Home made standard resistors using Vishay VHP100T and VHP101T - Page 1
Forum section: electronics / general

author: iisak
author_rank: Contributor
timestamp: July 16, 2018, 12:49:49 pm
content: My starting point: I work in a resistance lab. I have measured many old commercial (Tinsley, Guildine, MI, Fluke, ZIP, Siemens&Halske, ESI, Leeds&Northrup, General Radio, HP, Cambridge, Wolff, Hartmann&Braun, AOIP, Cropico, Sefelec, IMMS, Tettex, Göerz, Quadtech, Willow...) standard resistors. Some of them are very good even today. On the other hand some have TCs of several ppm/C. Even they are mostly good if kept in oil bath. Their drift is typically in the range of 1 ppm/year. There are much worse ones too, of course.In the sets of our own reference standards we have some values, for example 100k, that we actually have only one really good reference resistor. Some day it may break down and so we need some backup. It would be good to have one or two sets of secondary standards that I keep all the time in our +/- 0,1 C air bath. Then one or two more sets that I can give to my colleagues when they need a reference resistor. They may not have so good temperature control in their setups, maybe 23 +/- 2 C.As yoy know, nowadays there are resistors with amazing stability and TC specs. Why not to by a good Vishay resistor with $50 and put it in the box? An alternative is to buy several commercial standard resistors, maybe $4000 each? Not very attractive.I have already put some Vishay resistors in a box in 2012, but now I have a new 2018 design which I will now describe.* EnclosureDie cast aluminum box. Connector for grounding/shielding.* Component selectionFrom 100 ohm to 100 kohm I think that the best ones availlable are Vishay VHP100T (100 and 1000 ohm) and VHP101T (10k and 100k) oil filled hermetically sealed 0.005% tolerance resistors. I'm also interested in other decadic values between 1 ohm and 10 Gohm and there might be some use for up to 100 Tohm and down to 0,1 mohm, but they are so different worlds that they go off this topic.Data sheet:http://www.vishaypg.com/docs/63003/vhp100.pdf* Trimming valueVishay VHP100T and VHP101T I have are mostly +/- 50 ppm from nominal value, so I see no need for trimming. There's no difference if the deviation is 50 ppm or 0,5 ppm, as far as it's known. Also, even if the initial value would be trimmed to for example less than 1 ppm from nominal, it would drift out from that window over the years. Or maybe not? VHP resistors should be very stable also in long term.* Compensating TCIf the 0,05 ppm/C specified for VHP-resistors is true, there is no need for TC compensation. In a temp controlled calibration lab the room temperature variation would be 1 C, in air bath maybe 0,1 C and in good oil bath 0,01 C.* Attaching the component and selection of external terminalsCommon path for F(orce) (current) and S(ense) (voltage, potential) leads should obviously be short. Everything between sense terminals and actual resistor will be measured as a part of the resistor. So the layout for the connection is F1----S1---R---S2----F2. Sense terminals close to the resistor. So in practice there are 4 binding posts linearly.Sense terminals are gold plated, Force terminals are regular not-gold-coloured-material, I didn't check what was that.In practice, does the length of the leads from resistor to sense terminals matter? Some quick calculation: 2 cm of 0.635 diameter copper wire has a resistance of close to 1 mohm. So it's 10 ppm of 100 ohm. More important is the TC of the copper wire which is +4000 ppm/C so the value of my 1 mohm copper wire changes +4 uohm/C. Combined with 100 ohm resistor this changes the value of 100 ohm by 0.04 ppm/C which is in the same range with the VHP specs of 0.05 ppm/C. So it matters a little bit. For resistors higher than 100 ohm it doesn't matter any more.No soldering is used in sense connections to avoid heating the component during assembly (this was probably unnecessary, a component should be designed to be soldered) and to avoid any thermal voltages due to soldering junctions.Instead of soldering, the sense connections are just pressed under the gold plated nuts of the sense binding posts.Force connections are soldered.In my older design in 2012 I (gently) presser the resistor to the cover of the aluminum box with a strip of copper which was attached the the box by screws. That time I thought that it would be good if the resistor is connected to some large metal mass to stabilize temperatures. Maybe also some heat sinking ideas were present.Now in my 2018 design I decided that no other mechanical connection in addition to the binding posts are needed. It may be better that no mechanical stress is applied to the metal can of the resistor. No heat sinking is needed because the resistor is intended to be used in very low power. No larger thermal mass to buffer temperature fluctuations is needed because the TC of the resistor should be practically zero.* Temperature measurementIn my older design in 2012 I put a 10k thermistor which was (gently) presser against the Vishay component. I wanted to measure the actual temperature of the resistor directly from it. In my 2018 design there are several resistors in the same box and I would have needed same amount of thermistors and 2N more binding posts for them. Also some mechanical stress, which I want to avoid, would have been inevitable if I had attached termistors to the metal cans of the Vishay resistors.Now in my 2018 design I only have one 3 mm hole in the middle of the top cover of my aluminum box. An external 3 mm PT500 sensor is put through the hole and there is a simple mechanical guide structure inside the box that ensures that the PT500 touches the floor of the aluminum box. 3 mm PT500 is also practical selection because our resistance lab uses them in our countinuous ambient monitoring system.* Measuring the resistance value in generalFortunately our lab has a commercial resistance bridge made by Measurements International so repeatability of <0,1 ppm is possible and absolute uncertainty of below 0,4 ppm can be reached. Measurement power should be < 10 mW, preferably < 1 mW to avoid any self heating.* Long term stabilitySome tests how a fresh resistor behaves should be done. In the data sheet there is interesting discussion about the three steps of post manufacturing operations of Vishay: "The exercises that are employed are (1) temperature cycling (2) short time overload, and (3) accelerated load life." Operations 1 and 2 are done to all resistors by default. For number 3 it is written: "How much acceleration is a function of the application and should be worked out between our applications engineering department and your design team." So the operation 3 is not done for the components by default?Encouraged by this I actually sent an email to Vishay and asked could I do this accelerated load life by myself. (No proper technical reply for that email so far.) What is it in practice? I made a long 17 days continuous measurement for one of my 100 ohm resistors immediately after it was taken out from it's package and put in the enclosure. It was no surprise that there was more drift in the beginning and the rate monotoniously decreased. Total drift was -0,8 ppm over the 17 days. During the last day the rate was no more than -0,024 ppm/day (-8,6 ppm year). If I fit an exponential decay curve on the data it seems that the drift would settle to practically zero after about 200 days of loading.Total drift over that period would be almost 3 ppm, which is not in specs, but it doesn't matter if the resistor is stable after that. Maintaining resistance values is the work of tens of years, not days or months.So I started to think should I put some current over all my resistors for 200 days to age them to their most stable part of lifeThen I found some other info from here:https://www.rhopointcomponents.com/media/blfa_files/VPG_Design_and_Selector_Guide_for_High-Precision_Resistors.pdfThe text would seem to be in conflict with the data sheet, because it is written that "STO (Accelerated Load Life) is performed on all resistors during manufacturing, with a function of eliminating any hot spots if they exist." So according to this the PMO number 3 is also done to all resistors and I shouldn't see this 500 - 1000 hours of initial drift? Maybe the drift of 17 days I saw was due to mechanical stress during the assembly. That would be more convenient because I just have to wait. No need for putting some "ageing current" over all the resistors!After this initial 17 day experiment I have been measuring other resistors of 100, 1k, 10k and 100k which were all assembled at the same time, so in other words their assemblies were 17 to 22 days old when they were measured for the first time. During 3 to 16 hour measurements I have seen no drift more than 0,05 ppm which is just noise.Conclusion: 0,8 ppm drift over 17 days was most likely due to stress caused by assembly or just a bad individual resistor. More long measurements is anyhow needed to verify this.* Summary of 2018 designI ended up to a very simple, even primitive looking construction. But I hope that it's well resoned mostly because of the practically zero TC of the component. If the TC is not zero, then you have to start thinking bout larger thermal mass, maybe some double enclosure with thermal isolation between them, using thermistor attached directly to the resistor etc.* Long term stability of my 2012 design.In 2012 I made 4 boxes with one 100 ohm Vishay Z201 in each of them. I have given away 2 of them and don't know how they behave, but the two others have quite linear drift of -1 ppm/year and -2 ppm/year. So they are better than many commercial standard resitors which may have a price of $4000 or more!

