Reference/standard resistance DIY project (?...) - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: December 18, 2022, 01:33:58 am
content: Hi all,I searched the archives somewhat cursorily, so apologies if this has been explored in depth elsewhere (but please steer me the right way if it has, I can't find it though) - I'm finding it surprising there aren't a dozen stickies on this particular topic at the top of this category - but I've been considering building myself some sort of standard resistance kit for checking instrumentation. For reference, I currently top at 6.5 digits with my meters, but there may be more than that in my future.I posted on this elsewhere and Dr Frank (I believe he hangs out here) recommended the VHP series for the resistive element. I've been looking at the Z foil series prior to that (.005% and .2ppm). The price of the first is about $120 vs about $30 for the second, which may mean I can have several values of the Z foil for the cost of one VHP. Useful?...Another cost consideration - related to the other aspect above - is the binding posts which add about $13 if the Pomona 3770 series. So if I go multiple values the binding posts will add up quite quickly, particularly if going Kelvin on some of the values.Aluminum enclosure seems another good idea.Looking forward to thoughts and input - thank you!

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: December 18, 2022, 05:21:07 pm
content: Hi, welcome to the metrology section of the forum.There is a lot of information in the stickie at the top. Look at the section on resistance standards.Yes, buying commercial 0.005% resistors is a good way to go. Personally, I think VHP versus Z201T primarily gets you stability over time. Most of us (read *cheap*) just check multiple "standards" one against another to detect drift. Beware that the TCR of z-foil is not 0.2ppm it is more like 2-5ppm if you read closely and check actual real-world measurements. But, TCR is not highest priority for your stated purpose.Getting better than 0.005% is harder. I have ordered a 10K calibration standard from Texas components. I will review it when it arrives--it is NIST-traceable supposedly. That is $245.Hopefully, you have multiple 6.5 digit meters to check one against another.Maybe you should get a 7.5 digit meter?As for binding posts, you can usually find "cheapish" gold-plated posts from various sources if you do not want to pony up for the pomona ones. They seem to be the least expensive "guaranteed" source. Perhaps I am wrong (corrections will fly if so) but I think thermal emf is also a secondary concern since you can deal with it, in the case of a resistance measurement, by being patient and having a more or less constant temperature. It would seem tertiary as compared to the other issues already discussed. Also most good DMMs have offset compensation;https://xdevs.com/doc/Keithley/Appnotes/offsetCompensation.pdfFinally, I hope to make available some well-known resistances to USA cal club members in the near future. You could sign up for that, as well. They will be in the 0.005% range.Best,Randall

author: Rax
author_rank: Super Contributor
timestamp: December 19, 2022, 10:35:29 pm
content: Thank you very much, Randall. I'm getting a Z201 this week and can't wait to check it out a bit.While I'm also exploring out of the box solutions out there (such as the AB resistance standards, for instance), I've ran into this:https://www.ebay.com/itm/281749689480?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=2OEfdjvLQOG&sssrc=2349624&ssuid=gzj5kmvGQ3K&var=&widget_ver=artemis&media=COPY.I wonder what this community's thinking is on those - how they compare to other similar resistors (such as Leeds and Northrup types).One thing that jumped at me is that, even if they're placed in these massive enclosures, they're not immersed in an oil bath. Rather they seem to be in full free air under the case.

author: bsw_m
author_rank: Frequent Contributor
timestamp: December 19, 2022, 10:50:06 pm
content: Quote from: Rax on December 19, 2022, 10:35:29 pmThese resistance standards have good long-term stability, but a high TCR.If you are able to measure their actual resistance value after purchase, and you have a precision thermostat, these standards may be a good choice.

author: Villain
author_rank: Regular Contributor
timestamp: December 19, 2022, 11:16:08 pm
content: Quote from: bsw_m on December 19, 2022, 10:50:06 pmOr just put them in oil bath as intended

author: bsw_m
author_rank: Frequent Contributor
timestamp: December 20, 2022, 07:06:12 pm
content: To get a better understanding, here is an example of an individual passport for one of my resistance standards. The true resistance value, alpha-beta coefficients for each standard are given in its individual data sheet. In the absence of an individual passport, all of these values will need to be measured by yourself.P.S. I do not use these standards in any exact measurements, so I have not measured the true value of their resistance to date. I have these resistance standards more as a collection.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: December 20, 2022, 07:24:03 pm
content: Quote from: Rax on December 19, 2022, 10:35:29 pmWhat are "such as the AB resistance standards, for instance"?I don't see these (ebay standards) as cost-effective. As compared to purchasing a 0.005%Z201 they are more expensive and have no provenance. To me, these standards need to come calibrated in some way. For example the TXSFR I mentioned above comes with a NIST-traceable certificate. All such certification expires: so the fact that a standard was once calibrated in the past is meaningless.In your situation something like this makes sense perhaps:https://www.precisionresistor.com/MC-7-Calibrator.html

