[UK-SW] wanted: meter to "calibrate" a Weston Cell - Page 1
Forum section: electronics / general

author: tggzzz
author_rank: Super Contributor
timestamp: January 07, 2017, 06:32:30 pm
content: As noted herehttps://www.eevblog.com/forum/metrology/1000-my-(hi)story-of-the-weston-cell-of-the-volt-and-of-being-a-volt-nuts/msg1100033/#msg1100033I've recently picked up three Weston Cells that are working well as far as I can tell - but I only have a measley 5.5 digit voltmeter.Just for thesakeof it, I'd like to measure them with a >5.5 digit voltmeter. Is there anyone in the Bristol/Bath and vaguely neighbouring areas with such a voltmeter?Obviously I would come to you rather than vice versa, and I don't want to post the cells!Thanks

author: Assafl
author_rank: Frequent Contributor
timestamp: January 08, 2017, 12:24:38 pm
content: One of the difficulties I encountered in measuring a Muirhead I have with a 6.5 digit DMM (it is a K-231-A an ovenized temperature controlled cell, the DMM is a calibrated HP34401A) has to do with figuring out thermal compensation and establishing an accurate temp reading for the cell itself.It is greatly important as the K-231-A has a 64uV/C Tempco. That means that a 0.1C change spans a 6.4uV change in the reading (at uV resolution, or at a 6.5 digit resolution reading 1.018...V). Muirhead recommend that the thermometer have a precision of 0.01-0.02C - which isn't a problem - I use a PT1000 and the same 6.5 digit DMM.So I tried to adjust the temp controls to get a stable voltage and got the following:So the attempt to compensate for measured temperature failed.The problem lies with the temp well. It is a copper tube that is close to the heater at the bottom and gets to the top. The deeper your probe goes the hotter it gets. So what I was trying to compensate was some random temperature along the temp well.I would guess the temperature one is interested is the cell solution and septa temps. Those would be hard to measure. BTW - NIST (to alleviate the need to measure accurately) had the naked cells in circulated mineral oil baths to many decimal places of accuracy (using a mercury column thermostat).At this point I decided to use the cell as the temp sensor. It is probably wrong to do so but my hope was that if the cell is bad (noisy) the randomness of the reading will fail to improve the tempco. I recalibrated the potentiometers and am currently at the following tempco (at least for a few weeks worth of data):So - if you plan to take the cells to someone who has a long range DMM - figure out how you will compensate for temp - or the readings may be disappointing (or worse - make you suspect their accuracy - even if perfect).The worksheet I use is as follows. The reason I made it is that the potentiometers of the thermal control were erratic, so I had to rebuild them with faderlube. I then had to figure out how to calibrate the potentiometers which turned out to be pretty involved - especially if you want to try the full 12-32C range spec'ed by Muirhead....

author: HighVoltage
author_rank: Super Contributor
timestamp: January 08, 2017, 12:42:06 pm
content: One very important aspect of measuring these cells is to take the DMM in to a high resistance mode.The typical 10 MOhm resistance is loading the cell too much.An 6 1/2 digit Agilent 34401A in High R mode (> 10 GOhm) works really well.I have several Weston cells now and couple of them are really stable and good.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 12:48:21 pm
content: All the temperature related points are true, and will indeed limit the precision of the results. But ultimately this is only for fun; I amdeterminednot to become a voltnut. Realistically I have no reason to expect that a 68yo reference is within spec. Hell's teeth, I was surprised it worked at all!Irritatingly, my Cambridge Instruments Weston Normal Cell (age unknown, also appears to be within spec) does have a built in thermometer with 0.5C scale marks.I would just do whatever is possible within a limited time and accept the limitations of the measurements. I suspect my strategy for temperature compensation would be to make the measurement, then quickly expose the cell and measure its temperature with a non-contact thermometer. That would rely on speed and thermal mass, so wouldn't be ideal, but would be better than nothing.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 12:51:08 pm
content: Fortunately my HP3468A has an impedance of >1010ohms on the 3V range.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 08, 2017, 01:00:33 pm
content: Quote from: tggzzz on January 08, 2017, 12:51:08 pmIf you have a HP3468A, then you have 10 uV resolution at 5 1/2 digits at the 3V DC range.You probably don't gain much more insight with one more digit for a chemical cell.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2017, 01:36:54 pm
content: One problem with the old chemical cells is that they are somewhat sensitive to vibrations. So after moving the cell to another place, it may take a few weeks to stabilize again. At least you never know after moving it around.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 01:39:36 pm
content: Quote from: HighVoltage on January 08, 2017, 01:00:33 pmIndeed. Hence my comments about being determined not to become a voltnutBut more, for little cost, is fun

