ESI SR1 standard resistors - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: January 19, 2023, 06:07:28 am
content: Hi all,Just looking to stir a bit of conversation over these resistors, which are available very affordably out there for the metrology turbo-hobbyist (the only kind).I think they have accrued quite a bit of bad rap, from what I'm seeing based upon a critical post or two, and I'm just not sure I'm convinced this reputation is deserved.I've recently acquired a set from a Tektronix lab that (apparently) closed shop relatively recently and they were measured within a few, very few, ppms of nominal in the easy values. They're also obviously well aged, and btw, aging served them very well if I'm going by the values measured by the lab. Just about uncannily well.It's also what I'm seeing in my environment - they're within ppm of nominal (up to 20-25, depending on value) - again, almost uncannily so. By spec, tempco is 5ppm or less - again, very good.I got another one since the original batch (to get a complete set of values) from a large US instrumentation retailer - for a song and a dance, even if carrying a verification sticker by the retailer from 2020 - and it also seems to be pretty much dead on. More nefarious impact from bad reputation, if you ask me. Not that I'd mind getting these for such little dough.All this is measured with a calibrated 8502A, for anyone wondering. Checked with a "noncalibrated" (but exact, afaict) 8840A which confirms its readings.Why the bad name for these? Is this what others have seen with these ESIs? I'd be very surprised, but maybe I just got very lucky with my set.

author: DavidKo
author_rank: Frequent Contributor
timestamp: January 19, 2023, 08:25:18 am
content: The ones, that I have, are quite "used" and only one have both plastic bottom nuts OK. Some of them are only hold by the GND nut. Without measurement I cannot confirm if the temperature PPM is OK or not.From the mechanical point of view, the nuts are serious flaw of these units.

author: Rax
author_rank: Super Contributor
timestamp: January 19, 2023, 03:34:21 pm
content: I actually found a datasheet for the current production ESI SR1 (the one listed on the IET website for $735-$1656) that states it can be sold in "optional esi-style configuration" - which is exactly how the original resistors were sold.This tells me the original ESIs and the current production ones made by IET are not so radically different from each other.

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 04:35:50 pm
content: Quote from: Rax on January 19, 2023, 06:07:28 amWell, since you asked, criticisms of these could include:1) Their small size, which means they're easy to drop on the floor. I haven't seen all that many of these, but quite a few have been in very poor physical condition.2) They appear to not be truly kelvin-connected.3) They are arguably less robust both physically and environmentally because they use a manganin wire wound on a mica card (at least the lower values do) without any hermetic sealing or significant physical support.4) Since they came in at least two different paint jobs, if you try to assemble a set with individual eBay purchases you may not get a nice-looking matched set. The horror!Of course the L&N NBS-style models are also not truly kelvin-connected and also not hermetically sealed, and although the internals are oil-bathed and pretty well protected if the top is tight, the external terminals are very susceptible to the environment.I imagine the overall population of these SR1 resistors is probably doing pretty well and it would be a mistake to judge the product line solely by looking at examples that are for sale on eBay or wherever. The ones that have lived a peaceful life in a dry lab are probably all fine. Unless the clowns that work there have dropped them on the floor.

author: Rax
author_rank: Super Contributor
timestamp: January 19, 2023, 04:53:43 pm
content: Quote from: bdunham7 on January 19, 2023, 04:35:50 pmI've seen this stated before, and I'm not sure I understand it. They do have two sets of connectors - top and bottom - and, as far as I understand, a Kelvin connection essentially means two separate leads all the way (internal to the fixture) to the actual points of connection at the resistive element. If the resistive element is connected evenly to the top and bottom of the SR1, would that not qualify as a Kelvin connection?

author: bdunham7
author_rank: Super Contributor
timestamp: January 19, 2023, 05:21:09 pm
content: Quote from: Rax on January 19, 2023, 04:53:43 pmIt very well might and I forgot to include my intended disclaimer that I might be wrong about the kelvin issue. However, this photo shows that the two terminals are not independently connected directly to the manganin, there is a short, thick, presumably copper, conductor in between and then a tinned brass or copper strip on the mica. Those elements are all common to the source and sense circuits and probably amount to hundreds of µohms.https://www.eevblog.com/forum/metrology/teardown-standard-resistors/?action=dlattach;attach=207316;image

author: Kosmic
author_rank: Super Contributor
timestamp: January 19, 2023, 05:42:15 pm
content: Interesting post from Edwin G. Pettis about the SR-1.https://www.eevblog.com/forum/metrology/esi-sr1-standard-resistor-current-rating/msg1256635/#msg1256635Quote from: Edwin G. Pettis on July 15, 2017, 12:52:29 amAnd you can find the definition of a working standard over here:https://encyclopedia2.thefreedictionary.com/Working+StandardsQuoteSo a lot of those working standard had a rough life going all around a manufacture for example used to calibrate and adjust equipments and instruments.Personally I think SR-1 are, most of the time, overpriced for what they are. I had better luck collecting SR1010, which is a transfert standard.

