Weston Cells - Page 1
Forum section: electronics / general

author: JamesBee
author_rank: Newbie
timestamp: February 25, 2022, 08:30:36 pm
content: Hello, my first post.Recently purchased some electronic parts on Facebook and received these items in the photos.I didn't have any knowledge until I looked up but can no information on the "admiralty pattern" device.It seems to consist of a row of cells in series, a group of precision resistors, the left hand block are all 10k, and a device on the right of which I have no idea.Can anyone offer any insights.There were also several loose cells, I measured one at 1.0185 volts.A little too complicated for my first post!

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2022, 08:45:09 pm
content: Welcome,Are they what you were expecting to receive, or just thrown into the mix?Firstly you should know that the contents are highly toxic - MercuryandCadmium so if you don't have the skills, or intention, of using them then they need to be disposed of at the appropriate disposal facility.Muirhead D-845s are Saturated Standard cells, so they don't significantly degrade over time like unsaturated ones. Their lifetime is based on the finite energy drawn from them. Assuming that they haven't been exhausted, shorted even for a few seconds, or measured on a low impedance meter, they should still be ok.I've attached the datasheet (courtesy of another member)...EDIT: As you've added more photos, the main bank of cells appear to be D-550-As. I have attached the datasheet for those too...

author: alm
author_rank: Super Contributor
timestamp: February 25, 2022, 08:56:21 pm
content: A 10 MOhm DMM is probably already borderline re impedance, so I'd only measure it using a DMM with an impedance in the GOhm range. 5.5+ digit bench meters will often have a high impedance mode up to 3V or 10V. See the manual or datasheet.

author: JamesBee
author_rank: Newbie
timestamp: February 25, 2022, 09:16:45 pm
content: Many thanks for the quick replies, much appreciated.I have no test gear with that spec, though this very week I came by a Fluke 883A Differential Voltmeter, but a quick test with both 4 1/2 meters produced identical readings of 1.0185 volts on one of the free cells.The resistors in the block seem to be in spec, all exactly 10k.Better photo of the device.

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2022, 09:21:46 pm
content: That was lucky! Differential voltmeters, and other types of bridge measurement devices that have infinite impedance at balance, are indeed the traditional method of measuring and comparing cells.It looks as if you have quite a cell bank there!P.S. Note that the temperature coefficient of Saturated cells is relatively high - you need to take ambient temperature into account for precision measurements. The formula is in the datasheets. They will probably take a while to fully settle after transport too.

author: JamesBee
author_rank: Newbie
timestamp: February 25, 2022, 09:40:11 pm
content: The cells I got last night, the voltmeter on Sunday last though I've not powered up yet, busy week and no US lead that fits though I do have a Step up/down Xfmr.A quick inspection revealed no obvious problems and it seems to be as originally manufactured. I can't find a spec on power consumption of the meter.Temperature in this room where the cells are currently 19.8C and will probably not vary by much on any one day. That's lower than their calibration figure I'm sure.

author: Gyro
author_rank: Super Contributor
timestamp: February 25, 2022, 09:47:26 pm
content: One other thing to try. The standard way of comparing cells is to connect them back to back (negative to negative) and measure the voltage between the positive leads on your lowest measurement range. That should help you to weed out any cells that are suspect - again, assuming that they have stabilised to the same temperature.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 27, 2022, 05:43:00 pm
content: A few years ago I was lucky to have found a brand new (Never used) Muirhead cell.It even came with a certificate of test.In 1971 the measured voltage was 1.01864 V and when I measured it first, it was 1.01858 V

author: tggzzz
author_rank: Super Contributor
timestamp: February 27, 2022, 06:48:28 pm
content: Quote from: HighVoltage on February 27, 2022, 05:43:00 pmWell, 60µV is only 1.5ºC temperature change.https://www.eevblog.com/forum/repair/solartron7061/msg3008786/#msg3008786Didn't the volt definition change by 9ppm since 1971?

author: edpalmer42
author_rank: Super Contributor
timestamp: February 27, 2022, 08:28:35 pm
content: I'm surprised that nobody has linked in this document.NBS Monograph 84Standard Cells - Their Construction, Maintenance, and Characteristicshttps://nvlpubs.nist.gov/nistpubs/Legacy/MONO/nbsmonograph84.pdfEverything you ever wanted to know about the care and feeding of your Standard Cells.Ed

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 31, 2022, 05:41:18 pm
content: Hi guys,I own Guidline voltage standard built as 12 weston cells inside the oven.I have it with some historical data. Each cell was tested for internal resistance and the values are close to 600 ohm as they should be.However i don't find this setup useful .I thought about connecting 10 of cells in series and dividing down to 10.00000V.Is this idea reasonable?Is there a better opamp for this purpose than OPA140 or ADA4522 or older LT1012 (i'm talking about the first stage) ?I'm curious what do you think about my idea.