author: alm
author_rank: Super Contributor
timestamp: December 20, 2022, 07:25:59 pm
content: Quote from: bsw_m on December 19, 2022, 10:50:06 pmDo you have any insight in the sample variation in the long-sterm stability? There are some commentsherethat the Soviet-era standard resistors exhibit a large sample variation.

author: bsw_m
author_rank: Frequent Contributor
timestamp: December 20, 2022, 08:19:33 pm
content: Quote from: alm on December 20, 2022, 07:25:59 pmI have not observed my own standards of resistance of this class, I am not interested. Since there are much more stable resistance standards.As I have heard, the range of long-term stability of such resistance standards can be from 1ppm to 10ppm per year.In general, if you look at the results of measurements in the central metrology database of the Russian Federation, these values are confirmed. As an example, randomly chosen measurement results of one of the resistance standards. Annual stability in the last column.

author: alm
author_rank: Super Contributor
timestamp: December 20, 2022, 08:31:21 pm
content: Do those resistors in the database represent randomly picked resistors as you might order from eBay, or would they be cherry picked, like the NIST did back when they were using a pool of Thomas type resistors as reference for the Ohm?

author: bsw_m
author_rank: Frequent Contributor
timestamp: December 20, 2022, 08:49:37 pm
content: I don't know the history of the resistors from the base. There may be some bought and used, or some selected. The example given is just a random resistor from the base.What is sold on ebay.com - no one will give any guarantee here, including the serviceability of the resistor. If anything, I think the prices on ebay.com for these resistors are grossly overpriced.When it comes to buying resistance standards, I would recommend taking a closer look at the P3030, these resistance standards have better stability and a much lower TCR.

author: bdunham7
author_rank: Super Contributor
timestamp: December 20, 2022, 10:47:56 pm
content: Quote from: Rax on December 18, 2022, 01:33:58 amSo, you connect your ~50-100ppm 10K standard to your 6.5 digit DMM and this is what you see. Now what?

author: strawberry
author_rank: Super Contributor
timestamp: December 21, 2022, 09:52:03 am
content: I see 5.5 digits

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2022, 03:20:11 pm
content: Quote from: strawberry on December 21, 2022, 09:52:03 amOn the 8502A the 6.5-digit 'Hi-Res' mode displays the 6th full digit in the spot where the exponent would other wise be. So that '3' is the last digit, not the range exponent, which by coincidence would also be a '3' in this case. The only way to tell that on this meter and in this photo is to know that if the digit is an exponent, the '+' or '-' sign in front of it will be illuminated.But the problem doesn't change much even with a very good 5.5 digit meter.

author: strawberry
author_rank: Super Contributor
timestamp: December 21, 2022, 09:04:19 pm
content: I think 34401A last 6th digit is useful for relative measurement only

author: VintageNut
author_rank: Frequent Contributor
timestamp: December 22, 2022, 02:24:45 pm
content: For the 6 1/2 digit Dmms that I have, on resistance ranges, the bottom two digits are below the uncertainty of the range.Dmms are better at measuring dc voltage than they are at measuring resistance. That is why there were 6 and 7 digit resistance bridges way back in the day. Turn the resistance measurement problem into a dc voltage measurement problem.

author: mendip_discovery
author_rank: Super Contributor
timestamp: December 22, 2022, 06:13:40 pm
content: Quote from: VintageNut on December 22, 2022, 02:24:45 pmThey used to make high-resolution volt meter setups as well. Then some clever so-and-so thought maybe we could add more than one feature together and make a meter that could measure multiple things just to save space on the work bench.Quote from: bdunham7 on December 20, 2022, 10:47:56 pmYou measure it, regularly if you like. A 50-100ppm resistor might be 50-100ppm from 10k but it may well be very stable and give you some consistent numbers.Here is my 100k,Spec for resistance,0.005% + 4 Ω + 0.035 ΩUoM for the 34401,0.012% + 2.5 ΩThe limits are big but the measurements are tight. YMMV.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 22, 2022, 07:03:23 pm
content: Quote from: mendip_discovery on December 22, 2022, 06:13:40 pmHow did you measure the resistance simultaneously with two DMMs?

