How do you measure drifts of the order of 1ppm/year? - Page 1
Forum section: electronics / general

author: splin
author_rank: Frequent Contributor
timestamp: April 30, 2017, 02:22:39 am
content: Some posters here have equipment/references, including LTZ1000s and even LM399s with claimed drifts of around 1ppm/year. I'm not challenging the claims but I'm curious as to how you can measure this without regular access to JJA calibration facilities.An obvious start is to compare the UUT with a reference with a known and predictable drift which would presumably mean regular calibration with very low uncertainties. I believe it is very difficult to achieve 1ppm - eg. the hp 3458A spec states a 2ppm uncertainty from NIST in traceability to factory standards for a new unit - before any additional short term drifts during transport etc. I assume that affordable Fluke calibrations have similar uncertainties. And by the time you get your 3458A, 732B or whatever home there is the drift since calibration and additional uncertainties in comparing to the UUT.Is it acceptable to extrapolate from references with a long and stable calibration history (but perhaps not recently calibrated), to compare with the UUT and thus deduce it's relative stability if not it's absolute accuracy? Eg. if a reference has a history of (say) 10 or more calibrations, 2ppm uncertainty at each calibration can be averaged out to calculate a more precise annual drift - assuming the results show some degree of predictability.And whilst the LTZ1000s and LM399s have good tempcos, the 7 to 10 convertors are less good which means that you need very stable temperature sensors, to ensure measurements are made at exactly the temperature each time - perhaps a few 10s of mK at most?How long would you have to observe a reference before you could be confident that it is drifting at 1ppm or less per year?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: April 30, 2017, 04:13:58 am
content: Fluke has an application note that covers this topichttp://us.flukecal.com/literature/articles-and-education/electrical-calibration/application-notes/fractional-ppm-traceabiliI am on a quest to do something similar with a combination of Fluke and Datron 10v references. I have one 732B that is calibrated and the rest were bump measured against it. I think it would take a few calibration cycles over a few years just to have the confidence that my calculations are accurate enough to declare ppm drift values. It is very expensive and time consuming. I don't think there are any quick and easy ways to declare annual drifts without a few years of data.

author: Andreas
author_rank: Super Contributor
timestamp: April 30, 2017, 05:27:40 am
content: Quote from: splin on April 30, 2017, 02:22:39 amHello,If you have luck (= consistent readings) and access to 3-4 different calibrators and DMMs with a long calibration history in the same drift rangeyou will need at least around 5 years to get the drift value of the reference alone.But this does not include the voltage dividers and not the absolute volt of the reference.And yes it is very time consuming.I am also doing daily/weekly measurements of all LM399/LTZ references against each other to see if there are seasonal changes between the calibrations.With best regardsAndreas

author: ap
author_rank: Frequent Contributor
timestamp: April 30, 2017, 08:53:39 am
content: Actually here is the theoretical background:http://us.flukecal.com/literature/articles-and-education/electrical-calibration/papers-articles/maintaining-10-vdc-03-ppm-oAs can be seen, this is not trivial theoretically. There is some room for wrong assumptions to be made, potentially impacting the uncertainty result.Therefore I have decided, for the time being, that I will continue to take the conservative route and have one travel standard (732A) calibrated with a beltter than 0.5ppm uncertainty, transfer this to my others (4910 and 732B) and use the 4910 as the working standard, because it has the best specified uncertainty (1ppm/a, in reality it is much better, just as the 732A is). So at the start of the calibration cycle, my measurement uncertainty is better 0.5ppm, better 1ppm for at least 90 days, going up to about 1.5ppm after a year (or less if calibration is done more frequently).

author: brucehoult
author_rank: Super Contributor
timestamp: April 30, 2017, 11:11:59 am
content: 1 ppm is almost 90 ms per day.Any internet-connected PC with NTP will give you zero long term drift, with usually 100 ms or better absolute accuracy.If you can measure your 1 ppm source over a week or ten days then you can access its accuracy within 10% by comparing to any internet-connected PC's clock.A GPS time source would let you do it much more quickly and/or accurately.

author: e61_phil
author_rank: Frequent Contributor
timestamp: April 30, 2017, 11:27:28 am
content: Quote from: ap on April 30, 2017, 08:53:39 amCan you tell something about the drift stability of your Fluke 5440B compared to your nice 10V standards, please?

