Uncertainty Budgets and Decision Rules - Page 1
Forum section: electronics / general

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 09:05:29 am
content: As I have been spending a fair bit of my work time on these above subjects. Would people here like to know more about it? I'm no expert but I am happy to show people the stuff I know and hopefully it will help the next person. I often see people here going on about the 1yr manufactuer's spec of their bit of test gear or even quoting the 24h spec. But the measurement is much more than that.The new 17025 documents are putting more pressure on labs to have a decision rule and that is a good thing as there are lots of labs stating stuff is passed when actually given the barn door of an uncertainty they have it could well be undetermined. It is quite funny looking at mechanical calibration certs where the Unc is wider than the spec but even funnier is that labs had an Unc for length but they never had one for flatness or parallelism of that gauge, now they have to and it is causing lots of cuffufle. The alternative is labs just don't say it conforms to a spec but I get a feeling that won't slide for much longer as customers assume compliance of a gauge that has been calibrated.

author: e61_phil
author_rank: Frequent Contributor
timestamp: August 09, 2021, 11:31:11 am
content: Hi,I think that could be a real eye opener for many people here. Many people are thinking they can measure with lower uncertainties than most of the cal labs, just because they own a 3458A or something like thatBest regardsPhilipp

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2021, 12:25:09 pm
content: That would be valuable particularly if we can point people to it, thus avoiding hasty half-correct statements.Since there's no way you would be comprehensive, a "30000 ft" overview plus pointers to more detailed info might be appropriate.

author: FransW
author_rank: Frequent Contributor
timestamp: August 09, 2021, 02:04:19 pm
content: Generally a minimum awareness of absent knowledge is necessary or required to appreciate and understand the limits of a piece of test equipement used.It certainly pays to elevate a basic knowledge of physics to appreciate the limits of the individual in question, the test equipment in question and the DUT in question.Working in a certified lab with accreditation (Laboratory quality management system review) should prevent under-achievement.ISO / IEC 17025:It is an international laboratory management standard that guarantees the accuracy of laboratory data results.ILAC-MRA:ILAC (The International Laboratory Accreditation Cooperation) is a global association of accredited laboratories, inspection bodies, proficiency testing provider bodies and reference material producer, whose members include accreditation bodies and stakeholder organizations around the world. Accreditation bodies around the world are peer-assessed to be capable, signing agreements to enhance acceptance of cross-border products and services.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 02:17:26 pm
content: Measurement uncertainty is just defining the statistical probability of how good your measurement capability is to a 95% probability where k=2. UKAS has published some interesting documents talking about the subjects.M3003-Expression-of-Uncertainty-and-Confidence-in-Measurement-Edition-4-October-2019.pdf&LAB-48-Decision-Rules-Edition-3-June-2020.pdfThey are fairly accessible and show the importance of looking at Uncertainty and how the lab that calibrated your equipment influences your measurements and if anyone uses your measurements how that can affect others. Further away from government standards the worse it gets.It takes in lots of elements but once you get your head around it, it is not as complicated as people fear.Now there are some very good write-ups on it on websites such asISO Budgets - 7 Steps to Calculate Measurement UncertaintyJust so people are aware I am a Lab Manager for an ISO 17025 UKAS lab in the UK and I have had to pick up the pieces left by colleagues who have left the company so my know-how is from learning as I go. I am no expert in the matter, I'm just working it out for myself at times but we are a small lab and don't do a lot of UKAS work our bread and butter in is onsite work but many of our customers expect us to be UKAS accredited to do the work we do even if it is not to UKAS. I have just spent the past few months working towards getting 17025 in the mechanical lab and I much prefer electronics much easier.What I am thinking is to show you an example of a budget for a digital multimeter I have here at home (121GW) and explaining where all the bits I have, go. Therefore allowing other people to build up a budget for their needs.

author: FransW
author_rank: Frequent Contributor
timestamp: August 09, 2021, 02:41:10 pm
content: It is not only:"Measurement uncertainty is just defining the statistical probability of how good your measurement capability is to a 95% probability where k=2. UKAS has published some interesting documents talking about the subjects."But including the probalitity in perspective of physics today.The impossible is done immediately, wonders take a bit more time and on request there is magic. For example quantum uncertainty and entanglement.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 02:55:43 pm
content: Quote from: FransW on August 09, 2021, 02:41:10 pmNo need to get into quantums and it's entanglements we all know the answer is 42 but we just don't know the question. If pedantry is going to be the theme then I will just wander off now.

author: FransW
author_rank: Frequent Contributor
timestamp: August 09, 2021, 03:17:52 pm
content: That is not the point I am trying to make.\My point is that the assumption of in-depth knowledge is often over-sahdowed by lack of sufficient knowledge.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 04:14:23 pm
content: I would be interested in a discussion of the matter because while I don't fully understand all the issues, I think there is a wide range of misunderstanding about what you do and don't get from a calibration lab. And quite frankly, while perhaps UKAS is better at this, I see a lot of what I consider obfuscation and misdirection in the policies of a lot of so-called cal labs here in the US. That may seem harsh, but in my view the common no-data cal cert is just a way for underperforming cal labs to collect money for work they are unqualified for from customers that often don't care. I would like to hear from a non-OEM cal lab about how they actually deal with the exact topics you listed, as well as to what standards a device like a DMM should actually be calibrated to. For example, if I send one of my 6.5 digit DMMs for calibration, what I want is before/after data and all adjustments made to better than the 24-hour specifications with equipment demonstrated to have anaccuracyuncertainty better than a 5:1 TUR relative to those 24-hour specs or alternatively, at least 3:1 TUR with sufficient guardbanding used. And I'd like that in a 23C +/1 1C environment with a 72 hour acclimatization and a 2-hour warmup. A typical sales rep from a typical cal lab will flubber around for half an hour about their various services and accreditations without managing to actually address any of the issues.Here is something from a long-ago thread about a dodgy eBay voltage reference. The dodgy reference isn't the issue I'm interested in, this cal certificate is. Comments?

author: try
author_rank: Regular Contributor
timestamp: August 09, 2021, 04:28:31 pm
content: Hello mendip_discovery,maybe your contribution could make some folks rethink their quantitative postings that are flooded with insignificant digits.

