From the bin!-- weston cell +meas. data & lastly Muirhead Cat. data sheet - Page 1
Forum section: electronics / general

author: lowimpedance
author_rank: Super Contributor
timestamp: August 04, 2016, 06:59:48 am
content: While inspecting the dumpster for anything useful (as you do) I came across this unloved item from 'Muirhead'. (1960 vintage). Missing its cosy protective outer casing !.Quick test with an high impedance input DMM to minimize loading. Still a good reading ?? .....well who knows. Not about to give up on the somewhat more modern stuff for this old timer.Any way with the nasties contained inside it should be disposed of a little more carefully next time!.Final edit : I have added the Muirhead catalogue data sales sheet at the end of the thread.

author: Kilrah
author_rank: Unknown
timestamp: August 04, 2016, 05:53:56 pm
content: Someone who's found that thing in a box in the attic and has no clue about it.

author: edpalmer42
author_rank: Super Contributor
timestamp: August 04, 2016, 06:14:03 pm
content: FYI, here's more than you ever wanted to know about Standard Cells:http://digicoll.manoa.hawaii.edu/techreports/PDF/NBS84.pdfAccording to this document, an unsaturated cell has a life of 23 to 37 years under perfect conditions or 12 to 18 years under realistic conditions. If that '60' that I see is the year of manufacture, this thing is long past its best before date. They say that the minimum voltage is 1.01830 so you've got some room there.You should check the internal resistance of the cell. It should be in the 100 - 500 ohm range. The other thing that happens with age is that the temperature characteristics degrade. I've got a couple of Eppley unsaturated cells. They aren't useful anymore because the voltage isn't stable even at a constant temperature.Ed

author: Sbampato12
author_rank: Regular Contributor
timestamp: August 04, 2016, 07:00:45 pm
content: Quote from: edpalmer42 on August 04, 2016, 06:14:03 pmI have one from Eppley also. But I did read somewhere that we don't should connect direct on DMM inputs, I'm not sure about that.

author: Vgkid
author_rank: Super Contributor
timestamp: August 04, 2016, 09:55:43 pm
content: Surprised that it survived intact. Even more that it appears to be working correctly.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 05, 2016, 03:32:47 am
content: It was in the work bin and probably from an office that was cleaned out. Why and when it was removed from its casing I will never know.Took another reading after letting it settle overnight in the lab, (21 deg C), and it seems to be stable around 1.018560 V. (given my.. umm.. excellent low thermal connections)The date is 26/04/1960 so not bad for a 56 year old (older than me). I cant recall seeing many of these things in my time as they were quickly replaced with the cropico and other electronic standards.Even the cropico's were long ago retired and tossed, (see my other thread!) . Lets just say there are 'suitable' alternatives available.POSTSCRIPT :While I was having a quick look around to see if any other cells found their way to the back of a cupborad somewhere I did find a colleaque/collector of fine antiquity like me had a duff CAMBRIDGE INST. cell still in a case and kindly offerred the casing for the MUIRHEAD.The case is two cylinders separated with insulating wool for thermal lag, and usually the cell sits in oil too (I wont bother with the oil though).A hole in the top cap is for a mercury thermometer.So after a little bit of fitting see the pics of its new home. Now this old weston cell will stay in my lab at least till I'm gone anyway.Will measure after the weekend allowing it to settle again after all the handling.

author: TiN
author_rank: Super Contributor
timestamp: August 05, 2016, 04:44:33 am
content: I have one from Eppley tooPerhaps can rig it up too so we can "compare"? Have blue CAT5 cable as well.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 19, 2016, 01:26:31 am
content: Just to update the post with some measurement data of the absolute value versus the (work!) Lab temperature over approx. a week.Note the air con was being upgraded and the temp control was still being adjusted which shows at the start of the samples which was convenient to show the effect of ambient on the Cell.3458a at default startup conditions. Samples data logged with pencil and paper when I thought about itThe temp meter used was a PRT type panel meter with only 0.1 deg resolution. Also I moved the temp meter away from the cell for the measuremnts and placed some bubble wrap around the Cell and DMM terminals to reduce air currents.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 22, 2016, 08:06:16 pm
content: I wrote up a page from various sources a while back when I still had standard cells-http://www.conradhoffman.com/stdcell.htmIMO, all unsaturated cells are now junk, as they haven't been legal to manufacture in several decades. All my Eppleys eventually went unstable and couldn't be trusted. You'll have to record data for a couple months to be sure. Not so the saturated cells, though they need a temperature controlled oven and should never be inverted. They don't travel well. Ultimately I got rid of those too, as I didn't like having that much mercury around.The best thing to do with old Eppley and Weston unsaturated cells is take the nice thermally lagged case, send the guts to the hazardous waste collector, and install a nice solid state reference. Then you have something that looks cool and is still useful.CH