author: alligatorblues
author_rank: Regular Contributor
timestamp: January 20, 2023, 04:45:44 am
content: I have (2) SR104s. Those are pretty good. I had one calibrated by Keysight, and it had drfted <3ppm in 47 years.

author: Rax
author_rank: Super Contributor
timestamp: January 20, 2023, 05:14:09 am
content: Quote from: alligatorblues on January 20, 2023, 04:45:44 amThis is about SR1s, not SR104s... But even so (or even more so), one of my resistors drifted 2ppm in a few decades too, another one less than 4ppm (going by lab calibration sticker). I'm cursorily measuring them at even less delta from nominal than that (the entire range of resistors - 1 ohm through 10 Mohms), depending on the fluctuations I'm getting. I do have to watch the temperature, but that probably throws all my instruments out of whack when not proper. Given what I paid for them - which is quite a bit less than the typical ask you may see on the usual marketplaces - I think they're a steal.I'm not making them into primary lab standards, but for a hobbyist bench, they may be exactly what one needs and honestly much more.

author: bdunham7
author_rank: Super Contributor
timestamp: January 20, 2023, 05:31:19 am
content: Quote from: Kosmic on January 19, 2023, 05:42:15 pmI've looked at those, but it adds upt as you need a lot of SR1010s and all the accessories to cover a wide range. OTOH, you can get all the intermediate values like a Dekabox only less convenient and more precise. xDevs did a writeup on the SR1010 and the one he evaluated didn't do any better than the SR1s that I've seen. YMMV, but I think an unsmashed SR1 is probably close to as good as you get short of a primary standard like the SR104. What else is there? As far as overpriced, the trick is not to pay too much!

author: Kosmic
author_rank: Super Contributor
timestamp: January 20, 2023, 02:54:19 pm
content: Quote from: bdunham7 on January 20, 2023, 05:31:19 amI don't think we should talk about the SR104 here since it's clearly in another league.These days, putting a good Vishay resistor in a box would probably give better results than an old SR-1.

author: bdunham7
author_rank: Super Contributor
timestamp: January 20, 2023, 03:23:27 pm
content: Quote from: Kosmic on January 20, 2023, 02:54:19 pmYes, that's an idea and one I'm going to do for a 1R shunt/standard but I'm using a different resistor. The problems with going the Vishay Z-foil + box are that you have to make sure you get one that is environmentally protected (not the audiophool versions), you end up with lower current limits and it ends up costing more for a $35 resistor, four $15 binding posts and a box than for a decent SR1 on eBay. And then it still isn't truly kelvin-connected and that may matter for the 1R to 100R values.

author: Rax
author_rank: Super Contributor
timestamp: January 20, 2023, 04:21:30 pm
content: Quote from: Kosmic on January 20, 2023, 02:54:19 pmMy point is that you wouldn't (I mean cost vs. performance, at least). You'd need to go to a VHP101 which, by itself is - at least in the US - about $120. It's also a 50ppm part (from nominal). By the time you put it in a box (aluminum? some thoughtful temperature conditioning provisions?), get four or five good quality, low EMF binding posts, do proper wiring inside, you're well outside what the SR1s are going for (actual sale price - one can be had for less than $50). I am intently not including temperature probing provisions in this, as obviously the SR1s don't have that either.And for that cost you'd still be pretty far from the 2-4ppm from nominal than (at least some of) these seem to be at (the unabused ones). Tempco may add to this, but I think that's more of a non-issue this day and age.

author: Rax
author_rank: Super Contributor
timestamp: January 20, 2023, 04:41:28 pm
content: Quote from: bdunham7 on January 19, 2023, 05:21:09 pmOn their mechanical robustness - I think the image above may indicate why, although working standards and therefore prone to abuse and manhandling, they seem to have mostly withstood that with panache.The mica card + the unifilar winding likely have very little weight which together with the dimensional rotational arm would create relatively little angular momentum stressing the assembly in case of impact (against a very thick conductor that has a very strong mechanical commandment over the resistive assembly, opposing the angular momentum).