Callibration Resistor Standard - Page 1
Forum section: electronics / projects

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 10, 2012, 11:49:21 pm
content: Does anyone here have an experience with resistance standards? I've decided to try to get one to calibrate my micro-ohm meter, but new ones are a bit out of my price range...I found some old Soviet ones on eBay for a reasonable price here:http://www.ebay.com/itm/0-1Ohm-100mOhm-0-01-Resistor-Standard-Resistance-an-g-ESI-GR-GenRad-/230775231142?pt=LH_DefaultDomain_0&hash=item35bb4692a6These are old and are shipping from Ukraine to the US. Would the accuracy of these things be damaged in shipping or by aging? They will still be much better than a 1% current shunt resistor, but do you think it would be worth the price?

author: nukie
author_rank: Frequent Contributor
timestamp: December 11, 2012, 12:38:53 am
content: How do you know the used Soviet unit has not been overloaded?Vishay Z foil if price is right.

author: notsob
author_rank: Frequent Contributor
timestamp: December 11, 2012, 12:54:43 am
content: How about 1 of these - 3 tested 0.1% resistors & you can check you DMM as wellhttp://www.voltagestandard.com/DMMCheck.html

author: robrenz
author_rank: Super Contributor
timestamp: December 11, 2012, 12:56:24 am
content: Some questions that will affect what you may need:Is your micro ohm meter 4 wire Kelvin connection?Does it have current reversal or offset compensated ohms to minimize thermal emf errors?What are the ranges and resolutions and what is the current drive level for each range?Are the leads shielded and does it have a stated CMRR for the sense leadsWhat is the meters stated accuracy?

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 11, 2012, 01:36:54 am
content: It is not a commercial meter, but one I am designing. The current source is 100mA, controlled by a buried zener voltage reference. I'm not too worried about the source, as the components are very tight tolerance. I am putting a precision instrumentation amp to multiply the sense voltage by x1, x10, or x100 to increase the sensitivity, but this is the part that worries me. I have no accurate way to check that the multiplication is accurate. For a $30 IC, it had better be, but I'd like to know.There are 0.01% resistors in 100ohm and up (I'm using one in the current source) but this is too high for my design to measure. Once you start getting into the fractional ohm range, low tolerance resistors get very expensive.The eBay seller claims that the standards are tested working, but does not mention being checked for cal. His eBay reviews are positive, and one reviewer posted that the resistor was within 0.005% tolerance. So they seem legitimate. I just don't know much about these things.The leads are 4 wire and shielded. There is a current reversal switch. The meter will be a bench Fluke model, I'm not sure of the model number as it is not in front of me right now. I'm not expecting 0.1% accuracy on a meter that costs $100 to build. But I would like to be able to check everything.

author: Harvs
author_rank: Super Contributor
timestamp: December 11, 2012, 01:44:40 am
content: Sorry OT:QuoteWow, what part are you using that costs $30?I'm currently doing a similarish thing conditioning the current shunt voltage in an eLoad prior to ADC, and an auto-zero op-amp + analog mux auto-ranging and resistors will be < $6 (though I will be correcting for resistor mismatch in software.)If you're just trying to check the amp's acuracy, surely that's going to be easier by just input a stable reference voltage, and measuring the input and output?

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 11, 2012, 02:00:04 am
content: I don't remember the part number, and I don't have it with me, but it's a TI (or AD, don't remember) InAmp with pin-selected fixed gain of 10, 100, and 1000. 0.05% tolerance for the gain at x1000, lower for smaller gains. I'm keeping an open mind about using something cheaper, thoughI'll continue that search later tonight.I'm using a TI Ref102 10V, 0.05% voltage reference with a 100-ohm 0.01% foil resistor in the current source. Aging specs for the reference aren't too bad and the tempco is 2.5ppm. It will be operated only for short intervals without heating up much.

author: grenert
author_rank: Frequent Contributor
timestamp: December 11, 2012, 02:06:18 am
content: Texas Components will make Vishay bulk foil precision low tempco resistors to whatever specifications you need (within the limitations of their technology, I think they go down to about 0.5R):http://www.texascomponents.com/This set of DMM calibration resistors includes 1R and 10R ones:http://www.precisionresistor.com/product.php?productid=144&cat=507&page=1