author: Andreas
author_rank: Super Contributor
timestamp: March 31, 2022, 09:07:34 pm
content: Hello,I would use a low input bias current OP like the ICL7650 / LTC1050 or something like that as first stage.I think for power off protection you will need a N-Channel MosFet like BSS138 or similar.with best regardsAndreas

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 31, 2022, 10:35:28 pm
content: Andreas,thanks for the suggestion.I was referring to a well known schematic from LT1012 datasheet with PMOS- without thinking too much.Is it just a drawing error ? IMO the gate should be tied to -15V.Your NMOS solution will work indeed if I add some capacitor to ground at the buffer's input.BR

author: guenthert
author_rank: Frequent Contributor
timestamp: April 01, 2022, 02:57:09 am
content: Quote from: doktor pyta on March 31, 2022, 05:41:18 pmUh, why? Consider yourself blessed to have access to a stable (saturatedweston cells should still be superior to all solid-state references), virtually noise-free reference. If you need 10.00000V, get yourself a lead-acid battery and a reference divider.

author: Andreas
author_rank: Super Contributor
timestamp: April 01, 2022, 06:46:16 am
content: Quote from: doktor pyta on March 31, 2022, 10:35:28 pmHello,have a look at the LT1097 datasheet.There it is a 4 terminal depletion Fet.But it seems it is obsolete now.with best regardsAndreas

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2022, 07:05:13 am
content: Quote from: guenthert on April 01, 2022, 02:57:09 amSuperior in some senses, not in others. It is true that a saturated cell will be better when measuring the noise of the DMM's reference, and when measuring temperature.I've used my Solarton 7081 with two Trancell zener sources and a saturated Weston standard cell. Note the different vertical scales, and the 1500 reading is about 24 hours.Trancell with 1.5ppm/15µV popcorn noiseTrancell without popcorn noise:Saturated Weston standard cell acting as a thermometer, but without noise:

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 01, 2022, 09:14:35 am
content: @guenthertthe rationale for adding the buffer to 10 Weston cells is to:1. provide low output impedance2. provide protection against shorting the cells. It happens and it has bad effect.I know that main advantage of my ovenized setup is the low noise.Scaling 10.18 to 10.00000 should not introduce much of error /drift/ tempco.According to the calibration reports my set of Weston cells drifts about 1ppm/ year.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 01, 2022, 09:25:22 am
content: @AndreasGood point.The same application is in the LT1008 datasheet.So it is not a mistake but a depletion MOSFET.BR

author: tggzzz
author_rank: Super Contributor
timestamp: April 01, 2022, 10:47:54 am
content: Quote from: doktor pyta on April 01, 2022, 09:14:35 amAnd 40ppm/K

author: HighVoltage
author_rank: Super Contributor
timestamp: April 01, 2022, 10:52:00 am
content: Quote from: doktor pyta on March 31, 2022, 05:41:18 pmInteresting project!How did you test the cell's internal resistance without loading them up?Also, what temperature do you keep the cell at?

author: Andreas
author_rank: Super Contributor
timestamp: April 01, 2022, 11:09:25 am
content: Quote from: doktor pyta on April 01, 2022, 09:25:22 amHello,its getting more and more mysterious.according to page here it is a dual p-channel J-Fet:https://www.darisusgmbh.de/shop/product_info.php/info/p8578_2N3609-----P-Kanal--Si-Feldeffekt-Transistor--350mW.htmlmy ECA-List lists the 2N4352 as replacement typeThe replacement type should be a P-Channel MOSFET with substrate connectionhttp://www.calogic.net/wp-content/uploads/2019/03/2N4352_Datasheet_Rev_A.pdfI for my part will stay with the logic level N-Channel BSS138 and the gate tied to a higher voltage than input voltage + ~2V.with best regardsAndreas

author: MegaVolt
author_rank: Frequent Contributor
timestamp: April 01, 2022, 11:16:12 am
content: It is important to consider the resistance of a closed transistor. It should be comparable to the resistance allowed for the cell. 1GΩ is 1nA of leakage. It may be worth considering analog switches rather than individual transistors.