author: VintageNut
author_rank: Frequent Contributor
timestamp: April 30, 2017, 06:00:28 pm
content: The primary lab that I visited earlier this year has qty 4 of 732Bs that are sent to Fluke to measure against the Fluke JJ once per year.They have a large quantity of 10V references that remain in the primary lab sitting in a group. The total number of references is more than 10.One day a week, all of the references are intercompared and logged in a spreadsheet.The drift of the group of 4 traveling 732Bs is something like 1/3 ppm/yr.When my qty 2 of 731Bs were calibrated, they were installed into the large group of references and all of the group plus my 2 731Bs were all intercompared. The intercomparison is accomplished with an automatic system. It is rather slick. There are wires on the bench ready to connect the unknown 10V reference(s) to the data acquisition system.

author: pelule
author_rank: Frequent Contributor
timestamp: April 30, 2017, 06:17:28 pm
content: The factors of measuring the drift are time & a stable (traceable) reference - thus money and patience.I am measuring my tools since 3 years now (comparing each to detect potential issues).I measure (with same setup and procedure) the references each month with both meters.I log also temperature, humidity and pressure for later comparison and analysis.This measures give just a RELATIVE drift indication (detecting issues) - it gives NO traceable uncertainty and NO accuracy information.My references are:HP 735A DC Transfer Standard (1.019V spec: max. 120 ppm/year - so far measured -4.5ppm/year)Fluke 3330B DC Voltage & Current Calibrator (10V spec: max. 72 ppm/year - so far measured -8.5ppm/year)Guildline 9152T-P4 Standard Cell Enclosure, 4 cells (spec*: max. 4 ppm/year - so far measured -1.5ppm/year)(*I have found several different values regarding longterm drift, thus I am not 100% sure regarding the drift spec.)My measure equipmenthp3458A (8.5 digit, last calibrated 2014, spec max. 7.2ppm/year)Datron 1281 (8.5 digit, last calibrated 2008, spec. max. 3.2ppm/year)(both meters show a difference of 2-3ppm on the measurements).When I have collected enough budget, I will calibrate the 3458A and 1281 again.Thus I can conclude so far:Meters and references have no issue. The drift of the references is better as specified./PeLuLe

author: carl_lab
author_rank: Frequent Contributor
timestamp: April 30, 2017, 08:05:22 pm
content: Quote from: brucehoult on April 30, 2017, 11:11:59 amI think this thread is about drift ofvoltagereferences, not about time/frequency references.

author: brucehoult
author_rank: Super Contributor
timestamp: April 30, 2017, 10:07:17 pm
content: Quote from: carl_lab on April 30, 2017, 08:05:22 pmOh. Oops!

author: splin
author_rank: Frequent Contributor
timestamp: April 30, 2017, 11:09:25 pm
content: Some interesting responses - thanks. The Fluke references especially relevant.What would be most interesting is actual numbers, and thanks for those who have provided some. So what calibration uncertainties are generally achieved for the equipment/references that posters here discuss? I assume most aren't calibrated at primary standards laboratories.Then there is the temperature question. If for example the calibrated reference to which the reference which is being characterised (UUT) is being compared is a 3458A which has a long history and predicatable drift of quantified uncertainty, then all measurements, which will be taken over long time periods, will need to be taken either at exactly the same temperature or the TCs of both the UUT and the 3458A need to be known. In either case the temperature measurements need to be accurate requiring calibrated thermometers or thermometers known to be sufficiently stable over periods of perhaps many years. The accuracy requirement is of course inversely proportional to the reference/UUT TCs.The 3458A TC isn't specified for this role; I guess it will primarily be that of the LTZ1000 cct and may typically be .05ppm/K so not a big source of error but if you are looking at overall uncertainties of < 1ppm then these add up. To measure it will require another stable source and another source of uncertainty. So what uncertainties are being achieved or at least estimated for temperature and TC measurements?

author: VintageNut
author_rank: Frequent Contributor
timestamp: May 01, 2017, 03:01:28 am
content: The lab that I visited is kept at 23C. The uncertainty of the two 731Bs was 0.344ppm and 0.38 ppm. I have no idea how they calculate the uncertainty. I just was handed a report.

