Fluke calibration misconseption - worth knowing! - Page 1
Forum section: electronics / testgear

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 22, 2021, 04:24:54 pm
content: I have so often read posts where people talk about the calibration of the Fluke DMM's where it clearly indicated that people think of the calibration as a procedure where Fluke adjusts either hardware or software to compensate for drift.Here is how a Fluke calibration is performed:Fluke measures the DMM with high precision gear and note every measured data down on a calibration certificate.This is done on every calibration and after several calibrations is it possible to predict when a meter will be drifted too much for your needs.Calibration is NOT a calibration but an advanced status check of your meter's performance.

author: helius
author_rank: Super Contributor
timestamp: April 22, 2021, 05:08:05 pm
content: Calibrationmayinclude adjustment if it is possible and necessary to have readings within spec. Calibration is fundamentally a procedure comparing an instrument to a traceable standard. When your instrument comes with a cal certificate, the differences (offsets, errors) betwen it and the standard are written on the certificate. In the days of wood-cased instruments, these would be written on the paper inside the front cover and used as a matter of course when reading off measurements.Not all instruments can be adjusted, but anything that can be compared to a standard can be calibrated.

author: tggzzz
author_rank: Super Contributor
timestamp: April 22, 2021, 05:09:59 pm
content: Calibration is not adjustment. That is standard metrology terminology, and is common to all calibration labs and all equipment.You can send a steel ruler off to a calibration lab for calibration. Do you think they would ever alter its length?Usually in metrology, stability is more important than precision or accuracy. With calibration, you can see how stable an instrument is over time, and do simple arithmetic to convert a measure value to the "true" value. Adjustment destroys the history and makes stability calculations unreliable.

author: J-R
author_rank: Super Contributor
timestamp: April 22, 2021, 05:13:19 pm
content: Some Fluke-specific info and terminology:Typically a handheld Fluke DMM will come with a piece of paper that is an assurance of sorts from them to you stating the DMM met its basic accuracy specifications when it left the factory. However, that is not really a "calibration certificate". If you want a Fluke DMM with a calibration certificate it is typically about $150 more for that option. You can see this by looking at the Fluke web site and checking out the Fluke 289 and 87V for example. There is the regular DMM and then another entry with "CAL".However, for a bench DMM like the Fluke 8845A you will notice that it does come with "17025 accredited report of calibration included", so in that case it is a true calibration certificate.For the Fluke 287/289 and 80 Series V there is a "Calibration Manual" available for download. Fluke uses the terms "accuracy tests" under "Performance Tests" to explain the procedure to verify the DMM is working correctly and in within accuracy specifications.If the DMM does not pass those tests, then the "Calibration Adjustment" procedure is to be followed and "calibration adjustment steps" are provided.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 22, 2021, 05:15:14 pm
content: Quote from: tggzzz on April 22, 2021, 05:09:59 pmThis ^. Ninja'd while I was fetching the definition...http://www.websters1913.com/words/Calibration

author: threephase
author_rank: Regular Contributor
timestamp: April 22, 2021, 07:36:56 pm
content: The official definition of calibration comes from the International Vocabulary of Metrology, generally known as VIM. Currently on version 3, but those who are interested are lucky as there is a consultation on going over the next version and you can download this for free from the BIPM website;https://www.bipm.org/documents/20126/54295284/VIM4_CD_210111c.pdf/a57419b7-790f-2cca-f7c9-25d54d049bf6The official definition (section 5.18 on page 39 of the document above), is quite wordy, but it basically means that the term 'calibration' is indeed comparison of an instrument to a known standard, traceable to an official reference.Calibration labs and manufacturers will differ though. Some calibration labs will provide a form up front for you to select the services you require, which can be either 'calibration' or 'calibration and adjustment' with before and after values or just after adjustment values. Those that do not offer this, in my country, you can be certain that 'calibration' is just checking and no adjustment is made.Some of the more expensive apparatus that I have, the manufacturers do automatically adjust the instrument to the best performance they can get, it is quite rare for that to be done automatically though and it is always stated up front that they do this. It also comes with a hefty price tag.

