multimeter 1 year accuracy specs - Page 1
Forum section: electronics / general

author: taydin
author_rank: Frequent Contributor
timestamp: December 14, 2019, 08:01:26 pm
content: I am in the market to get a Keithley DMM 7510 and currently looking at the accuracy specs. The accuracy is stated for 24 hours, 90 days, 1 year, 2 years etc. And as the time increases, the accuracy spec gets worse.Let's assume that I bought the 7510 and after 1 year, I take it to a calibration lab and have it properly calibrated. Once that is done, will the accuracy "restart" from the 24 hour spec? I think this is true, but just wanted to make sure that the volt nuts confirm it

author: MegaVolt
author_rank: Frequent Contributor
timestamp: December 16, 2019, 05:31:57 pm
content: Quote from: taydin on December 14, 2019, 08:01:26 pmJudging by the Calibration Data Report, the instrument is checked for compliance with 1-year tolerances.

author: niner_007
author_rank: Frequent Contributor
timestamp: December 18, 2019, 09:38:45 pm
content: If it is adjusted, then I imagine the 24h spec would apply

author: TiN
author_rank: Super Contributor
timestamp: December 19, 2019, 04:30:26 am
content: Adjustment is irrelevant for calibration period. Calibration means that your device isverifiedagainst standard with known uncertainty. If such standard have complete chain of verifications up to SI unit, then such calibration is also traceable. Period of calibration (with adjustment, or verification only without and adjustments) is up to the user/owner of the device under the test.If you don't specify it, calibration labs will usually go for 1 year manufacturer spec, to determine if device is meet datasheet spec or not. Cheap calibration will tell you exactly that "Device IN SPEC" or "Device OUT OF SPEC". More expensive calibrations will often provide you with measured, expected reference, and uncertainty values on each test point and you could get exact idea about particular device behavior.It does not mean that device will become out of the calibration after X hours/days/months, it is only shows you "expectation" spec, provided by the manufacturer, based on their product stability/quality assesment. How stable each specific unit is depends on many factors, including environment, shipping stress, use cases, age, used components, settings and so on.Calibration is also valid only at the time of the verification. Moment device leaves the calibration lab - it becomes a statistics excercise. Hence most of lab, who reallyneedtraceability and accurate measurements have own standards/calibration labs to perform verification (and adjustment if needed) at same location, or maintain multiple instruments on rotation at nearby calibration lab.

author: taydin
author_rank: Frequent Contributor
timestamp: December 20, 2019, 12:54:51 pm
content: Thanks for the responses guys. So there is a choice between having calibration done to meet the 1 year spec and having calibration done to meet the 24 hour spec, with extra expense I presume.If the extra expense is not an issue, would it make sense to go for the 24 hour spec when calibrating? Here is the rationale that I am thinking:When the device is manufactured and post-manufacturing calibration is done, the part will be drifting quite a lot, because no aging has occurred on the unit as a whole. So I think the unit will lose its 24 hour accuracy spec quite quickly.But when I take the unit for calibration 1 year later, some aging will have already occurred. So when I ask the lab to do 24 hour calibration, the unit will hold the 24 hour calibration for much longer, and when it finally loses it, it will not have drifted too far off of that spec.Does this way of thinking make sense?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: December 20, 2019, 06:04:09 pm
content: Quote from: taydin on December 20, 2019, 12:54:51 pmOnly in a year will it become clear how much he will drift. And only after many years, having collected statistics, we can begin to consider the drift to be constant and predictable.

author: mzzj
author_rank: Super Contributor
timestamp: December 20, 2019, 06:07:22 pm
content: Quote from: taydin on December 20, 2019, 12:54:51 pmYour reasoning makes sense in a way but it's not how manufacturers have meant the specs.24h or 1 year specification is just a number to compare and predict your unit's stability.Imagine a system that drifts +0.001mV per day constantly to one direction: After 1 day you have 0.001mV error and after 365 days you have 365 times bigger error.(in reality the manufacturers would do their best to balance the drift so its random plus minus zero. Since its never perfect it still drifts somewhat but the error doesn't just simply accumulate at constant rate)Fresh off the calibration you can expect your equipment to havedriftedless than 24h specification, one year after calibration you can expectdriftto be within 1 year specification.(also note that calibration is not same as adjustment, there can be still large error even if you got your instrument calibrated just recently)