author: try
author_rank: Regular Contributor
timestamp: August 09, 2021, 05:04:44 pm
content: Hello bdunham7,Quote from: bdunham7 on August 09, 2021, 04:14:23 pmDISCLAIMER:please be aware that I don't work in a cal lab and that I am not certified in any way!1. Nearly 54 ppm measurement uncertainty with k=2 is not something I would pay any dime for.2. Maker and models of the calibration instruments used are omitted - obviously for a reason. They only show their IDs.3. M. Smith did not sign this certificate.4. Furthermore there is no description of the measurement setup.A good lab will provide you with a picture that shows you even the wiring setup during measurement and the id of the cable used.5. There is no reference to the id of the document containing the calibration procedure.Let me guess, the ebay voltage reference was sold with an uncertainty of +- 2ppm slightly ignoring the 54 ppm shown above?I can't see anything documenting NIST traceability:https://www.foxvalleymetrology.com/fvm-advantage/quality/accreditations/The next lab from where I live is Perschmann, located in Braunschweig, where the PTB (German primary lab) is residing as well.You can download their accreditation if you want to:https://www.perschmann-calibration.de/wp-content/uploads/Akkreditierung_Anlage_DAkkS_DIN_EN_ISO-IEC_170252018_D-K-15089-01-00_2019_E2_12.01.2021.pdfLook at page 9 for the voltage figures.I did not find any similar NIST related document with Foxy Valley.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 05:54:32 pm
content: Quote from: bdunham7 on August 09, 2021, 04:14:23 pmThis I admit is common, most customers don't know and don't look at a cert. They just accept it as a necessary evil that has to be done. You do get some that look and you get the odd question or two. I have just had to explain to a test lab what Uncertainty is and isn't. He assumed it was the specification. I think its partly down to information about it being kept to the nerds and not spoken about in public. Most customers just want to know is it in spec.Quote from: bdunham7 on August 09, 2021, 04:14:23 pmThe whole idea of ratio should be downgraded as this is an old school method of thought and often lead people along the line of assuming as this is more accurate its better and it often doesn't take other things into account and in many cases is just not possible if you do a proper budget. I remember chatting to a friendly UKAS auditor about this and they did say it was a very strong thought process in America.The Certificate should make it clear the method used to do the calibration. For example "The voltage source was left in a temperature controlled environment and allowed to stabilise for a minimum of 24 hours, then the equipment was turned on and allowed to warm up for 1hr prior to the calibration where it was measured using digital multimeter" this is so that if it was sent to another lab then the method could be reproduced. Because you may have compared it to a known voltage source using a DMM etc etc.Our lab is 22C +/-2 and for electrical that is perfectly fine and its common for UK elec labs to run at this temp because of the yanks and the cheapness on the aircon, that was the story told me, I think the 22C comes via Fluke. Otherwise, it would be 20C like we do in the mech labs.From what I have seen it is the 1yr spec that labs aim for as the 24hr spec is almost what I would call repeatability in the absence of a repeatability study. Think of it this way, unless you get it calibrated every 24hours then that spec is worthless.If no adjustments have been made then the cert should say that its not been adjusted. If it has then you need to show before and after adjustment results. Because you need to work out the risk its had to your prior measurements let's say it's 0.5V out of spec, you would need to check that you hadn't had anything passed or failed because of that error then maybe look at recalling those jobs to rectify your measurements. Yes, it is a big can of worms if that happens.Quote from: bdunham7 on August 09, 2021, 04:14:23 pmThat is an interesting cert in that they don't make it clear if it was compared to a reference standard though there are hints about it but in a confusing manner. They say before and after adjustment but the results are the same so no adjustment has been made. The Temp looks a little chilly and hum is nice and dry not sure if that is a little too dry, also it doesn't state the lab tolerance on temp, it might drift by 5C over 24hrs so the kit could have been in 14C and only 1hr before taken up to 19CUpper and lower specifications are confusing as hell as they have the wrong specifications there. I would be aiming for a measurement in the 6ppm for the general measurement. Also, they should measure the other outputs. Now I would assume there is a separate Unc for the 24hr constant measurements as that would differ to a single measurement.Now the interesting thing is they state compliance and yet the Unc would say they can't make that statement in today's rules with decision rule. See attached, which is going on a 6ppm Tolerance.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 06:04:25 pm
content: Quote from: try on August 09, 2021, 05:04:44 pmWuss. But it only takes a moment to see issues, but to an average customer, there is plenty to say its ok.Quote from: try on August 09, 2021, 05:04:44 pm1. Yup that is shocking.2. Not a requirement, I don't show the kit used. I do need to record what kit was used but the customer doesn't need to know. But they do need to know the method, aka measured using a DMM.3. Yeah not a good sign.4. Again not a requirement to show the way it was connected, the uncertainty will be based upon their procedure and leads used. Another lab might have a lower Unc due to a better setup. I have gone to PTFE coated and twisted wires to reduce my Unc.5. Yes it does, FVE-000 and FVE-011.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 06:37:24 pm
content: Quote from: mendip_discovery on August 09, 2021, 05:54:32 pmWhat would you replace TUR/guardbands with? It's not just a school of thought, it is both manufacturer and official policy still, AFAIK. I happen to agree with the concept that more accurate is better and I understand that it can be hard on the 'budget'--both the uncertainty budget and the bank budget.QuoteYes, a 1 year cal is often done to 1 year specs. The problem with that is the 1-year specs include a years worth of drift and typically +/- 5C worth of tempco. So you have 24-hour +/-1C specs vs 1-year +/-5C. At the factory, the meter will presumably be adjusted to the 24-hour specs in a +/-1C environment and the reason the OEM doesn't just list that as the specification for a year is that they are including the additional drift and tempco as well as a confidence interval. It isn't about any guarantee that the meter will retain its 24-hour specs--for that you would indeed have to calibrate every 24 hours. It's about assuring that the meter will remain within the 1-year specs for a year and from 18C to 28C. The reason that the 'typical' performance of a good DMM is much better than the specified performance is simply that it is necessary to insure that k=2 or 2.58 as the case may be. If you test at at just one controlled temperature and accept any value that falls within the 1-year specs, you have not assured that the meter will remain accurate for the calibration period or over the specified temperature range nor do you have any sort of confidence interval, no matter how accurate your references are. Somebody correct me if I'm wrong about any of that.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 06:58:22 pm
content: Quote from: bdunham7 on August 09, 2021, 06:37:24 pmYes, the 1yr is what you can safely assume it will be within a year to year. The 24 hrs is what that claim it will do over 24hrs. The idea I have seen is that the 24hr performance is what you would use if you were building a budget for its ability to measure a for a whole 24hours. Let's say you wanted to measure the output of 10V source over 24hours taking readings every 10mins, you would take that 2ppm spec and add that to your Unc because over 24 hours it would be expected to drift within that spec. I would say any measurement Unc for a 24hour measurement would be higher than a normal quoted Unc as it has extra issues. Most devices even at 24hr will have a spec including the +/-5C drift in temp as the insides will wander as well as the external, thermal effects and all that.Quote from: bdunham7 on August 09, 2021, 06:37:24 pmIt is saying on that day in these conditions and with my Unc that it measured this when that was applied. Often certs will say this does not imply long term stability. Its a check that the meter is still in spec. You need to look at it over the years to be confident it won't drift out.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 07:24:19 pm
content: Quote from: mendip_discovery on August 09, 2021, 06:58:22 pmIndeed, most calibration is just a check to make sure the device is still in spec. However, although years of data may be nice to have, it is the manufacturers design and specifications that provide the assurance that drift and tempco will not cause it to go out of spec within the relevant time/temp/etc limits, to a certain level of confidence. And that is predicated at least partially on the meter being dialed in right at the center at the beginning of the calibration interval. Observation and testing over time may give you the ability to assign an uncertainty lower than the OEM original specs--which is how most labs achieve their uncertainty numbers, right?QuoteIt may not be an ISO 17025 or whatever requirement, but this is what I was referring to by obfuscation. Why would you not want your customers to know this? And what would your best guess be as to what was used in this case?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 07:41:03 pm
content: Quote from: bdunham7 on August 09, 2021, 07:24:19 pmYup that is how you do it.Quote from: bdunham7 on August 09, 2021, 07:24:19 pmIt is not required and not even a recommendation. It shouldn't matter to the customer what I used, they just need to know the method and that I have a good Unc. I think part of the thinking behind it is to prevent armchair experts that go pah they use a puny meter, not knowing that lab has had it for 10 years and it is rock steady etc etc. also what do you do if you are using a custom bit of kit that you have proven works, for example, an inhouse resistance standard.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 07:59:59 pm
content: Quote from: mendip_discovery on August 09, 2021, 07:41:03 pmWell, I hope this remains a friendly conversation, but as a customer if you don't think I'm worthy of this information or if you think it is OK to not reveal it because I might not interpret it correctly, we aren't off to a good start. This just reinforces things I've said previously about cal labs. If you have a homemade contraption that works, show it to us and show us the data that proves it works--and let the customer decide. If the customer wants their stuff calibrated only by techs wearing a suit, tie and leather shoes, that is their call. In my case I would just want it to be accurate, no matter how it gets there.Here's an example from a presumably reputable (not known to me personally) independent cal lab.https://www.esscolab.com/uploads/files/8508A_Example_Cert_w_Data.pdf

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 08:01:07 pm
content: My first step towards a budget would be to get more than 1 calibration on your device so you can prove to yourself that it is within spec and that over a period of time it isn't drifting between calibrations.Below is a picture of the kind of thing I do this. I have just chosen the 50V DC linearity as an example but when you build a complete budget you would have to cover the whole of the DC, AC etc I think this is good practice as it makes you look at the results and then you can start to work out the mistakes if there is any on the cert. As you can see my 121GW isn't too brilliant. I think I might take it into work and do a good job on it and even run through the adjustment procedure to get it back inside the limits. The good thing is in 1 year the drift between calibrations isn't too bad. So it is repeatedly inaccurate.But this won't affect my ability to calibrate it as I don't plan to measure using the negative side of the meter so it's within specification, just.But over time if it's well behaved you can get a graph like this and then you can start to think about reducing the Spec to bring your Unc down.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 08:14:53 pm
content: Quote from: bdunham7 on August 09, 2021, 07:59:59 pmQuote from: bdunham7 on August 09, 2021, 07:59:59 pmI have a few customers who like to know the kit when it is due for cal etc. But there are only a few. Stating a recalibration date for my kit on a cert is just daft. I do regular checks crosschecks and depending on those results I may extend or reduce the calibration frequency and therefore it's my risk to manage.I don't need to list the kit as UKAS as the accrediting body is happy with the work I am doing. Why should you think otherwise? It's a bit like saying to the people at NPL I am not sure you know what you are doing is right.But you are as a customer or even potential customer welcome to come and audit us and see the lab, the equipment, the records etc. Under ISO9001 its expected for you to audit your suppliers especially if the work you do for them is classed as a high risk with regards to failures of products.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 08:23:52 pm
content: Quote from: mendip_discovery on August 09, 2021, 08:01:07 pmWell, even with 1 cal you still have the factory cal to rely on, so that's 2 right? If your gear is characterizedspot onto a certain uncertainty, you should be able to calculate the upper and lower bounds of drift, right?QuoteFirst, if you haven't recorded an accurate temperature, how can you know how much of what little drift there is might be due to tempco? Perhaps I harp on tempco a lot, but I've observed it to be the single largest cause of deviation in the mid-grade bench DMMs that I work on and play with. Second, isn't the 121GW 0.05% + 5 counts on that range (not 0.01%)? So your meter appears to still be in spec?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 09:04:52 pm
content: Quote from: bdunham7 on August 09, 2021, 08:23:52 pmBut I don't know the acceptance limit for the manufacturer as I didn't get a cert as such, I have done it from Nominal for the first year. This is why kids its important to get a cert with your toys and if you buy used its really nice if they can give you the old certs.Quote from: bdunham7 on August 09, 2021, 08:23:52 pmOn the worksheet, I recorded the temp and hum for both tests and I see in the manual there is a temp coefficient I can apply to the budget. I am going to do the same as Keysight gear and assume a 5C swing.Ooh the Spec has changed, it was 0.01% in the Sep 19 edition. Them crafty Aussies.When you say accurate temperature, you want to know I had to do an Unc budget for the thermometer used in the lab so that I could use that error in the error I use for a coefficient calculation.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2021, 09:38:23 pm
content: Quote from: mendip_discovery on August 09, 2021, 09:04:52 pmGood point--I would think that anyone seriously concerned about accuracy would get a cert with purchase, but in fact even my 8846A did not come with any data. They do now. I would presume at least the 24-hours specs for an acceptance limit for those manufacturers that list a 24-hour spec.QuoteThat must be a misprint, the original manual from 2018 and the Kickstarter page also show 0.05%. 0.01% would probably make it the most accurate handheld DMM in existence by a wide margin.https://www.eevblog.com/files/EEVblog-121GW-Manual.pdfQuotePerhaps, but at some point if you can demonstrate that the effect of an error is much less than other uncertainties, I would think you could drop it. If your thermometer is accurate to 0.5C, I wouldn't worry about it unless you are required to. After all, you can start throwing in electric field, gravitational pull, sound level and background radiation if you like--all of which theoretically could have an effect--but at some point they don't matter enough to worry about.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 09:51:24 pm
content: Next, I would look at the repeatability of the device and build up some standard deviations for the measurement. I did this with the 5V source I have here and took a reading every 5 mins. Would prefer it to be at 50V but this is just an example so meh.E8 =COUNT(E2:E6)E9 =AVERAGE(E2:E6)E10 =STDEV(E2:E6)E11 =(E10/SQRT(E8))E12 =(E11/E9)*1000000E13 =(E11/E9)*100The limiting factor is the resolution and that does give you a bit of a hit with regards to your imported ppm.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 10:06:06 pm
content: As I am someone who likes to show my working out I often will have a tab just for the Contributions, lots of labs write it straight into the Budget but I like to do the maths then at least when I get it wrong I can correct it and the budget gets filled in as well.As you can see it is fairly basic and I haven't had to take in the magnetic flux caused by bdunham7 spinning in his sleep. You can add more, if you can warrant it. I get the info from the specs etc then I convert it to the same resolution as I will be working in ppm and mV otherwise the % and ppm stuff can get confusing. I think the only slightly complicated one is the Temp Coefficient, and that is from the manual and with most good multimeters they are happy with a +/-5C but I don't have that so I did the % at 18C and at 26C and minus one from the other getting me the error I have from 22+/-4 if you can control your space even better then you can get this down more. Just be wary of saying you are at +/-0.5C as I bet you your thermometer doesn't have that good of an uncertainty and yes I have had to calculate that before.If you are doing pressure gauges you have to take into account local air pressure, and local gravity. Thread gauges, rake and compression. Let us just say electrical/digital is much nicer.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 09, 2021, 10:33:59 pm
content: Now time for the Budget,Now all you need to is bring in your info into the budget. This again isn't that mental, the hard bit is working out what kind of probability you are bringing in. All you are doing is Dividing by 2 or 1 or by the square root(3). Then you =SQRT(SUMSQ(YOUR_READINGS)) then times that by two to give you the +/-. Then to finish it off you round up to a nice number.*note please ignore the 5.000 in the mV next to Man spec, I was tinkering.*Its not a scary thing, but it shows the importance of getting a good lab to calibrate your stuff and getting enough data that you can stat to get away from Man Spec. The resolution just allows you to get better repeatability but only if your machine repeats.If I have time I will post something about decision rules tomorrow.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 12:31:15 am
content: OK, I'm not quite understanding all of that so perhaps you could help with the lingo and a few other things?'Imported uncertainty'--is that the uncertainty of your lab or the device used by your lab in this test? Or something else?Could you explain the terms 'RSS' and 'dominance check'?Why do you assume a rectangular probability distribution for the manufacturer-specified uncertainties? And if you do assume that, why would the divisor be greater than one? I would think that a rectangular distribution would represent a larger spread than even a k=1 normal distribution, thus requiring a divisor of less than 1.Edit: No tails, so I guess that's not right. I was visualizing a k=2 or more distribution--a k=1 distribution has a lot outside the bars.I have other questions but I'd be interested in those first so I can think about it.

