How are precision multimeters calibrated by manufacturers ? - Page 1
Forum section: electronics / testgear

author: michaeliv
author_rank: Frequent Contributor
timestamp: August 06, 2015, 04:44:31 am
content: Hi,I was wondering what do manufacturers use for calibrating high precision multimeters like the KeySight 3458A 8½ Digit multimeter.I've found various scenarios similar to "I've calibrated my multimeter to a voltage reference that was calibrated to a precision multimeter that was factory-calibrated and still in spec". But it never goes beyond that, how do manufacturers actually calibrate them and if they use a refference, how is that calibrated ?Is there a process which generates an exact guaranteed voltage, like Casium atoms oscillations can be used to exactly define a second ?Thanks!

author: daqq
author_rank: Super Contributor
timestamp: August 06, 2015, 04:51:03 am
content: QuoteThere are some processes that give you a known voltage. In ye olde days Weston cells were used for such reasons:https://en.wikipedia.org/wiki/Weston_cellToday, the best voltage reference available is a Josephson junction array:http://www.hypres.com/products/voltage-standard/

author: LaurentR
author_rank: Frequent Contributor
timestamp: August 06, 2015, 05:25:49 am
content: Very nice read, totally accessible:http://smile.amazon.com/Calibration-Philosophy-Practice-Fluke-Corporation/dp/0963865005/

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 06, 2015, 05:42:56 am
content: The 3458A is special as it only needs two references for complete traceable calibration, 10V and 10kOhm. (Artefact Calibration, see hp journal 4-1989)For 10V, you would need a Fluke 732B, which previously had been calibrated against a Josephson Junction Array quantum standard.For 10kOhm, you would need an ESI - SR104, which previously had been calibrated against a von-Klitzing quantum hall effect standard.All other DMMs (like the Fluke 8508A) need a "gold standard" calibrator like the Fluke 5720A, which itself is an artefact standard also (itself calibrated from 10V, 10k, 1Ohm, see above).Due to the needed TUR (Test Uncertainty Ratio) for these high grade , it will need to be verified to tighter specs. The source of this are again the mentioned quantum standards, plus some ratio standards like the 720A, the 752A, and AC/DC converters as thermocouples or the 792A.All these DMMs need to be calibrated mode by mode and range by range.P.S.: The JJA and the Hall effect quantum standards are supposed to become the official S.I. references for the Volt and Ohm in about 2018, if some of the S.I. definitions will be modified.Then, the Volt and Ohm will be based also on quantum standards, and as uncertain as the Second which is by the Cesium clock.Frank

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2015, 05:59:26 am
content: This post and general discussion in this recent thread:https://www.eevblog.com/forum/testgear/auto-calibration-in-modern-digital-scopes-and-digital-multimeters/msg722152/#msg722152

author: acbern
author_rank: Frequent Contributor
timestamp: August 06, 2015, 07:02:50 am
content: Quote from: Dr. Frank on August 06, 2015, 05:42:56 amWell, what you refer to is adjustment. Calibration, as I am sure you know, means comparison of values measured by the DUT against standards. Keysight calls it performance verification in their calibration manual. The 3458A actually makes it easy to adjust it by its artifact method (Flukes 5700 and 5720 support this too), but the validation still requires a check of every measurement value, with details given in the 3458A calibration manual (tests to do and limits and so on). And you need to do it after you have done the adjustment. There is a military document somwhere on the web, that states validation every second time is ok with a 3458A, in the end this is a question of probabilities. So there is no way arround precise standards for resistance, voltage, and AC, more precise than the 3458A, to do a reasonable calibration. It must be mentioned that the 3458A when calibrated by Keysight has a 2ppm tolerance of the reference used allone, so when you get your 3458Aback from Keysight, so e.g. the 100V is not precise more than 4.8ppm against national standards.For DC, using a reference divider like self-calibration Fluke 752 resistive Hammon divider together with a 732 reference and a non-calibrated voltage source (5440 or 5700) will do. For AC, a Fluke 5720A or a cal'ed Datron 4920 and a stable source can be used. Another way, if you have one stable and precise AC voltage, is to use this and a Dekatran AC voltage divider for the lower frequencies, which by design is very precise. And for ohms, appropriate ohms references are needed.So as you see, there are some ways to circumvent a Fluke 5720A in certain areas, but still you need some precise gear.

author: Timur Born
author_rank: Regular Contributor
timestamp: August 06, 2015, 07:27:09 am
content: Interesting how these terms vary. With displays "calibration" usually means "adjustment" (of LUT tables), while "profiling" means "comparison measurement against standards".