author: EEVblog
author_rank: Unknown
timestamp: July 16, 2018, 12:52:39 pm
content: Awesome first post!Welcome to the forum

author: iisak
author_rank: Contributor
timestamp: July 16, 2018, 12:55:52 pm
content: Here's a pic, (not very good one).

author: TiN
author_rank: Super Contributor
timestamp: July 16, 2018, 01:07:15 pm
content: Kinda strange to hear all these question from one who works in resistance lab, but anyway.QuoteThis is no short question, but the main reason why those commercial standard cost 4K$ or more, is not due to some voodoo tech, but amount of testing, validation and uncertainty accounting is done. Even if you buy very good 50$ (or even 200$, not the point here) resistor element, you would still need to invest time and resources to be able of saying "this standard has x.x ppm/annual uncertainty with x.x tempco and x.x stability". That nice MI bridge you have access to is not free to operate, so there is your time = cost.VPG resistors are NOT specified at 0.x ppm/C. You will have to buy lots and use your equipment to sort them.Or you can ask VPG to pretest and bin tempco for you, but then you will pay much more (manyfold) than 50 bucks per resistor.And VPG customer support (at least for me in asia region) is horrible, all I can say about that. I'm still waiting on resistors ordered in February, still without any solid ETA date.Based on photo, design does not look good to me at all, sorry. Some people, including me, like to tighten posts nicely to break possible oxides on surfaces. That will cause little, but non-avoidable wiggle of the post, and all that stress will transfer to resistor body.Your concerns about not soldering sense terminals (to avoid thermal stress to package) might be worthy, but then you solder force terminals and completely null the whole point. You either solder them all, or you don't solder them all.17 days is not even short-term, it's like initial warm-up time period. Long term study starts from 6months, better 1 year.QuoteUsually length does not matter, if you don't sink/source any current into these leads. That is whole point of kelvin connection.Now what would be useful is to see actual data from your 2012 boxes, then we can play some numbers. Just keep in mind, that comparing 1 or 2 random samples (which could be lucky golden, or the junkbin-grade) to the commercial standards (I'd assume you compare to official spec, not actual measured drift, eh?) is not even apples to oranges, but monkeys to cucumber. Good proven commercial standards show drift much lower than manufacturer specification.P.S. don't let all this stop you from trying though, we definately would enjoy seeing some photos of that MINTL bridge you talking about in actionAnother side question, you are focused on zero tempco so much for colleagues use at relaxed temperature conditions, okay. But do those folks also have stable enough equipment to actually use that 0 tempco resistor standard? E.g. 3458A have it's own tempco on resistance measurement function in range of 1...3ppm/K, so that will be major error contributor (given even that the DMM is calibrated to very low uncertainty) to your friends measurements.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 16, 2018, 01:38:13 pm
content: Maybe you can use an airbath you mentioned and measure the real tempco of your reference-resistors. Since Vishay-datasheets basically say "1 device in a quantity of 100 might be awesome with zero TC, but chances are good that the TC is just meh".Also: shipping to you (temperature variation, mechanical shocks) and mechanical stress applied while mounting the resistors can lead to the observed drift.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 16, 2018, 05:39:39 pm
content: I must ask, what is a "resistance lab" and how do they make any money? Part of a national lab?The only thing I can contribute is that bolted connections on any tinned wire eventually fail, or at least change resistance. Solder or gas tight crimps are the only thing I trust.