author: RYcal
author_rank: Contributor
timestamp: August 10, 2021, 02:54:26 am
content: Well well wellWelcome to my favorite topic....Let me start by outlining my experience in this field. I have over 15 years in accredited calibration labs in multiple disciplines including pressure, temperature and DC/LF electrical parameters. I currently own and run a DC/LF accredited cal lab and DO NOT offer any non-accredited calibrations period, I see no value in them and it devalues my lab.There are typically two types of uncertainty budgets1. CMC Budgets (these are what your scope of accreditation should be)2. Uncertainty budgets for a UUT (unit under test).In my lab we make a statement to specifications but part of this is including the measurement uncertainty into this statement i.e. If the error plus the uncertainty is greater than the spec we cant say whether it passes or fails so we have another term we state called a W.O.U (window of uncertainty) so you can either have a pass ,fail or a W.O.U. Its as simple as guardbanding via measurement uncertainty. Most of my calibrations are done by remote control of my instruments and reporting is done in the same software package (MET/CAL MET/TEAM) so all this is hard coded into my procedures.So my CMC budget is typically how good my reference equipment is now there is two way to tackle this sensibly.1. You could just be conservative and use manufactures specs provided that it has a calibration cert attached to it and that cert doesn't involve any W.O.U's ( if so it does you blow the spec out to account for it). Manufactures specs typically include any expected drift, resolution etc. HOWEVER I must also monitor my gear to make sure it isn't drifting out of spec between calibrations this is typically done every three months via an automated procedure that puts the data into an excel file for me to plot.2. You could apply corrections and uncertainties from the accredited calibration report that came with your reference gear as it definitely should have one to maintain traceability. This however can get VERY messy with having to interpolate corrections and uncertainties, but it is doable. It all depends on your target audience whether is work is worth it. You will still need to drift check your reference gear using this approach as well.A typical UUT uncertainty budget at very minimum includes any uncertainty bought through from my reference gear, the uncertainty due to the resolution of the UUT and a type A contributor. I can break it down more if people want. Its the whole structure around the chain of traceability (A cal lab cant calibrate anything better than its own capabilities and I have seen some that apparently can)This approach really throws the whole 4:1 ratio out the door, say I have a 3:1 ratio then without adding anything else into the mix I can roughly say that I can afford an error of around 60% of spec with inducing a W.O.U, a 2:1 then would bring that allowable error down to around 40% (these are all guesses at this stage as I don't know the other contributors values yet) but it gives me a good idea on what to expect from a calibration I personally start to crunch numbers and look carefully at expected results at around 2.5:1 ratio.To answer a few questions so far RSS means "root sum square" its an accepted way to combine all the uncertainties in your budget provided they are at 1 sigma the formula is as follows in excel "=sqrt(uc1^2+uc2^2+uc3^2)" uc is the separate contributors at 1 sigma. This gives us what is a called standard uncertainty then we can just multiply that by the k factor to get it to an expanded one. I personally calculate my k factor using the welch-satterthwaite formula.My main pieces of equipment are a Fluke 5522A and a Fluke 8588A.I love talking uncertainties this is just the tip of the iceberg on them - I also offer consulting and contracting to labs seeking to become 17025 accredited or just uncertainty matters.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 03:37:08 am
content: Quote from: RYcal on August 10, 2021, 02:54:26 amI would be interested in how you do it in practice. I have additional questions formendip_discoverythat I should let him answer, but I'm sort of baffled by the treatment of the UUT manufacturer specifications. Is there something obvious in his approach that I'm missing?QuoteOK, got it! Given that, I'm not clear on what the 'dominance check' is or what sums would be less than the RSS, but I'll leave that for the OP.QuoteSo what does that give you in terms of your 'CMC budget', which I presume is the best uncertainties your lab is capable of? And if you can squeak by with a TUR of 2.5:1 and a lot of guardband, what would be the most precise instruments that you would accept for calibration? Do you have separate 10V, 1 ohm, 10k, etc standards as well?

author: RYcal
author_rank: Contributor
timestamp: August 10, 2021, 04:15:31 am
content: Quote from: bdunham7 on August 10, 2021, 03:37:08 amAll very good questions.There is nothing going into the uncertainty budget that comes from the UUT specs. I have no idea what they mean by dominance check either this is a new term to me as well.Here's a decent read on the very basics of how I do it NOTE the "S1" formula is wrong in the article but it gets corrected in the comments.https://support.flukecal.com/hc/en-us/articles/115005666563-Implementing-ISO-IEC-17025-Measurement-Uncertainty-Requirements-in-MET-CAL-Version-9-XI'm very conservative at the moment and my terms "loosely" reflect the manufacturers specs for my 5522A nd my 8588A in there respective classes. realistically I feel around 6.5 digit instruments is where I feel comfortable at the moment using the 8588A as a transfer standard for most test points.

author: Irv1n
author_rank: Newbie
timestamp: August 10, 2021, 04:53:09 am
content: At first glance, a budget of uncertainty is thrown; it is undesirable to combine relative and absolute values. If the resolution of the device is 0.001 V, then calculating the uncertainty budget should be done as follows: 0.001 / 2 / sqrt (3) or 0.001 / sqrt (12). There are a couple more questions about the budget, I'll ask you later.Added:Not entirely clear with the temperature coefficient. Does it change by 4 degrees during the calibration process? How many degrees are you above normal calibration conditions?Uncertainty by Type A:This value is based on YOUR Measurement Process and Reporting Procedure. If your measurement process takes a measurement and reports the value, then you need to take only (std div) . Otherwise, if your reported "Measured Value" is based on a mean of a specified number of repeated measurements, then yo can use std of mean. Ensure your experimental data consist of individual measurement values and not mean values!

author: RYcal
author_rank: Contributor
timestamp: August 10, 2021, 05:11:05 am
content: Quote from: Irv1n on August 10, 2021, 04:53:09 amCorrect but UUT resolution is only one factor contributing to the budget.

author: tszaboo
author_rank: Super Contributor
timestamp: August 10, 2021, 07:37:59 am
content: Quote from: bdunham7 on August 10, 2021, 12:31:15 amYeah before you go: "I want this and this and this on my cal certificate", you should look up the difference between standard and accredited calibration.If you don't specifically ask for it, then they will not give you that, because it is a lot more work, and a lot more expensive, and most of the time companies need the cal to place a checkbox in their ISO 9001 or other certificate.