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 06, 2015, 07:28:19 am
content: Quote from: LaurentR on August 06, 2015, 05:25:49 amNice book! Strongly consider to buy it. Nice addition to my personal book library!Thanks for pointing out! Will show this to a friend as well. So maybe that will be 2 books on Amazon

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 06, 2015, 08:53:10 am
content: Quote from: acbern on August 06, 2015, 07:02:50 amDear acbern,you are completely right about the verification, completing the calibration process, although HP once called that mechanism artefact calibration, and claimed full traceability to national standards, only by this.Now I wonder, how they can offer a real "calibration" at all, as they probably won't use a 752A, 732A, etc. for verification.They instead use a 5725A for verification (I got a verification report on that going with my new 34465A), but even a Gold Standard instrument would not provide enough TUR for verification of the 24h specification, as it's having approximately the same level of uncertainty than the 3458A.So how would that really work in reality?Do they check against 1 year or 90 days limits only?Frank

author: codeboy2k
author_rank: Super Contributor
timestamp: August 06, 2015, 08:58:55 am
content: Quote from: pascal_sweden on August 06, 2015, 07:28:19 amFYI: The first edition is readable online via the Internet Archive Libraryhttps://archive.org/details/Calibration-PhilosophyInPractice

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 06, 2015, 09:31:38 am
content: Quote from: pascal_sweden on August 06, 2015, 07:28:19 amThe very old, 1st edition can be downloaded via goggle books, I think.The 2nd edition can not be purchased, as it's been distributed by Fluke exclusively on their calibration trainings.It is available only as 2nd hand.Anyhow, both are highly recommended.Frank

author: acbern
author_rank: Frequent Contributor
timestamp: August 06, 2015, 09:40:54 am
content: Quote from: Dr. Frank on August 06, 2015, 08:53:10 amThats interesting information. The calibration manual calls for a 3458A opt 002 with 90 day cal. as the gold standard. The 5700 that they use as a transfer standard may in your case be replaced by the amp for generating higher voltages, together with something else, just guessing. (What it was should be listed in your cal cert and it would be pretty interesting to know).So that golden 3458A referenced in the cal manual means an uncertainty of 2ppm + 2.6ppm + 0.05ppm for 10V (I have not checked the error calc that they use in the cal manual, potentially their golden 3458A uses a better than 2ppm voltage reference, that should still be checked). That is not very exciting. The 3458A can do better, if properly adjusted and checked. E.g., my standard 732a is calibrated to 0.3ppm, and can be used to verify all DC voltages together with a 752, so you can do much better at home on DC when you do it right. Of course this takes time, and time is money, so an external cal is probably always some compromise, unless you want to pay a lot. Thats maybe also why Kexsight, as I read somewhere, has a higher level calibration available as well. Details of which unclear to me.There is a lot of mentioning of how precise the Keysight standard cal is in many blog entries, also on voltnuts. I have always had some doubt about that. If you can shed some light on this as per your cal report that would be great.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 06, 2015, 12:17:43 pm
content: Quote from: acbern on August 06, 2015, 09:40:54 amSorry, about the different 3458A calibration levels at Keysight, I also can make an educated guess only.. That exactly was the discussion on volt-nuts.For my 34465A, the verification of the initial production calibration, which came with the instrument, does not specify the verification limits at all!So they simply verified several "main" ranges and noted the deviation, but only wrote the 1 year limits beside that.So nobody has a clue, what their internal acceptance levels on this verification test would be..All DCV ranges were spot on, 1V .. 1000V at or below the 1ppm level, 100mV at about 15ppm (due to thermal voltages), but everything about 10 times better than 24hr. specification, as expected.So I could snatch another 10V verification point for my volt reference bank, demonstrating that my 3458A is still spot on in the 10V range.In fact, I repeated the verification of the 34465A, and found it being off 9..15ppm in the 100V and 1kV DC ranges, whereas the 100mV, 1V, 10V were still spot on (thanks to Quarks).The 10kOhm also measured my 10k standards to exactly 10.00000k (thanks again, dear abern!)At recalibration, I found out that there is a fault with the 34465A and 34470A HV ranges, in conjunction with the ACAL command, which busts the calibration, and also with the input impedance flag (set to 10M), which erratically introduces 3ppm error.So Keysight did the verification w/o ACAL.. FAIL!I strongly suggested a firmware error, and my instrument will be sent to Keysight for analysis..The new / replacement instrument already indicates that it has the identical errors.OK, but that's another story..Frank