author: Andreas
author_rank: Super Contributor
timestamp: July 16, 2018, 06:51:58 pm
content: Hello,Quote from: Echo88 on July 16, 2018, 01:38:13 pmWith the experiences from here:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg1458463/#msg1458463I would also test every single resistor for T.C. if I need below 1 ppm/K.with best regardsAndreas

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 07:52:02 am
content: Thanks for the feedback! You have some good points.Quote from: TiN on July 16, 2018, 01:07:15 pmCertainly it's true what you write in here. On the other hand if I buy any standard resistor I still would have to do all that work again. I would measure TCs myself and would start to collect stability data. So my alternatives are (1) $50 for resistor + $200 for my assembly work + $2000 for measurement and calculation work over next 5 years. Total of $2250Or (2) $4000 for standard resistor + $2000 for measurement and calculation work over next 5 years. Total of $6000.On the alternative (1) there is a risk that after some work I find out that the individual resistor was bad. I hope that having 5 pcs per value is enough that there some good ones too. Also my results from those that I have measured since 2012 are encouraging.Quote from: TiN on July 16, 2018, 01:07:15 pmHave you seen any statistical data how good they really are? It would be most interesting!I now have some preliminary data from 8 individuals and they seem to be 0.3 ppm/C or better. I will do more accurate measurements after they are stabilized. After some months maybe.Quote from: TiN on July 16, 2018, 01:07:15 pmWhat makes you think they are not well tightened? Or was there something else that does not look good at all?Quote from: TiN on July 16, 2018, 01:07:15 pmSeems that I have to explain this on a bit more detailed. I first tighten the sense points under the nuts. After that I do the soldering for force terminals. Force terminal are not so close to the element and also the nuts and binding posts of the sense terminals act as heat sinks placed in a good position between the force terminals and the element and hopefully most of the heat goes there.Quote from: TiN on July 16, 2018, 01:07:15 pmAbsolutely! Initial behaviour is the only thing I can study at this point!Quote from: TiN on July 16, 2018, 01:07:15 pmEverything that is between the sense terminals is measured as a part of the resistor. When you place the sense terminals, you define where your resistor begins and ends... As you can see from my calculations, 2 or 3 cm of copper leads matter on this level of accuracy.Quote from: TiN on July 16, 2018, 01:07:15 pmNow I have these 20 individuals, lets see how they look like. I'm comparing to actual measured drifts to our lab's own standards. Some of them have now almost 50 years of history. There are actually some individuals from the beginning of the 70's that are really good.Quote from: TiN on July 16, 2018, 01:07:15 pmYou are probably right: in most cases a 3458A is the best they have. But if it has 1...3 ppm/C, then the reference resistor should not have more than 1/4 or preferably 1/10 of it, so that it would not add any uncertainty. So I could set the target to 0.1...0.3 ppm/C.

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 07:54:51 am
content: Quote from: Echo88 on July 16, 2018, 01:38:13 pmI will certainly measure TCs at some point. You are also right in here: what I see at this point is probably due the stress of transportation and assembly.

author: TiN
author_rank: Super Contributor
timestamp: July 17, 2018, 08:01:51 am
content: QuoteI have tested over 200 pcs different VPG resistors, only few (<5%) show less than 0.3ppm/K. Some of results are public on my site.About "sense resistance". If no current drawn from those lines, what will be the voltage drop to measure? DMM calculate resistance from ohm law...

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 08:10:14 am
content: Quote from: Conrad Hoffman on July 16, 2018, 05:39:39 pmAmong many other things we do resistance calibrations for our customers. I would not like to define more accurately where I work, even though I believe that I haven't revealed any business secrets so far...Quote from: Conrad Hoffman on July 16, 2018, 05:39:39 pmThat is one of my biggest worries. How should they be connected. Data sheet says the leads are "solder coated copper". In the past I have sometimes scratched the coating away and made the crimp on the pure copper found under coating. My 2012 Vishay resistors are assembled that way. My experience was that scratching was a bit fierce and maybe caused some extra mechanical stress for the element.I have to concider if I still should take the resistors out from the box and do the scratching + crimp of just soldering also for sense terminals.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 17, 2018, 09:06:23 am
content: Quote from: iisak on July 16, 2018, 12:55:52 pmThat indeed does not look so good.For what you are after, you need to start with the right quality binding posts.Look at "LowThermal" products here:http://www.lowthermal.com/Each of their binding posts cost as much as your Vishay resistor, but they are worth it!