author: Anders Petersson
author_rank: Regular Contributor
timestamp: August 10, 2021, 09:03:12 am
content: Quote from: bdunham7 on August 09, 2021, 06:37:24 pmThis is my problem with calibration services. Calibrating once a year with the 1-year specs as criterion, especially if not providing hard numbers on the measured margin to those specs, that leaves the UUT possibly outside the 1-year specsfor the whole year. Well the next calibration, the year after, should catch that the UUT is no longer in the 1-year specs and thus indicate that the measurements during the past year were less accurate than assumed. This makes calibration mostly useful to verify past measurements, not guarantee the performance in the coming year. In contrast, naively reading the datasheet would have you believe that a 1-year calibration schedule awards you the 1-year accuracy specs. Anyone care to comment if this weakness is well-known?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 10, 2021, 10:49:03 am
content: Quote from: Anders Petersson on August 10, 2021, 09:03:12 amThe problem is more manufacturers specify a spec, doesn't say it's 1 year, though its implied 12month frequency is good. Its only the likes of the high-end unit that they start with the 24h, 30d and 12m stuff and even then they are no guarantees it will be within that spec. The labs are just doing a periodic check and maybe adjustment if its starting to wander out of specification. If the UUT is +20% of spec then why adjust because the next year it may be +40%, if its 75% then there is an argument to adjust as that would make it possibly 140% out the next year. Part of the issue is if you take the labs Unc into account that could be 20% of the spec so if they adjust they may not be improving things at that level but at 75% they have higher confidence that it will be improving.This is when having a reference standard in your place of work helps and you use it on a monthly basis. For example with scales I will often suggest to a customer they get a cast-iron weight and use it daily or weekly to verify the scales, that if they start to wander they can get us in to adjust them. So in the case of electronics, it can be just a case of plugging two meters into the mains and checking the measurements between them or using a resistance standard etc.I think some people like to shift the problem of accuracy of their equipment to an external lab and don't take steps to reduce the risks and carry out internal periodic checks, these don't have to be high standard checks but they do help monitor essential equipment isn't going outside of the spec. Do a spreadsheet with the results year on year of your device, is it well within spec, if it needs adjustment each year then why is that? Do you need to take it from 12m to 6m? Is the equipment often being used outside of the temperature conditions?Quote from: tszaboo on August 10, 2021, 07:37:59 amOther than the privilege on a non-profit firm visiting you and charging an eye-watering amount to watch you measure stuff the only difference between 17025 and 9001 calibration is I have to write down my work, maths and prove it. This often means you have a worksheet and a cert. So if you have to do corrections or calculations its done there. I also have to prove that the maths and procedures work. Its more work at the end of the day but a better way to work. Its nice to have someone who knows the subject come and see your work and make comments on how you need to improve.I am on holiday this week and I am just about to make a journey across the UK to St Neots, I will try and get back to ya all tomorrow but I never know as I am helping my mate get ready for a couple of days at a motorcycle trackday at Cadwell Park where I am the photographer (my actual qualified job).

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 10, 2021, 11:11:24 am
content: Quote from: bdunham7 on August 10, 2021, 12:31:15 amThat will be the uncertainty of the equipment used to test it, so in my case its the Unc for the Transmille I have at work. You would often find that on the certificate.Quote from: bdunham7 on August 10, 2021, 12:31:15 amThe Dominance check is some thing that is just on the Unc budget I have, I have never really paid much attention to it. It basically adds up the Value Relative ppm and checks it against the k=2 to ensure its greater. Its just a sanity check I think.Quote from: bdunham7 on August 10, 2021, 12:31:15 amI must admit this is where I get a little fuzzy on it. I have always known it to be Square and therefore its divided by the squareroot(3). I do suffer with the mathematics and probabilty stuff. I will take a look and see if I can find a referance in my notes to as why its done like that.Quote from: Irv1n on August 10, 2021, 04:53:09 amThis combination thing I think is there because it was on the template I have. Its not used, its is just there for reference.Quote from: Irv1n on August 10, 2021, 04:53:09 amI think you are referring to the resolution being 1/2 the LSD hence the 0.001 divided by 2. I must admit on some of the new budgets I have done for the mech stuff I have done this. I just didn't spot that I didn't do it with this. Though TBH its not a big part of the budget, its the damned spec and its +/-5lsdQuote from: Irv1n on August 10, 2021, 04:53:09 amI am stating I am working at a 22C +/-4C now I may be working at 20C and during cal it only changes by +1C but the 1C isn't what I am accounting for, I am accounting for error of where I am not at 22C. If I controlled my lab to 2C or 1C then I would get better ppm. My lab at work is 22C +/-2C.Quote from: Irv1n on August 10, 2021, 04:53:09 amI showed the way I worked the standard Deviation. I just take it into a ppm of the range as that allows it to scale up and down with the range. At work, I am lucky as my repeatability is usually comparing the measuring kit I have to the generation kit I have. So I can use the same tests for both sides. There maybe other ways to do this to reduce your numbers but this is trying to be practical.

author: ITTSB.Europe
author_rank: Contributor
timestamp: August 10, 2021, 11:53:52 am
content: Most Calibration labs in UK they are a bad joke.This conversation it is a bad joke.Someone started a topic to advertise globally his skills?UK has two sources within the country to buy calibrators from, and the how to use them this is given from the manufacturer.I got from UK an Agilent multimeter, with damage at calibration circuit, it does not retain the stored as calibration value, and slightly drifts at the last two digits at DCV.The lab tried 80 times unsuccessfully to store the calibration, they did not inform the customer about the damage, they simply print a calibration certificate and they get paid.By me receiving the DMM as used in Greece, I did found the calibration sticker and the calibration lab.I did contact them so to request a copy of the certificate if they had kept one, which they did.Now I know their name and address.While they did offer to me a copy of the calibration report, I do accuse them for lack of ETHICS and professionalism.Such opportunists they should end up to jail.Now I own three identical of that Agilent/Keysight HH DMM, two are in pristine condition and the third from UK, I am using them as three phase voltage monitor with Bluetooth and for remote review to 7” tablet.Therefore despite the issue the DMM this is still useful to me.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 03:48:03 pm
content: Quote from: mendip_discovery on August 10, 2021, 11:11:24 amHere's an explanation without derivations.https://www.isobudgets.com/probability-distributions-for-measurement-uncertainty/Clearly at least some of these are just accepted approximations. For example the correction for the 'U-shaped distribution' will vary with its shape, approaching one as the shape changes to having all the samples on one wall or the other. According to some chicken scratching I did while doing the this morning's crossword, the correction for a continuous rectangular distribution may well the square root of three. That still doesn't quite explain why that is the appropriate fit for incorporating a manufacturer spec into your budget. I also find it odd that metrology practice would assume bounded distributions.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 03:57:17 pm
content: Quote from: ITTSB.Europe on August 10, 2021, 11:53:52 amIf you reach that conclusion on the basis of one sample, your statistical skills are the bad joke.My only real complaint with cal labs is lack of transparency--without complete data and transparency, a calibration certificate is just paper and proves nothing. However, they do what their customers want and if that isn't what I want, then I'm not a customer, simple as that.The OP has decided to discuss these issues and I applaud him for that and think the discussion is worthwhile, even if I might not be a potential customer. He's clearly not advertising and is quite openly acknowledging his and his labs limitations.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 10, 2021, 04:04:13 pm
content: Quote from: bdunham7 on August 10, 2021, 03:48:03 pmPresumably since the OEM has removed those outside of some limit from delivery. While uncertainty in the manufacturers measurement equipment makes the value of this limit somewhat uncertain, the effect on the shape of the delivered distribution is not changed significantly under typical production rules of thumb which suggest that testing equipment have uncertainty less than 10% of the production limits.While this once would have been a good assumption, the assumption will be wrong for manufacturers who have achieved TQM goals on their production (very high yield on testing). In this case the distribution will probably be closer to normal, but as you move up the number of digits of precision TQM goals become less and less likely to be achieved.At the very top end it is difficult or impossible to achieve the 10x measurement uncertainty and the bounded distribution becomes less and less appropriate.The bottom line, the assumption of a bounded distribution is yet another rule of thumb which is often appropriate, but requires examination for best quality work.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 10, 2021, 06:10:39 pm
content: Quote from: bdunham7 on August 10, 2021, 03:48:03 pmIts from theGUMQuoteSo if the spec has a Specification and they say its where k=2 it would be normal and you use the divisor of 2 much like an imported Unc.*but*QuoteSorry, some of the Formulas have not come through but it basically says if there is a random chance it's likely to be anywhere in between an upper and lower limit then it is likely to be a rectangular distribution. It does later state there is an option if it is normally spot on but might deviate off between these limits you can apply a triangular probability.ohhIso Budgetguy has also posted about this and has reminded me about the microvolt on connection bit, bloody EMFJust so people know, I am just talking about this as I would like people to talk about it, and try and get the hobbyists [well any end user] looking at what they are doing and what is coming in. I have customers who use items such as a CMM to measure mechanical parts and when I ask about Uncertainty they just look blank and if pressed will just say the tolerance from the manufacturer.For the Cal Clubs, I think this can add a new level of fun as you can measure the same thing and state your measurement uncertainty and do a comparison between your uncertainties, often referred to as EN ratio. Labs are encouraged to do inter-lab checks to verify that if another lab was to test a meter just after I had done it the results would be the same if our uncertainty was taken into account.I have not mentioned my lab as TBH we are a small lab and not up with the big labs yet. Part of me is also using this to get feedback from others that also do this and just reaffirm what I know and maybe learn a thing or two along the way. Its quite amazing when you look outside your bubble on how different countries do their calibration or even inspection. I trained in photography and digital imaging and now I work in a calibration lab so I will be the first to admit this isn't what I trained in, I learnt it the hard way.

author: try
author_rank: Regular Contributor
timestamp: August 10, 2021, 07:52:06 pm
content: Quote from: RYcal on August 10, 2021, 02:54:26 amFrom somebody who is owning and running a cal lab a statement like[...RSS means "root sum square" its an accepted way to combine all the uncertainties...]I would have expected a different, lengthier and deeper explanation that allows to understand the formula instead of learning it by heart and applying it without understanding.That is disappointing!Kindergarten somehow.