author: mendip_discovery
author_rank: Super Contributor
timestamp: December 22, 2022, 07:16:04 pm
content: 34401A on top of a Tranny calibrator which has fixed resistors.

author: Rax
author_rank: Super Contributor
timestamp: December 23, 2022, 07:34:29 pm
content: Whoa! I blinked and here's the thread flaringQuote from: RandallMcRee on December 20, 2022, 07:24:03 pmhttps://www.ab-precision.de/products/electrical-standards/Are these known quantities in this community? Not sure who's used them, closely reviewed the datasheet, maybe measured their performance. I haven't been able to find a ton of posts on them.

author: Rax
author_rank: Super Contributor
timestamp: December 23, 2022, 07:46:38 pm
content: Quote from: bdunham7 on December 20, 2022, 10:47:56 pmI assume you're mentioning the 50-100ppm relative to the Z foils? But if we're talking instead about the AB RS1 (https://www.ab-precision.de/products/electrical-standards/), for the RS1/P or H, except for the "deviation from nominal," I see a lot of single digit ppm values on the datasheet.Just to make sure I understand your point in its correct context.

author: Rax
author_rank: Super Contributor
timestamp: December 23, 2022, 08:42:31 pm
content: But for Z foils - that's just $30 spent on fun...

author: bdunham7
author_rank: Super Contributor
timestamp: December 24, 2022, 04:23:40 am
content: Quote from: Rax on December 23, 2022, 07:46:38 pmNo, I was referring to the 'deviation from nominal' as the issue. So by 50-100ppm I'm referring to the .01 or .005% tolerance, not TCR. You can get reasonably low TCR (<10ppm/C) and keep the temperatures reasonable, but if you don't have a known accurate way of getting the absolute value you still don't know the actual resistance any closer than your meter will already tell you. A good sanity check for larger errors, a standard for lower-spec meters, but not as helpful with your 6.5-digit models.Still, I'll be interested to see how your project with the z-foil versions turns out.

author: Rax
author_rank: Super Contributor
timestamp: December 27, 2022, 02:10:53 pm
content: Quote from: bdunham7 on December 24, 2022, 04:23:40 amThese AB-Precision resistors apparently come with calibrated measured absolute values (I'd assume by 8.5 digit or so lab standard ohmmeter).

author: The Soulman
author_rank: Super Contributor
timestamp: December 28, 2022, 01:23:31 am
content: Quote from: Rax on December 23, 2022, 07:34:29 pmAdrian is a respected (by me atleast) member here (AP)Iwouldn't hesitate buying from him. YMMV

author: Rax
author_rank: Super Contributor
timestamp: December 30, 2022, 04:30:07 pm
content: Quote from: Rax on December 27, 2022, 02:10:53 pmI have confirmed this. The kit comes with documentation where each R section presents actual 8.5 digit measured resistance value. Thank you, Adrian!

author: Rax
author_rank: Super Contributor
timestamp: December 30, 2022, 04:48:02 pm
content: Quote from: Rax on December 19, 2022, 10:35:29 pmI promised to get back here with some impressions after playing with the Z foil. This is a .005% 100 ohms resistor.If I use this in 4 wire measurement with offset with my 8840A, I get essentially dead on 100.000 ohms readings. I assume using an offset dialed in with all 4 wires shorted to each other is the correct way of doing this.Just going 4 wire w/out an offset was getting me somewhere around 99.98X ohms or so. There's a bit of a wobble in any case scenario.I have a pretty eclectic mix of meters here, from 3.5 to 6.5 digits. My 8840A is the one I currently have here and trust the most, but they're all in very good shape.If anyone's wondering, my bench is in my garage and the wall thermometer (not a precision instrument) says 74F. Geographically, this is in Southern California near the coast, where the weather is typically relatively stable and mild, both temperature and humidity-wise. The garage is not tightly air conditioned (I do have both heating and cooling portable units in here but can't probably reach a very stable temperature as I would if my workspace would have a central AC type of environment).

author: Villain
author_rank: Regular Contributor
timestamp: December 30, 2022, 05:29:19 pm
content: Quote from: Rax on December 30, 2022, 04:48:02 pm4 wire ohms verification is done without offset math correction, this is also what 8840A cal manual says.

