeSo, it was kind of dead on arrival.Well, in one of these many papers that I read on Weston Cells, it said tokeep it in an oven at 60 Degree C for a few hours and it may revive. That iswhat 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 warmerWill be interesting to see, if this one will finally settle to a fixed value.Also, interesting, it had a sticker on the cell, reading:1.0177001/1/91Fluke 883ASo, 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 21, 2016, 06:05:21 pm
content: The relatively large temperature dependence suggests this is a saturated cell. Thus the voltage should be higher, not just 1.0177 V.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 21, 2016, 06:13:08 pm
content: 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.

author: Dr. Frank
author_rank: Super Contributor
timestamp: October 21, 2016, 10:29:40 pm
content: 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

author: johansen
author_rank: Super Contributor
timestamp: October 21, 2016, 10:47:15 pm
content: 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 06, 2016, 08:15:09 pm
content: 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.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 06, 2016, 10:57:40 pm
content: Quote from: HighVoltage on November 06, 2016, 08:15:09 pmIn 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.......

author: tggzzz
author_rank: Super Contributor
timestamp: December 27, 2016, 02:51:45 pm
content: Quote from: Dr. Frank on October 02, 2016, 10:17:06 amPah! Mere newbies, both the personand the cell - as shown belowI 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 cells1.01859V @ 20C implies a 0.1N solutionserial number is almost illegible, but it 1702date is almost illegible, but is 19-1-49!And there was me thinking that a 68 year old standard cell would be worse than uselessI 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.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 06, 2017, 06:24:39 pm
content: 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.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: January 06, 2017, 11:36:14 pm
content: What a beautiful find, tggzzz... You were certainly inb the right place at the right time! Thanks for sharing!

author: HighVoltage
author_rank: Super Contributor
timestamp: July 14, 2017, 03:51:42 pm
content: 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. BeckenhamModel:D-402-C/PDate:18.1.71Number:37500Voltage:1.01850 VOf 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!

author: beanflying
author_rank: Super Contributor
timestamp: February 27, 2018, 01:40:38 am
content: 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?

author: Macbeth
author_rank: Super Contributor
timestamp: February 27, 2018, 02:04:06 am
content: Quote from: beanflying on February 27, 2018, 01:40:38 amReally 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!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 27, 2018, 02:59:46 am
content: 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.

author: beanflying
author_rank: Super Contributor
timestamp: February 27, 2018, 03:39:32 am
content: I have done a little reading on these so already onto the HiZ input measurement but thanks anywayAs 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

author: tggzzz
author_rank: Super Contributor
timestamp: February 27, 2018, 09:01:34 am
content: Quote from: Conrad Hoffman on February 27, 2018, 02:59:46 am...or use a saturated Weston cell as a slow-acting limited-range thermometer.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 27, 2018, 09:50:17 am
content: Quote from: beanflying on February 27, 2018, 03:39:32 amI 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/°CEven 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.QuoteThat 735A looks really nice for its age!QuoteYes, a Null-Meter is the next step.You are on a good way to become a voltnut.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 01, 2018, 07:25:49 pm
content: 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

author: beanflying
author_rank: Super Contributor
timestamp: March 04, 2018, 01:01:15 am
content: I actually removed my second one from it's original plastic bag today after 50 yearsThe 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

author: HighVoltage
author_rank: Super Contributor
timestamp: March 04, 2018, 07:46:19 pm
content: Quote from: beanflying on March 04, 2018, 01:01:15 amAll my cells have shown around 40 µV/°C in TCInteresting, how come yours is listed at 5 µV/°C?

author: zhtoor
author_rank: Frequent Contributor
timestamp: March 05, 2018, 01:55:56 am
content: Quote from: HighVoltage on March 04, 2018, 07:46:19 pmbeanflying's cells are un-saturated ones, which have a lower tempco than the saturated ones.-zia

author: beanflying
author_rank: Super Contributor
timestamp: May 08, 2019, 03:34:03 am
content: 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

author: HighVoltage
author_rank: Super Contributor
timestamp: May 08, 2019, 07:15:15 am
content: That is impressive with a cell from '43Some of these cells are just amazing, how long they stay stable.What is the input impedance of the advantest DMM in the mV range?Also interesting that the advantest shows above 1000 mV in the mV range.