author: RYcal
author_rank: Contributor
timestamp: August 10, 2021, 08:03:44 pm
content: Quote from: try on August 10, 2021, 07:52:06 pmAnd this is where I leave the conversation.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 08:06:08 pm
content: Quote from: try on August 10, 2021, 07:52:06 pmI literally just wanted to know what the letters meant since it wasn't obvious to me given where they were found. I understand the principles, and for those that don't they might be better served by a link to a good explanation of the relationship between variance and standard deviation, as well as a derivation of why you take the square root of sums of standard deviations to combine them instead of just adding them. The answer, of course, is that variances are what you are actually adding.Here's the kindergarten version!https://www.mathsisfun.com/data/standard-deviation.html

author: try
author_rank: Regular Contributor
timestamp: August 10, 2021, 08:30:16 pm
content: Hello bdunham7,I am not critizing you, nor any question you asked, but the one statement I quoted.When you were asking questions about the lingo I totally agree with you. Never read the term "CMC" before and was able to find that using google.Is "CMC" a term I am supposed to know?But anyway, I have no expectation anymore.

author: bsdphk
author_rank: Regular Contributor
timestamp: August 10, 2021, 08:39:54 pm
content: Quote from: mendip_discovery on August 09, 2021, 09:51:24 pmI wouldn't call it a "limiting factor" as much as a "invalidating factor".In your example you calculate the standard deviation using formulas which assume a bell-shaped noise process, but your lack of resolution, relative to the noise in the signal gives you a step-noise process (ie: "±1" noise).When resolution is insufficient for the noise, the stddev thus determined is useless, because it depends on magnitude of the measured artifact.Assume a perfect digital voltmeter which measures volt with 100mV resolution, it can show 4.8, 4.9, 5.0, 5.1, 5.2 and so on, and it rounds perfectly.If you measure a 5.0000…V reference, it will constantly show "5.0", it will be doing that all the way from 4.9500…1 to 5.04999.....9 volt input.Congratulations: You have a meter with zero stddev!If instead you measure a 5.05000… reference, the meter will show "5.0" half the time and "5.1" the other half and your stddev is now 0.534 V.That is both a statistically unsound and a practically useless result: The uncertainty (estimate) should not vary cyclically across the range.There are a number of possible workarounds, but they all boil down to not just measuring a single voltage.The easiest is to sweep the voltage you measure across at least the full range of the last two digits of the DUT, with an extra digit of resolution (ie: at least 1000 measurements) and calculate the stddev of relative measurement error (ie: (Vmeasured - Vactual) / Vactual).If you cannot do that (need K-V divider + lots of knob-fiddling-time), you can instead (carefully) superimpose a (good!) AC signal on top of your reference voltage, in order to provide the bell-shaped noise process required by the formula you use, but you then have to compensate for the noise you added etc.It sounds absolutely bonkers first time you hear it, but it is *so* much easier to find the stddev for a 3458A than for a handheld 3½ digit meter...

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 08:57:51 pm
content: Quote from: try on August 10, 2021, 08:30:16 pmI didn't, but it was apparent what it was generally from the context. I think the block for me with 'RSS' was where it appeared and the fact that RSS means something entirely different to me, in a vastly different context--as does CMC! That's the problem with acronyms, often they are insider talk and have different meanings in different fields. The person using the term always knows what it means, of course!

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2021, 09:34:56 pm
content: Quote from: bsdphk on August 10, 2021, 08:39:54 pmI thought this was the point of adding an additional uncertainty based solely on the resolution limit? And according to his calculation, that works to about what your worst case measurement would be? And, of course, this is in addition to any observed or specified uncertainty in the readings.

author: bsdphk
author_rank: Regular Contributor
timestamp: August 11, 2021, 06:18:04 am
content: Quote from: bdunham7 on August 10, 2021, 09:34:56 pmIt probably is, but that seems to be to just be a crude fudge to avoid having to do the necessary measurements.Look at my example again: Depending on what precise voltage the cal-lab happens to use, you either add that additional uncertainty to a measured stddev of zero or 0.0534, giving very different results, depending on which precise voltage was used for calibration.In practice it is probably not much of a problem, because nobody really cares about the actual stddev of low resolution instruments, and for high resolution instruments there will generally be sufficient electrical noise to satisfy the statistical assumptions.I reacted to this because the example you used was almost a school-book example of the problem: When your measurements have ±1 nature, ie: digital presentation, it should have no effect on the resulting uncertainty, if you change any single measurement one step up or down and your example failed that test.As a general rule of thumb: If you only have one, two or three different sequential numbers in your data, stddev is not what you want.This topic is borderline into "Uncertain Numbers", and if you want to understand it better, I can highly recommend these two reports:Sensitivity in Risk Analyses with Uncertain Numbers:https://www.osti.gov/biblio/886899/Constructing probability boxes and Dempster-Shafer structures:https://www.osti.gov/biblio/1427258They are heavy, but you do not need to understand it all, much less follow all the math, to learn how to spot the quick-sand.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 11, 2021, 08:20:26 am
content: mendip_discoverythank you for this thread. I read it with great interest.Can you give an example of a complete calculation for example for calibrating 10V exit Fluke 732a (1) using 3458A and other Fluke 732a (2) to which there is data from the calibration laboratory of the top level.Those. So we took and received 100 measurements that show the difference between both standards. We know the parameters of the Laboratory Fluke 732a, the temperature in the laboratory, and the 3458A passport data.Can you make an example of how we get the data for the calibrated Fluke 732a (1)?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 11, 2021, 10:08:51 am
content: So with the resolution it's about the fact you maybe at any time 1/2 a digit away from the next digit. Let's say you have 10.001V on the display, is it measuring 10.0006 or 10.0014 so there is always 1/2 a digit in there. So my Unc I did is hedging on safer saying 1mV but I could improve it but saying we are looking at 1/2 of a digit so therefore 0.5mV. You will often see in a cert a statement that additional to the Uncertainty there is a furter 1 of 2 lsd that am sure if there to give a little unc to the UUT.Sorry about the abbreviations that some dont know I am happy to explain them, it's easy to get confused as it depends on what industry you are used to.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 11, 2021, 10:27:24 am
content: Quote from: bsdphk on August 10, 2021, 08:39:54 pmI'm on the phone so cant do a neat quoting.Remember am not looking at its error from Nominal but if it repeats that error and that could be caused by my 5V source having a bit of wobble but that is why you need to repeat these tests every year or so to see if you can improve on them with more stable kit. I could do it over more tests to give myself a lower ppm of error, or I could do a sweep and take an sum of all the errors but see below about the risks of driving it low, but there is another issue is that its weighting the budget but it's not the biggest thing on the budget, go for the big numbers and then you can chase the small ones.I did it with 5V as that is the only reference I have at home. Tbh when at work we use the top of the range as if it's going to get any wobble it's going to be there and this is testing our ability to repeat reading. But if our repeatability was 0ppm because we have such a low resolution device the adding of 1/2 digit should stuff give us about some cover for errors. There is lots you can add but at the end of the day if you quote an amazing uncertainty then you will be expected to back that up and if it goes wrong it could be very damaging for the lab.

author: FransW
author_rank: Frequent Contributor
timestamp: August 11, 2021, 11:40:56 am
content: "I would have expected a different, lengthier and deeper explanation that allows to understand the formula instead of learning it by heart and applying it without understanding.That is disappointing!Kindergarten somehow."I rest my case.Regards, Frans

author: bdunham7
author_rank: Super Contributor
timestamp: August 11, 2021, 03:21:45 pm
content: Quote from: bsdphk on August 11, 2021, 06:18:04 amYes, I think it would be pretty unusual for the assumed (worst-case) uncertainty based on resolution to be a large portion of the UUTs specified tolerances. I have an old 4.5-digit DMM that is specified to +/- 1 count offset plus +/-1 count at full range on it's 1V and 10V scales. In one part of the calibration procedure they actually have you provide a small stimulus halfway between two counts and verify that it flickers back and forth between the two with approximately equal time (intensity). Apparently there's just enough noise or other random variations for that to sort of work, and I can actually get it so that it meets the specification. Even in this extreme case, having a lower STDDEV than an assumed 0.5 digit just barely moves the needleon the specified uncertainty.QuoteWhich example are you referring to? Is it something I linked? In any case, yes STDDEV on data like that has issues but is generally presented just to demonstrate the arithmetic involved.

author: Echo88
author_rank: Frequent Contributor
timestamp: August 11, 2021, 04:35:05 pm
content: Please elaborate on the role of modern physics in uncertainty calculations of testgear FransW.

author: FransW
author_rank: Frequent Contributor
timestamp: August 12, 2021, 10:54:51 am
content: This is probably not what you would expect.It is however my world.Physics:There are no absolute measurements. All measurements relate to a traceable path of “modern” physical phenomena.The basis of this can be verified and validated.International & national standards are in the custody of i.e. BIPM and NIST.All standards are attempts to unify the measurement approaches. Nothing more.Physics today (as we perceive it) is an open subject. We have to realise that we, mankind, is not all knowing. There are too many subjects we do not have a complete understanding of.We have to keep this in mind when we select a piece of measurement equipment which should fulfill realistic basic requirements of the person that does the measurements and has a thorough understanding of exactly what and why he/she is trying to measure.To unify physical forces has sofar proven impossible. Due to mankind’s shortcomings.We use our own limitations to explain our actions. Therefore everything needs a “beginning” and an “end”. Why? Again, our own limitations.Wikipedia:In metrology, measurement uncertainty is the expression of the statistical dispersion of the values attributed to a measured quantity. All measurements are subject to uncertainty and a measurement result is complete only when it is accompanied by a statement of the associated uncertainty, such as the standard deviation. By international agreement, this uncertainty has a probabilistic basis and reflects incomplete knowledge of the quantity value. It is a non-negative parameter.Uncertainty:Uncertainty surrounding the values of the input variables compounds the difficulty of determining the accuracy of the output.Verification and validation :Verification and validation are independent procedures that are used together for checking that a product, service, or system meets requirements and specifications and that it fulfills its intended purpose. These are critical components of a quality management system such as ISO 9000. The words "verification" and "validation" are sometimes preceded with "independent", indicating that the verification and validation is to be performed by a disinterested third party.Limitations of mathmatics:Nature is not mathematico-logically perfect.Gödel proved that any (sufficiently complex) formal system cannot prove its own consistency, and that any such consistent system contains statements that can neither be proved nor disproved. That may be a limitation of formal systems, but only in a way that is a limitation on everything.By formalising mathmatics one can create one’s own prison (all historical physical forces unification attempts).Thinking and believing:Einstein: “The word God is for me nothing more than the expression and product of human weaknesses, the Bible a collection of honorable, but still primitive legends which are nevertheless pretty childish.”TEA:A subject I thoroughly enjoy.Frans