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 09:51:33 am
content: Quote from: HighVoltage on July 17, 2018, 09:06:23 amThanks for that link! So far I have found most of the gold plated copper contacts good enough. Not all of them.Their Model 770 Standard Resistors also look interesting. Often specs start to get worse after 10k, but they give the best specs up to 1M. With those specs I could consider buying at least 100k and 1M. For other values we don't have so much needs right now. But this thread is for home made standards, maybe I have to start a new thread about comparing the standards commercially availlable.For low thermal cables our lab uses silver plated copper wire crimped on these:http://metasweb01.admin.ch/euromet/copperlugs/copper_lugs.pdf

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 10:01:18 am
content: Quote from: TiN on July 17, 2018, 08:01:51 amGreat! Can you give me a link there?Quote from: TiN on July 17, 2018, 08:01:51 amI'm talking about the short parts of the leads that are common for force and sense paths. The parts from the resistor element to sense terminals. There is a voltage drop because the force current goes through them.

author: dl1640
author_rank: Regular Contributor
timestamp: July 17, 2018, 10:03:55 am
content: Above link returns Not Found..通过我的 PRA-AL00 上的 Tapatalk发言

author: HighVoltage
author_rank: Super Contributor
timestamp: July 17, 2018, 10:22:07 am
content: Quote from: iisak on July 17, 2018, 09:51:33 amI was not aware of that company, thanks for the info.Their gold plated lugs look great, but ... Swiss price of 12 CHF each at 100 pieces ... WOW!But to be honest, their crimp lookshorriblefor a professional cable.They should invest in a real crimping tool to get a gas tight crimp!

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 10:54:56 am
content: Quote from: HighVoltage on July 17, 2018, 10:22:07 amThey (METAS) are national standards laboratory of Switzerland, so I hope they know what they are doing. Can you trust that, I don't know. They are one of the top resistance laboratories in the world, together with NIST and couple of other big countries.The same pdf shows their custom made crimping tool, so at least they have had some thoughts how the tool shoud work if they have decided that the commercial ones are not good enough and they have to design their own instead.If the lugs are around $10 each that should be tolerable compared to those tellurium copper binding posts of Lowthermal company you linked, if they are >$50 each (more than my Vishay resistors, as you wrote).

author: HighVoltage
author_rank: Super Contributor
timestamp: July 17, 2018, 12:15:46 pm
content: Quote from: iisak on July 17, 2018, 10:54:56 amThey probably have changed their tooling by now, the PDF is pretty old.The lowthermal tellurium copper binding posts are around 23 USD eachLowthermal also has some nice spade connectors and here are a couple pictures of my crimp of these terminals.The wires are silver plated copper with PTFE insulation.I like how METAS added the extra strength to their terminals

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 12:54:43 pm
content: Quote from: iisak on July 16, 2018, 12:49:49 pmI started to think that partly because Vishay writes how differently bulk metal foil resistors behave compared to other technologies:QuoteBut the aging of my fresh resistor has continued at the same rate also when it was not under measurement (gap in data). It's good news, now I just have to wait some couple of months.See the another pic for showing the 6 years history of my 2012 resistor "D". It shows similar (?) larger drift for the first 3 months and then slower drift. It's now only -0.8 ppm/year from 2016 to 2018.I also found the individuals "A" and "B" and I am measuring them right now. (I had given away only 1 of 4, not 2 as I wrote before.)

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 01:08:36 pm
content: Quote from: iisak on July 17, 2018, 08:10:14 amI just discussed with a colleague during our lunch and he was also worried about the solder coated copper wires. (This guy has made some great ACDC shunts in the 90's and many other things.) I now seriously consider taking the resistors out from the box and carefully scratching the solder until enough pure copper is visible and then tightly pressing the leads between those gold plated nuts.

author: iisak
author_rank: Contributor
timestamp: July 17, 2018, 03:03:22 pm
content: Quote from: HighVoltage on July 17, 2018, 12:15:46 pmGreat! I think I'm ready to pay $23 per sense terminal binding post instead of $5 to avoid any risks. But do you see any reasons why I couldn't use cheaper connectors for force terminals? Thermal voltages shouldn't matter there, only on the sense terminals. On the other hand the connection panel maybe just looks silly if there are different looking terminals mixed together. Maybe I can again use some extra $ just for better looks.But on theoretical point of view, the quality of force terminals is not important, or is it?Quote from: HighVoltage on July 17, 2018, 12:15:46 pmI just have enough strength in my hands to crimp them with their tool... One other guy complained that he can't crimp those. The tool should have longer lever arms or something.

author: quarks
author_rank: Frequent Contributor
timestamp: July 17, 2018, 03:36:32 pm
content: about METAS crimp, actually they have a very good crimping tool (see link)https://www.eevblog.com/forum/metrology/diy-low-emf-cable-and-connectors/msg545213/#msg545213