author: tautech
author_rank: Super Contributor
timestamp: April 22, 2021, 07:58:03 pm
content: Quote from: FriedMule on April 22, 2021, 04:24:54 pmCalled Performance Verification and the first step in a calibration process that may only measure/record measurement drift if the instrument still meets manufacturer accuracy spec.Adjustments to meet manufacturers spec are a more involved process as most modern instruments only support closed box automated adjustment processes.

author: boggis the cat
author_rank: Regular Contributor
timestamp: April 23, 2021, 04:14:38 am
content: Quote from: tautech on April 22, 2021, 07:58:03 pmThis is an area where Fluke and other 'big brands' can provide the user value.You can download verification and adjustment processes for most Fluke and Agilent instruments (and some of the other big name manufacturer's). In most modern multimeters and similar, Fluke typically have 'hidden' calibration commands for automated verification and adjustment (it's all unavailable / under NDA)but alsoallow the user to run through an in-built adjustment process if they wish to do so.The really cheap stuff (from China and elsewhere) make it essentially impossible to maintain calibration accuracy as no process to adjust is available. You can still monitor it, and if it is stable enough then correct for inaccuracy, but it is less usable due to no way to adjust the indications. Claimed specifications often range from very optimistic to ludicrous.It's always worth checking out whether something you are looking to buy has any documentation available to allow adjustment and other maintenance.

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2021, 04:32:44 am
content: Quote from: FriedMule on April 22, 2021, 04:24:54 pmYou asked them? Fluke offers a huge range of calibration services for a wide variety of instruments. The metrologist's definition of the word 'calibration' is not binding on the calibration service provider, so if you care, you should ask first.For a typical bench DMM, a typical cal lab might offer three or four different services and either ANSI Z540, ISO 17025 or no accreditation as you request. The cheapest services are 'no data' or 'OOT data only', where they will verify that it is within specs (typically the one-year specs) and either not give you any 'as found' data or only give you the ranges that were out of spec and adjusted. The most expensive service might be ISO 17025 accredited with 'as-found' and 'as-left' data and adjustment, hopefully to 24hr specs. Policies and details vary (a lot) from lab to lab and from instrument to instrument.FWIW, on standard DMMs up to 6.5 digits, Flukes listed calibration services will all include adjustment AFAIK.

author: boggis the cat
author_rank: Regular Contributor
timestamp: April 23, 2021, 04:46:28 am
content: Quote from: bdunham7 on April 23, 2021, 04:32:44 amOnly if Fluke consider it necessary. It isn't clear what criteria they use.When dealing directly with Fluke it is difficult to get information on their process and criteria. They have a 'take it or leave it' approach.A note on the 'service levels' stuff: if you are going to pay for a calibration service, then you are almost certainly going to want / require the data. Possibly you can get by without the accreditation logo stamped on the certificate, but a piece of paper claiming that the instrument meets specifications is worthless. Providers know this, so they're likely just using this option to low-ball a price to potential customers.

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2021, 04:55:55 am
content: Quote from: boggis the cat on April 23, 2021, 04:46:28 amSince their calibrators are used very widely in the test equipment business they actually offer more services than most might think.Gold Support ProgramTo keep pace with the fast changing oscilloscope market, Fluke Calibration is continuously writing new calibration procedures.For a one-off, low-cost payment (less than it would cost you to have three or four DSO oscilloscope calibration procedures written), you can buy into our MET/SUPPORT Gold Software Support Program which gives you access to every new procedure written by Fluke’s software support team during the next 12 months.On current performance, that’s upwards of 100 new oscilloscope calibration procedures per year. And you won’t have to request updates or wait for delivery; every new procedure we write is available for download from our web site.In addition to the free procedures library,you will receive 60 days priority support to get you up and running with MET/SUPPORT Gold.You also have the option to buy into our MET/SUPPORT Gold program, which offers you various advantages, including the opportunity to download new procedures from our web site or even obtain custom written procedures. If the oscilloscope calibration procedure you require is not available and you need it quickly, we can write it for you at very competitive rates as part of our fast-track procedure writing scheme.https://us.flukecal.com/products/electrical-calibration/electrical-calibrators/9500b-oscilloscope-calibrator?quicktabs_product_details=1