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 01:43:08 pm
content: Quote from: Kleinstein on January 08, 2017, 01:36:54 pmInteresting. I haven't seen that mentioned, so either I haven't looked deeply enough or it is so obvious as to not be worth mentioning.What's the physical/chemical mechanism? The Muirhead cells are fused crystals, but I don't know about the Cambridge Instruments cell.

author: TiN
author_rank: Super Contributor
timestamp: January 08, 2017, 01:54:36 pm
content: QuoteSorry for telling the truth, but it's already too late. You got infected.Stage 1 - denial phase.All visible symptoms already in the thread..QuoteQuoteQuoteQuoteIt's not too late however, but you need to act quick. Dispose the cell as HAZMAT, and never ever think about it again, as all normal people do...Now, I know there is no cure, so I agree withKleinstein. Just moving my EPPLEY cell from one shelf to another cause output to shift ~20uV, slooowly going back after few days. So there is little to no point taking cell anywhere and expecting meaningful measurements hours-days after stress. So you need someone to take their long-range meter, bring to you, have meter sit there warming up for x hours, then take some readings and compare them to your 5.5d one.

author: Assafl
author_rank: Frequent Contributor
timestamp: January 08, 2017, 02:22:51 pm
content: Quote from: TiN on January 08, 2017, 01:54:36 pmThe K-231-A Muirhead (if moved) doesn't sway by more than 2uV (if at all). I mean moved upright and slowly. Not shaken, dropped or disconnected from the heater power supply (if you do disconnect it temporarily, it takes about an hour to re-stabilize the temp). Or maybe it is the sintered septa?If you hold the cell directly in your hands maybe you are warming it a bit? Or maybe you are moving it to a place with more draft/heated area?

author: Assafl
author_rank: Frequent Contributor
timestamp: January 08, 2017, 02:30:54 pm
content: Quote from: tggzzz on January 08, 2017, 12:48:21 pmSince the Tempco is high (actually Very High) - resolving to 1C will leave an uncertainty of +- 64uV. 0.5C to +-32uV. That makes the 6.5 digit measurement rather useless....Non contact (IR) type will also be a bit low on accuracy especially since the cell itself is glass. Perhaps use a PT100/PT1000 chip pasted to the H cell using thermal adhesive and measure that?Also, as others have suggested - If you drive with it you may want to let it rest a month before measuring...

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 02:58:07 pm
content: Quote from: TiN on January 08, 2017, 01:54:36 pm<grits teeth>Please shut up. That's. Not. Funny.</grits teeth>QuoteAre the Eppley's saturated or non-saturated? There appears to be a significant difference at these levels.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 03:18:55 pm
content: Quote from: Assafl on January 08, 2017, 02:30:54 pmTrue.QuoteYes, but... I prefer not to trust the last digit in any instrument, and I don't really know the state of my HP3468A's calibration, other than to know it survived my replacing the battery.QuoteCertainly true with a reflective surface, but there are non-reflective surfaces in there. The accuracy of an IR thermometer would have to be validated anyway.Using a decent contact thermometer, as you suggest, definitely exceeds the "fun" part - and I'm going to use that as a mental obstacle to becoming a voltnut.QuoteAnd I remember using Weston cells at school to calibrate NiFe cells using a 1m long resistance wireWe were warned not to take any current, but I'm sure that was not achieved!

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 08, 2017, 06:17:05 pm
content: Quote from: tggzzz on January 07, 2017, 06:32:30 pmI suppose a better way of doing it would be to source a known good 1V (or even 1.018V) reference and do a differential measurement between this reference and your Weston cells. Your HP3468A has a good high impedance 30mV range, this way about 10ppm accuracy would be possible and no need to move the cells or meters.CheersAlex

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 07:11:42 pm
content: Quote from: Alex Nikitin on January 08, 2017, 06:17:05 pmUnfortunately I don't have such a reference - except for the other Weston cellsThe differential between the two cells in the Muirhead is pleasingly small, but gives no extra information!