author: beanflying
author_rank: Super Contributor
timestamp: May 08, 2019, 07:29:44 am
content: It is a 2 million count meter and 10^10+ Ohms on the lower ranges as standard. The cells were removed from a local Uni Physics Lab (RMIT Melbourne) in 1975 and haven't been used since.Just a cool thing to add to the collection and I am rolling an 3DP/Laser Enclosure to have a clear Perspex side for the un cased Tinsley one for display.

author: tggzzz
author_rank: Super Contributor
timestamp: May 08, 2019, 08:27:36 am
content: Quote from: HighVoltage on May 08, 2019, 07:15:15 amI have 1949 cell that is similarly in spec.Saturated cells last but have a 40uV/C tempco around 20C. Unsaturated cells have a lower tempco, but don't last anywhere as well.Here's some information from the NBS ("The construction and characteristics of standard cells"http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS84.pdf):

author: beanflying
author_rank: Super Contributor
timestamp: May 09, 2019, 06:38:08 am
content: Just because it is a little different I cracked the case on the Cambridge Cell to see what made it not tick.While it is a single tube outer it appears to have a second inner glass with the base of it containing liquid Mercury and then into what looks like some sort of flock barrier and back outside the inner tube to the Amalgam.$0.02 Teardown and also so it doesn't get lost in TEA

author: ArthurDent
author_rank: Super Contributor
timestamp: May 10, 2019, 01:46:54 am
content: Some may find this interesting.The story of standard cells has some footnotes. Eppley tried what they thought was a innovation in cell enclosures and temperature control with their model 119 three cell enclosure. Instead of being heated, it used peltier effect coolers to maintain the temperature just a few degrees above zero degrees C. The theory was that, if you look at a graph of the voltage vs temperature of a cell, rather than heat the cells up, the enclosure cools the cells to the turning point in the temperature curve where the change is basically a flat line, or zero. What they apparently found, from what I heard, was that there were condensation problems and the idea was scrapped. I don’t know how many of these units were made but there is at least one which I own.Attached is a temperature curve showing the zero point and a photo of the outside front of the 119 enclosure that has two heatsink enclosures with the power supply and peltier effect controllers on each side plus a large heatsink on the back of the center unit. The inside has a beer can size cylinder holding the 3 cells surrounded by rigid foam. There is a thermometer well hole in the middle and a matching hole in the cover plate.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 10, 2019, 03:00:38 pm
content: Here are some photos of a more conventional standard cell enclosure/oven, my Guildline 9152-R4. You can see that the leads from the cells (inside the clear tubing) goes around each of the three concentric housings one complete turn to prevent heat creep on the leads. The leads are small and you’d think they would want to keep them as short as possible to minimize drops, but when measured using a bucking voltage to balance the cell voltage, you are drawing no current so there is no voltage drop on the leads.

author: ArthurDent
author_rank: Super Contributor
timestamp: May 10, 2019, 05:39:14 pm
content: Here is a photo of a dead Muirhead saturated cell. although it doesn't say, I'm pretty sure it is a saturated cell.