author: iisak
author_rank: Contributor
timestamp: July 19, 2018, 01:50:16 pm
content: Some updates about my project.As I wrote before, after reading many useful comments from you, I got worried about the contacts between my resistor leads and the binding post. Solder plated copper leads of the Vishay resistors pressed between two gold plated nuts. Can that be stable in long term and is there too much thermal EMF?I removed some gold plating and found out that binding posts are gold plated brass. Not so good, but this is just the best I have right now. Need to buy Pomona 3770 or Lowthermal 2758 or something else later.I scrached the solder off from the resistors leads from that area that goes under the gold plated nuts and tightened the nuts again. As I thought it was difficult to do the scraching without shaking and bending the leads and resistor elements. Some unwanted mechanical stress.My 100 ohm individual "A" jumped +2.5 ppm during this operation. After some time I will see if this was a permanent jump or will it drift back towards it's old value. Haven't re-measured other individuals yet.I have been reading this interesting thread "DIY Low EMF cable and connectors" but it is a bit long to easily find the best binding posts from there. Anyway, I have got some thoughts:1) Pure copper binding postsOutside connection should be ok, if the pure copper is kept clean. Of course the lugs and cables also need to be low thermal EMF, but I do have such stuff.Inside connectios are permanent and they don't need maintaining. Maybe they shoud be done by scratching the solder from the resistor leads, crimpin the leads to METAS or some other good lugs, and tightening the lugs between copper nuts of the binding posts. Or maybe lugs need not to be used? Resistor leads could be tightened directly between copper nuts?2) Binding posts with gold plating directly on a tellurium copperOutside connections: cleanign is not so important as it is with the pure copper. This is good if the user is not so careful and forgets the cleaning.Inside connections: generally same considerations as with the pure copper stuff.My general experience is, that the inside connections are the weak point of many designs. For example Pomona 3770 here:https://www.mouser.fi/datasheet/2/159/d3750_60_70_1_01-34263.pdfIt looks like the component is intended to be soldered. Not good. If some lug under the nut and washer is used, that's not good either, because the nut is brass and washer is steel. I feel that they ruin the good design by having these materials.Low thermal 2758 looks better if the nuts and washer are also low thermal EMF materials. Are they?http://jswilley.com/files/45782675.pdf

author: TiN
author_rank: Super Contributor
timestamp: July 19, 2018, 02:57:48 pm
content: Quote from: iisak on July 19, 2018, 01:50:16 pmThey are. I spent quite some time to build little 1 ohm box with Low Thermal posts:However after using that project, there are few nasty flaws in it, but those would not matter for 10Kohm box.Which ones? Well, I'll remain that to be guessed, sorry, no freebies here.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 19, 2018, 03:30:08 pm
content: If you are using these bridges, or a DMM with 4W and Offset compensation , like the 3458A, the 34465A/470A, the Fluke 8508A and similar, the question about the connection technique and material of the connection jacks is quite useless.The said instruments will completely compensate for all kind of errors like parasitic resistances, and also e.m.f. of jacks, solder junctions, surface metallization (i.e. pure copper, or not).Your current brass jacks are just fine, in this aspect, as is the pure tinned surface of the resistors.CuTe jacks might help with other instruments, lacking Offset Compensation, and might make the measurements with the above mentioned instruments more stable when temperature changes occur.The really bad idea in your setup is the unstable and undefined Kelvin clamping on the inner connections of the low Ohm resistors.This connection defines the exact value of your reference resistor, and if there is mechanical stress like vibration or shock, or if the screws will change their force onto this connection, you will encounter changes on the nominal value.So either you invent a more solid clamping mechanism, or you just solder a sense line to that point; should be 10mm apart from the resistors body, because Vishay defines the nominal value at that location, if I remember correctly.These BMF resistors are very sensitive to big temperature variations, and if you heat or cool them more than about +/- 20°C from room temperature, you will see more and more of hysteresis, which may also be static, i.e. will not vanish over time, but by thermal cycling only.Therefore, a heat clamp is required, if you will solder this crucial Kelvin connection.Vishay tests their resistors at -40°C and +125°C, so you won't know in which state they will be delivered.What you see as a 1ppm drift, is probably the relaxation from this hysteresis, but not the timely drift, which they specify to be 2ppm/6yrs. typical.So that's the 2nd bad part of your design, the lacking thermal management of your resistors.You must connect a thermometer / sensor in direct contact to each of your 4 resistors, to determine their individual temperatures.At best, you do that by adding a 'thermal mass', i.e. a solid metal block from copper or aluminium, at least 20x20x20 sq mm is sufficient, inside which the resistor and the sensor were inserted.That way you get the direct thermal coupling, but also a thermal inertia, so that resistance measurements are much more stable.That allows you in a first step to determine the individual T.C.s, which will be <0.3ppm/°C per specification of the VHP10x, but probably not as low as 0.05ppm/°C.Then by making temperature cycling experiments, you will also determine hysteresis of the resistors themselves, or your thermal setup / coupling.As the T.C.s are quite constant over time, you then are able at all temperatures, to reproduce the nominal value by simply measuring the resistors temperature.As you are working in a cal lab, you for sure know the SR104 standard resistors, which are made up exactly this way... especially they have a quadratic correction curve imprinted on the case.My setup can be found here:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg464413/#msg464413It does not look so good, either, but it contains a lot of my (low) temperature physics experience.Only by such a setup you will achieve around or below 1ppm stability and accuracy, and by frequent monitoring of their values with temperature correction a drift prognosis.Frank

author: iisak
author_rank: Contributor
timestamp: July 19, 2018, 07:56:36 pm
content: Frank, you must be right. If even in the adored SR104 design the force and sense terminals are just soldered together with a piece of thick copper wire right below the box cover (see the pic in the first message of the teardown thread), it can't be so critical. Although maybe we can't be sure if it is some cadmium solder or something else exotic.I also have measured many kinds of self made resistor boxes from colleagues and customers which have been stable even if they are just some stable resistor soldered directly to bnc or other heretic connectors...Maybe I try just simple soldering with good heat clamping in my next version to get mechanically stable sense points.Adding that thermal mass is now also in my to-do list.

author: IanJ
author_rank: Unknown
timestamp: July 22, 2018, 11:59:25 am
content: Hi,This thread has me interested in constructing a box with 4 resistors in it, looking at Vishay Z-series @ 0.2ppm/degC for the availability/cost and I have the perfect die-cast enclosure for it.My question is regard to oil filling as has been mentioned, so can I simply completely fill my enclosure with a low dialectric oil?.......any ideas on the oil type to use? Off the bat I see transformer oil to IEC 60296:03.......suitable?Ian.

