Multimeter calibration DIY - tools needed? - Page 1
Forum section: electronics / general

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 27, 2022, 09:57:40 am
content: Is it realistic to keep my multimeters calibrated myself?I have never tried, so for those that have, what tools are needed? I heard a quip about it, if you have a 5.5 meter, you need a 6.5 to calibrate it, etc. Is this true?What other equipment is needed? Is an electronic load considered accurate enough to calibrate multimeters? A search didnt turn up much.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 27, 2022, 11:02:49 am
content: Quote from: mapleLC on April 27, 2022, 09:57:40 amBy and large you need calibration equipment that is an order of magnitude better in terms of precision than the device to be calibrated, so yes, a 6.5 digit to calibrate a 5.5 digit etc.Depends on the load; most of the ones I've seen (in mortal price ranges) are 4.5 digit at best, so no good for calibrating meters better than 3.5 digit.Then you need to consider, how do you calibrate your calibration equipment..?If you're just doing this for home hobbyist sanity checks on your meters, you might want to think about getting a decent 6.5 digit meter and having it professionally calibrated, then using it to check your lesser meters.Bear in mind the uncertainties involved; they add together, so a 6.5 digit meter with say, 0.002% +/-5 digits could give a reading of 9.99975 to 10.00025 when measuring a voltage of exactly (good luck with that) 10.00000 volts, so when you use your calibrated 6.5 digit meter on anunknown voltage source, with the 5.5 digit in parallel, the uncertainty is 0.002% +/-5 digits + the uncertainty of the 5.5 digit meter, say 0.005% +/-3 digits, so your total uncertainty would be 0.007% +/-8 digits (it isn't exactly, but that's a close enough approximation without giving yourself a headache with the actual maths).

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 27, 2022, 11:35:46 am
content: Quote from: AVGresponding on April 27, 2022, 11:02:49 amThanks, I understand the error factor adding up, it must get frustrating keeping up with this when you are at the edge of measurement science.So an electronic load is not suitable for use in calibration? This confuses me. The 5.5 and 6.5 quip aside, what do you measure against when calibrating? Its my understanding that a calibrator does not fill this role, perhaps I am mistaken.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 27, 2022, 12:31:22 pm
content: What measurement did you want to use the load to calibrate? I'm guessing current?A multifunction calibrator like a Fluke 5XXX-series will be a voltage and current source, both AC and DC, and also a decade resistor, and have the range to cover the extreme ends of the ranges of the meters it needs to calibrate, so, say for example you have a generic 3.5 digit handheld DMM. It probably measures 200.0mV to 750VAC @20Hz to 20kHz, 200.0mV to 1000VDC, 200.0uA to 10.00A AC/DC, 200.0Ω to 20.00MΩ.In order to calibrate the meter, the calibrator must provide V, A, and Ω at the ends, or near the ends of the meter's ranges. The calibrator needs to be able to source 20.000mV to 700.0V AC at 20.0Hz to 20.0kHz, 20.000mV to 900.0V DC, 20.000uA to 9.000A AC/DC, and 20.00Ω to 18.000MΩat leastin order to do this.A common electronic load would not be able to do this, except for maybe the 10ADC range.When you calibrate a meter, the calibrator provides a source measurement, say 10.000V. The meter must display this value (10.00V for a 3.5 digit) within the specified uncertainties to pass calibration; if it doesn't it might have to be adjusted (that's a whole other can of worms).The calibrator will have been calibrated against a more precise calibrator itself; the chain continues until you reach the national standard for your country, this is what is meant by "traceable calibration", it can be traced back, with all the uncertainties specified, to the national standard, and ultimately the international standardhttps://en.wikipedia.org/wiki/International_System_of_Units#Introduction.

author: mapleLC
author_rank: Frequent Contributor
timestamp: April 27, 2022, 12:42:48 pm
content: Quote from: AVGresponding on April 27, 2022, 12:31:22 pmOk so how does this square with the 5.5 and you need a 6.5 digit meter thing?If this calibrator has the feature set, is there no other tool needed?