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2021, 05:10:39 am
content: Quote from: boggis the cat on April 23, 2021, 04:46:28 amActually quite a swath of calibration customers want nothing of the sort. The calibration certification is required for one reason or another (ISO, regulation, etc) and they just want the cert at the lowest cost and least inconvenience.QuoteThat is true for devices that are not out of spec to begin with. And without the data, I don't think you would ever know. It's very hard to reach anyone knowledgeable, in my experience as a not-very-important customer.

author: james_s
author_rank: Super Contributor
timestamp: April 23, 2021, 05:47:33 am
content: Quote from: bdunham7 on April 23, 2021, 05:10:39 amThat was my experience working at an ISO9001 certified company. All equipment had to have up to date calibration, half the time the stuff came back with faults it didn't have when it was sent out, and calibration seals on the housing so I couldn't easily get in there and fix them. We had one very expensive device that came back with a screw missing from the inside, I tried to plug a cable in and the whole input board came unplugged from the motherboardNone of that mattered though, there was a valid calibration sticker on the thing so we were good to go as far as the suits were concerned.

author: FriedMule
author_rank: Frequent Contributor
timestamp: April 23, 2021, 06:49:18 am
content: Sorry I was not specific enough, what I meant by DMM was the handheld DMM's.And as tggzzz wrote Its fare more important to know how a meter is drifting between calibrations than resetting and correcting for errors. If Fluke was capable to calibrate a meter, would you expect each meter to be calibrated to nearly perfection and a deviation certification for each DMM instead of a general standard printed in the manual.It's a bit like a crystal, you keep an eye on how it drifts and do everything to never bump / reset it because resetting a crystal does that you have no chance of knowing it's drift pattern.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2021, 07:22:10 am
content: Quote from: boggis the cat on April 23, 2021, 04:14:38 amReally? There's an endless stream of Fluke fans here who are proud that their gear "never drifts".Quote from: boggis the cat on April 23, 2021, 04:14:38 amNone of my cheap Chinese gear appears to be drifting.Maybe "drifting" isn't a thing any more now we got rid of the trim pots and use digital calibration instead.Quote from: boggis the cat on April 23, 2021, 04:14:38 amMy $25 Anengs agree with much more expensive meters to within a single digit, even the 22000 count ones (AN870). Maybe digital calibration took out the "worker in a sweatshop twiddling trim pots" factor.Ifone ever goes wildly wrong then they cost less to replace than a single Fluke calibration.

author: tautech
author_rank: Super Contributor
timestamp: April 23, 2021, 07:23:08 am
content: Quote from: FriedMule on April 23, 2021, 06:49:18 amNo.Every instrument drifts from the instant it is turned ON until it meets equilibrium operating temp where it only needs to meet the manufacturers spec and if this % (level) of accuracy is insufficient for your needs you need a better instrument.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2021, 07:33:26 am
content: Quote from: FriedMule on April 23, 2021, 06:49:18 amYep.Purely anecdotally: Has anybody here had a digitally-calibrated handheld DMM that "drifts"?(ie. with age, not with ambient temperature)

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2021, 07:39:11 am
content: For a lot of devices, if they're outside of their spec that means the device is faulty and should never be trusted from that point on.At least not trusted for any important readings that need some level of confidence.Even if you did tweak the calibration adjustments to bring it back into spec, would you now trust it, knowing it has a history of going out of spec in the past?Usually what you care about is confidence in your readings.You can never be totally sure a device didn't have a fault and go out-of-spec 3 seconds before you used it.The trendline of the drift over the last n cal checks gives you confidence it's still in spec or when it might drift out of spec.If the device has been in-spec for its last 10 calibration checks and not drift much from center then you can be quite confident it is likely still in spec when you're taking a reading now. It has a history of being accurateOf course there are some exceptions, some devices need to have their calibration adjusted because drift is unavoidable.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2021, 07:45:19 am
content: Quote from: Psi on April 23, 2021, 07:39:11 amThis is one of the reasons for always owning multiple DMMs - you can check them against each other and get to know/trust them.Quote from: Psi on April 23, 2021, 07:39:11 amDevices with a simple voltage reference are very unlikely to go out of spec with time, eg. handheld DMMs