author: beanflying
author_rank: Super Contributor
timestamp: May 11, 2019, 07:44:20 am
content: Interesting info on the cold option.There is a couple of Heated Cabinets on evilbay for more than I would consider so I will make do. The curio factor and stability over decades is still fascinating.I still have no real plans for my lot and if more come up for not much I may add to the fleet just because they are a cool toy

author: ArthurDent
author_rank: Super Contributor
timestamp: May 11, 2019, 01:39:32 pm
content: The problem with buying a standard cell enclosure now is you never know the condition of the cells inside. When I bought my Guildline 9152-R4 a few decades ago, the seller said one of the cells was iffy and the voltage on that cell is a little low. I just remeasured the internal resistance of all four cells and 1, 2, and 3 are between 900 and 1000 ohms, which is near the high end of acceptable, but the 4th one reads almost 4000 ohms.The way the internal resistance is measured is indirectly by checking voltage drop with a known load resistor. Although some references say use a 1 megohm resistor, a 10 megohm resistor is better. I use my HP735A as a bucking reference for the cell under test and an HP419A as the difference indicator. I also monitor the HP735A voltage with an HP3457A for a double check. You can use this formula to calculate cell internal resistance.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 11, 2019, 03:40:20 pm
content: Subscribing to a fascinating thread.I have visited Newport, RI previously. Surprising that a quaint resort town better known for its ultra-wealthy mansions and Jazz festivals, is also home to Eppley Labs.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 12, 2022, 05:16:09 pm
content: lets pump this thread back to life :-)I just been at one of the best vintage parts collectors in my contry, truely a walk thru a time-machine,and see what i was lucky to bring back home..I did bring my keysight and test them all :-) ha ha.. all deliver the written voltage 1.01859my two muirhead standard cell d-550 got a date handwritten on the back : 21-4-70so they are 52 Years old !

author: tggzzz
author_rank: Super Contributor
timestamp: June 18, 2022, 10:24:23 am
content: It looks like I've stumbled on a workingunsaturatedWeston standard cell.I picked up a couple of Solartron DVMs with the giveaway "CAL 1.019" and "CAL 1.0185" buttons and trimpots. After being on for a while, and twiddling the trimpot so the display matched that of my Agilent 34410A, one read 0.04 (i.e. clearly dead), and the other 1.019. What? It isn't dead?!I'll ignore the dead one, and concentrate on the apparently good one.It is inside a Solartron 1420.2BA. A datasheet is stamped "received 2 Feb 1966". The service manual states "internal Weston standard cell (unsaturated)" and "Weston Cell Temperature Coefficient +-0.0005% per degree C", but does not state the specific type. There are no ICs nor valves to hint at manufacturing date; thedatasheeteven lists the number and type of diodes and transistors! However, the nixie displays are marked 69xx, so it looks like the meter is ~1970 vintage, i.e. over 50 years old.When spelunking, I used my Agilent 34410A on Hi-Z setting to measure the standard cell voltage, and it was 1.018725V when the unconditioned room temperature was 25C. According toConrad Hoffmann, the voltage should be 1.0190V @ 20C, decreasing by 5µV/C as the temperature rises, so if "new" my cell should be 1.018975V. Cells tend to decrease voltage by 25µV/year (with quite a wide range), so my 250µV fall equates to 10years. Not bad for something with a 7-18 year lifespan.The cell is inside the black plastic case in the centre of this picture; I can't see any identifying information and it looks like it is simply a box to protect the glass phial. I'm not going to attempt to desolder it to read the other side.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 18, 2022, 11:03:56 am
content: A few in my collection :

author: dseyst
author_rank: Contributor
timestamp: June 21, 2022, 11:59:55 pm
content: I measured my Eppley model 100 standard cells which are naturally all unsaturated. The highest six voltages I recorded were:1.0189791.0189961.0189961.0190301.0190611.019328Strangely enough, the highest voltage came from the oldest cell - serial #748240. My cheap thermometer was reading 21.8 degrees C. The voltages were all measured with an Agilent 34401a set to 10 gig input inpedance. I have checked all of them with a Fluke 8505a and a Keithley 196 also, and all meters read within 10 microvolts of each other. The other four cells I have all read around 1.018300 or so.Dave

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 22, 2022, 04:55:30 pm
content: I had a collection of Eppley model 100s and what you need to do is cross compare them on a regular basis for a year or three. Even though mine appeared to be in reasonably good health, their long term stability was poor. Saturated cells can go nearly forever if properly cared for, but trusting unsaturated cells is risky. IMO, one of the nicest looking cells is the Weston model 4.