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 08, 2017, 09:04:15 pm
content: What about a 4.096V reference like the MAX6341? The output could be divided by four to get you close or you have the trim pin to get you closer. It is a buried zener design with low tempco. It should be able to run off a small battery for traveling to/from someone who can help measure it. I am not sure if a LTC1043 would be a good choice but parts cost here is ~$20US.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 08, 2017, 09:50:37 pm
content: Quote from: tggzzz on January 08, 2017, 07:11:42 pmI can make you one, if you are not in a hurry, perhaps closer to the end of this month.CheersAlex

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 08, 2017, 10:54:50 pm
content: FWIW, saturated cells have a high tempco and are always temperature controlled. Usually one doesn't worry too much about the accuracy of the setpoint, but will keep track of the variation. I think the JRL oven system also had a built in bridge so at the same time as a measurement or intercomparison was made, you could null the bridge and know the temperature variation. The voltage at a particular temperature would have been measured against NIST or other standards, not assumed or calculated by temperature.Any of the small portable cells like the Eppley 100 or very pretty Weston cells, are unsaturated and have a relatively low tempco, at least compared to the saturated ones. They'd be used as a bench standard but are only moderately good, and only if measured on a regular basis. Typically not temperature controlled. IMO, there are very few good ones left, as they have a shelf life of a few decades at best, though sometimes you get lucky!

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 10:56:04 pm
content: Quote from: ManateeMafia on January 08, 2017, 09:04:15 pmI try to avoid anything from MaximI already have an ADR4550 with nominally the same initial accuracy (0.02%) and 2ppm/C.Its voltage, after ageing for several weeks, is 5.0008/5.0009V, i.e. just within spec of 1mV. Dividing it down with a array resistor with a relative tolerance of 0.05% gives 2.50045V and 2.50059V (i.e. a total of 5.00104V).So without more digits, it is the two clocks problem

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 10:59:39 pm
content: Quote from: Alex Nikitin on January 08, 2017, 09:50:37 pmThanks for the generous offer. If there's anything more accurate than my (newly disclosed!) ADR4550 0.02% initial accuracy reference, then I might well be interested.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: January 08, 2017, 11:02:24 pm
content: I started off with similar measuring "issues". All it leads to is more gear, more references, and a lighter wallet. It's only a matter of time

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 08, 2017, 11:14:14 pm
content: Quote from: tggzzz on January 08, 2017, 10:59:39 pmI can calibrate a reference to about 10ppm accuracy on the HP3458A Opt 002. My plan was to make a small portable reference for exactly this kind of application, something to send out for a while. It should provide 10V and 1V DC, 10K and 1K. I have a couple of hermetic 10K+1.111111K Vishay BMF dividers and some 10V LT1021CM metal can buried zener references (other options are LM129 or LM399). I'll build one and test it for a while. If the reference is good and stable, I will be happy to send it to you for some weeks so you can measure the cells accurately.CheersAlex

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 11:17:45 pm
content: Quote from: Conrad Hoffman on January 08, 2017, 10:54:50 pmMakes sense.QuoteMy Cambridge Instruments "Weston Normal Cell" is 1.01858V at 17C. I suspect, but cannot be sure, that it is also a saturated cell. It does have an inbuilt mercury thermometer.I don't know its age; I only know the serial number. Although I acquired it at the same time as the Muirhead, there's no evidence it came from the same source (MEL for the Muirhead)

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 11:19:32 pm
content: Quote from: ManateeMafia on January 08, 2017, 11:02:24 pmHence my attempting, probably unsuccessfully, to use other people's equipment to calibrate my cells! Yup, I'm a chicken, and I will not become a voltnut. Honest.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 11:29:17 pm
content: Quote from: Alex Nikitin on January 08, 2017, 11:14:14 pmThat's very generous, and I would of course cover any costs.I'm glad that the Fluke 332B that's coming up for auction on the 26th January is too heavy (30kg) for me to want to liftA narrow escape indeed.

author: VintageNut
author_rank: Frequent Contributor
timestamp: January 08, 2017, 11:40:39 pm
content: Quote from: tggzzz on January 08, 2017, 11:19:32 pmMy Epley cell required over a month to settle down after it was shipped to me. Taking one measurement is just a start. Several measurements with some months between are required to establish a trend. This "somebody else" would have to visit you where your cell was sitting quietly for some time.When NIST used these cells, they lived in a quiet, temperature controlled environment undisturbed for many years. That is how these cells should be treated.

author: tggzzz
author_rank: Super Contributor
timestamp: January 08, 2017, 11:46:48 pm
content: Quote from: VintageNut on January 08, 2017, 11:40:39 pmSo I now understand!QuoteAccepted. Given the provenance, a shut-down school that hadstuffin random heaps from other schools, I'm amazed they even work at all!

author: HighVoltage
author_rank: Super Contributor
timestamp: January 09, 2017, 09:57:15 am
content: Quote from: tggzzz on January 08, 2017, 11:46:48 pmYes, you should be amazed, this is a real good thing.I bought a few of ebay and had them shipped to me and they do not work at all or have such low voltage that their life is gone.