author: zapta
author_rank: Super Contributor
timestamp: August 09, 2015, 03:31:10 pm
content: Quote from: Dr. Frank on August 06, 2015, 05:42:56 amVery interesting reading (including the other articles). Thanks.BTW, it says that "Calibration is a process in which individual gain and offset values are adjusted, manually or electronically..." which is different from the definition I saw here in a few threads.

author: macboy
author_rank: Super Contributor
timestamp: August 09, 2015, 05:46:01 pm
content: Quote from: zapta on August 09, 2015, 03:31:10 pmSome here suggest thatcalibrationis exactly and onlyverificationand thatadjustmentis a separate process. I strongly disagree. Calibration for me has always been a two-fold process: verification followed by optional adjustment (and another verification). For good reason, adjustment is avoided if the instrument is in spec, even if it is reading a little high or low andcouldtechnically be tweaked just a little to be made "better". The main reason is to ensure consistency. If the instrument goes for calibration and comes back as "in spec", you should be able to repeat a measurement done before calibration to get the same result. If the instrument were adjusted, you know that you would not get the same result.

author: quarks
author_rank: Frequent Contributor
timestamp: March 22, 2016, 06:42:07 am
content: I totally missed this, but it still is interesting to read.

author: nidlaX
author_rank: Frequent Contributor
timestamp: March 22, 2016, 08:19:39 am
content: Does anyone know how much those Hypres JJA chips retail for?

author: tszaboo
author_rank: Super Contributor
timestamp: March 22, 2016, 09:37:37 am
content: Quote from: macboy on August 09, 2015, 05:46:01 pmYou dont really need toguesswhat calibration and verification means. It is an established term. Every engineer ever using a meter should read this.International vocabulary of metrology — Basic and general concepts and associated termshttp://www.bipm.org/en/publications/guides/vim.html

author: tggzzz
author_rank: Super Contributor
timestamp: March 22, 2016, 11:01:21 am
content: Quote from: macboy on August 09, 2015, 05:46:01 pmI too disagree. For me calibration means ensuring I have enough milk in my coffee. Of course I accept that other people might be confused by my non-standard use of terminology.

author: macboy
author_rank: Super Contributor
timestamp: March 22, 2016, 12:38:49 pm
content: Quote from: tggzzz on March 22, 2016, 11:01:21 amThanks for contributing.In any case,NANDblog is right, no need to guess, just read the "dictionary":QuoteIn simple terms and in the present context: verification to specifications, followed by adjustment if not in spec.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 22, 2016, 04:16:04 pm
content: Quote from: macboy on March 22, 2016, 12:38:49 pmNo, there's still one point missing: you need again a verification to specifications, after adjustment.Reason is, that the adjustment process might fail, or might not work as expected.I'm a burnt child, with my new 34465A. (see :https://www.eevblog.com/forum/testgear/keysight's-new-34465a-(6-5-digit)-and-34470a-(7-5-digit)-bench-multimeters/msg889215/#msg889215.)Keysight adjusts each DMM coming from factory, or after 3 month of storage, and verifies the result DIRECTLY after the adjustment.That's contained in each Test Report, which claims <1ppm deviation for every DCV range.Anyhow, the 100V and 1kV do have a systematic +10ppm shift / failure, if the verification is done at a LATER verification time, e.g. after engaging ACAL.So, even the best "calibration" practices have to be challenged.That 34465A issue taught me several lessons in calibration / adjustment.Frank

author: retrolefty
author_rank: Super Contributor
timestamp: March 22, 2016, 10:33:31 pm
content: Quote from: Timur Born on August 06, 2015, 07:27:09 amIt's Dave's fault, he stated firmly in one of his videos that "calibration is not adjustment".