author: dseyst
author_rank: Contributor
timestamp: June 22, 2022, 10:26:05 pm
content: I do plan to check these Eppley cells on a somewhat regular basis but I don't have a real use for them other as a kind of curiosity. I was born and raised in Newport, R.I. and at one time lived about a two minute walk away from Eppley Labs. I wasn't much into electronics years ago or I think I would have bought some of their saturated standard cells when they were still being manufactured.Dave

author: artag
author_rank: Super Contributor
timestamp: June 26, 2022, 10:03:30 pm
content: I've been after a Weston cell for a long time, and when I saw one offered for peanuts (thanks to mikeselectricstuff!) I snapped it up. It's the Muirhead D-845-F as commonly found above and has a 1971 cal sticker from Ferranti's Wythenshawe cal labs. The voltage then was 1.01924. Mike mentioned that he thought it was still in spec. Serial is 461647 so that was probably close to the date of manufacture, judged from the dates of other people's cells.I dont (yet!) have a 3458A and the DVMs I do have (HP34401A and K2015) are out of cal, but they do agree to a few uV that it's 1.018636V, albeit at 26C. So if correct, that would be only 11uV per year fall. Too good to be true so I guess it's likely that my meters are actually reading a bit high (they were set to be equal about 5 years ago).

author: dseyst
author_rank: Contributor
timestamp: July 03, 2022, 12:40:46 am
content: Well, I was interested to find out what my Eppley standard cells (all unsaturated cells) would measure at 20 degrees C. I measured them with a Agilent 34401a and the thermometer was just a cheap digital from Amazon.The first eight cells are model 100 cells and the last two are miniature cells.1.0192161.0190291.0190621.0190001.0190291.0193251.0188541.0189431.0183411.018193The third cell and the two miniature cells seem to vary the least with temperature. Amazing if you ask me.Dave

author: HighVoltage
author_rank: Super Contributor
timestamp: July 03, 2022, 08:10:23 am
content: You have 10 pieces Eppley standard cells?And they are all above 1.018V and stable.That is impressive after so many years.One of my standard cells has taken a sudden dive and went from 1.0181V to 1.0164V within 6 month.All the others are also very stable.

author: dseyst
author_rank: Contributor
timestamp: July 03, 2022, 08:02:35 pm
content: Yes, I do have 10 Eppley standard cells. Three of them are very stable. The other 7 are stable but are more temperature sensitive. The three most stable only vary by microvolts as the temperature rises and falls. I also have 4 Weston model 4 standard cells but they all have problems.Dave

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2022, 05:25:24 pm
content: Infected by this topic I got my first black box:Muirhead D-845-DReference CellNo: 459872I did a measurement immediately after its arrival (34401A in HighZ) and got 1.018418V. That is on the low side from what I see here. It was shaken during the transport and also it was pretty hot outside, so the voltage will change a little bit after the cell stabilize itself, imho.Is there a chance to estimate the production date based on the serial number?

author: grizewald
author_rank: Frequent Contributor
timestamp: August 09, 2022, 08:33:39 pm
content: This is my Muirhead D-402-A Standard Cell - A saturated Weston cell in a beautiful black Bakelite case which I picked up from eBay several years ago. It was a bit of a gamble really, probably totally illegal to ship and when I went to collect it from my local post agent, I wasn't looking forward to having to tell them that they needed to call a hazmat team if I could see that the package was wet.Thankfully, it survived an international journey through the postal system. I can see a few drops of mercury on top of the cadmium sulphate crystals in the negative leg, but other than that, it is unscathed. These particular cells were designed to withstand being transported in any orientation, given suitable packaging, so it's not really that surprising that it was still OK.The cell itself is just two years younger than I am and is probably in significantly better condition!From time to time, I measure the cell voltage with my calibrated 7.5 digit Solartron meter and the reading is usually around 50µV higher than the temperature adjusted expected reading and well within the meter and cell's specifications. The thermometer that I keep in the thermometer hole in the cell's case isn't calibrated, so there is also some uncertainty in my calculation of the temperature adjusted voltage.It has shown no signs of the voltage changing during the just over three years it has been in my possession, so even after some 55 years, it is still performing remarkably well! The latest entry in my log is:Date Temp Expected Actual Delta2022-05-22 25.25 1.018351 1.018400 0.000049While it might be more useful these days as a thermometer rather than a voltage reference, it's still a great piece of history which I am very happy to own and I'm amazed that it still seems to work just as well as it did when it was new.