author: Assafl
author_rank: Frequent Contributor
timestamp: January 09, 2017, 12:29:54 pm
content: While I cannot claim to understand the chemistry well enough, from reading the NBS booklets and Eppley lab stuff posted on this site, it becomes very clear that saturated cells may live an accurate life as long as there is an ample supply of ions in the liquid electrodes (hence - DO not squander coulombs).When they fail, it is mainly due to contamination and/or disintegration of parts (like the septa - AFAIK the septa is the seal or "sponge" that prevents the anion/cation liquid electrodes from mixing if the cell is inverted or tilted; the NBS ones used very pure materials and no septa - so tiliting would mix the liquids and kill them on the spot). The Muirhead assumedly has a "sintered" (sort of a foamed metal) septa which is claimed to allow it to work even after it being moved or jostled about. I am not a chemist though...What makes them so temp sensitive is that the saturation point is highly dependent on temp - so the supply of Ions changes in direct relation to temperature.Unsaturated cells, however, do not have an excess of Ions, and hence, as these are depleted, the voltage decreases.So I would summarize that finding an unsaturated cell in good working condition would be surprising, whereas finding a saturated should not be as surprising (especially if kept in storage, away from stuff shorting it...).

author: Assafl
author_rank: Frequent Contributor
timestamp: January 09, 2017, 12:46:43 pm
content: Quote from: tggzzz on January 08, 2017, 03:18:55 pmJust reread the thread and I was like you but this Jurassic battery has made me rethink that approach. The HP34401A is rock solid to the last digit at 100PLC and even at 10PLC the last digit at max oscillates between two successive digits. Even the extra 300nV mode via GPIB is useful albeit noisy. Note that I don't mean that I could assume accuracy and ignore the 10uV uncertainty of the calibration (at the 1.2V range) - but it does mean I can use the last digit to do comparative readings.Furthermore, if the digits reading is now jumping around (as with the Datel calibrators) I can now perfectly place the "blame" on the DUT being noisy or drifting about.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 09, 2017, 06:16:22 pm
content: I'm not a voltnut- I can stop any time I want. I just don't want to right now.

author: VintageNut
author_rank: Frequent Contributor
timestamp: January 09, 2017, 06:44:25 pm
content: Quote from: Assafl on January 09, 2017, 12:29:54 pmI bought a saturated Muirhead and it is dead at 0.X volts. My unsaturated Epley is aging gracefully and is at 1.0187XX volts. Its decay is mostly linear and its temperature sensitivity is low. Both were purchased from eBay.My thinking is that the Epley cells survive better in transit. If I had a source locally for the saturated cells that I could pick up and drive home, I would happily show up with a good multimeter and choose the working specimens.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 09, 2017, 07:56:36 pm
content: Quote from: tggzzz on January 08, 2017, 11:17:45 pmIs that a measured voltage or something written on the unit?An unsaturated cell (e. g. the Eppley 100) has a 'traditional voltage' of 1.0192x V. My two came from the factory at 1.01924 and 1.01925. I also have a couple of Muirhead D-550-A cells that have values of 1.01859V @ 20C printed on the label. The original documentation states that the tolerance is 0.01% and includes a temperature compensation chart. Over a temperature range of 10C to 40C, the voltage drops by 1.41 mV. The value of 1.018x and the temperature dependence are typical of a saturated cell, although that term isn't mentioned in the original documentation or in the Muirhead info posted earlier somewhere on eevblog.

author: tggzzz
author_rank: Super Contributor
timestamp: January 09, 2017, 09:24:13 pm
content: Quote from: edpalmer42 on January 09, 2017, 07:56:36 pmA measured voltage, which confirms the saturated suspicion.(Why hadn't I picked up on that specific unsaturated voltage?; I knew it was higher. Sigh)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 09, 2017, 09:56:31 pm
content: Change temp slowly- if the voltage changes about 40 ppm/C it's saturated. If it changes about 5 ppm/C it's unsaturated.The voltage on unsaturated cells can vary slightly, depending on if they're acidic cells and the concentration used.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 09, 2017, 10:30:26 pm
content: Quote from: tggzzz on January 09, 2017, 09:24:13 pmIt might not be saturated. You could have an unsaturated cell that's on its last legs. If the value had been printed like mine was, it would be more likely to be a saturated cell. It's very odd that the value at 17C just happens to be the same as a typical saturated cell - which is used in an oven at an elevated temperature.Conrad's temperature testing suggestion might tell you more. According to NBS (now NIST), as an unsat cell ages the temperature dependence typically increases so it's hard to say what value you'll measure. NBS Monograph 84, Section 7.1