author: ap
author_rank: Frequent Contributor
timestamp: May 01, 2017, 10:51:28 am
content: Quote from: e61_phil on April 30, 2017, 11:27:28 amWill do, but the next cal cycle is just comming, so need some time to have data complete.Quote from: splin on April 30, 2017, 11:09:25 pmOne would use the 3458A only in relative mode if one wanted to do precise measurements (as related to the topic of this thread), comparing two standards, so the absolute uncertainty of the 3458A is irrelevant. Also temperature drifts of the 3458A are irrelevant, this would be short term measurements (comparisons). It could even be an uncalibrated 3458A. The most accurate way is to null the DUT out against the standard (provided this is doable). A 3458A is too unprecise to do absloute measurements in that accuracy range. It must be noted that when you switch on a 3458A and do an ACAL after thermal stabilization, you always have slight drifts (hysteresis...).

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 01, 2017, 03:58:01 pm
content: Hello,in general, to determine the drift rate / stability of any artefact standards (e.g. prototype kilogram, Weston cells, prototype meter, Thomson or Reichsanstalt Ohm, OCXO, MASER or Cs clock, etc.) you might always use at least one other reference standard of the same stability class.From the relative drift of these two artefacts, you can always estimate the drift rate of these references, you might assign about half of the total drift to each individual reference.The more artefacts you have in your group, the better the estimate of the drift rate.3 references lets you decide, which of the three is the one which drifts the most.The drift mechanisms and systematic errors should be well understood, and additional drift influences, like temperature coefficients and so on should also be considered or mitigated.So it is not necessary to have a superior standard to determine the drift rate.That procedure is good metrological practice, especially in a certain case, where artefacts are still in use, and no superior standard exists.That is currently the prototype kilogram, where the metrologists simply compared "Le Grand K' against its about 30 copies, and determined, that the whole group drifts apart, and especially, that the prototype kilogram in Sèvres is the most unstable (rate -5*10^-8 in 100 years)So that self-monitoring procedure I have used for the last 8 years, on my voltage and resistance references.I maintain 4 different voltage references, 2 LTZ1000 @ 45°C, continuously powered (systematic drift: typ. -0.8ppm/yr.), the LTZ1000A @ 65°C reference inside my 3458A, not constantly powered (systematic drift: typ. -2ppm/year at continuous power, therefore typ. < 1ppm/yr. due to long non-powered state), and a double SZA263A reference @ 55°C inside my 5442A, also not continuously powered, (typ. + 1..2ppm/yr.)These typical drift rates are taken from different investigations or datasheets of similar references, like Spreadbury, Pickering, Fluke, Metron, etc.The 3458A is taken as the baseline; all absolute measurements are done relative to it.The difference to the 5442A was always less than +1.5ppm over the last 4 years, and the difference relative to both of the LTZ1000 was always less than -3ppm over that time. Therefore, the drift rate inside the whole group is for sure < 1ppm /yr.A similar procedure is done on 5 VHP202Z, 10.000kOhm resistors, which all stay inside a +/-0.5ppm window over the last 2 years, so the drift of this group is also < 1ppm/yr.As the 40K resistor inside the 3458A is not temperature stabilized, it can't be taken as an additional artefact reference. Anyhow, the correction factor (reading of 3458A relative to the median of the group) is inside a +1.8ppm / -0.5ppm window over the last 4 years.My 3458A already has a 40kOhm, VHP101, which should also drift as low as the 5 other VHP202Z (typ. 2ppm/6 yr.).The low variation of this correction factor evidences such a low drift rate.Frank

author: lars
author_rank: Regular Contributor
timestamp: May 23, 2017, 04:40:25 pm
content: In the LTZ1000 thread I asked for LTZ drift but also for the drift of Vishay hermetic resistors I gets confused. The only “proof” of 2ppm/6 years I can see are the test of just two samples compared to a QHRhttp://www.vishaypg.com/foil-resistors/case-studies/study/vsl_1/. One of the samples seems to be about 2ppm/6years but the other much worse at 2ppm/year!I recall that Andreas in some thread saw up to 10ppm? drift just due to temperature cycling 10-40C of VHP resistors I think he bought from Digikey.My own experiences are that the five VHP resistors I got really brand new from Powertron many years ago drifted about 5-10ppm the first year, but less than a ppm/year after that. I also got 2 pieces of the similar HK series from Alpha and they drifted less than a ppm the first year (as far as I could guess compared to GR and L&N standards) but I don´t know how new they were even if I got them from Alphas sales representative (that is: could of course have drifted on the shelf). Any other that have experiences of the drift of VHP? Anything more published?So If I have nothing more to compare to what can I expect from Vishay VHP drift? (I don´t think 2ppm/6years are the correct answer).Of course it would also be interesting to know the same for other resistor like Econistor or Ultrohm WW or non hermetic BMF but when we probably are far from 1ppm/year?Lars