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2022, 08:58:22 pm
content: Quote from: grizewald on August 09, 2022, 08:33:39 pmI used that to beat my supplier down in price to £0 (but maybe £5 for a couple of D-402-Bs, some Na, some Mg, some KMnO4, and some NaClO3. A local 16-18yo college was shutting and selling off all its contents. A local clearance company was dealing with it, and the young girl knew a bit about disposing of hazardous materlals.I gave them a reasonable but low price for a couple of very useable stereo microscopes; normally I can't stand the things. I also gave them tips about flogging boat anchors that didn't interest me. (That was back in 2016, before I became a *nut, abd before the TEA thread had exposed/developed our vices)QuoteSee pictures of mine in reply #35QuoteI haven't seen any info to that effect, but that is absence of evidence, not evidence of absence. Any refs?

author: grizewald
author_rank: Frequent Contributor
timestamp: August 10, 2022, 08:01:14 pm
content: Quote from: tggzzz on August 09, 2022, 08:58:22 pmIn the attached document from Muirhead, they say: "The construction of the cell elements is such that, with suitable packing, they may be transported in any position."I remember seeing your much older examples of the same cell. Are they still as stable as mine?

author: tggzzz
author_rank: Super Contributor
timestamp: August 10, 2022, 09:10:52 pm
content: Quote from: grizewald on August 10, 2022, 08:01:14 pmThanks for the PDF; saved.I certainly haven't done any "formal" measurements of stability, but I have informally showed it is a good temperature sensor, e.g.https://www.eevblog.com/forum/metrology/weston-cells/msg4093978/#msg4093978andhttps://www.eevblog.com/forum/testgear/muirhead-kvd-and-internal-corrosion-ignore-it-or-fix-it/msg2894446/#msg2894446

author: trobbins
author_rank: Frequent Contributor
timestamp: December 12, 2022, 05:26:10 am
content: A chance to test a Weston cell recently was thwarted by the Fluke 801BR I was restoring being the /AG version with a zener. But I now have two cells to ponder over - a Tinsley Normal that I understand is a saturated version with crystals visible, and an Australian made version made by Munitions Supply Labs (MSL operated from 1922 to 1948) that I can't easily identify if it is saturated or not.The Tinsley measures 1.0124V at 19C with a Keithley 197 (>10G rated, and about 1nA of measured bias with a 10Meg load), so may have had a hard life given it came from a technical college, and had a nameplate 1.0183V value at 20C. Post #28 from HighVoltage in 2016 indicates that some oven heating may be worthwhile, so I will try that in a few weeks if there has been no change. Btw, HighVoltage you commented that one of your cells degraded badly earlier this year - was that cell the same as the cell you recovered by heating back in 2016?The MSL measures 1.0187V and so may be nominally ok, and has a makers label with 1.0184V at 20C. Given this unit is circa 79 years old, that suggests it is a saturated version although the base portion of the cell glass is coated in black so I can't confirm.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 12, 2022, 08:43:35 am
content: Quote from: trobbins link=topic=75140.msg4576474#msg4576474Yes, this was the cell that arrived badly, was revived and stayed stable for some time but now I would consider it dead.All my other cells are still well.