author: mzzj
author_rank: Super Contributor
timestamp: August 12, 2021, 02:25:10 pm
content: Quote from: Anders Petersson on August 10, 2021, 09:03:12 amI'm not sure if this is what you were after but this is quite well covered in 17025 standard and 17025 accredited labs.17025 accredited labs are not allowed (or should not automatically) mark next calibration or recommended calibration intervals in the calibration stickers or in the certificates as this is something that customer should define by themself depending on case.(customer should base the decision on risk/reward ratio of drifting outside specifications and specific use ie. how heavy-handedly the instruments are used)Depending on case you can adjust your calibration perioid to shorter or longer or "over-specify" your equipment so that they stay within your requirements for longer perioids.And after all its still probabilities: ie your meter stays within 1 year specifications with 99,8% probability, calibration results are correct with 95% probability and so on.You wont get anything absolute but you can squeeze the probabilities to a level that is acceptable for your operations.Sometimes it might mean 10x 3458A DVM's in parallel and calibrated every 3 months. Or only one, calibrated once in 5 years.

author: FransW
author_rank: Frequent Contributor
timestamp: August 12, 2021, 03:55:13 pm
content: So what is the probability of finding the true value?

author: bsdphk
author_rank: Regular Contributor
timestamp: August 12, 2021, 08:59:16 pm
content: Quote from: FransW on August 12, 2021, 03:55:13 pmPractically nill, because the true value depends on a lot of noise-processes for which we generally do not control, even in calibration.For instance all grid powered instruments are sensitive to the grid at some level, it may be the voltage, the frequency, the harmonic distortion, random noise, shot noise, EMC etc. etc. etc.In a well designed instrument, these effects are at least an order of magnitude below all the effects you calibrate for, but that does not mean that they stop existing, merely that they do not matter.But because they are still there, the least significant digit dithers, telling us, that there is no "true value".

author: bdunham7
author_rank: Super Contributor
timestamp: August 12, 2021, 10:02:36 pm
content: Quote from: FransW on August 12, 2021, 03:55:13 pmAre you positing that thereisa true value?

author: FransW
author_rank: Frequent Contributor
timestamp: August 13, 2021, 08:44:03 am
content: Yes, I do.The significance might escape us today (& tomorrow) but behind it all there is an explanation.As an example I often use the Planck time which can be calculated as defined.As a consequence time-nuttery seems misplaced and irrelevant as the definition of 1 Volt.The agreement for usage is clear. We need to talk about the same thing if we want to understand each other.It is however quite a task to extend the current limits.The tesseract in Interstellar is another example for stretching the limits but certainly challenging (Thorne, theoretical physicist).

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 16, 2021, 08:46:30 pm
content: Quote from: MegaVolt on August 11, 2021, 08:20:26 amSo attached is a crude Budget for a 10.0000000V source. I am not sure the Load Regulation is a factor to take but I am assuming it is the worst case scenario when you hook up a load. A bit like when you load a DC power supply. The Imported Unc is the bit that weights up the reading. Without data to prove that the 10V source is going to remain in spec, I can only assume it will do. But if it did you could keep a similar Unc just adjust the ppm stuff for the 10.0000472V (rather than 10V) that you will calibrate to, its what we do with standard resistors.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 17, 2021, 08:08:41 am
content: Many thanks for the expansion answer. If you do not mind I will ask moreQuote from: mendip_discovery on August 16, 2021, 08:46:30 pm1. Number 536.9 μV describes the measuring laboratory? Those (lab source standard + NULL Meter) has a 536.9 μV uncertainty?Quote2. Where did the number of 10.0000472v come from? I do not see this number in calculations?3. Why do we consider noise and drift having a rectangular distribution?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 17, 2021, 11:05:18 am
content: Quote from: MegaVolt on August 17, 2021, 08:08:41 amThat is from the lab that tested the source. It is their Uncertainty that is very large and so therefore it makes a big impact on the readings. It makes their measurement a little bit of a joke as they can't say its in or out of spec.Quote from: MegaVolt on August 17, 2021, 08:08:41 amThat is the measured value from the certificate. If you have it adjusted to be within the specification and it keeps within the spec then its easier to use 10V and apply the Uncertainty above. But if it drifts to 10.00000472 V and let us say and it can't be adjusted to be 10.0V you can still use it, you can use it at 10.0000472 V but you would need to recalculate the ppm values to 10.0000472 V. It has 50 μV of adjustment but it might overtime drift so far you can no longer adjust it.Quote from: MegaVolt on August 17, 2021, 08:08:41 amIt is a fairly safe assumption most things are Square unless they are otherwise, M3003 does give some examples and from the top of my head its because its a limit of + and - of the centre point and there is an equal chance of it being +1ppm or -1ppm with noise. The same goes for Drift, it could drift up as well as down, from day to day or year to year. Noise could be triangular as its potentially going to be the right reading but occasionally not but its hardly a number to worry about.I would be chasing another lab to get the Imported Uncertainty down.If you can prove your line voltage is <10% of swing then you could even drill down that ppm, if its 1% then you can take the 0.5ppm and take a stab at saying its 0.05ppm, or if its battery-operated then maybe run it off the mains remove the issue.If you have an 8.5digit meter with a really good Uncertainty then you could use that and just compare the reading from your source to the Unit Under Test and that would remove the 6ppm load regulation straight of the bat. The 10V source then just becomes a transfer standard and can in theory not be calibrated. But I still would just so if you use it without another meter then you can.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 17, 2021, 11:46:20 am
content: Quote from: mendip_discovery on August 17, 2021, 11:05:18 amI'm a little confused. We have a standard owned laboratory. Let's call it lab_standard. And there is a user standard. Let's call it Custom_standard.Lab_standard = 10.00000472 v with UNCERTAINTY 536.9 μV (k = 2). This is data from the calibration certificate.If I use this standard for a year, I must use the uncertainty calculated in the table and equal to 545.86 μV (k = 2)And all the calculations that were made while relate only to lab_standard? I understood correctly?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 17, 2021, 12:23:42 pm
content: Quote from: MegaVolt on August 17, 2021, 11:46:20 amYes, this Uncertainty relates to that Lab Standard. You can then use it the calibrate measurement devices. So you would say the Customers Multimeter measured this 10V source with an Uncertainty of 545.86 μV (k = 2). You could also use it to calibrate the Customer Source by Nulling/Zeroing using the lab's multimeter and lab standard then measuring the customer's unit but that wouldn't my prefered option but it can be done and you would have to possibly add some extra in for the Lab Multimeter.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 17, 2021, 12:27:46 pm
content: Thanks for answers. All clear.Quote from: mendip_discovery on August 17, 2021, 12:23:42 pmWhich calibration method do you choose?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 17, 2021, 12:44:16 pm
content: Quote from: MegaVolt on August 17, 2021, 12:27:46 pmAt work, I have a Multiproduct calibrator as a source which I use to calibrated customer meters. I also have a 6.5digit multimeter for the measurement of sources. On the 17025 schedule, the best capability is quoted even if you have other kit available.It gets fun as I have calibrated a few shunts and the budget for that is in the file but I know I need to give it a damned good stare as I have a feeling its not quite right, but the only shunts I calibrate are our own so Iam not too worried yet. For the budget I use the 10A out of the calibrator with the multimeter measuring the voltage drop, but I need to rethink as I also need to take into account of the calibrator getting warm and the 10A dropping over the calibration period, to do that I would need to use another 6.5digit meter and a suitable shunt. Then there is an issue around 10A being used and you are not testing the shunt at an A that is suitable, ie 10A to test a 4000A shunt is a bit of joke as you are not warming it up etc.

author: mzzj
author_rank: Super Contributor
timestamp: August 17, 2021, 04:42:18 pm
content: Quote from: mendip_discovery on August 09, 2021, 09:05:29 amAround here it seems quite common to skip the specifications and pass/fail statements altogether and only provide the measurement results (and uncertainty) for the customer.Certainly that way in our lab: Out of the few thousand temperature calibration certificates that I have signed maybe five! had any sort of comparison to specifications.For anyone interested the ILAC G8 is the current "bible" on decision rules:https://ilac.org/latest_ilac_news/revised-ilac-g8-published/Requirements for decision rules vary quite a lot by field: DMM calibration with 95% uncertainty is good enough for most uses but you might want to consider much larger guardband if you work in drug lab or calibrate police radar gun.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 17, 2021, 05:23:34 pm
content: Quote from: mzzj on August 17, 2021, 04:42:18 pmYes its pretty much the same here, but I just get a feeling it is not the best option and in the future, they will push labs to have it in place. I have a simple one set up for the mechanical one, if the Measurement and Uncertainty is outside the spec then fail, if its in then pass, if the result is in or out but the Uncertainty is a little out then class as undetermined. I need to get my head around the maths and maybe work out a probability of it being a pass or fail, and report that. I am currently going though getting mechanical and lets just say there are some of it that I am really not enjoying.