author: ez24
author_rank: Super Contributor
timestamp: March 23, 2016, 01:47:29 am
content: Quote from: michaeliv on August 06, 2015, 04:44:31 amI had this same question about a year ago when I bought a voltage standard for about $100. I believe he used a 3458A to calibrate my unit. On his web site, he stated that his unit was calibrated by Keystone in Colorado. I got on their web site and after digging around, found they use a proprietary Josephson Junction. I believe Tin set it up for them. ha haThe US Standard Lab (sorry forgot the proper name) uses JJ for the US national voltage standards. My guess the only member here that has built one is Tin (I believe he is into liquid helium ?). If I remember right there was an article on groundbreaking price reduction that got the price down to $1 million and only required one PHd to set it up. And they export them to other countries.Being that $1 million is US government price, I would imagine Keystone's price to be less. I wonder if anyone here knows for sure? But regardless I know it would be out of my range even if I sold my house.Here is a 4 page summary that has the proper names for more research.http://nvlpubs.nist.gov/nistpubs/sp958-lide/315-318.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: March 23, 2016, 07:34:46 am
content: Quote from: retrolefty on March 22, 2016, 10:33:31 pmMaybe he was talking about a reference, not a meter. With a reference, you just adjust the label, and that is it.

author: Towger
author_rank: Super Contributor
timestamp: March 23, 2016, 08:12:32 am
content: I think there is some confusion over the calibration of a new meter and of a old/used meter. I would fully expect calibration of new meter in the factory to require adjustments, hardware and/or software depending on design. However, once it is in the customers hands 'ideally' it should stay calibrated and calibration is just an annual check of this, tracking long term drift.If it does require later adjustment, the significance of this depends on the quality of the meter. A 5 year old handheld, not a big deal, however with a 5 year old 8 digit bench meter there is something serious wrong with it.

author: nctnico
author_rank: Super Contributor
timestamp: March 23, 2016, 10:35:06 am
content: Quote from: Timur Born on August 06, 2015, 07:27:09 amAccording to Webster's dictionary 'calibration' means checking (verifying) and adjusting.

author: tggzzz
author_rank: Super Contributor
timestamp: March 23, 2016, 11:05:00 am
content: Quote from: nctnico on March 23, 2016, 10:35:06 am“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” “The question is,” said Alice, “whether you can make words mean so many different things.” “The question is,” said Humpty Dumpty, “which is to be master—that’s all.”LEWIS CARROLL (Charles L. Dodgson), Through the Looking-Glass, chapter 6, p. 205 (1934). First published in 1872.

author: billfernandez
author_rank: Regular Contributor
timestamp: March 23, 2016, 06:32:41 pm
content: Quote from: Timur Born on August 06, 2015, 07:27:09 amTrue. These terms are used differently in different contexts.

author: Wytnucls
author_rank: Super Contributor
timestamp: March 23, 2016, 08:35:59 pm
content: International vocabulary of metrology:2.39 (6.11)calibrationoperation that, under specified conditions, in a firststep, establishes a relation between the quantityvalues with measurement uncertainties providedby measurement standards and correspondingindications with associated measurement uncertaintiesand, in a second step, uses this informationto establish a relation for obtaining a measurementresult from an indicationNOTE 1 A calibration may be expressed by a statement,calibration function, calibration diagram, calibrationcurve, or calibration table. In some cases, it may consist ofan additive or multiplicative correction of the indicationwith associated measurement uncertainty.NOTE 2Calibration should not be confused withadjustmentof a measuring system, often mistakenlycalled “self-calibration”, nor with verification of calibration.http://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2008.pdf

author: TiN
author_rank: Super Contributor
timestamp: March 24, 2016, 01:00:32 am
content: QuoteYikes, now I'm revealed, uh-oh.But No, I'm into liquid nitrogen stuff and the best references I poses are array of LTZ1000-based gear and modules around, just like any other volt-nut here. JJA chip itself cost about 50K$, and complete PJVS standard system with closed-loop refrigeration system is 400K$, according to NIST site. I covered thisbefore in overview here.