author: trobbins
author_rank: Frequent Contributor
timestamp: December 13, 2022, 02:40:06 am
content: Ta, that seems to be a typical outcome from any rejuvenation attempt on a grossly discharged wet battery cell. Do you recall where the 60C heating scheme came from, or was commented on?A 1907 paper from the Royal Society adds some context to the historical development of the saturated Weston cellhttps://royalsocietypublishing.org/doi/pdf/10.1098/rsta.1908.0010

author: HighVoltage
author_rank: Super Contributor
timestamp: December 13, 2022, 11:05:15 am
content: I had one old documentation that mentioned this oven re-conditioning. I would have to look, to find it again.The problem is probably also, that we have no idea what has happened to these cells over so many years. Many have most likely been handled by people who do not even know what this is.However, I find it impressive that some old cells are still so stable after 50+ years.

author: eppley
author_rank: Newbie
timestamp: January 07, 2024, 02:16:33 am
content: Just FYI. The Standard cell division of Eppley Labs in Newport RI was shutdown around 2000 and the contents were donated to the Vintage Communications Museum in Windsor CT. The material and instruments sat in storage for about 20 years and are just now being organized and planning is underway to make a significant museum display from all of it. All the Eppley documentation is in the collection. There is data on every cell produced by eppley going back to the 1920's. If someone needs original cell manufacture data (date of manufacture, final voltage, chemistry ......., please email me.

author: eppley
author_rank: Newbie
timestamp: January 07, 2024, 02:28:32 am
content: Sorry, I am a newbe here. Please send a message, not an email.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 07, 2024, 03:16:06 am
content: Back in 2009 I contacted the Vintage Communications Museum and was able to get copies of the original data for two of my Eppley 100 cells. Unfortunately, they couldn't find records for 3 bare cells. I've attached a copy of the info that they sent so you can see what's included.Ed

author: trobbins
author_rank: Frequent Contributor
timestamp: January 07, 2024, 03:23:41 am
content: WRT the 1961 factory record, would 40C be part of a forming or initial preparation process?

author: eppley
author_rank: Newbie
timestamp: January 07, 2024, 04:25:08 am
content: Material is a bit more organized now. If you want to forward the 3 bare cell serial numbers, I will take a more detailed look for the history. I will also look a bit further into the tests done at 40C. That may be specific to a very small group of cells. Testing at that temperature is not typical per the records.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 08, 2024, 01:34:06 pm
content: FWIW, I had a Julie standard cell oven with Eppley cells. Julie was odd in that they used a higher temperature for the inner oven, 37C. Maybe higher temperature testing was for people like that.

author: iMo
author_rank: Super Contributor
timestamp: April 04, 2024, 08:08:27 am
content: I made a measurement (1 day long 34401A, HiZ input) on my Muirhead Ref Cell and got TC between +6..+9uV/C.Is that typical for my cell (unsaturated perhaps)? It seems to me the TC is much lower, there is another mechanism influencing it (discharging in the HiZ DMM input?).My cell is close to EOL based on the voltage, also I tilted it flat for a while before the measurement (grhh), anyhow below the data.Muirhead D-845-DReference CellNo: 459872PS: added the graphs with first 3.5h cut off..

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 04, 2024, 10:41:05 am
content: Quote from: iMo on April 04, 2024, 08:08:27 amHello,the 845-D is an unsaturated type, i.e. lesser T.C. (than the -40.6 µV/K of saturated ones), but higher annual drift.If you search the Internet for "muirhead standard cell 845", you'll find a datasheet on eevblog, which specifies these types to have +/- 5µV/K.So it's about the ballpark, what you measured, but you have to take the age (does T.C. increase, when a cell is really too old?), and your measurement method into consideration. Latter might show more hysteresis, than the real, lower T.C.. That's hard to decide from your measurements.Have you properly ruled out the T.C. of your 34401A?FrankPS: I think the link to saturated / unsaturated types are here:https://www.eevblog.com/forum/metrology/weston-cells/msg4031467/#msg4031467

author: iMo
author_rank: Super Contributor
timestamp: April 04, 2024, 11:42:07 am
content: Hi Frank, the TC of my 34401A is aprox +0.5ppm/C at 10V range, in the above graphs you may see raw data.Moreover, I do not know my TC at the 1V range, I never did the measurement of my TC at 1V range..Let us assume the TC is the same at 1V range- see below the corrected TC (and with some light median filtering).