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 22, 2018, 05:51:49 pm
content: Quote from: IanJ on July 22, 2018, 11:59:25 amIan,you obviously misunderstood that oil-filling item..All VHPxxx resistors themselves are oil-filled, and that's all you need.That provides a direct thermal coupling between resistor element and case.If you then put that case tightly into a metal mass, maybe with thermal conducting grease, you have a perfect coupling with the temperature sensor and the out case.You don't need any further oil filling. That's required only, when you want to thermally couple a bunch of resistors, like in the 720A, or when you want thermal coupling and oxygen protection of a naked resistor element, like in the Reichsanstalt or Thompson type Normal Resistors.In this case, it would simply be a big mess w/o any additional benefit.Another thing: the VHP20xZ resistors have a T.C. of about < 2 ppm/°C , guaranteed, typically below 1ppm/°C, but do not at all expect 0.2ppm/°C .. my 5 EA have 0.3, 0.5, 0.7, 0.7 and 1ppm/K..If you want guaranteed < 0.2ppm/°C, then you need the VHP101.Frank

author: IanJ
author_rank: Unknown
timestamp: July 22, 2018, 06:08:39 pm
content: Quote from: Dr. Frank on July 22, 2018, 05:51:49 pmI understand the oil filled VHP series, but I was looking at the Z-series, which are not oil filled...........Just thought it might help keep them more thermally stable and removal of air would help against humidity changes......poor mans hermetically sealed.But I guess have them in a closed enclosure and insulated will be enough.............Ian.

author: branadic
author_rank: Super Contributor
timestamp: July 22, 2018, 06:45:25 pm
content: QuoteUsing a small oil film instead of thermal grease makes a better thermal contact. This is at least what temperature sensor companies will tell you.-branadic-

author: Dr. Frank
author_rank: Super Contributor
timestamp: July 22, 2018, 07:22:39 pm
content: Quote from: IanJ on July 22, 2018, 06:08:39 pmThe Z series denotes a different resistor alloy only, there's C, K and Z alloy, with different nominal / typical T.C.s, i.e. +2ppmm/K, -1pp/K and +/-0.2ppm/K, respectively.The VHP101 contains a series combination of two resistor elements, with C and K alloy, to get a compensated T.C., below 0.3ppm/KAnd you can as well have the Z alloy inside a hermetic, oil filled case, I have used the VHP202Z at that time, and they perform extremely stable over time, apart from the mediocre T.C.So you know that also, and probably mean that you had a look on a molded Z-resistor.That's also no good idea, if you buy cheap, you will end up with a cheap result.The mold compound itself is a thermal resistor, and the oil will not do any good to improve that.Also it would not protect the resistor element as it would inside a hermetically sealed case.That would not make any sense at all, but a big mess only.Frank

author: mzzj
author_rank: Super Contributor
timestamp: August 15, 2018, 06:59:24 pm
content: Quote from: iisak on July 17, 2018, 07:52:02 amI have measurement results for 4x 100 ohms and 4x 25 ohms VHP202Z resistors from last several years. Long term stability has been reasonably good but 25 ohm resistors havehorribletempco around room temperature ( looked like close to 2ppm/cel)Compiling the measurements to one clean excel file is another project to do...Vishay VHA -4Z series with 4-wire connection has been disappointing in long-term stability and 100 ohm resistor also had "larger than hoped for" self heating effect.BTW: If I'm guessing right who you are look for calibration reports xxxx12E001 and xxxx12E002--Matti

author: mzzj
author_rank: Super Contributor
timestamp: August 15, 2018, 08:24:00 pm
content: For comparison our 100ohm Tinsley 5685A total drift has been smaller than the 0.5ppm calibration uncertainty for last 10 years so I would say you get what you pay for...

author: Echo88
author_rank: Frequent Contributor
timestamp: August 15, 2018, 08:32:25 pm
content: Did you compare the said resistors against the Tinsley 5658A and was the Tinsley calibrated periodically against a better standard, like a QHR?

author: mzzj
author_rank: Super Contributor
timestamp: August 15, 2018, 08:47:17 pm
content: Quote from: Echo88 on August 15, 2018, 08:32:25 pmExactly.The 25 ohm and 100 ohm resistors are all connected in series and I just use "the box" to periodically verify our Hart Scientific/Fluke 1590 "super thermometer" (or resistance bridge if you wish) linearity. For that purpose I don't need spectacular stability or tempco so VHP202z's work reasonably well.