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 30, 2019, 08:48:03 am
content: As I understand TS question, he doesn't need to calibrate 'in 24h', because that "24h" period starts every 24h. It's not for calibrating, but for predicting device tolerance next 24h.

author: tggzzz
author_rank: Super Contributor
timestamp: December 30, 2019, 09:50:36 am
content: Quote from: taydin on December 20, 2019, 12:54:51 pmIt sounds like you are confusing calibration and adjustment.As noted above, with a cheap calibration the only difference between a 24hr and 1year would be an increased chance of failing the 24hr "calibration".If you want a meter adjusted, it is often better to send it back to the original manufacturer.

author: taydin
author_rank: Frequent Contributor
timestamp: December 30, 2019, 03:10:01 pm
content: Quote from: tggzzz on December 30, 2019, 09:50:36 amI don't understand why you are making that distinction ... If I send the multimeter to a cal lab and they bring it to the 24h tolerance level it is called calibration, but when I send it to the manufacturer and they bring it to the 24h tolerance level, it is NOT called calibration, but it is called adjustment? Why? Both have the same objective, which is to meet a certain accuracy spec.And it isn't clear to me what you mean with cheap calibration. I won't tell the calibration lab a "target price" when I ask them to calibrate my meter. I will tell them a "target accuracy spec", which is the 24h spec. And it is quite reasonable that this is more expensive than calibrating towards the 1 year spec.

author: taydin
author_rank: Frequent Contributor
timestamp: December 30, 2019, 03:17:26 pm
content: Quote from: Vovk_Z on December 30, 2019, 08:48:03 amSorry, I don't understand what you mean. But here is a summary what I want to achieve: If you look at the Keithley DMM 7510 datasheet, you will see 24h spec, and 1 year spec (and some other timeframes that are not relevant for our discussion). The 24h spec is the best one.The 7510 is calibrated to meet the 24h spec when it leaves the factory. But because it is new, it will be drifting fast and will soon lose the 24h accuracy spec. But if I have the 7510 recalibrated 1 year later TO THE 24H SPEC, it will be drifting less (at least that is my assumption and something that I want to discuss and get opinions about) and so it will have a better accuracry compared to calibrating it to the 1 year spec.Hope this makes it clear ...

author: ap
author_rank: Frequent Contributor
timestamp: December 30, 2019, 04:42:06 pm
content: Yes, after one year of aging, it will probably be better in drift than initially. However you do not know how much. So you can still only apply the data sheet drift spec. Or you start to establish a drift history. That is a lengthy process, because you cannot assume that it drifts less or the same every successive year. And you would have to establish it for all ranges that are of interest to you. And you will probably see inconsistencies that make the process more difficult. In other words, for this type of gear (precise, but not metrology), its probably not worth it (annual 24h spec drift cal is not cheap). You are probably better of by buying a few low drift standards if they cover your ranges of interest such as e.g. resistors or a voltage standard.

author: tggzzz
author_rank: Super Contributor
timestamp: December 30, 2019, 05:05:17 pm
content: Quote from: taydin on December 30, 2019, 03:10:01 pmSee TiNs respond w.r.t. cheap.Do your research w.r.t. calibration and adjustmentin metrological context.

author: taydin
author_rank: Frequent Contributor
timestamp: December 30, 2019, 06:04:53 pm
content: Quote from: ap on December 30, 2019, 04:42:06 pmI see. So statistically it is likely that the drift will be less after one year, but there is no guarantee.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 31, 2019, 12:29:29 am
content: A DMM isn't a primary voltage/resistance reference. But it can be used to compare voltage/resistance references. And this is where the short term (24h etc) specs are important.If I have like a uncalibrated 10V source, I can go to a friend who has one, with my favorite stack of DMMs. There, I measure the reference and get a average reading of like 10.000'15V on the DMM stack. If I know that the reference is dead on 10'000'00V, I have an average 15ppm long term error on my DMMs, but I don't have to worry about that.Now if I'm a real voltnut and brought a stack of 9 DMMs with me and they have a known 10ppm 24h accuracy (usual 24h error of good inexpensive 6 1/2 DVMs) , I can adjust the unknown voltage referece at home to an average 10.000'15V reading on the DMM stack and will AFAIK get 10V with a standard deviation error of 10ppm/sqrt(9)=3.3ppm on my voltage reference. Which should be good enough to calibrate something good/expensive such as a 8 1/2 digit DMM or a reference such as Fluke 5440B.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 01, 2020, 10:27:13 pm
content: Quote from: taydin on December 30, 2019, 03:17:26 pmI cited above the paper with which the device leaves the factory. You can check what range its accuracy falls from the factory.Tolerances are there for 1 year.