author: lowimpedance
author_rank: Super Contributor
timestamp: August 23, 2016, 12:08:17 am
content: Thanks Conrad, I actually did come accross your page as one of the first when searching for information and was very useful so thanks for that.And yes this cell is only for curiosity now and as you say the nice thermally lagged enclosure is most likely the most useful part!!.I also posted a thread having a look inside an electronic replacement from Cropico which is ironically another dumpster recovery.https://www.eevblog.com/forum/metrology/cropico-esc1-electronic-standard-cell-a-look-inside/I have not had a chance to repair it and then log some data.......yet.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 23, 2016, 07:32:17 am
content: Very nice find & assembly.Anyhow, the standard value given in 'international Volt' has to be adapted to SI-Volt, or to 'absolute Volt', as it was called, then.(In former times, there existed also the differing NBS - Volt, see the NBS document)I found in my old documents following relationship for the output of a saturated cell at 20°C:1.018300 Vint = 1.018646 Vabs (SI)The first order T.C. of about -39.9 ppm/K (= -40.6 / 1.0183 ppm/K) matches your measurements.Your Weston cell is still in very good shape, as it differs about -30ppm from the standard value @ 20°C only.A negative drift over time is also typical; yours would be -0.5ppm/yr., if it had exactly the nominal value in 1960 (which was probably not the case, anyhow)Frank

author: jesuscf
author_rank: Frequent Contributor
timestamp: August 24, 2016, 09:48:33 pm
content: Quote from: Conrad Hoffman on August 22, 2016, 08:06:16 pmAny way to recover the platinum wire?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 25, 2016, 11:25:51 pm
content: I'm not sure on the wire. The ones I've seen keep it very short, with a splice, and you'd have to break the glass, risking a mercury and cadmium spill. IMO, not worth any possible gain. The thing I found so appealing about standard cells is they're the only "easy" method to create a voltage standard without access to calibrated comparison standards. Even that's not true unless you can purify the materials and happen to be a chemist, but it's the thought that counts! You'll do better with solid state references, rather than unsaturated cells. Maybe not true with saturated cells, but they're really a pain to maintain at perfect temperature and not damage them during use.

author: lowimpedance
author_rank: Super Contributor
timestamp: September 14, 2016, 05:55:44 am
content: While trawling through some old trade catalogues in a filing cabinent I came across some Muirhead catalogues and wouldn't you believe it the cell I found is listed with all the specs.And looking at the data measured and comparing to the catalogue specs. the cell from the bin is still in excellent shape !. Also the suffix letter indicates this cell was supplied without a housing (just as found in the bin!!).So attached is the scanned data/sales sheet from the catalogue to complete the story.

author: Gyro
author_rank: Super Contributor
timestamp: September 15, 2016, 04:53:19 pm
content: Now that you have probably put all the catalogues away again, any chance that there is a page on the Muirhead K-375-C standard cell? I've noticed that there are a few floating around on ebay. They seem to be a smaller, single tube design and I'm wondering how they did it. More curiosity than anything.

author: VintageNut
author_rank: Frequent Contributor
timestamp: September 16, 2016, 07:03:50 am
content: My cursory reading about weston cells is that you do not want to source any current from the cell. Even a very high impedance DMM will draw some current.The cell is supposed to be measured deferentially. The way I measure my Eppley cell is with a 10V Fluke 731B, a Kelvin Varley divider and an ordinary DMM as the null meter. The DMM is a 6 1/2 digit KE2000 on the 100.0000 mV range. It can resolve 0.1 uV.My Eppley cell is from 1975. The most recent voltage measurement is 1.018731 in June of this year. A faded sticker on the cell states that the cell was 1.01918 in 1985. It drops 1 to 2 uV per month. I have had this cell for 1 year.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 16, 2016, 07:22:53 am
content: With a high impedance meter like the K2000 it does not make a big difference whether you do a normal measurement or do it differentially. The input current is something like less than +-50 pA plus an parallel resistance of more than 10 Gohm (often way more), which would be less than 100 pA at 1V. So one can not even be sure about the sign of the current at less than 1-2 V. Currents in the pA range should not be a real problem - when the standard cells where up to date, the voltmeter currents where more in the µA to nA range. If you are that paranoid about currents, keep the cell really dry ( < 40% RH) to reduce surface leakage at the cell itself.It would be a good Idea to check the input current before, as leakage can be higher if contaminated.