author: bdunham7
author_rank: Super Contributor
timestamp: August 17, 2021, 07:50:05 pm
content: Quote from: mendip_discovery on August 16, 2021, 08:46:30 pmIs this based on the certificate that I had posted or on your own lab?One of the things that would concern me is the ad-hoc assembly of your uncertainty budget without an explicit, sanctioned guide to doing so. As a result I think you've made some errors, please comment. In any case, unless all labs follow similar procedures, they won't get consistent results--and if calibration labs don't give consistent results, then what's the point??1. There should be no need to analyze the various details of the stated 6ppm uncertainty from Fluke if that is what you are going with. That is an all-inclusive k=2.58 (99% CI) number (not rectangular distribution) and includes such things as 10C worth of tempco, line regulation and so on. You shouldn't need to add anything else in there that is not related to your lab or ability to do repeatable measurements.2. If you really want to give an uncertainty at cal temp and use only the drift spec, that spec is 3.0ppm/year, k=2.58 (AFAIK--CI isn't stated explicitly in the document I have but is for later and other models)3. Line regulation is already 0.05ppm.4. Load regulation is a predictable characteristic, not an uncertainty. That has nothing to do with calibration, which for a voltage standard should always be done at high impedance.Now those are nits that have to do with your process, which is what is being discussed here. However, the real issue for me is that neither your lab nor the one in the certificate I posted has any business letting a voltage standard like this in the door--unless you are buying it for your own use. You can be 'honest' and list your actual UNC numbers all you want, but what is happening is that somebody is getting a calibration certificate that allows them to say 'calibration certificate' but is only slightly more useful than a 9 volt battery. However, when the customer sees "10.0000472 v with UNCERTAINTY 536.9 μV (k = 2)" read straight off an old HP3458A, they likely only care about the first number and consider rest to be boilerplate CYA stuff that they can ignore. I would like to see a policy--or even regulation or law--that requires the sticker, not just the calibration certificate, to note any instrument that is calibrated to less than the manufacturers specifications.I'm guessing you mostly agree with all that, so I hope your lab isn't among the ones doing this sort of nonsense. So what would it take to actually calibrate a 732A for real? And who can do that?

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 07:58:56 am
content: Quote from: bdunham7The cert you posted along with the spec sheet.Quote from: bdunham7There is no hard and set rules on what you put in or leave out of the budget, it is audited and the auditors will pull you up if they believe you have made an error. The main thing they are concerned with are you erring on the side of caution rather than overestimating your ability. The aim is that labs are getting consistent results within their claimed Uncertainty if our interlab EN ratio is >1 there is a serious issue with the budget because you have an Unc lower than you are actually getting, if it is <0.2 then there is an argument that our budget needs looking at because your Unc is higher than it needs to be.Quote from: bdunham7Not sure where you get the k=2.58 from as I pulled up a spec sheet for the Fluke and there is no mention of the K figure. I will admit I hadn't looked up the full spec sheet I missed on some of the nuances, I did wonder about the gap between 18 and 28C didn't get it in my head at the time. In that case it would be Normal and a dividing factor of 2.58.Quote from: bdunham7I missed its Normal operating temp is in the expected lab conditions, I would only look at it more if it was outside of that.Quote from: bdunham7Yup, another mistake by me, I did say it was a crude one.Quote from: bdunham7I was just looking at sources of potential error. If for example the meter drops or raises voltage due to 10Mohm or 1Gohm measurements then it will add to the uncertainty.Quote from: bdunham7Its the customer's responsibility to check the lab has a suitable Unc and capability to fit their needs. If I had that send it I would do it, I would quote my Unc and that would be that. Its a bit like saying I bought a dollar store multimeter and it is not as good as an HP 8.5digit meter, who is at fault, the store for selling a crap meter to the customer or the customer not doing research on what they need. Manufacturer's don't always explain how they managed to get the results they get they tend to keep some of that data to themselves, some items come with good supportive documentation but that isn't the norm. Often its for the customer to decide what they want, they might only want to know its value so they can use it for correction, some want to know its within 2% even if the spec is 0.5%, just because you need is manufacturer spec then don't assume its the same for others. With regards to an Uncertainty that is larger than the Spec that I feel is going to happen, it is where the emphasis on decision rules is coming in because labs won't be able to say pass or fail if the customer asks them and their uncertainty is larger than the spec, they would lose their accreditation.My lab would quote an unc of 60ppm + 1mV as I only have a 6.5digit meter at the moment.

author: binary01
author_rank: Contributor
timestamp: August 18, 2021, 11:36:52 am
content: Quote from: mendip_discovery on August 09, 2021, 09:05:29 amPersonally, I am in favour of this push for labs to consider compliance for most customers. In the pressure calibration world (at least in Australia) we have been required to assess instruments for compliance to a specification for many years, where the specification is provided by the customer (based on their usage, risk, etc) or a default specification is adopted based on the manufacturer's 12 month accuracy.In my region, most instruments are sent for routine calibration to see if the instrument still performs within the original specification, and if not, ask that it be adjusted to do so - in this case, assessment of the instrument for compliance to a specification is the most important aspect for the general industrial customer. It can be dissapointing for us metrologists, who are usually focussed on minimising uncertainty, but this is a reality for many customers who may not understand uncertainty. Pass/fail is easily understood and metrologists are well placed to assess this. For digital pressure instruments, the arithmetic addition of uncertainty and error/correction at each point must be equal or less than the specification. We have carried over the same compliance rule in our electrical lab.Unlike pressure labs, electrical labs in Australia have often historically not considered any compliance/decision rule and simply reported a measurement and uncertainty, which in some cases was disturbingly large, and inadequate for the test instrument. Poor labs were able to do this with little recourse, relying on the customer ignorance. With the emphasis on decision/compliance rules in 17025, these labs are now obliged to assess if their uncertainty is adequate during contract negotiation. This is an excellent outcome. Of course, there are still labs that ignore this with the "barn door uncertainty" and give a 100 ppm uncertainty to assess a 50 ppm accuracy specification, and say that it "passes", but I hope these are picked-up by our accreditation body over time.Although it is ultimately the user's responsibility to contract a laboratory with a capability that suits their needs, it can be very hard to navigate or understand a scope of accreditation, and I think we also have a duty of care to make sure our capability is likely to be suitable.Of course, for metrologists seeking calibration of their reference equipment, they understand uncertainty, errors, corrections, drift, etc, very well and can easily ask their reference lab to assess the instrument to lowest possible uncertainties, without adjustment or consideration for compliance to an accruacy specification, and this is permitted based on specific customer request within 17025.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 12:47:38 pm
content: Quote from: binary01 on August 18, 2021, 11:36:52 am

author: bsdphk
author_rank: Regular Contributor
timestamp: August 18, 2021, 01:54:20 pm
content: Quote from: mendip_discovery on August 18, 2021, 12:47:38 pmThe worst calibration I have ever heard about, is the BIPM's Watt Balance.They told me they would have to calibrate out the gravity gradient, that is, not thedirectionof the force of gravity, but thecurvatureof the force of gravity, because BIPM is literally located on the side of a mountain.How ? No idea, but they knew they would have to.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 03:07:27 pm
content: Quote from: bsdphk on August 18, 2021, 01:54:20 pm17025 is similar to when you do a maths test, most of the marks you get are for the working out of the sum, not just the answer. I am sure the 17025 accreditation bodies are full of pedantic people...you know...forum members. They spend the day wondering about things that might affect the reading and then ask the lab to work it out. Electrical is fairly easy in the scheme of things.

author: bdunham7
author_rank: Super Contributor
timestamp: August 18, 2021, 06:05:49 pm
content: Quote from: binary01 on August 18, 2021, 11:36:52 amYes, and yes...but it goes further. Some customers are well aware of what they are getting and use subsequent obfuscation to hide the fact that their calibration certificate is worthless. It isn't just companies needing to meet some ISO requirement that all devices used for any purpose, like testing 9 volt batteries, have to have an annual calibration. You see this in the sale of test equipment, the calibration of subsequent devices and so on. The scammer states that the device is calibrated and lists the calibration certificate number, but no mention is made of the fact that it is calibrated with an uncertaintytwo orders of magnitudeworse than what most people would expect. And I suppose in many cases they can get away with this because they know that devices like voltage standards and long-scale DMMs will probably not drift that far in any case--thus relying on the quality of the original instrument rather than the most recent calibration. This is, in fact, what the guy was doing with that cal certificate that I posted--there was a long previous thread about the guy who sells "2 ppm" voltage standards on eBay.This is why I would like to see the rule that any calibration done to less than OEM specs be denoted as such on the sticker and the certificate, with perhaps an additional requirement that anyone listing the certificate number has to disclose that as well, with failure to do so being deemed actionable fraud.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 06:36:11 pm
content: Quote from: bdunham7 on August 18, 2021, 06:05:49 pmOther than that won't happen. Who verifies that the spec published is actually realistic? I can see the likes of Fluke and Keysight making it virtually impossible for external firms to calibrate stuff as they are the only ones with the funds and equipment to calibrate the stuff (plus they won't see the secret box), then everyone will winge that it takes 6 weeks and costs a gazillion bucks to get the meter calibrated. Killing off competition and preventing any new labs from coming onto the market unless they are willing to spend half a billion on kit to get into the market.

author: bdunham7
author_rank: Super Contributor
timestamp: August 18, 2021, 06:56:35 pm
content: Quote from: mendip_discovery on August 18, 2021, 03:07:27 pmIf you are encouraged to throw whatever you can think of into the equation and just give a measurement and uncertainty, no matter how inappropriate for the device at hand, then that method of operation works. If your only fear is accidentally understating your uncertainty, then perhaps it is the way to do it. The only downside would be that in some cases, the calibration results become relatively meaningless. You've gone through the steps correctly but you haven't arrived at a helpful answer That sack of potatoes is 10kg +/- 50% +/- 2kg! Oh dear!However, I can't imagine actually operating that way in practice! Surely at least some of your customers demand a go/no-go decision? And if you unnecessarily increase your uncertainty, you are increasing the possibility of unwarranted false rejects or conditional results, which AFAIK are regarded as just as undesirable as false acceptances. Or isn't that true for you?QuoteIf I haven't beaten you up too badly, could you share how your lab gets such a seemingly terrible uncertainty result? Assuming your 6.5 digit DMM is typical ~35ppm/reading + ~5ppm/range sort, where does the additional uncertainty come from???