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 11, 2012, 02:32:02 am
content: Grenert, those links look good. I'll give them a call tomorrow. That DMM calibration set would be good to have on hand anyway.

author: robrenz
author_rank: Super Contributor
timestamp: December 11, 2012, 02:49:00 am
content: If you are using a 8846A or 8845A I have a suggestion that will probably give better accuracy and cost less.Make a low precision 1 amp source and spend all your money on a 0.1 ohm precision high wattage 4 wire shunt with low TCR. Pass the one amp thru the shunt and and DUT with the source leads and use the shunt voltage drop as the reference voltage into the fluke inRATIOmode. Then input the sense voltage from the DUT sense leads into the input voltage ofRATIOmode. Ratio mode will do the ratio of input V to reference V to 1ppm. That means you could theoretically resolve 0.1µ ohm with this setup. The beauty of this setup is the current does not have to be precise or even stable. You are making a direct ratiometric comparison to the precision shunt. Your error will be the ratio mode error plus the shunt error. This way you get -140 db of CMMR on the sense leads of the meter and Increased accuracy of being able to take readings at 100 NPLC for increased stability and effective resolution. You still need a current reversal switch to remove thermal emf errors.You could use reduced current levels for higher resistance but at 10 ohms and up the 8846A is very precise so not much point in that.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 11, 2012, 03:05:02 am
content: Unfortunately, I do not have an 8845 or 8846 (though I wish I did). Mine is quite a bit older. Still, this is a very clever and useful procedure. I'm not sure that it would work with my equipment, though. I don't have anything to precisely divide signals.PS- I saw your video on making the Kelvin clips. Very nice work! Clearly this is an area you know a lot about. I'm certainly grateful for your input.

author: nadona
author_rank: Regular Contributor
timestamp: February 10, 2014, 09:34:11 pm
content: This is old thread but instead of PM to robrenz, I decided to continued it. Maybe someone can use of this same information in the future.1) "spend all your money on a 0.1 ohm precision high wattage 4 wire shunt with low TCR."Like these?http://bit.ly/1gnovOkbit.ly/1dCQupP2) Any other DMMs have the "Ratio" functions? The "Ratio" function of HP 3456A & 3458A, are they same with Fluke's?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 11, 2014, 05:22:40 pm
content: Possibly the technique described athttp://conradhoffman.com/HamonResistor.htmlwill be of use. Once you have a master divider to compare against, choosing resistors for another is easy. Just set it up as a bridge and compare the two dividers with the most sensitive meter you have. Never let an old thread die!

author: nadona
author_rank: Regular Contributor
timestamp: February 11, 2014, 08:29:11 pm
content: Quote from: Conrad Hoffman on February 11, 2014, 05:22:40 pmThank you.Above thing can be used to measure 1mm ohm resistor to 10ppm or 100kohm to 10ppm at 1KHz?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 12, 2014, 01:00:11 am
content: Stray capacitance and probably even skin effect will throw things way off at 1 kHz. It doesn't seem like that high a frequency, but when you talk ppm, everything matters. You might find that ratio transformers are a better tool for AC, or at least could be used to cross check results. Good ratio transformers will be accurate to a couple PPM, look just like resistive dividers and will have 6 or more decades of adjustment.

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 01:09:26 am
content: According to the GR 1689 manual, to calibrate the meter 4 resistors calibrated to 20ppm at 1KHz are required.I bought the 7 decade Gertsch RT to get the 95.3K ohm measured at 1KHz. The problem is to begin with at least I need 1 resistor measured at 1KHz accurate to at least 20ppm.Thank you.

author: Vgkid
author_rank: Super Contributor
timestamp: February 12, 2014, 01:19:07 am
content: The only place for that is to send it off to the npl, that is what iet quotes for their calibration measurements.