author: trobbins
author_rank: Frequent Contributor
timestamp: April 17, 2024, 06:35:47 am
content: Further to posts #79 and #83 by eppley, I contacted the museum and Barry M responded with some history, advice and photos of the relevant Eppley factory records for a 'MIN 1' model standard cell that is in an RFL model 829G AC/DC Calibration Standard I am part way through restoring.The Eppley records confirm the calibrator was manufactured circa late 1974 to early 1975, which is also confirmed by datecodes on some internal parts. One could suggest from those records that the batch of 25 cells ordered by RFL were for a nominal month of calibrator production, and further suggest that about 500 calibrators per annum were being produced, as the cell was an optional extra. That sort of aligns with serial number information in the 829G manual, so a customer like RFL could well account for circa 1500 cells for a product line over 5 years.The museum's donation of Eppley records involves about 20 shelves of three-ring binders and covers about a million cells, so that makes RFL just a minor user, given that Barry indicated they made circa 20-30 cells per day (eg. 6,500 per annum ballpark).The cell in the calibrator is still functional at about 1.01811V, indicating a nominal 24uV loss per year, which is consistent with the 20-40uV/yr advice in NBL Monograph 84 from 1965, and is not behaving erratically so far. The cell can stay in the calibrator, which luckily includes terminals for an external reference, and enjoy its retirement now that it has a copy of its birth certificate.

author: Nanitamuscen
author_rank: Contributor
timestamp: May 09, 2024, 08:28:38 am
content: Here is such a normal element that came into my hands as part of a large two-circuit thermostat. Surprisingly, he survived the delivery of the shipping company, most likely thanks to a thermostat weighing 20 kg.Labels on top:normal elementGrade 1 T 20 degrees Celsiusmanufacturer 3-DThe Teplokontrol plant in LvivAssembly in 1956.The inscription on the side: "Initial" "Exemplary"

author: czgut
author_rank: Contributor
timestamp: October 12, 2024, 08:56:44 am
content: Some notes on measuring Weston Cells, using today's Hi Input Resistance VoltmetersMentioned multimeters usually have High Input Resistance due to feedback, keeping Guard Voltage equal to Voltmeter Hi Input Voltage and lowering input bias current of Hi input below 50pA.Usually, (after fixing 1V range) residual bias input current is of the order of 5..50pA (reverse current of diodes protecting input against overvoltage and insulations leakages).1. I would not leave Multimeter connected to Weston Cell unattended: Input Resistance is high until Multimeter gets Power Supply. In case of break in Power Supply, current sourced by Multimeter from Weston cell at 1V level by protecting diodes may be quite high and may destroy Weston Cell.Or I would use Low EMF relay, disconnecting Hi and Guard of Voltmeter in case of Power Supply break.Some multimeters shut down their input (and Source Voltage) temporarily (through safety resistor in series with input 40kOm) at time of autoranging or zeroing. Therefore2. It is good to have another 1.018V source, and conect Multimeter to this Source, and fix manually range of multimeter to 1V DC. Thist will load input capacitors, and get Hi input at 1V. Then disconnect multimeter (Guard and Hi first) and connect input of Multimeter (Lo first) to Weston cell.3. Weston cells are sensitive to temperature change (40uV/deg @ 20'C). They were usually kept at Mobile thermostats at Tref +-0.01deg, and +-20mK at National Laboratories.4. They are also sensitive to load. Temporary shortage =>1.2 mA current (1V/800 Ohm internal resistance) can ruin cell as reference cell. Recovery of cell's Voltage after temporary shortage may take months.