Practical TUR and the 732B - Page 1
Forum section: electronics / general

author: martinr33
author_rank: Frequent Contributor
timestamp: March 18, 2022, 10:09:35 pm
content: I am looking at a current 732b calibration certificate from Fluke Everett.So my question is, am I looking at this the right way?The meat of it is the 732b measures at 10.000001V TUR 6.7:1.The equipment used is a set of 4x732A on what looks like a four month rotation.The 732B is specified at +/- 0.3ppm in 30 days, which I assume is the appropriate specification for calculating the TUR, therefore in this case +/- 3uV.Therefore, the reference system must have an uncertainty X that solves 6uV/X= 6.7.Which leads to a reference uncertainty of 0.9uV, 900nV, or +/- 450nV.As there are four units in the reference standard, we can therefore assume that, on average, each unit has an uncertainty 0f +/-900uV.

author: RYcal
author_rank: Contributor
timestamp: March 19, 2022, 12:24:57 am
content: I'm not sure your math is correct in regards to the ref uncertainty I work it out to be 4.5E-7 Volts or as you said 450nV, So why are you doubling it to 900nV and why are you doubling the UUT spec?...All reported uncertainties should be expanded uncertainties so no need to double.As for the uncertainty conclusion I would hope that Fluke are looking at the whole measurement setup and combing all appropriate contributors to the whole budget so if they did use all four references then all four contributions should be included in the budget. You will be surprised at how accurate these cells can be when you have good history on them and are using a JJ array to verify them.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 19, 2022, 03:17:17 am
content: Thanks, I appreciate the confirmation, we are on the same wavelength.I used a calculation that could accommodate asymmetric errors, so the numerator and dominator are effectively doubled. Which as you noted, leads to the same result.All four references are listed, but there is no further breakdown than "6.7:1".And yes, on the references. What may be a little surprising is that they used 732a units, not b or c devices. I assume that they pick their best references for calibration.Also, here's a question about the process.- Assume I have a JJA system that for all intents and purposes has no noise.Now, I can measure each 732a against the JJA and derive an output value and standard deviation of the noise from the 732a.Then, I measure the UUT under each 732a, capturing voltage and SD for each one.Finally, I average the readings and root sum square the standard deviation of all four, giving me a smaller SD than I have for any individual unitAnd that gets me to the 6.7 UUT. No mean feat on a .1ppm measure.

author: Villain
author_rank: Regular Contributor
timestamp: March 19, 2022, 08:19:55 am
content: It is not surprising at all that they would use fluke 732A as drift gets lower over time, the history of the units gets more known and they are simply older than 732B and 732C.

author: mendip_discovery
author_rank: Super Contributor
timestamp: March 19, 2022, 09:34:43 am
content: Just look at the 17025 schedule,https://clients.intelex.com/Login3/Fluke/DocLink?id=wnzSSc3TwN6Jd7VxdokeUbnYmoqUXa13u36FtyLLLW8o48W44wGTSee6rp0GMAty0I see,10 V Direct Voltage Maintenance Program Measure – Range 10.00 V, 0.06 μV/V using Fluke 732B and DMMWell Isolated DC Sources or Voltmeters: DC Voltage – Source - 0 V to ±10 V, 5 nV/V + 50 nV using Josephson Voltage System (JVS)If in doubt just ask them for the Uncertainty. They won't bite.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 20, 2022, 10:32:12 am
content: Quote from: martinr33 on March 18, 2022, 10:09:35 pmPlease share your calibration certificate!No, you're not looking at this the right way, as you confuse uncertainty with stability.At first, T.U.R. (test uncertainty ratio) is roughly defined (*) as the ratio of the (anticipated) uncertainty of the D.U.T. over the uncertainty of the reference.(Stability parameters of course contribute to these uncertainty figures.)Please, carefully study the specification of any voltage reference (732X, 7000, 4910, etc.) and you'll see that they are all specified for stability, i.e. drift parameters ONLY, but never for uncertainty.That means your 0.3ppm/30days is the timelydriftof your 732B, but NOT its uncertainty. Other stability parameters are its T.C. (<0.04ppm/K), noise (short and mid term), retrace after RT changes, etc.The uncertainty of your 732B depends mostly on the uncertainty level of the references Fluke uses internally, and on the comparison/transfer method. As this may vary from cal lab to cal lab, it's evident that this can not be part of the specification, but it should definitely be part of the calibration certificate.Therefore, Fluke should have specified, which uncertainty their bank of four 732A delivers, and with the TUR of 6.7, you can then calculate the uncertainty of your 732B.Read carefully, if Fluke already includes any drifts, or if this has to be added from the specification.If these information are missing, the cal report is useless. (*)There should exist a similarThe accreditation report for Fluke Everett, as mendip_discovery provided, should be referenced in the cal. report.Let's say, that Fluke can guarantee these 0.06ppm uncertainty for their 10V DCV, (see page 30 of the accreditation) then the initial uncertainty of your 732B would be about 0.4ppm, which is a quite reasonable value. (*)Frank(*) A much better and more detailed description of uncertainty calculation can be found in "Calibration: Philosophy in Practice", 2nd edition, chapter 31.Maybe the online 1st version already might contain this.