author: splin
author_rank: Frequent Contributor
timestamp: August 15, 2018, 11:52:02 pm
content: mzzj, your results are a bit dissapointing to say the least. The 25R results especially look terrible - Edwin Pettis claims his wirewound resistors typically drift less than 3ppm/year whereas yours appeared to take 5 years or so to settle down to < 3ppm/year.Your results are well adrift (pun intended) from Vishay's claims. According to figure 11:http://www.vishaypg.com/docs/49789/vfrguide.pdfBoth a 103R and 157R VHP101 moved less than 1 ppm over 8 years, in a random walk rather than the monotonic curves you measured. Perhaps more importantly, Vishay specify drift at typically less than 2ppm in 6 years in their datasheets.So either:a) VHP201Z drift performance is not as good as the VHP101 (I'd be surprised if there is much difference)b) Your devices suffered some environmental conditions (shock, temperature extremes etc) either before, or after you received them, which caused large shifts in values requiring a few years to recover. What temperature ranges have they experienced during your tests?c) You were unlucky (8 times or twice from a batch POV),d) Vishay got lucky with their test samples.e) Vishay published the results for the best performers out of the batch of many on test (surely not!)f) Vishay's results are for parts that had been around for a year or three before they started the testg) Vishay's test had been run for rather longer than 8 years and they only showed the last 8 years measurments (surely not!)h) Some combination of the above or something else.To be fair to Vishay, Dr Frank and zlymex had very good results with VPR hermetics WRT drift. zlymex had drift << 1ppm over several years, but they may have been well aged parts - see reply #30:https://www.eevblog.com/forum/metrology/teardown-standard-resistors/25/The above resistors are also much higher resistance and it may well be that the extremes of values, high and low, may drift rather more than those in between. It may well *not* be the case of course, in the absence of more data.Since long term drift performance of Vishay hermetic foil resistors is a key characteristic and selling point I assume they will have hundreds, if not many hundreds of parts on long term tests including each of the different models and parts from different batches. Given the importance of the drift characteristics I'm surprised that they seem to have been *very* economical on the amount of data they have made public. Perhaps I have been looking in the wrong place but I can't recall seeing results for more than a handful of individual specimens.It would be marvellous if they would publish much more drift data but it clearly ain't gonna happen - commercially sensitive or embarassing perhaps.Big customers may get sight of the data but surely small to medium customers will account for a significant percentage of their production given that these parts are expensive and relatively specialised. I wonder how many of the latter customers take Vishay's claims on trust and how many run their own long term and expensive qualification testing? added comments on 25R performance.

author: mzzj
author_rank: Super Contributor
timestamp: August 16, 2018, 06:13:06 am
content: Quote from: splin on August 15, 2018, 11:52:02 pma) could be that the VHP101 goes trough some special "PMO"b) impossible to say what happens between factory and our lab but it was custom order (not NOS or recycled..) Ever since soldering the resistors in the box they have been sitting in room temperature 23+-2c calibration lab.did my best during construction not to shock the parts. ie heatsink the leads with flat pliers during soldering and not stressing the resistor body during bending the leads (no idea if its needed but tried to stay on safe side)d) I usually read the component datasheet "typical" values as "lies, lies, bigger lies"Vishay is especially good with their tempco specmanship

author: iisak
author_rank: Contributor
timestamp: August 16, 2018, 07:16:44 am
content: mzzj: interesting data! And some of it really was availlable closer to me than I expected...You are getting the same -1...-2 ppm/year for 100 ohm Vishay resistors than we have since 2012. At least for us the predictable dirft at that level is not a big problem. High TC would be more problematic if and when these are being used outside the most stable air baths.

author: mzzj
author_rank: Super Contributor
timestamp: August 16, 2018, 12:10:21 pm
content: Related rant: Just got information that Fluke* can't sell Tinsley standard resistors to europe because of ROHS!"#%"#¤%#"¤ Dammit!*) Yes apparently Fluke has grabbed also tinsley's standard resistor businessThey tried to offer Fluke 742 as an alternative, and even that one is close to 5000 eur/usd

author: mzzj
author_rank: Super Contributor
timestamp: August 17, 2018, 07:39:10 am
content: Quote from: splin on August 15, 2018, 11:52:02 pmSounds like one plausible explanation.http://www.vishaypg.com/docs/63171/TN104.pdfVishay also says: ".An additional mechanism concerns ohmic values below100 . The ohmic value of internal connections canchange with time and temperature by a few milliohms — adrift equivalent to 10 ppm in a 100  resistor and of 1000ppm in a 1  resistor. Very low-value foil resistors requirespecial construction of their internal connections. Forvalues below 0.5  the “Metal Strip” style of resistorsincludes very robust internal connections and an option ofa four terminal external connection---"Correction factors for drifts in lower values and thermalresistance data can be obtained from the ApplicationsEngineering Department"Haven't really paid attention earlier but now noticed this tidbit:"ZERO TCR OF FOIL RESISTORSUltimately, the TCR of the foil resistor is affected by twoopposing physical phenomena, which depend both on theresistive element on its own, and its relationship to thesubstrate to which it is bonded.Resistivity of the free foil changes directly with temperature.After bonding, the difference of the temperature coefficient ofexpansion (TCE) between the foil and the substrate willcause compression or expansion strains on the much thinnerfoil, directly affecting the resistance change with temperature(strain gage effect).These two effects occur simultaneously on the resistor withtemperature changes, and can be detrimental to theperformance of the resistor. They can however, also be usedto negate each other toward improving the overalltemperature characteristics of the resistor."I have to say that I don't necessarily like the idea

author: meandeev
author_rank: Regular Contributor
timestamp: August 18, 2018, 09:37:12 pm
content: Hello Frank,the construction of your setup makes sense to me, so I will "copy" it.which binding Posts did you use in your setup? Is the resistor glued in the alu-block, clamped in it or held by the thermal grease (and the wires)?Where did you get the box (or which dimension did you use)?RonnyQuote from: Dr. Frank on July 19, 2018, 03:30:08 pm