author: AVGresponding
author_rank: Super Contributor
timestamp: April 27, 2022, 01:36:00 pm
content: You need an order of magnitude (minimum) so that the uncertainties in the more precise meter don't have an impact on the less precise one.Let's say a 6.5 digit meter reads 10.00000V from the calibration lab (which will have its own level of uncertainty) at the end of it's upper limit, as 10.00007V (I got the dp wrong on my uncertainty in my previous post). Then you use it with your own voltage source to calibrate a 5.5 digit meter.You would set your voltage source so that your 6.5 digit reads 10.00000 (not 10.00007 as this could be "actually" 10.00000 to 10.00014). This means the "actual" voltage could be from 9.99993 to 10.00007.The 5.5 digit meter would read 10.00000 volts as 9.9999(5) to 10.0000(5) +/-3 digits, so 9.9996 to 10.0003 (the bracketed 5 represents the % uncertainty, on this meter, a Fluke 8840A, it's too small to affect this reading, the +/-3 digits is the relevant factor here).If the "actual" voltage was between 9.99993 to 10.00007 due to the uncertainty of the 6.5 digit meter, the 5.5 digit meter would read from 9.9995 to 10.0004, which is in fact just outside the specs of the 5.5 meter example here... which goes to prove my Keithley 2000 isn't good enough to be used as a transfer standard for calibrating my Fluke 8840A!It has the precision, but not the accuracy (I used the 90 day accuracy numbers, it doesn't seem practicable to me that you could get it back from the lab, and warmed up within a 24 hour period).

author: Fuzzy Star
author_rank: Contributor
timestamp: April 27, 2022, 03:03:11 pm
content: Quote from: mapleLC on April 27, 2022, 09:57:40 amNot exactly what you're asking but you might find this tread interesting:https://www.eevblog.com/forum/testgear/non-precision-multimeter-function-tester-which-properties-are-required/

author: alm
author_rank: Super Contributor
timestamp: April 27, 2022, 06:28:46 pm
content: A 6.5 digit meter may not be enough to calibrate a 5.5 digit meter. The uncertainty doesn't usually go down by an order of magnitude as the resolution goes up by a digit. For example the Keithley 199 is 90 ppm/year basic DCV, the Fluke 8840A is 50 ppm/year, and the HPAK 34401A is 35 ppm/year. So if the 34401A is on a yearly calibration interval, it won't be able to verify a Fluke 8840A is within its 1 year specifications. You really need to do the math for every point to calculate the test uncertainty ratio, the ratio of the uncertainty of the device under test to the uncertainty of the reference. The lower the test uncertainty ratio, the more careful you'll need to be in terms of uncertainty calculations, guard banding, etc.

author: J-R
author_rank: Super Contributor
timestamp: April 27, 2022, 06:39:46 pm
content: There is a lot of good info to be found in the Metrology threads, just be careful not to go too deep.In my opinion, unless you're a true volt-nut, it's not particularly realistic for the average hobbyist to try to calibrate a 6.5 digit DMM due to the necessary "ducks in a row" (equipment/money, knowledge, experience), but 5.5 and lower are relatively trivial if you have a "good" 6.5 digit DMM.Personally, I obtained an older, used 34401A, then had it calibrated and obtained the data. It required no adjustment from the previous calibration from many years prior, so this built a bit of confidence in it.I also obtained some standards, such as the DMMCheckPlus along with the measured values.Armed with that information, I reproduced the conditions of the cal lab (time from power on and temperature) and then compared the readout with the calibration data and verified that everything lined up between the DMM and the standards. As an example, DC voltage matched to the very last digit exactly (yes, in 6.5 digit mode).Now you are pretty well set up to use the 6.5 DMM to calibrate/adjust a 5.5 or lower DMM.For some equipment you most likely need to generate the precise output that the equipment expects, which can require some effort, but for others you can simply enter the value as displayed on your 6.5 digit DMM and that is adequate.Every year or so I mail my standards in and that is far cheaper than having the 34401A calibrated.Beyond this, there are perhaps some other items that you will probably find useful in order to generate the various necessary values, such as resistor decade boxes + 10-turn potentiometers, function generator + random transformers/ratio-tran, high voltage DC-DC converter, etc. The metrology folks will balk, but I think for the hobbyist this is all fine.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 07:04:41 pm
content: Quote from: mapleLC on April 27, 2022, 09:57:40 amNo. Well...If you spend a few thousand dollars on some old calibrators and an accurate 6.5-digit DMM, then spend a few weeks or months hunting parts for your old calibrators and fixing them up and trying to adjust them with your 6.5-digit DMM, then perhaps you can reasonably calibrate--or at least check calibration--on your4.5-digit and lessmeters. But many handhelds will require a separate precision high current transconductance amplifier, so you'll need some more money and time for that, unless you happen to have spend a pile of money on a newer, better calibrator with high enough current capacity.If you want to pretend to be able to calibrate longer scale meters, then you start skulking around trying to score a large variety of resistance and voltage standards, K-V dividers and maybe even an old, broken high-performance calibrator that just theoretically might be fixable, as well as a few old HP 3458As (apparently the minimum is 3 so that they can be compared) and set aside at least two rooms in your house for this new obsession. Note that basements and garages typically aren't temperature controlled enough for this venture, so bedrooms and living rooms must be sacrificed. After 3-5 years (minimum) and a few tens of thousands of dollars, you might begin to think that you can probably calibrate a 6.5-digit DMM, but you won't be completely sure.So yeah, totally doable.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 27, 2022, 07:31:29 pm
content: Quote from: alm on April 27, 2022, 06:28:46 pmYes, thus illustrating the difference between precision and accuracy. In fairness the 8840A and 8842A haveverygood accuracy specs for their precision, better than many other 5.5 digit meters.Quote from: bdunham7 on April 27, 2022, 07:04:41 pmI think you overestimate the costs involved by quite a bit. Even here in the UK I've paid way less than that for my accumulation, which includes AC/DC V/I sources from uA to A and uV to kV. I've numerous 5.5 and 6.5 digit meters (Iamdue for some expenditure on calibration, managed to get it paid by work last time...) and I can promise you the total costs so far will be in the low-few-thousand dollars, not tens of thousands.From what I see in my ebay searches, the US is usually cheaper for many items, notably HPAK, Keithley, Tektronix and Fluke all being cheaper on average. Some of it is so cheap that it's worth the cost of shipping to the UK, though not often...No idea on cal lab fees in the US though.