author: Dr. Frank
author_rank: Super Contributor
timestamp: May 23, 2017, 05:25:08 pm
content: Quote from: lars on May 23, 2017, 04:40:25 pmAs far as I can tell from my group of 5 VHP202Z, they drifted less than 1ppm apart of each other, during 6 years.Several reference measurements in between showed < 1ppm/year uncertainty for my measurement-setup.This BMF technology is sensitive to big temperature excursions, so you may easily get big hysteresis effects , when the resistors are heated or cooled more than 30°C apart from RT.I got 2 of these resistors checked by Powertron, and they cycled them to -55°C and 125°C to determine their T.C.sWhen I got them back, both showed a +6ppm shift.This hysteresis very slowly crept back on one resistor. It took over a year to nearly reach the original value.The other resistor showed a permanent shift, no creeping back, so I temperature cycled it, and succeeded in bringing him back within a few tenths of ppm.Soldering heat will have the same effect.So you should not confuse this hysteresis effect with an undisturbed, virgin timely drift.Therefore, I'm convinced, that these 2ppm/6 years is a substantial and real specification of the hermetically sealed and oil filled types.Frank

author: Moon Winx
author_rank: Regular Contributor
timestamp: May 24, 2017, 01:46:22 am
content: Our lab measures DC reference standards (mostly 732Bs) on a conventional JVS and we have history on some of these references going back a couple of decades. I think it was around the mid-90s when we started using the JVS for these measurements on some 732As that we still use as check standards (so they are measured way more often than necessary).If you guys want to know the drift rate of these types of zener references, and a legit predicted value formula that was derived from a study using empirical measurement data from 140 zener references, please seehttp://vmetrix.com/ZenerP.pdf. It's only 3 pages, a quick read.Humidity is actually the biggest contributor to seasonal swings of the measured values.

author: CalMachine
author_rank: Frequent Contributor
timestamp: May 24, 2017, 02:09:10 am
content: Quote from: Moon Winx on May 24, 2017, 01:46:22 amMay I ask what lab you work at?

author: lars
author_rank: Regular Contributor
timestamp: May 24, 2017, 04:59:20 pm
content: Quote from: Dr. Frank on May 23, 2017, 05:25:08 pmHello Dr. Frank,sounds like you are a Vishay seller. I fully agree that the VHP series are really good resistors but am still confused that the 6.45kohm resistor drifted 12ppm in 6 years. I have also, since I saw the curve for the 12.9kohm, wondered if two different drifts cancels in that curve. It seems that after six years the drift gets higher than in the beginning.Lars

author: Moon Winx
author_rank: Regular Contributor
timestamp: May 25, 2017, 03:49:41 am
content: Quote from: CalMachine on May 24, 2017, 02:09:10 amI work at a US gov't primary standards lab... Since I'm new here on this forum, I don't want to really get more specific than that.

author: CalMachine
author_rank: Frequent Contributor
timestamp: May 25, 2017, 01:12:01 pm
content: Quote from: Moon Winx on May 25, 2017, 03:49:41 amI completely understand! I was figuring as such. We do a significant portion of our work with the Govt.

author: kj7e
author_rank: Frequent Contributor
timestamp: May 27, 2017, 01:33:48 pm
content: Quote from: Moon Winx on May 25, 2017, 03:49:41 amMaybe you can answer the age old question, exactly how close is "Close enough for government work"?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: May 07, 2019, 09:33:00 pm
content: Quote from: Moon Winx on May 24, 2017, 01:46:22 amUnfortunately the link is no longer working. Maybe someone saved a copy? And he can lay it out?

author: 2N3055
author_rank: Super Contributor
timestamp: May 07, 2019, 10:04:40 pm
content: There you are..