author: macboy
author_rank: Super Contributor
timestamp: March 24, 2016, 01:40:41 pm
content: Quote from: Towger on March 23, 2016, 08:12:32 amI think the reverse is true. If a 5 year old handheld requires adjustment, it is defective (or abused), plain and simple. There is absolutely no reason for a handheld to drift out of spec, since the specs are relatively loose to begin with. A good handheld might be e.g. +-0.025% which is 250 ppm. But that 8.5 digit metrology grade meter is spec'd to be within 8 ppm of actual. They can drift several ppm per year. So after 5 years of continuous use (many are powered continuously for stability) the drift adds up and they instrument is unlikely to be within spec.

author: ez24
author_rank: Super Contributor
timestamp: March 24, 2016, 08:12:51 pm
content: Quote from: TiN on March 24, 2016, 01:00:32 am

author: Hobby73
author_rank: Regular Contributor
timestamp: March 29, 2016, 07:00:37 am
content: <prepare for dumb question>I looked at a few articles/videos where DMM owners calibrated their own meters. It looks straightforward. I noticed the adjustment is always performed on the voltage measurement.If the voltage is calibrated, does that imply that the current and resistance measurements are also calibrated?I didn't see any separate steps for the other two. I know ohms law, and it seems like you would need to calibrate at least two of the three measures to ensure they are all accurate but all the calibration work was done for voltage only.

author: Fungus
author_rank: Super Contributor
timestamp: March 29, 2016, 07:48:34 am
content: Quote from: Hobby73 on March 29, 2016, 07:00:37 amYes.(usually - it obviously depends on the meter)Quote from: Hobby73 on March 29, 2016, 07:00:37 amThe other two can be done by having very high-precision resistors inside the meter and measuring voltage drop across them. If voltage is calibrated then they're automatically calibrated too.

author: FireDragon
author_rank: Regular Contributor
timestamp: April 01, 2016, 07:08:06 am
content: I have what is, perhaps, a stupid question.If an 8 1/2 digit meter has an error greater than 1 ppm then what is the point of 8 1/2 digits vs. a 6 digit meter? If the extra 2 1/2 digits are essentially noise in the measurement error, then everything that I know indicates that they are a waste of time.What is the actual difference in measurement accuracy between a 6 1/2 , 7 1/2 and 8 1/2 digit meter? Surely the price difference has to buy something! Likewise, if the 6 1/2 digit meter has a measurement error measured in 100s of ppm then, again, what is the point? Or is just all marketing?

author: michaeliv
author_rank: Frequent Contributor
timestamp: April 01, 2016, 07:21:12 am
content: Quote from: FireDragon on April 01, 2016, 07:08:06 amThe point is resolution, not necessarily accuracy. The last digits are not noise, they are not random numbers, but numbers that correlate to the measured value.Ex: If you want to measure the PPM of the LTZ1000 (0.05ppm/°C) or even the PPM of some precision resistors, you can't do that with a 6 1/2 voltmeter as the reading basically won't change over a 20C temperature delta. Here you don't care if the voltage is 100% accurate, but you care that you get an accurate voltage delta at different temperatures.Also an 8 1/2 digit multimeter will have a better ppm spec than a 6 1/2 digit multimeter + extra features probably.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: April 01, 2016, 09:30:12 am
content: Quote from: Fungus on March 29, 2016, 07:48:34 amOnly if one assumes these "high-precision resistors" have not drifted outside specification.