author: Psi
author_rank: Super Contributor
timestamp: April 23, 2021, 07:50:42 am
content: Quote from: Fungus on April 23, 2021, 07:45:19 amYeah.Other things like torque/load sensors often need to be adjusted during calibration because the act of using them (applying a torque/load) will, over-time, physically twist or deform them and cause drift.You can monitor how bad this is by looking at how much adjustment is needed and shorten the time between calibration so it's always being adjusted before it drifts too much.

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2021, 08:04:04 am
content: Quote from: Fungus on April 23, 2021, 07:45:19 am...and if they do, they need to be binned/repaired, not adjusted.A voltage reference that goes out of spec over time isn't to be trusted at all.

author: Arhigos
author_rank: Regular Contributor
timestamp: April 23, 2021, 11:28:11 am
content: Quote from: Psi on April 23, 2021, 07:39:11 amNo. Devices having guaranteed drift only for a specific period of time. Normally one year, some can be extended for 2 years, for example if you check fluke 754 specification:1 year 3vdc is 0.02 % and 2 year is 0.03 %. Nothing can have a lifetime calibration spec.If you need trusted device to make sure everything is ok for all period of calibration you should get ISO/IEC certificate 17025 with accreditation. If you just need to check perfomance then ANSI/NCSL Z540.1 should be enough. (i'm talking about US, not sure about other countries)

author: Fungus
author_rank: Super Contributor
timestamp: April 23, 2021, 11:56:31 am
content: Quote from: Arhigos on April 23, 2021, 11:28:11 amThat's just ass-covering. Does it happen in practice?I've seen 30-year old meters that agree with brand new ones down to the last digit. This makes me suspect that most calibration certificates are just ass-covering for legal reasons.(for devices that are based around an internal voltage reference)

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 23, 2021, 02:46:39 pm
content: I sent a number of Fluke meters to Fluke for other problems and they did the calibration check and they adjusted none of them. That makes me think I wouldn't need to send them my meters for calibration yearly as very likely no adjustments is needed.

author: tggzzz
author_rank: Super Contributor
timestamp: April 23, 2021, 03:01:40 pm
content: "Need" must be in terms of "for the purpose of ...".Often calibration is done so you could demonstrate in a court of law that our measurements were correct (within the stated limits).If only one person/organisation is involved, adjustment is easy: use a screwdriver to make the values agree. If more than one, then open your wallet!

author: jonpaul
author_rank: Super Contributor
timestamp: April 23, 2021, 04:30:17 pm
content: All:This discussion seems bizzare, We Have a fine Fluke 8842A 5 1/2 d DVM, the CAL button allows procedures for ADC, DCV, ACV, ohms 2 wire Ohms 4 wire amps.Every function can be CALed and opon completion of each routine the constants are stroed in an EEPROM or NVRAM.After the CAL there is a noticeable improvement in accuracy.Perhaps the lower accuracy DVMs do not have a CAL feature.Kind Regards,Jon

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2021, 04:51:31 pm
content: Quote from: Fungus on April 23, 2021, 11:56:31 amTypical performance will be much better than the specified uncertainties from a reputable manufacturer. That's just the natural result of including the environmental limits and a 95% or 99% confidence interval in the specifications. You can try and estimate the typical performance from specs from someone like Fluke/HPAK/TeKeithley, whereas specs from some 2nd tier manufacturers may just be aspirational.Long-term drift is a real thing, but varies greatly with the deviceand how it is selected and burned in. In most non-metrology devices, other factors will outweigh reference drift.

author: bdunham7
author_rank: Super Contributor
timestamp: April 23, 2021, 04:56:41 pm
content: Quote from: jonpaul on April 23, 2021, 04:30:17 pmI think most of them have a cal feature, although it may be practically inaccessible in some cases. Every modern Fluke that I'm aware of--and every one I own that doesn't have trimpots--has a method similar to your 8842A in which you enter CAL mode, select a mode and range and then present the meter with a calibrated stimulus. Handheld DMMs typically won't allow you to use custom values like the 8842A as this is less necessary with lower precision.