author: J-R
author_rank: Super Contributor
timestamp: April 27, 2022, 07:55:17 pm
content: Will have to disagree with some, I've been through the full calibration procedures on two Fluke 8840A units and an 8842A and this is easily done with a stable 6.5 digit DMM like the 34401A assuming you have recent calibration data and at least one other way to verify it before starting. In my case I have a Keithley 2015 and 2010 as well as quite a few transfer standards/references to cross-check things. I will say the PDVS2mini is a big help for a lot of this, but not required.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 08:58:32 pm
content: Quote from: J-R on April 27, 2022, 07:55:17 pmYou can do that and with a great deal of thought and care, you may get decent results--at least on DC volts. However, your TUR and other requirements for a 'real' calibration are nowhere near being met. The 34401A and 8842A are in about the same class accuracy-wise and there are some points where the tolerances of the 8842A are actually better. You are relying on your reference meter to be 'characterized' as much better than the stated specifications, especially if you are doing the ADC calibration procedure on the 8842A. A recent calibration, even with data, just isn't enough to support that conclusion.

author: alm
author_rank: Super Contributor
timestamp: April 27, 2022, 09:10:23 pm
content: Quote from: AVGresponding on April 27, 2022, 07:31:29 pmYes, but the main point is that the improvement of uncertainty slows down at high resolution. Another example with current models:3.5 digit: Fluke 117 0.5%4.5 digit: Fluke 87V 0.05% (ratio with previous model: 10)5.5 digit: Fluke 8808/Keysight 34450A 0.015% (ratio with previous model: 3)6.5 digit: Keysight 34461A 0.0035% (ratio with previous model: 4.3)7.5 digit: Keysight 34475A 0.0016% (ratio with previous model: 2.2)8.5 digit: Keysight 3458A 0.0008% (ratio with previous model: 2)For the test uncertainty, keep in mind that the (type A) uncertainty due to the setup (e.g. noise in the source) will be added to the specified uncertainty of the instrument (type B error) (seethis explanation), so the ratio will generally be worse than the ratio between the specifications. Seethis paper by Flukeon how to calibrate a DMM with a reference that is only borderline using guard banding and other techniques.Quote from: J-R on April 27, 2022, 07:55:17 pmWhat was your worst-case test uncertainty ratio? We are talking about calibration here, right, so that would involve the evaluation of measurement uncertainties. Of course you can do a sanity check that makes it plausible that a unit is at least close to specs, like things Dave does in a video about turning something on, not letting it warm up, look at the value for a second and say that it appears close enough. But otherwise I'd think the best you can get from the 34401A is it's 24h uncertainty (15 ppm on its best DCV range), while the 8842A's best 1 year uncertainty is 30 ppm. So you'd need quite a lot of guard banding to say with 95% confidence that the 8842A is in spec.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 09:14:35 pm
content: Quote from: AVGresponding on April 27, 2022, 07:31:29 pmI'm basing that on having 3 HP 3458A with at least one calibrated, a Fluke 5700A, at least one if not more Fluke 732A/B standards and and some high-end standard resistors. And a bunch of other stuff, of course.

