Thought experiment with regard to 6 1/2 digit DMM calibration - Page 1
Forum section: electronics / testgear

author: SharpEars
author_rank: Unknown
timestamp: September 26, 2015, 10:08:19 pm
content: Imagine I have three HP 3456A units and that they are within 1-2 counts of each other at 6 1/2 digits on the 0-10 V measurement range, all across the range (i.e., 1V, 3V, 5V and 10V). Let's also assume that these units came from three different sources and were calibrated at different times over the course of their lives by different people with different calibration equipment at different times (assumption here, but most likely valid), with the latest calibrated one being circa 2010 and the rest calibrated at unknown times. Their serial numbers also vary wildly.The question:Because they are so close to each other in their readings, can you consider them as well calibrated (i.e., to 1-2 last digit uncertainty/2-4 ppm) for the 0-10 V measurement range?

author: bson
author_rank: Unknown
timestamp: September 26, 2015, 10:37:59 pm
content: You can personally regard them as calibrated if you wish, and they're probably close enough for most development work.However, they're not NIST traceable, which means they're not calibrated to a measurement grade. You couldn't use them for instance to electrically characterize a component for a data sheet, for research purposes (where others need to reproduce you results), or any otherobjectiveuse that requires absolute accuracy.However, you can use them for relative measurements, like a before-after DUT. The max drift spec still applies here, so if you take two measurements a couple of hours apart the 24 hour spec applies to the two measurements. If there are shorter drift specs, like 5 minutes, 60 minutes, then these apply as well. Note that these specs also set bounds on environmental conditions. So you could use it say to produce a histogram of a bag of samples over a 60 min period, and know what the max measurement deviation is; get a hygrograph that plots humidity along with temperature and sometimes air pressure. So you could use them to measure distribution of properties within a sample population, but you won't ever know what the exact values are unless you get one of them calibrated in a way that traceable to the SI standard.

author: bson
author_rank: Unknown
timestamp: September 26, 2015, 10:42:41 pm
content: Hmm, on second thought this assumes linearity. I'd just get them calibrated and not worry about it and focus on more meaningful problems.

author: SharpEars
author_rank: Unknown
timestamp: September 26, 2015, 10:43:59 pm
content: Quote from: bson on September 26, 2015, 10:37:59 pmSo, in a nutshell I am wondering: They could not have all drifted exactly the same amount in the same direction over the last 5+ years. This is extremely unlikely. Therefore, they must be very close to a reference standard within several ppm, with quite a high degree of certainty. Am I wrong in making such an assumption?

author: cncjerry
author_rank: Unknown
timestamp: September 26, 2015, 11:55:20 pm
content: I have around 8 or more 6.5 digit meters that all have read close to each other at one time or another. That means that they have all read close to each other at one time or another and nothing else.It's highly likely your meters are close to the standard but you can't assume anything. I've been debating this myself for a while with meters as well as components. If you buy a bag of 1% 10k resistors and plot the distribution you could assume that the median resistors are closer to 10k vs the outliers and you would be wrong again.I have 2 x 3457a and 2 x 3456a meters that since I purchased them have read within a two counts like yours. I also have 3 frequency counters that were all close to each other. I then bought a GPSDO and they were all a few counts off.It would be great if us hobbyists setup a coop where we could bring our equipment to a members' lab and test or calibrate our equipment for free. To start, if anyone in the SF bay area would like to test against my GPSDO 10Mhz, 15Mhz or 1PPS standard, send me a PM. Just bring your 1k, 100K, or 10V .0001% standards with you...

author: smgvbest
author_rank: Unknown
timestamp: September 27, 2015, 12:07:00 am
content: This is soley my opinion on your Thought experimentBeing close in counts to each other only infers they are close to each other not to any reference standard.If you had said they are 10.00005VDC +/- 2 or 3 counts and your source was a know calibrated source then I think you could infer they are well calibrated to a reference. To simply be +/- a few count and not know what the reference was I think you can only infer they are close in cal to each other. And in either case with out a proper calibration they are only good for hobby use not to hold any professional standard.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 27, 2015, 12:25:46 am
content: That's the general 'man with 3 clocks' problem, and can be discussed on any group of standard for any metrological unit, like voltage, resistance, time, etc.Without having access to a superior standard, you can anyhow calculate a probability for the current uncertainty of your three standards, taking into account that they were once calibrated to a known uncertainty, and their specified drift over time, and the number of members in this group.So your key idea is correct, if they today agree to a certain amount they might still be accurate.On the other hand, the 3456A is specified for about 20ppm / 90d, that's 20 calibration periods in total, each adding 6ppm, if I remember the spec. correctly.And you ONLY have 3 artefacts, that's only about SQRT(3) improvement in variation.Further on, the history of the instruments is not known.. they usually drift up to the mentioned limits, when always powered, but will drift much less, down to zero, when left unpowered.You also might have some systematic drift of either the voltage references inside (I assume the LM399H is inside ?) and the FineLine resistors maybe.(The LTZ1000 for example has a typical systematic drift of -0.8ppm/yr., similar to the SZA263 which might have +1ppm/yr.)So in the end, the probability of really finding 2-3ppm uncertainty of your group of instruments is very low, with these boundary conditions.Joe Geller once measured 78x LM299 references over 1000h.. and this is exactly the same problem with the probability of the mean uncertainty, see there:http://www.gellerlabs.com/LM299AH-20_Case_Study.htmhttp://www.gellerlabs.com/LM299/LM299AH_Corr_Time_Drift%20b.pdfFrank