author: Arhigos
author_rank: Regular Contributor
timestamp: April 23, 2021, 11:06:27 pm
content: Quote from: Fungus on April 23, 2021, 11:56:31 amOr course it's happening. Why do you think calibration labs even exists lol?What kind of 30 year old multimeters you had and how you compared them?

author: Fungus
author_rank: Super Contributor
timestamp: April 25, 2021, 07:11:01 pm
content: Quote from: Arhigos on April 23, 2021, 11:06:27 pmFor verification and issuing of paperwork?Where's the example of a meter that went out of spec?

author: bdunham7
author_rank: Super Contributor
timestamp: April 25, 2021, 07:40:25 pm
content: Quote from: Fungus on April 25, 2021, 07:11:01 pmhttps://www.eevblog.com/forum/testgear/hp-34401a-reads-a-little-low-on-ac/msg3241784/#msg3241784

author: Brumby
author_rank: Unknown
timestamp: April 26, 2021, 12:40:39 am
content: Just my thoughts.......I will go out on a limb here and say that, unless you are working at quantum levels where the laws of physics ensures unwavering quantities, EVERYTHING drifts. The question is by how much and over what time period.If you have a device that drifts two orders of magnitude below its finest resolution, then you will never see it - and, clearly, there are some devices which we have encountered that appear eligible for that category!The whole purpose of "calibration" is to ensure confidence in the measurements taken. When you get into metrology level measurements, from what I have encountered, the word "adjustment" sends shivers down spines. In these situations, the HISTORY of comparison to a standard on how a device performs is at least - if not more - important than the point-in-time accuracy. Even if a device falls out of spec, if we have a record of the history, then we can still have a high level of confidence in what the actual values were at the time of measurement.For example, if you have a meter which is used to measure 1V to within a 50nV tolerance and the calibration history (comparison to a standard) shows it read 4nV low three years ago. 8nV low two years ago, 12nV low last year and 16nV low right now, then you can be pretty confident you have a linear change - and that this will allow you to determine the actual voltage at any time between these points with a very high degree of confidence. Importantly, it will also indicate theexpecteddeviation over the following year. In the above example, you will have a high level of confidence that the unit will read 18nV low in six months. You will be keen to see the calibration report in a year's time to ensure this was correct, but in the mean time, you will have reasonable confidence in the measurements taken.Adjust anything and you have changed the system, rendering the previous history highly suspect, if not useless. In the days of trimpots, any adjustment changes the mechanical state of the contact between the wiper and resistive element where mechanical settling through pressure, vibration and/or thermal cycling would ensure you need to start a new history record and it would need a number of calibrations (comparisons to a standard) over a period of time to establish the pattern of change. Certainly, the modern process of storing calibration data digitally reduces many such susceptibilities, but not everything.

author: Fungus
author_rank: Super Contributor
timestamp: April 26, 2021, 06:18:43 am
content: Quote from: bdunham7 on April 25, 2021, 07:40:25 pmYour own posts suggest that meter was somehow damaged (overvoltage?) and replacing some resistors put it back into spec with no other adjustment needed:https://www.eevblog.com/forum/testgear/hp-34401a-reads-a-little-low-on-ac/msg3301010/#msg3301010

author: bdunham7
author_rank: Super Contributor
timestamp: April 26, 2021, 02:21:54 pm
content: Quote from: Fungus on April 26, 2021, 06:18:43 amMaybe that was the cause, I never got enough information to really know. But the point is that I had an apparently nice working meter that was just a little out of spec. Does the cause matter? The point is that if accuracy is critical, just assuming that meters don't go out of spec, or even extrapolating assumed drift from previous observation, is insufficient.