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 01:28:35 am
content: What is that npl stands for? Wiki has no entry for it.All I need one. From there I can measure the others. And I have another GR RCL machine which need same resistors.IET sells calibration kit which is DC measured and accuracy isn't good as manual says.

author: Vgkid
author_rank: Super Contributor
timestamp: February 12, 2014, 05:46:10 am
content: NPL is the UK version of NIST,here is the advertisement datasheet for those calibration resistors.http://www.ietlabs.com/pdf/Datasheets/1689-9604.pdf

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 06:07:44 am
content: I have more accurate resistors than their calibration resistors.With their cal set I can't cal 0.01pF to 0.02% accuracy. May be 1pF to 0.05%Their cal set. "Accuracy: 0.1% of nominal value" puzzle me.1) Manual say it requires 20ppm accuracy measured at 1KHz. IET Cal set is measured with DC2) GR 1689 meter's spec is 0.02%.3) More accurate resistors are required to compensate on board four 0.1% resistors.

author: Vgkid
author_rank: Super Contributor
timestamp: February 12, 2014, 06:59:34 am
content: Here is the paper I was looking for, on the calibration of digibridgeshttp://www.ietlabs.com/pdf/application_notes/OntheCalibrationofDigibridgesandtheVerificationHPH.pdf

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 07:21:04 am
content: Thank you very much for the PDF.It looks like it doesn't have to be measured at 1KHz.Do you have URLs pointing to the Appendices?A. “Frequency Response of Standard Resistors”B. “Resistance Measurements: AC vs DC”C. “ Using Digibridges for “DC” Resistance Measurements”D. Values of HF Standards

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 06:15:14 pm
content: Thank you for the 3458A ratio explanation.1) Is that the shunt in the URL is right type? How precise it should be?2) 3456A's ratio is different from 3458A's?

author: nadona
author_rank: Regular Contributor
timestamp: February 12, 2014, 10:15:06 pm
content: DiligentMinds.com: I am very happy with your answer! Thank you.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 13, 2014, 03:00:06 am
content: A silly thing, but I sent a L&N resistor out for cal and the lab wanted to reject it because it was too far from its nominal value. What they forgot is that between the time the resistor was built, and the calibration, NIST changed the definition of the ohm slightly. I can't remember what year, but a lot of the used L&N stuff is easily that old. Personally I prefer older standards- any drifting has probably already happened! Do change the oil in standards, as the oil can become corrosive over time and change the resistance.

author: CaptnYellowShirt
author_rank: Frequent Contributor
timestamp: February 13, 2014, 05:40:45 am
content: Quote from: Conrad Hoffman on February 13, 2014, 03:00:06 am1990 was the change over in the US. The Ohm is now based on the Quantum Hall Standard (K_90 -http://en.wikipedia.org/wiki/Quantum_Hall_effect).Before K_90 (1884-1990) the world had the Mercury Column Ohm. It required someone to build a glass capillary with a cross section of 1mm^2 and a length of around 1m long.*If you've ever had the "pleasure" of working with Hg you'll know this must have been a read headache. Hg tends to either not wet solid metals or wet them so well it forms an amalgam of unknown properties.Its times like this that make me wonder: How did we ever get to the Moon?*I say "around" because it could have been 100cm, 106cm, 106.3cm depending on what 'type' of ohm you were trying to produce. But I think the world settled on the 106cm version by the 1900's.

author: Vgkid
author_rank: Super Contributor
timestamp: February 13, 2014, 06:40:58 am
content: AC vs DC Resistancehttp://www.ietlabs.com/pdf/application_notes/ResistanceMeasurementsAcVSDc.pdfvalues of HF standardshttp://www.ietlabs.com/pdf/application_notes/Values_of_HF_Standards_HPH.pdfcoax resistor measurements, includes the 1822, first piece of gear I almost ever bid onhttp://www.ietlabs.com/pdf/GR_Experimenters/1969/GenRad_Experimenter_March-April_1969.pdffreq response of resistorshttp://www.ietlabs.com/pdf/application_notes/OntheFrequencyResponseofResistorsHPH03-08v6.pdfstandard resistorshttp://www.ietlabs.com/pdf/application_notes/OntheFrequencyResponseofStandardResistorsHPH8-94.pdfhave fun reading

author: nadona
author_rank: Regular Contributor
timestamp: February 13, 2014, 08:53:38 am
content: Vgkid: I owe you a big one.

author: Vgkid
author_rank: Super Contributor
timestamp: February 14, 2014, 05:24:39 am
content: No problem, I own a 1689 as well. It started this test gear hobby, thats why I'm here.@ dilligent-minds, what are your thoughts on the VHZ series?

author: EEVblog
author_rank: Unknown
timestamp: February 14, 2014, 11:33:06 am
content: Quote from: DiligentMinds.com on February 14, 2014, 09:17:43 amI still find this a bit hard to believe. The worlds best precision resistor maker surely can't be wrong (or that far off) on their typical figures?They must have measured them?