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2022, 06:22:48 pm
content: Quote from: Villain on December 30, 2022, 05:29:19 pmCalibration adjustment of the meter is a separate issue. The measurement in the 20/200R resistance ranges on the 8840A/AF/8842A series are essentially just a measurement of low voltages in the 20mV and 200mV ranges with additional gain stages. The specified accuracy for those ranges--both voltage and resistance--is only acheived using the offset control for zeroing. That is right in the specifications. Using 4-wire doesn't change this, that only eliminates the need to compensate for lead resistance.

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2022, 06:27:47 pm
content: Quote from: Rax on December 30, 2022, 04:48:02 pmThat would be the correct way to do it. If you put your 4-wire short on and then switch to the 200mVDC range (without offset) do you get a small negative voltage, about -.020mV?

author: Villain
author_rank: Regular Contributor
timestamp: December 30, 2022, 07:17:32 pm
content: Quote from: bdunham7 on December 30, 2022, 06:27:47 pmIndeed fluke mentions that all resistance specs are using offset control. I am a bit surprised they do not specifically tell the user to do that during the verification procedure after adjustment.

author: bdunham7
author_rank: Super Contributor
timestamp: December 30, 2022, 07:41:57 pm
content: Quote from: Villain on December 30, 2022, 07:17:32 pmThe specs require you to do a zeroing with the offset control within one hour of the reading. During the calibration adjustment, you are setting new zero and gain constants in the calibration memory, so as long as you do your performance check within the hour, no additional zeroing should be needed to meet the specification. The specs are what they are guaranteeing, the calibration process is what they want you do to to make sure that happens. Also the calibration and performance check are done at 23 +/-1 C, the specs are guaranteed to 23 +/- 5C. You expect better results when you control the temperature more closely.

author: jhoffman
author_rank: Contributor
timestamp: April 29, 2023, 02:56:20 pm
content: Quote from: bsw_m on December 20, 2022, 07:06:12 pmI thought it might help the audience to translate your"individual passport for one of my resistance standards"into English.I used Google Translate to do this, so please don't shoot the messenger if there are any mistakes. Much easier now to understand what is going on for the English speaking audience.

author: gamalot
author_rank: Super Contributor
timestamp: April 29, 2023, 03:23:09 pm
content: This is for my 1 milliohm resistor, and it just so happens that I made a digital backup of it the other day.

author: Nanitamuscen
author_rank: Contributor
timestamp: April 29, 2023, 05:11:21 pm
content: I will add information about the stability of the resistance coils produced by the USSR models p321 and p331. I am attaching a screenshot from a document containing data on the stability of several coils of different denominations. Stability data is the penultimate column.

author: MiDi
author_rank: Frequent Contributor
timestamp: October 08, 2023, 06:20:31 pm
content: Translation

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 13, 2023, 05:14:47 am
content: The VPG HZ Series are most accurate in the world. 0.001% 0.2ppm/C. Vishay even touts them as resistance stamdards..

author: Mickle T.
author_rank: Frequent Contributor
timestamp: October 13, 2023, 09:10:27 am
content: 0.2 ppm/°C

author: alm
author_rank: Super Contributor
timestamp: October 13, 2023, 10:40:30 am
content: Quote from: alligatorblues on October 13, 2023, 05:14:47 amWhy do you consider accuracy important? Accuracy is just about the value the day it was manufactured, not the value it may have a year later. Accuracy is pretty much irrelevant for a resistance standard which would have regular calibrations. I'd say drift and tempco are way more important (and humidity coefficient, pressure coefficient, etc). Regarding tempco, don't count on "typical" specs from a Vishay datasheet. Those are should be called "exceptional specs that you may never encounter". Seethis previous post of minewhere I give links to real data and experience from multiple members of the community. They are certainly good resistors, probably the best you can readily buy, but if you expect "typical" performance you'll be sorely disappointed.Regarding their use in standard resistors. Wekomm started offusing off-the-shelf VPG resistors(we've probably all seen the pictures), but had problems reaching performance targets and yield, so in the end moved toa custom process. IET is also using a number of (no doubt selected from a large pool) VPG resistors and is having worse performance than back when ESI were using their own custom precision wirewound process. So don't count on off-the-shelf VPG resistors rivaling a good commercial resistance standard, though depending on your requirements you could make a standard out of any resistor if the goal is to calibrate a 3.5 digit multimeter.

author: alligatorblues
author_rank: Regular Contributor
timestamp: October 16, 2023, 04:17:08 am
content: Vishay Precision does have some stipulations in the HZ series datasheet. They are not end all to be all. The TCR is shown on a chart, and it is different for a number of temp. ranges.