author: TiN
author_rank: Super Contributor
timestamp: April 27, 2022, 10:45:14 pm
content: QuoteDepends on multimeter, but in typical scenario answer is no. Unless you have academic/volt-nut interest in going thru calibration/ppms it is not cost or time effective to calibrate your DMM instead of paying $100-300 for external accredited calibration lab work with data (typical price for 4.5-6.5d meter).QuoteEvery good manufacturer lists equipment and procedures recommended to perform calibration and/or adjustment of the instrument. Those who don't list such information, well, not worthy spending time for calibrationQuoteDepends. Usually for typical 5.5d meter you do NOT need another DMM, uncertainty of typical calibrators (5500A/5520A/5700A, etc) for job is more than adequate to perform direct adjustment/calibration. But if you talking about calibrating long-scale meter, doing more complex setups, then additional equipment like characterized 3458A, nanovoltmeters, picoammeters, resistance bridges, sensors might be required.Even if you have 8.5d DMM and want just to calibrate 5.5d meter, just a DMM itself is useless, because you need good enough and wide-range source to generate various DCV, ACV, resistance, DCI, ACI, temperature, capacitance and similar stimuli signals. If you don't have 1kV source that can maintain stability better than 5.5d meter spec for duration of test, you cannot calibrate 5.5d 1kV range to meters specifications, no matter how many other DMMs are available ;-).Sure, one crazy enough could DIY all needed sources/references/etc but it would eat thousands $ and months of the time until you reach even 5.5d specifications on all points and ranges. Or one could buy used broken calibrator for $4000-30000, then repair, test and calibrate calibrator now, and all for what? To do $200 5.5d calibration.QuoteElectronic loads at best offer 0.1-0.05% accuracy and often lack of stability and resolution to perform anything useful with 5.5d DMM. Typical workhorse for DMMs calibration is Fluke 55xx series multi-function calibrator.

author: J-R
author_rank: Super Contributor
timestamp: April 27, 2022, 11:54:26 pm
content: The performance tests and calibration procedures for the 8840A are relatively simple and straightforward. Section 6 of the manual:https://assets.fluke.com/manuals/8840A___imeng0300.pdfThe 8840A/8842A also have the helpful variable input feature which allows you to enter the exact value from your references, with some limitations.Using various electronics 101 methods it is not overly difficult to create all of the required inputs, no calibrator required.

author: bdunham7
author_rank: Super Contributor
timestamp: April 28, 2022, 12:12:17 am
content: Quote from: J-R on April 27, 2022, 11:54:26 pmThe procedures are straightforward, but do your sources meet the accuracy requirements in that manual? Even if you used the variable input w/reference meter method, they still have to be very stable and your reference meter needs to meet the requirement if you want a proper job. That requirement is listed as 10ppm or better for DC volts. You can get by with worse, but with no assurance of an in-spec meter when you're done.As far as inputs go, I struggle with the 200VAC @ 100kHz. I've used 100VAC @ 50kHz on occasion and it's better than nothing.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 28, 2022, 10:31:23 am
content: Quote from: bdunham7 on April 27, 2022, 09:14:35 pmI wasn't talking about setting up my own calibration lab!I was just saying you can have very good sanity checking to a reasonable degree of precision and accuracy without spending 10's of thousands. Obviously if I use one of my traceably calibrated 6.5 or 5.5 digit meters to calibrate one of my handheld 3.5 or 4.5 digit meters, it doesn't qualify as "traceable calibration", but it's good enough for my personal use.

author: J-R
author_rank: Super Contributor
timestamp: April 30, 2022, 10:38:34 pm
content: Pages 6-4 and 6-5 of the Fluke 8840A manual helpfully show the allowed min/max values and the AC voltage especially allows a wide berth. For the verification tests I used a function generator and a transformer to step up the voltage.A big point being missed here is that if you want traceable accuracy you can pull out your calibrated 6.5 digit DMM and use that. For the 5.5 and lower, you are not going to grab them and demand the very last digit be spot on, that is just being intellectually dishonest with the conditions of the argument.

author: bdunham7
author_rank: Super Contributor
timestamp: May 01, 2022, 03:20:33 am
content: Quote from: J-R on April 30, 2022, 10:38:34 pmOK, if good enough is good enough, then there you go. But you might not have the 6.5 digit meter handy, or in my case, I might be selling that 5.5 digit meter to someone who actually does expect it to be spot-on. And being reliably spot-on for years on end is pretty much the only reason to have an 8842A, especially at todays prices. Its feature set is pretty sparse, although it is quick and easy to use.In most cases with the 8842A, an ad-hoc calibration procedure is likely to make it worse than it was before. There are exceptions such as if the calibration data is lost or the AC board is swapped, but there aren't that many other reasons to be doing anything more than a performance verification, in which case there's no harm done.