author: splin
author_rank: Frequent Contributor
timestamp: August 18, 2018, 10:40:09 pm
content: Quote from: mzzj on August 17, 2018, 07:39:10 amInteresting - I'd missed that. So if the early years drift rate of low value parts is rather higher than middle value parts then why doesn't the datasheet say so - they do it for the TCs after all? They show differing drift rates in Vishay Beyschlag precision thin film datasheets for different resistor values for MMA0204 and UXB0207 resistors.Assuming that your results are reasonably typical that means the "typically < 2ppm drift in 6 years" claim in the D/S is extremely misleading given your 25R's 30ppm/6 year drift. I guess they don't state which 6 years they mean.I've just found another Vishay document which includes the same 157R and 100R VHP101 drift graphs I linked earlier (with < 1ppm total drift over 7 years) but this time they are attributed to a Swedish customer, Metrima AB.http://www.vishaypg.com/docs/49568/purtol.pdfLook at the title (page 4);"Documented performance from a customer for hermetically sealed VHP101 Foil resistors which have been on test for over 10 years."But the graph only shows 7 years data!.There could be an innocent explanation such as 'the final three years data were invalidated due to some experimental flaw' but the cynic in me says there has to be a strong possibility that they are cherry picking by leaving out large drifts in the first three years.Why publish customer data rather than their own of which they must have plenty? Perhaps they think it may carry more weight because of a semblence of independance and that people don't trust their own very much?

author: mzzj
author_rank: Super Contributor
timestamp: August 19, 2018, 06:14:12 am
content: Quote from: splin on August 18, 2018, 10:40:09 pmSo that they can say "it's just typical case and we even didn't measure it by ourselves" if there is liabilities.VHP101 "window method" tempco is just another show of specmanship if you ask me.. no guaranteed max of absolute tcr anywhere. Could be S-shaped curve with 2ppm/c slope around room temperature and still fit inside the windowGraph says "typical" 0,3ppm/C (that would be worse than "typical" for VHP202Z)

author: mzzj
author_rank: Super Contributor
timestamp: August 20, 2018, 10:47:21 am
content: Did some digging on lab:Vishay 100 ohm VHA516-4Z (Z-foil 4-wire hermetic resistors) 6 year total driftsn 1: -23ppm (this one has been transported around and has probably seen some temperature swings during transit)sn 2: -9ppmsn 3: -13ppmOne of them is pretty muchspot on at the moment:

author: mzzj
author_rank: Super Contributor
timestamp: August 20, 2018, 11:07:08 am
content: And some other values from VHA516-4Z set:200 ohm -16 ppm /6y300 ohm -5 ppm /6yAll of "my" vishay resistors seem to drift down. 10, 25, and 50 ohm resistors have been also going down by various amounts.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 20, 2018, 11:54:52 am
content: Quote from: meandeev on August 18, 2018, 09:37:12 pmAt best, you would use CuTe binding posts, from Pomona.Most important is the high isolation of the plastic, because other manufacturer (multi contact, now obsolete) used lower grade plastic, which caused ~1ppm error @ 10kOhm.CuTe has low thermo couple voltage, important only for DMM w/o Offset Compensation.The holes in the aluminium block (or copper, if you have access) are hand-made, so it's more a loose fit.I have used the usual thermal grease for transistors. Better would be the low temperature physics thermal grease, called Apiezon.Either grease should be good enough for thermal contact and mechanical fix.As some of by boreholes were too wide, I had to use one or two layers of aluminium foil to fix the resistor.The temperature sensor is also attached with thermal grease.I used Hammond aluminium boxes, 92 x 38 x 31 (mm), maybe mod. 1590A, because I also needed very good thermal contact to the aluminum block (again thermal grease used between block and case), and a very good thermal conduction material around the whole assembly. So that serves three purposes, first a thermal short all around the resistor and the aluminium block, additional thermal mass for thermal inertia, but also good thermal conduction, to bring the whole box to a controlled temperature.. I simply place these boxes on top of the 3458A, so you can stabilize the temperature to 25°C, depending on the location on the top case.I would use bigger aluminium blocks, and also bigger aluminium boxes, to have even more thermal inertia.I would also use Pt100 sensors, instead of the precision NTCs.Frank

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 20, 2018, 05:44:14 pm
content: Remember that thermal grease isn't a very good thermal conductor. It only works well for transistors because we use a very thin layer, hopefully no more than a few ten thousandths of an inch. Loose fits with a gob of grease won't couple as well as close fits with a small amount.

author: IanJ
author_rank: Unknown
timestamp: August 24, 2018, 07:58:01 pm
content: Hi all,I've started a wee build, hopefully I will get some VHP100/101 resistors eventually, but for now I am using Vishay Z-series (0.2ppm/degC).Biggest problem I found was getting stable banana sockets, i.e. ones that didn't require D-shape holes, didn't break the bank and also didn't physically have issues that affect the contact thus resistance........and in the end I opted for what you see in the pics below.Once I get the VHP100 series resistors I'll solder them directly to the posts, but for now the z-series are fitted to the tabs. I'll also solder jumpers from main post to sense post instead of using the tabs.Enclosure is a Hammond 10758PSLA (die-cast)PS. Still waiting on the 100k, and still to characterize the ones fitted.Ian.

author: usagi
author_rank: Frequent Contributor
timestamp: August 27, 2018, 10:30:33 am
content: similar to build I did a while back.reminds me, i need to get them checked again.