author: tggzzz
author_rank: Super Contributor
timestamp: January 09, 2017, 11:05:05 pm
content: Quote from: edpalmer42 on January 09, 2017, 10:30:26 pmIt is a possibility, but I don't want to believe itThat's supported by it having an integral thermometer, which my Muirhead doesn't.Alsohttp://physicsmuseum.uq.edu.au/weston-normal-cellwhich is about something with the same name and manufacturer, says:"The Weston Standard cell was used as a standard of voltage in laboratories. Thety typically stabilised at around 1.018638 Volts. A number of cells of the same age and manufacture will agree wihin microVolts. Cells were checked annually and discarded when the EMF dropped to 1.0183 Volts. No current was normally drawn from the cell. The active components are a cathode of 10% Cadmium amalgam in Mercury, an anode of pure Mercury under a plug of Mercury and Cadmium Sulphates paste, all in a saturated solution of Cadmium Sulphate."

author: Assafl
author_rank: Frequent Contributor
timestamp: January 10, 2017, 08:52:39 pm
content: Lowimpedance was kind enough to share with us datasheets for many Muirhead models. See the following thread:https://www.eevblog.com/forum/metrology/muirhead-std-and-ref-cells-all-catalogue-info-uploaded-!-(ayce)/Since my cell did not come with a measurement sheet, the above thread is where I found out that my specific cell had a -64uV/C slope at the target working temp of 37.5C and what the voltage at that temp was. At that point I was able to validate that my cell was indeed functioning properly (I suspected it was not).Also - NBS Monograph 84 quoted by edpalmer42 is a great read.- AssafPS - I don't have an LTZ1000 in da house and am therefore not a volt nut (a volt not?).Edit: corrected the link.

author: tggzzz
author_rank: Super Contributor
timestamp: January 13, 2017, 12:37:47 pm
content: I've gone and done it again...I went back to my "source" and found asecondCambridge Instruments "Weston Normal Cell". This one has clearly been mistreated, since the integral thermometer has been broken. In addition, I think I've seen some of the crystals moving - but I've no intention of seeing if I can turn it into a toy snowstorm.So, does it work?The first Cambridge Instruments "Weston Normal Cell" a few days ago was 1.01858V at 17C. Today it is 1.01870 at 14.5C. Over-interpreting the readings gives a delta or 117ppm for a 2.5C temperature change. That corresponds with the 40ppm/C Conrad mentioned for saturated cells.The second cell is 1.01859V. Yesterday it was ~10C, so it it probably approaching 14.5C now. Wiring the two cells in inverse-series gives a delta of ~85-95uV, or 88ppm/0.0088%. (I wonder what it will be in a few days time).So, is the second cell working? Yes. Is it metrology grade? Schools' grade metrology or enough to calibrate a basic handheld DMM yes, otherwise "hell no"!Now I really don't need 4 cells (2 CI plus a dual Muirhead); having so many is greedy. So, what should I do with it? One possibility is to donate it to a local HackSpace, along with dire warnings about misuse and hazardous chemicals

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 13, 2017, 02:46:13 pm
content: That was my dilemma a year or three ago. I had about 6 Eppley 100 cells, a Weston and a fully functional JRL oven with saturated cells. None of the un-saturated cells were good enough to use as references, and the JRL oven bothered me because it ran so hot (37C) and used primitive thermoregulators with yet more mercury. What I did was remove all the un-saturated cells from their housings and turned them in as hazardous waste. The housings will make nice solid state standards, being thermally lagged with copper or aluminum and having good output posts. Heck, it would even be possible to ovenize one with little trouble. A discrete wall-wart jack for power could be installed on the back. Cringe if you want, especially given what they go for on eBay, but I gave the saturated cell bank to a fellow engineer and scrapped the JRL oven, saving only a few bits and pieces. Now the only mercury I have in the house are a few lab thermometers and two Fortin barometers in the garage. Not zero, but better than the several pounds contained in the standard cells. I'm not completely reference-less, having found three Fluke 731s surplus and an Analogic/Data Precision 8200 reference. I had a Fluke 332B, but that's more useful as a boat anchor than standard, so I gave it away. Way too heavy and took up too much bench space.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 31, 2017, 04:38:16 pm
content: Quote from: tggzzz on January 08, 2017, 11:29:17 pmIt is the end of the monthand perhaps you should drop me a PM with your coordinates in a week or so.CheersAlex