author: alm
author_rank: Super Contributor
timestamp: May 01, 2022, 08:08:45 am
content: Quote from: J-R on April 30, 2022, 10:38:34 pmWhen talking about multimeter calibration, this is generally assumed to be a verification that the meter meets its factory specifications (1y if your generous, 24 if you're strict). This will have to consider the specified uncertainty of the instrument, the uncertainty of the reference and the process itself. If you say I want to verify that the instrument is within 0.1% of nominal, then this will of course make things much easier. But as bdunham7 says, what's the point of this. Any 8842A that has not had a part replaced that affects accuracy should be much better than that as is.

author: J-R
author_rank: Super Contributor
timestamp: May 02, 2022, 06:53:41 am
content: Quote from: bdunham7 on May 01, 2022, 03:20:33 amObviously if you need any of that, then ship it off for calibration!Quote from: bdunham7 on May 01, 2022, 03:20:33 amI'm not sure what you mean by "an ad-hoc calibration procedure", but if only a specific function and range are out of spec the A/D calibration can be skipped and you can perform the offset and gain calibration alone. This is mentioned in both the 8840A and 8842A manuals. I specifically went through this on one of the 8842A units (primarily the 20 Ohm/mV range) and the result was a major improvement. On one 8842A I did NOT touch the calibration and it's quite a bit off in the 20 Ohm 4 wire resistance range, although still barely meets the 2 year spec. The only repair I performed on that was to re-solder the transformer to fix a flaky display.Of course calibration procedures vary between DMMs so there could be further nuances to discuss in those cases. The Brymen BM869s is a DMM that is easy to calibrate/adjust. I've done this a few times to better take advantage of the 500,000 count mode.Quote from: alm on May 01, 2022, 08:08:45 amIn the case of the Fluke 8840A/8842A, specific language that is used by Fluke is "performance test" in order to verify specifications and "calibration procedure" to adjust anything if necessary or desired in order to meet your requirements.

author: alm
author_rank: Super Contributor
timestamp: May 02, 2022, 08:55:01 am
content: Quote from: J-R on May 02, 2022, 06:53:41 amThat may have been the terminology back then, but it's not how it's used these days in the context of DMM calibration:EEVblog #420: What Is Calibration?For adjustments it's especially important to be mindful of uncertainties, or you may very well make things worse.

author: bdunham7
author_rank: Super Contributor
timestamp: May 02, 2022, 02:02:31 pm
content: Quote from: J-R on May 02, 2022, 06:53:41 amBy 'ad-hoc' I mean using proceduresand equipmentthat doesn't meet the manufacturers official requirements but is deemed good enough for the purpose at hand. I'm not criticizing your use of these methods and I do this myself. I'm just a bit more circumspect about how good I can claim that the results are. Yes, you can adjust one range and that is exactly what I do when repairing/swapping AC boards. As for your 8842A low ranges, were they out of the spec with offset control or were they exceeding the number of counts allowed in the manual for operation without offset control? The one thing that wouldn't surprise me is having the offset drift a bit in the two low ranges with the separate gain amplifier. But most users wouldn't notice because they would use offset/zero in these ranges anyway. I hardly check this because the offset number without zeroing typically drifts around with temperature and on-time.QuoteIt would be clearer if they used the terms "performance verification" and "calibration constant setting". One oft-missed point about this distinction is that many of these instruments specify values for the performance verification that do not match the points used to set the offset and gain constants. The idea is that the calibration constants are set at zero and either full range, FS or near-FS (1.9xxxx). Then the performance verification may be done at some other points, often the most difficult points that will expose any problems with the instrument. So in theory, a cal lab should be doing a complete, separate performance verification after setting the constants.

author: TiN
author_rank: Super Contributor
timestamp: May 03, 2022, 01:20:47 am
content: QuoteIn practice any good lab will do at least two calibrations (which is measurement only of the unknown DUT vs known reference)as received, and calibrationas shipped.Customer may want to have adjustment performed which is separateoptionaloperation, which will go in between two calibrations. Some customers would always prefer to lab never touch adjustments on instruments, since they might be doing in-house history analysis of the device or do other studies, like stability of the instrument under shipment/stress/different labs conditions, etc. By default one should expect that calibration lab wouldNOTdo any adjustments without explicit customer say so.