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 14, 2014, 01:23:08 pm
content: Quote from: EEVblog on February 14, 2014, 11:33:06 amI couldn't believe that either.Dr. Zandman's company Vishay for me was the pioneer of resistor technology, especially metal bulk, but also metal film (Beyschlag, Philips, and others).Maybe, I've got a bad batch of 5 VHP202Z (0.3 .. 1.0ppm/K), but Vishay did not accept my complaint. Everything within spec. That's right.But that also proves, that you can forget about the typical values. Too optimistic.You now have four 10k resistors, 2EA of VHP100, 2 of Z type.Build a Wheatstone bridge with them, monitor the bridge output voltage, and vary the temperature of either one of the resistors types, +/- 5..10°C, perhaps, monitor the temperature change also, while keeping the others on constant room temperature.I bet, the T.C. can be measured very easily that way, and the different technologies will have clearly different T.C. orders of magnitude.Frank

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 14, 2014, 02:01:02 pm
content: You have to be careful with thermocouple voltages skewing the measurements. At this level you need to match materials as well as possible and never have the two leads at slightly different temperatures. I'd think a stirred oil bath would do the trick for the part, but pay attention to the rest of the chain as well. The resistors may well meet spec, but getting that performance in a real system is a whole different matter.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 14, 2014, 06:13:13 pm
content: Quote from: DiligentMinds.com on February 14, 2014, 04:37:45 pmAll five have a neg. TempCo, and were nearly linear. 2nd order is very weak.I won't change / compensate that, because now after I determined the individual TCs, I can very well live with it.The resistors seem to be repeatable to a few tenths of ppm, after backward calculation to 25°C.I have no VHP100. This type and its special construction had been suggested by a Vishay representative, who is a real technology wizard on those resistor types.His statement was, that the VHP100/101 are the superior ones concerning T.C.Additionally, their datasheet lacks this limit parameter of (0.2 (typ.) +/-2 (max)) ppm/K.They are solely defined by the 10ppm/30°C or 60ppm/180°C box.That guarantees 0.3ppm/K maximum, although in a box.. But the Z type resistors are also box-wise specified.Frank

author: quarks
author_rank: Frequent Contributor
timestamp: February 14, 2014, 09:09:34 pm
content: when I looked for the best possible TC for resistors (for LTZ1000), I was also told by VPG to go for the VHP101

author: quarks
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:18:48 am
content: Quote from: DiligentMinds.com on February 15, 2014, 12:37:07 amThat is good confirmation of my own research and communication with VPG and I can add, they use the following alloy types with different TCR behavior:C-Foil alloy has negative parabolic response 2ppm/KK-Foil alloy has positive parabolic response 1ppm/KZ-Foil and Z1-Foil C-alloy have positive parabolic response 0.2ppm/K (or about 5 times lower as K-Foil)Quote from: DiligentMinds.com on February 15, 2014, 12:37:07 amThat is interesting. Now I have a good explanation, why Dr. Franks VPG documented values seemed to be off, when we compared it to my SR104.In my comparisons the Keithley 2002 was the worst 8.5 Digit Ohmmeter.

author: quarks
author_rank: Frequent Contributor
timestamp: February 15, 2014, 06:54:27 pm
content: Quote from: DiligentMinds.com on February 15, 2014, 06:28:49 pmattached are the specs I used and a chart comparing your mentioned DMMsDo you have better specs for the 2002?

author: robrenz
author_rank: Super Contributor
timestamp: February 15, 2014, 07:34:42 pm
content: Nice Chart

author: robrenz
author_rank: Super Contributor
timestamp: February 16, 2014, 02:50:41 am
content: Below 1 Ohm the Agilent 34420A is much more accurate than the 3458A