author: bdunham7
author_rank: Super Contributor
timestamp: August 18, 2021, 07:13:25 pm
content: Quote from: mendip_discovery on August 18, 2021, 06:36:11 pmI'm not sure what you mean about published specs being realistic--do you think they are too conservative or too optimistic? Fluke and Keysight have tons of published information detailing how they justify their specifications, although I don't know how that squares up with UKAS. If that isn't good enough, we can all just examine the products once they are in the field. MY observations are that typical closed-case calibration Fluke or HP DMMs easily meet their specifications the vast majority of the time unless they are broken, regardless of how long ago the last cal was. I have numerous such Fluke bench meters, and one HP 34401A, that meet their 24-hour specs for decades on end. As for Fluke making it impossible to calibrate their stuff, they publish full calibration manuals telling you exactly how to do it and they sell the very equipment needed to to the job. There's no secret formula that I'm aware of.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 07:18:24 pm
content: I have just had a quick read of the service manual and it does throw up some thoughts on some potential issues the effect of the use of the guard or not, Thermal EMF Errors, Leads, Stability of the oven etc etc. but at the end of the day, the lab imported is going to be a big part of it, until you get to what I thinkFluke UKclaims of 0.75uV.Though I will admit I am interested they just don't use an 8.5digit meter to measure the reading and record that, it is more of a comparison between two 10V sources and a null meter. A few more items added to the long time wish list for the lab.

author: mendip_discovery
author_rank: Super Contributor
timestamp: August 18, 2021, 07:50:24 pm
content: Quote from: bdunham7 on August 18, 2021, 07:13:25 pmI can see some firms, not necessarily Fluke coming up with some interesting specs just to sell a product and its not until later we find they are not. Fluke and Keighsight are currently making data available but what if they didn't, Clare Flash testers now Seaward don't produce calibration gear to the same standard as they used to. I have a force meter in the lab at the moment that I have to pay a firm each year to get a code to get it to work again as it bricks once the cal is out. I have had to turn away gas alarms as the company that makes them want 4k for the cradle and codes to recalibrate them, and yes they brick after the cal runs out. Just because a few people do play nice don't expect it to be the case always.Quote from: bdunham7 on August 18, 2021, 06:56:35 pmMy UKAS auditor does groan when I refer to our Unc being a safe Unc. Its big but I am just getting my head around things and trying to understand the elements that are there and which ones I need to look at. I never had a handover from the previous 2 lab managers and I happen to be the lucky bloke who can have a go at almost anything so I get all the fun jobs onsite so my time to drill into it is limited but I am getting there.QuoteMost of it is that I am using an Agilent 34401A and the 10V source is on the threshold of two ranges of my Unc so I went for the 10V-100V range (safe mode). But that has a DC Spec of 45ppm (35ppm for the 1-10V range) so I need to get some data of its history so I can then start to use my spec rather than the manufacturer spec. The same goes for the mV as that again is the spec for linearity holding me back. 21ppm is from the lab that calibrated my kit. Now I can get 0.712mV but it has been rounded up to 1mV in the past and I haven't taken the effort to go down to uV and start pounding at that. I have tweaked the Unc this year a little as I did drop the ppm down to 57ppm. If I use the 1-10V range I would be 46ppm+60uV. I have just sent the meter off to another lab with better customer service and lower unc so I will need to plumb that in, I also need to have a proper look at the specs and how well my meter has done over the years.

author: binary01
author_rank: Contributor
timestamp: August 19, 2021, 12:51:29 am
content: Quote from: mendip_discovery on August 18, 2021, 12:47:38 pmYes, we have to consider lots of environmental factors and uncertainty contributions when using deadweight testers. However, they are excellent references and it is worth the effort for the brilliant long stability and low uncertainty. We calibrate DWTs and have written and sell a software package for our customers to simplify it as much as posisbleWorking at high pressure is scary at first, but as long as you use the correct high pressure fittings and tube, it should fail "safely" - you should only get a spray of oil and loud bang of dropping masses if a fitting doesn't hold. The loud noise can certainly cause a surprise and good reason for the brown trousers.Quote from: bdunham7 on August 18, 2021, 06:05:49 pmThis is precisely what we do whenever the accuracy specification used for compliance is different than the OEM specification. This ensures it is clear to any user of the instrument, even if they don't have immediate access to the calibration report. We do this for pressure and electrical instruments. If we cannot confirm the desired/OEM specification due to instrument performance or UoM, we discuss it with the customer to see if the performance that we can confirm will still meet their needs. We don't find this difficult or onerous to do, and we can then be confident that our calibration will be useful for them.

author: bdunham7
author_rank: Super Contributor
timestamp: August 19, 2021, 01:22:54 am
content: Quote from: mendip_discovery on August 18, 2021, 07:50:24 pmMany smaller specialty manufacturers have funky calibration setups that are very specific to their device. It may be that they want to keep the revenue in house and it also may be that their small volume means that their support capability is limited--sometimes it is one person and one calibration setup in the whole world. If they will sell you the necessary stuff, then then it is just a matter of whether you are willing to pay the price. There's not a requirement that equipment be serviceable with generic or ad-hoc methods. In some cases they may be worried about liability from measurement tools that have been inadequately tested. Whether customers accept that probably depends on the product.QuoteIt sounds like you are learning by being thrown in the deep end of the pool right off the bat.QuoteThere appear to be some specific errors in that process that it might be helpful to review.First, the 10V range on the 34401A goes to 12V, so there would be no sane reason not to use this range--and high impedance mode should be selected for voltage standards, although it will make no discernable difference in this case.Second, the specs on a 34401A are neither drift nor linearity and unless UKAS rules are insane or they are being misapplied, there's no point in breaking it down like that. The 34401A specs, as received or if calibrated properly to manufacturer specs (as in by Keysight), are inclusive of all errors--drift, tempco, linearity, calibration reference uncertainty, etc. And, if the manual I have is to be believed, they are tok=4. At least that is what this manual says, which is different from the k=2 I always presumed to apply to HPAK gear....Third, while calibrating a 34401A with an 'imported uncertainty' of 21ppm on the DC ranges seems like a very poor choice (why??), if you do go this route the errors break down in to scale, offset (range) and INL (linearity) Drift only accounts for a certain part of the first two. Drift, tempco within the 18-28C range, random jumps and so on are not separated out. Only INL, which is specified as 2ppm-reading/1ppm range, is separately listed. While you may think (and UKAS may allow you) to use the device history to come up with a lower uncertainty, you should be careful. Perhaps you can infer the in-bounds tempco from the listed out-of-bounds figure and so on, but I don't know if you can make a solid theoretical case for doing it this way. It may be stable to 0.1ppm for years, then one day when you power it on it jumps up 5ppm for no particular reason. That is accounted for in the OEM specs, but possibly not your historically-derived uncertainty. OTOH, it appears you are unlikely to underestimate your uncertainties anytime soon.

author: TiN
author_rank: Super Contributor
timestamp: January 14, 2022, 07:46:58 am
content: Been playing a little with recently publishedSandia PSL's Uncertainty Calculatorand it's quite powerful thing.Also have prebuild / precompiled versionhere.Was looking for something like this quite a while. It's also available in source code and using open toolchain, which is yet another plus.

author: bsdphk
author_rank: Regular Contributor
timestamp: January 14, 2022, 05:55:22 pm
content: I can highly recommend this Sandia Tech Report as companion reading:https://www.osti.gov/biblio/886899-sensitivity-risk-analyses-uncertain-numbers

author: TiN
author_rank: Super Contributor
timestamp: January 16, 2022, 08:40:51 am
content: Played with resistance data and Sandia's toolkit.So far pretty happy with capabilities. Have found few very minor bugs.Here's dataset I've been massaging with resistance measurements. Summary in graph form and report below.PDF with math and uncertainty models and analysis.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 17, 2022, 12:09:16 pm
content: Quote from: TiN on January 16, 2022, 08:40:51 amHow can I compare resistors with uncertainty 0.1ppm?

author: mzzj
author_rank: Super Contributor
timestamp: January 17, 2022, 12:33:09 pm
content: Quote from: MegaVolt on January 17, 2022, 12:09:16 pmTypically with resistance bridges. Some examples would be ASL F18 or various MI modelshttps://mintl.com/productcategories/metrology/resistance-measurement/resistance-bridges/These offers accuracy as low as 0.015ppm

author: bsdphk
author_rank: Regular Contributor
timestamp: January 17, 2022, 01:02:38 pm
content: Bit TiN, please read the document I posted a link to, because "Gaussian" is not the only game in town, and you likely underestimate your uncertainty by assuming/postulating gaussian noise processes.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: January 17, 2022, 01:19:34 pm
content: Quote from: mzzj on January 17, 2022, 12:33:09 pmhttps://www.ebay.com/itm/1544938408613000$)))

author: TiN
author_rank: Super Contributor
timestamp: January 17, 2022, 11:44:19 pm
content: bsdphkValid point, I'm still learning all this. Yet to find clear guidelines or documents on which distribution should be applied to what. For time being I get pretty uniform gaussian results from actual measurements, so the analysis above should be still in correct ballpark. I'd like to add tempco corrections next so analysis can be done also for setups without thermal chambers.MegaVoltMore like32k$. I plan to repeat measurements with more typical 8.5d DMMs too once I have analysis template with tempco of DMM and DMMs own errors. Started with a bridge because it's easier (with bridge don't have to worry about tempco, linearity, drift etc.).