author: Villain
author_rank: Regular Contributor
timestamp: March 20, 2022, 10:51:55 am
content: Regarding Fluke Everett CMC:https://clients.intelex.com/Login3/Fluke/DocLink?id=wnzSSc3TwN6Jd7VxdokeUbnYmoqUXa13u36FtyLLLW8o48W44wGTSee6rp0GMAty0Be careful though, fluke has two service locations in Everett, the CMC above is from the Fluke Park Laboratory which offers best uncertainties.The two different locations:Fluke Park Laboratory6920 Seaway BlvdEverett, WA 98203 USUS Customer Support Service Center1420 75th Street SWBuildling 4Everett, WA 98203 US(Did not find the CMC for this location)

author: martinr33
author_rank: Frequent Contributor
timestamp: March 20, 2022, 02:50:41 pm
content: Thanks again for the comments and helping me dig into what should not be an obscure topic.I believe the cert to be from the US support center. They are not my certs, so I cannot share them on line.However, I can confirm that the 732b cert includes just the voltage, the TUR, and the standards used - in ths case, 4x 732a on a 4-month cal cycle. For my initial approach, I have to assume that that is sufficent data to be useful.1) I am not confusing uncertainty with stability (sorry if I gave that impression). The problem is, the ONLY accuracy data I have on the cert is the TUR. Therefore, I have to assume that published specs are sufficient as a reference, which forces the assumption that the Fluke 732b specification is for both stability and accuracy.1b) and yes, the TUR is all I get on the Fluke certificate.2) I have to assume that the calibration chain is unbroken back to NIST (including validation of the JJ array). This logic imples that the 732a bank cited in the certificate has been carefully characterized against Fluke's reference standard, providing a TUR for the bank.3) Under these assumptions, I can impute the TUR for the bank of 732as.4) Here's where it gets tricky for me. The TUR does not appear to be an absolute measure, but is instead dependent on the accuracy specified for the target instrument. So if I were to double the acceptable error on the 732b to 1 ppm from 0.5 ppm, then the TUR would also double.Another implication is that, were I to express the result as 10.000000V rather than 10.000001V, the TUR would also decrease. Not by much in this case, but the two-sided form of the calculation would naturally reduce the value of the TUR. Put another way, the distribution would shift off center.That leads to another complication, which is if my device were biased to the high end of the range - 10.000 049 V, 1uV off the spec limit, I would get a TUR of 3.33. Not great, but not an indicator of imminent out of spec behavior. This edge condition, however, reflects the nature of the root sum square approach over stacked tolerances.

author: mendip_discovery
author_rank: Super Contributor
timestamp: March 20, 2022, 05:51:45 pm
content: Quote from: Dr. Frank on March 20, 2022, 10:32:12 amI think you are also confused. TUR = (UUT upper specifications - UUT lower specification) / (2 x Uncertainty).As we know what the specifications are then we can determine the Uncertainty but I would advise against relying on this as the rounding errors could cause issues eg.https://xkcd.com/2585/I have been playing with TUR on some of my worksheets as a sanity check that I have entered in the correct Uncertainty Values into the Decision Rule calculation.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 20, 2022, 06:20:25 pm
content: Unfortunately, the cartoon in the link has massive arithmetic errors - but I appreciate the idea.I always like sanity checks in spreadsheets.You raise another interesting point, though. On a voltage standard, we can set the goal posts where we like. This is not true for, for example,a DMM.

author: mendip_discovery
author_rank: Super Contributor
timestamp: March 20, 2022, 06:52:48 pm
content: You can specify what you like even for a DMM. I have customers that just want 2%. Manufacturers spec is also a choice and its the most common one, even if 95% of customers do nowt with the info anyway. I have a label and a certificate = happy.It why assuming a spec for a bit of gear used for calibration is a bit pointless. If the lab has some gear with some very extensive histories and very good checks they could easily outperform the manufacturer's specifications and therefore lower the Uncertainty.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 20, 2022, 08:06:12 pm
content: Early on, I looked into local calibration for my K2002 and K2001. I was quoted $270 for one, and $70 for the other.In both cases, I would have received a cal sticker.However, when I looked at the equipment, calibration would have involved older calibrators, with no standards in sight. Furthermore, the quality of the calibration is dependent on the capabilities of the technician.The bottom line is, the implications of a cal sticker and the price paid may have no relationship to each other. However, if you want your lab to have certified instruments, the stickers are all you need in the vast majority of cases.

author: bdunham7
author_rank: Super Contributor
timestamp: March 20, 2022, 08:10:18 pm
content: Quote from: Dr. Frank on March 20, 2022, 10:32:12 amQuote from: martinr33 on March 20, 2022, 02:50:41 pmTo generally address the above comments:Certain types of calibration certificates, such as the ones typically listed as "ANSI Z540..." only list limited information on the certificate. That's just the way it is, and whether that makes it 'useless' or not depends on what your needs are.TUR = Test Uncertainty Ratio, which if I understand it correctly in this instance would be the output uncertainty of the 732B divided by the calibration uncertainty (all factors) of the lab and the instrument they use. Thus there is no single TUR for the 734? bank of standards, that number is only the denominator in the TUR ratio.This now gets a bit tricky, because according to the 732B manual, its total output uncertainty (k=2.58) is simply the specified stability plus the calibration uncertainty (k=2.58). This implies that these two numbers include all factors. That's derived from the manual, but since the CMC document seems to imply k=2.00, there may be some additional complications. The result is that the numerator in the TUR, the total output uncertainty of the DUT, has the calibration uncertainty as part of it. So TUR = (S + U)/U, or if we use the 30-day stability spec of 0.3ppm and the Fluke CMC spec of 0.06ppm, TUR = (0.3 + 0.06)/0.6, or 6 and the implication is that the total output uncertainty for 30 days is 0.36ppm. Since they list a slightly different TUR, they are using somewhat different numbers for something--most likely related to the difference in confidence intervals.It is a bit annoying that they can't just straight out tell you the absolute calibration uncertainty (k=2.58, 99% CI) so that you can simply add it to the stability specification and get your total uncertainty for any period out to a year. But that is just not provided with that level of service.