author: mzzj
author_rank: Super Contributor
timestamp: January 03, 2020, 07:27:20 am
content: Quote from: MadTux on December 31, 2019, 12:29:29 amOnly with the assumption that your 9 DMM's drift in total random ways. In reality they are somewhat correlated.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 04, 2020, 10:18:47 am
content: Quote from: taydin on December 30, 2019, 03:17:26 pmI don't know exact, but I think when we do calibrate we do it with zero error, not with 24h or 1Y tolerance. I may be wrong, and there can exist different types of calibrations.(And such tolerances mean max drift in last (next) 24h or 1Y, was it calibrated or not.)

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 01, 2022, 12:02:45 pm
content: I’ve read through this a few times and I still don’t understand what 24h, 90 day and 1 year values represent. Google is providing slim pickings. In fact, this was the only relevant hit from a total of 2 pages of results!My understanding is that all these values say is that the instrument will at most change by the stated value within the respective timeframe. No?Why do the 24 hour figures exist. Surely no one calibrates daily?Why would calibration to 24 hour values be more expensive? The tolerances aren’t vastly different between the three?

author: Grandchuck
author_rank: Super Contributor
timestamp: November 01, 2022, 12:07:51 pm
content: Do the 24 hour specifications relate to usage as a transfer standard?

author: alm
author_rank: Super Contributor
timestamp: November 01, 2022, 12:33:58 pm
content: Quote from: PixieDust on November 01, 2022, 12:02:45 pmCorrect. Usually this includes the calibration uncertainty itself, so technically it's the uncertainty of calibration + the maximum drift within that time frame. The exception is some high-accuracy instruments that specify calibration uncertainty separately. HPAK 3458A and Datron 4000 series calibrators for example.Quote from: PixieDust on November 01, 2022, 12:02:45 pmIf you have an instrument that supports artifact calibration, then you certainly could. Or maybe you're just using one range, and compare that one range every morning to a standard. As Grandchuck mentions, transfer, where you do several measurements within 24h to compare several values, is another application. Say you had one really good reference resistor, and then measured the reference resistor and 10 other resistors within 24h while keeping the ambient temperature within the set limits, then the uncertainty of your comparison to the reference resistor would relative to the 24h uncertainty.Quote from: PixieDust on November 01, 2022, 12:02:45 pmThe original question was about theKeithley DMM 7510. The 1y uncertainty for the 10 V DC range is 14 ppm of reading + 1.2 of range, or 15.2 ppm for a 10 V input. While the 24h uncertainty is 2 ppm of reading + 0.7 ppm of range, or 2.7 ppm for 10 V. Even if you had a high-endFluke 5730A multi-function calibratorand calibrated it every 24h, its uncertainty for sourcing 10 V DC is 2.25 ppm, which is not at all good enough to calibrate a meter with a 2.7 ppm uncertainty. So instead of hooking the meter up to your multi-function calibrator and pressing the button, you need additional steps and equipment like aFluke 732C DCV standard. So I could certainly imagine calibrating the 7510 to 24h specs would be more expensive than the 1y specs that they can do with the MFC without any extra human intervention.

author: PixieDust
author_rank: Frequent Contributor
timestamp: November 10, 2022, 04:17:36 am
content: Thanks alm, I'll file this away for future reference. Still unclear on a lot of things. Just don't know enough at the moment for everything you said to make sense.I did find this article:https://au.flukecal.com/literature/articles-and-education/electrical-calibration/application-notes/artifact-calibration-theo& this video:which was quite interesting but I need to do more reading to understand everything you mentioned.