author: Gyro
author_rank: Super Contributor
timestamp: April 01, 2022, 11:28:14 am
content: Surely an analogue switch would load down the output of the cell(s) when the supply is removed.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 01, 2022, 01:27:47 pm
content: @HighVoltageThe procedure is measuring voltage across the Weston cell with and without 10Meg loading resistor. The loading phase should last few seconds and the cell will recover within hours without any harm.The temperature is held at 30'C +/-0.1'C. Which not good for me because in summer months temperature in my lab is higher. But I can live with that.It is interesting how Guildline made very stable oven using 5 transistors. It is a LC oscillator which operates at the edge of oscillation. The loop gain is determined by the divider made out of NTC and hermetically sealed resistor. See the attachment.@tggzzzfortunately it is ovenized

author: Andreas
author_rank: Super Contributor
timestamp: April 01, 2022, 05:39:38 pm
content: Quote from: MegaVolt on April 01, 2022, 11:16:12 amHmm,I quickly measured a BSS138 (which I use to protect unbuffered LTZ1000 references).With ~14V battery and in power off state I get ~19 mV across the 10 Meg input resistance of the 34401A.So around 2 nA at room temperature.So for a Weston cell I would perhaps try a BSS123 which is specced with a magnitude lower leakage current.With best regardsAndreas

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 01, 2022, 05:56:45 pm
content: An analog switch is rather not an option due to ESD input diodes. That's why LT uses discrete transistor.

author: guenthert
author_rank: Frequent Contributor
timestamp: April 01, 2022, 10:51:19 pm
content: Quote from: doktor pyta on April 01, 2022, 09:14:35 amWell, while I can't really argue against the argument of protection, as I'm exactly that clumsy guy who'd cause such an accident, the use with a reference divider would limit the chance of such.https://www.ebay.com/itm/325039540790?epid=1501497714&hash=item4badddca36:g:kcIAAOSwNHRh9xba&shqty=1&isGTR=1#shId(not affiliated)At that price I was thinking of buying it myself, but then, I don't have a standard cell.

author: 1audio
author_rank: Frequent Contributor
timestamp: April 03, 2022, 07:48:32 pm
content: Why not a good relay running from the same power as the buffer? Off would be limited by contamination. Only connects when the opamp is under power.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: April 04, 2022, 09:46:34 pm
content: I have concerns about using a typical non-latching relay and coil heating the contacts. So rather latching relay or small signal SSR like AQY221N2M.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 05, 2022, 06:33:29 am
content: A latching relay would need extra measures to make sure it really turns off relialy when the power shuts down. A photo-mos SSR could indeed be a viable choice, though I would not consider the metioned type, as there is no need for a low R_on, more like get a type for low leakage and slightly higher voltage rating.

author: ramon
author_rank: Regular Contributor
timestamp: April 05, 2022, 06:53:37 am
content: Which ones has better specs?

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2022, 07:21:28 pm
content: On the face of it, the AQY221N2M as it appears to have the lowest typical leakage at the highest voltage, although they all seem to match by the time you get below 5V... However the CRITICAL spec is the 10nA Maximum. There is little information there, so this might apply at the upper end of the operating temperature range, but might not.It looks as if you can get the manufacturer to select for <1nA maximum leakage, but your best bet would probably be to purchase a few standard parts and hand select yourself for the lowest leakage at ambient temperature and target voltage.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2022, 07:36:50 pm
content: Quote from: 1audio on April 03, 2022, 07:48:32 pmThis application is pretty muchthedefinition of dry switching. Very few contact materials could be relied upon to provide reliable contact closure. About the only type of relay that would be guaranteed to provide reliable switching would be a Mercury wetted type. Of course these aren't manufactured any more due to the toxic content, but are still available surplus on ebay.Given the toxic content of the Standard cell itself, a Mercury wetted relay would be rather appropriate. The only really serious issue would be that the relay would have to be maintained vertical to avoid accidental closure (a few are position independent), however you would probably want to avoid inverting the standard cell anyway to avoid shifts in voltage.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 05, 2022, 07:42:46 pm
content: The actual leakage is usually way lower than the maximum specs. A weston cell amplifer is more like a one off part, so testing the part before use should not be an issue.The temperature effect is documented for quite a few parts: expect the leakage current to go up about exponentially.The specs for the usual small MOSFETs are somewhat similar: the limit on the leakage even tends to be worse due to being more price sensitive parts.

author: TimFox
author_rank: Super Contributor
timestamp: April 05, 2022, 08:40:03 pm
content: Yes, the formerly-available Hg-wetted contact relays are great for dry switching; I use them for low-level audio.Be careful about the orientation however, which was mentioned above.If you tilt the unit too far from the indicated direction, the mercury will flow internally and short out all the contacts.This reverses quickly when you re-orient the relay to near the correct direction.Of course, you are not supposed to tilt a standard cell, either.

author: Gyro
author_rank: Super Contributor
timestamp: April 05, 2022, 08:47:38 pm
content: QuoteMe too.