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 01, 2016, 10:13:26 am
content: Quote from: Hobby73 on March 29, 2016, 07:00:37 amAt first, you obviously always mean ".. adjust their own meters", instead of "...calibrate..."Most DMM need adjustments for each separate range and mode, that's on the order of about 30 different Cardinal Points (100mV, 1V, 10V,..) to adjust.The same is true for calibration or verification of a DMM, there you also need all of these different Cardinal Points, i.e. you need a calibrator.So, generally, it's a "No" to your question. Volt and Ohm ranges are independent from each other, and therefore, Volt adjustment or calibration does not adjust or calibrate resistance or current ranges.This statement from Fungus is not correct!I doubt, what you have seen on these videos, really provides a complete adjustment.Many people buy these simple 5V /10V / 1mA / 10k "references", e.g. this DMMCheck, which can only serve for check of a few of the ranges of a DMM.There only exist a few DMMs or calibrators, which need only 10V, 10kOhm, and 1 Ohm to fully adjust (AutoCalibrate) all of their ranges and modes. But here, again, Ohm and Volt are independent, and current can be derived from Ohm's law.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 01, 2016, 12:21:12 pm
content: Quote from: FireDragon on April 01, 2016, 07:08:06 amNo, these are not stupid questions, but you have to distinguish and correlate more carefully these cruicial parameters:- resolution- uncertainty- stability / noise- linearity1. If a meter with 8 1/2 digitresolutionhas anuncertaintygreater than 1 ppm then what is the point of 8 1/2 digits vs. a 6 digit meter?Answers:6 digit DMMs resolve 5 digits only at the lower end of their scale, which is equivalent to about 10ppm only. Example: 01.200 00V on 10V range of a 34401A.7 and 8 digit DMMs provide enough resolution to sustain full uncertainty at these low values also.As the best zener references usually have uncertainties (and stabilities) only a bit lower than 1ppm, more than 7 digits are not necessary regarding absolute uncertainty.If you do high precisionrelativecomparisons between two external DUTs , the stability of the DMMs reference is of importance, and the linearity of its A/D. That allows to use the full 8 digits and really achieve up to 0.02ppm oftransfer uncertainty, which lower grade DMMs simply can't do.Examples can be found in this forum like in the Resistor Stability thread, or the Ultra LTZ 1000 thread.2. If the extra 2 1/2 digits are essentially noise in the measurement error, then everything that I know indicates that they are a waste of time.Answer:That's a different aspect, namely stability.Noise can obscure uncertainty, but an instrument can be noisy and however have low uncertainty, e.g. by averaging.Therefore, the sampling time (NPLC) also determines the noise and (direct) usability of the last digits. It's disturbing on Keithley DMMs, that they display too many digits for small NPLC numbers.In practise, 8 1/2 digit DMMs usually have much less noise than 6 digit instruments. (See our big Noise Comparison thread.)The 3458A delivers stable 8 digits readings, when using NPLC 100 and an evenly stable DCV source.3. What is the actual difference in terms of measurementuncertainty, stability, linearity and noisebetween a 6 1/2 , 7 1/2 and 8 1/2 digit meter? Surely the price difference has to buy something!Exactly, 8 1/2 digit meters have better references for lower uncertainty and stability, better amplifiers for less noise, and usually have more linear A/Ds.Look inside the 3458A, what kind of effort they have done to achieve that.The 7 digit nV DMM, the 34420A, has much less noise than the other DMMs, including the 3458A, that explains its high price. But it's not better in uncertainty and linearity than any other 6 1/2 digit DMM.The price difference also buys reputation, i.e. a 7 1/2 digit DMM automatically sells for double the price than the equivalent 6 1/2 digit type.See my comparison between the 34465A and the 34470A. Their only HW difference is the better LTZ1000A reference in the '470A, about 350$ spare part cost, and a SW enabled 7 digit display. (FW is identical, also). They are completely identical in specification, besides DCV.Anyhow, this 7 1/2 - digit - touch simply costs 1500$ more.There is absolutely no technical reason for this 7 1/2 digit resolution concerning linearity, uncertainty, stability.Frank