author: Fungus
author_rank: Super Contributor
timestamp: April 26, 2021, 05:29:42 pm
content: Quote from: bdunham7 on April 26, 2021, 02:21:54 pmI'm not saying that meters don't ever go out of spec, it's the slow, constant accuracy drift that needs a yearly correction that I'm doubting.There's a whole legion of Fluke handheld owners on here who are always proud to tell us that their meters are still bang-on after 20/30 years.(and I believe them!)There's also a few 1980s Radio Shack meter owners who say the same thing.As for Anengs? They haven't been around that long yet but my money would be on them staying in spec even without laser-trimmed resistor arrays. We'll see...

author: alm
author_rank: Super Contributor
timestamp: April 26, 2021, 07:37:21 pm
content: Do DMMs and their components experience a constant drift? Yes. The most well-studied references are the calibration lab standards. For example, here is data for the 10V output of a Fluke 732A voltage standard from "PREDICTION OF THE OUTPUT VOLTAGE OF DC VOLTAGE STANDARDS", 2009, by Ilić et al:And here is the 1.018V output, which is a resistive divider driven from the 10V output:And here is data for the ESI SR104 standard resistor compiled by zlymex, that I grabbed fromhere:It's even common do use linear regression to try to predict the value of the reference more accurately than assuming the last calibrated value. As described in the paper I cited. This would be a waste of time if the references did not have a constant drift.A DMM will obviously have a less predictable drift, because there are more components affecting the accuracy, all with their own drift characteristics. Here is data from a HP 34401A calibrated three times in 12 years. I believe I got this data of the PMEL forum. Clearly the DMM is drifting, although based on this limited data it does not seem linear.If the components in a handheld meter would drift less, the engineers designing these instruments would have done a horrible job. So clearly the handhelds will drift. Is this drift enough to be noticeable given their resolution? Maybe not.Also, very high or low ranges will generally drift more, and ACV will drift more than DCV. I think the statements about the meter still being bang-on after decades is usually on something like a 9V battery, not a 10 MOhm resistor, or a 700V 20 kHz AC signal, which based on another report on 34401A meters that I pulled from PMEL was found to be out of tolerance 8 times out of 250 tests (the point with the highest number of OOT events):Do you have any data on the meters being 'bang on'? Number of meters, range they were tested on, uncertainty of the test? How big is a 'bang'?Are we talking about agreeing to a least significant digit? The 24h tolerance? 1 year tolerance?

author: bdunham7
author_rank: Super Contributor
timestamp: April 26, 2021, 07:38:46 pm
content: Quote from: Fungus on April 26, 2021, 05:29:42 pmDrift depends on many factors, but one big one, especially for heated references, is power-on time. So a meter which is specified to drift less than a certain spec for a year might mean that they specify that the drift will be less than that amount even if the meter is on 24/7, and that with 95 or 99% confidence. So the meter that is on for less than 100 hours per year probably won't experience that. Yes, the vast majority of 3-4 digit DMMs will not go out of spec even over a very long time--and drift is probably less likely to be the issue at that level of accuracy.. Its not a conspiracy, it is just statistics.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2021, 02:59:36 am
content: Quote from: alm on April 26, 2021, 07:37:21 pmI'm not claiming that they don't drift, but if they aren't powered up much, drift may be the least important component of their actual uncertainties.QuoteIt's probably true that the commonly measured 10.00000 volts is the most accurate and stable range on the meter--as one might expect--but you need to include the uncertainties of the calibration equipment, especially when you look at those more difficult ranges. The single 34401A example really shows this. Also, your last example actually shows pretty much the expected result--a 96.8% rate of within-limits vs a specified 95% confidence interval. So simple statistics would indicate that two thirds of the meters would actually be off by less than half of the specified, which for that signal is what I would call "bang on".EDIT: I noticed later that 700V @ 20kHz seems to be a nonstandard calibration point and I didn't get the same uncertainty from the 1-year specs.QuoteI have 8 Fluke 8840/8842A type 5.5 digit meters here in various conditions ranging from mint to war ravaged and each of them at least meets their 90 day spec on the 2 and 20V ranges and probably the 24-hour spec. I haven't verified the other ranges yet but I haven't found any out of spec to the best of my ability to measure them. None of the are anywhere near recently calibrated. These are probably a bit exceptional in that regard due to the type of reference they use.