author: SharpEars
author_rank: Unknown
timestamp: September 27, 2015, 01:05:53 am
content: Quote from: cncjerry on September 26, 2015, 11:55:20 pmWith regard to your resistor example, the resistors are most likely from the same batch/run - they are correlated and may have all drifted by the same amount.With regard to my units, I am trying to show that there is no correlation between them, other than that they share a lot of common hardware. Once again, there are three units of different ages (by a factor of years if not decades), from three (different) parts of the country (USA), calibrated by who knows who, who knows when (with the exception of the one labeled unit stating a 2010 calibration date), with VERY different serial numbers (i.e., not one production run by a mile). For all of them to be so close to each other, unless the HP 3456A as an entire series over two decades is known for systematic drift up or down, is one hell of a coincidence!!For example, let's say each of them could drift +/-20 ppm/yr. That means that the chance of all three of them being within 2 counts, well you do the math, it's very very small, given that they are all at least a good five years out of calibration. An argument can be made that they drift out and back towards the zero point (the calibration point) and there is a much greater probability of finding units close to the calibration point and it is not anomalous to find three units, therefore, close to the calibration point. Great, that makes sense, the probability curve is weighted towards the calibration point and my units are therefore calibrated.I am asking all of this strictly for hobby reasons, but I find it hard to believe that the fact that they are this close to each other, given their stated drift characteristics of 20 ppm/yr, the fact that none of them has been calibrated within the last five years, is sheer chance and means nothing with regard to the accuracy of their readings...I should also add that I have a Fluke 8846a with an unknown calibration state, with which they all agree at the 1 V range (+5 ppm Fluke) and 10 V range (+35 ppm Fluke).

author: deadlylover
author_rank: Frequent Contributor
timestamp: September 27, 2015, 01:51:41 am
content: Not that it will help much, but I'll throw in my experience from buying used 6.5digit meters.I have six 6.5 DMMs and they all read within ~10ppm of each other when I bought them, none of them have any calibration stickers at all and they were mostly made in the late 90s.Values are referenced to an R6581 right after calibration;34401A#1 = +19ppm (never adjusted)34401A#2 = +20ppm (never adjusted)34401A#3 = +26ppm (possibly adjusted from cal count)3457A = +27ppmR6871 = +27ppmR6561 = +30ppmR6581 = +9ppm before calibration, ~ 2.4ppm uncertainty(95%) after cal + 90day spec of 3ppm.At one point I was the man with 6 clocks and I also thought the same as you, the bottom 4 meters of different models agreed within 4ppm and also, 2 are from the US and 2 are from Japan so they must be bang on, right?Noope.

author: Smith
author_rank: Frequent Contributor
timestamp: September 27, 2015, 06:21:43 am
content: 1. I have never ever had a meter out of calibration without it having serious damages/repairs, empty backup batteries, bad input connectors or serious measuring errors. This includes handheld and bench meters. Most people think it will immediately out of cal after the calibration period expires.I think (correct me if I am wrong) in America calibration means adjusting the device to the specs of a calibrated machine. In Europe calibration means comparing it to a calibrated machine. Here we (roughly translated) adjust the machine to specs if needed. But it very rarely does.2. I don't think NIST traceable calibration is needed for 99% of the people. It is if you have to comply to certain certifications, or make measurements with different kind of people (for comparison).It IS nice if your meters at home are calibrated against each other, so you can swap and compare measurements. It can make you aware of measuring errors too, and its nice to compare your device in case you don't trust it after it fell, had an overload or something similar.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 27, 2015, 09:05:21 am
content: I usually keep one of my 34401A meters calibrated to a traceable standard with official calibration certificate once a year.And compare the rest of my 34401A, 34410A meters to it.What is really astonishing to me is, how close they stay to each other over the years.In your case with the 3 x 3458A meters, all you need is a transfer standard that you compare to a known calibrated value to your meters for comparison.

author: SharpEars
author_rank: Unknown
timestamp: September 27, 2015, 01:37:16 pm
content: Quote from: deadlylover on September 27, 2015, 01:51:41 amThank you for an actual real world example. I must say that I am very surprised that all of your meters erred forward in terms of ppm shift. The question I would have for you is: How close in serial number were your 34401 units and did they come from the sample place (i.e., did you buy them all from one source at about the same time)?

author: deadlylover
author_rank: Frequent Contributor
timestamp: September 27, 2015, 02:50:31 pm
content: Quote from: SharpEars on September 27, 2015, 01:37:16 pmThe serials are;US36068772US360300163146A70762 (this one drifted a bit more than the others)I haven't peeked inside for the date codes yet so I'm not sure what vintage they are, they were bought separately at auction within a few months span. Even after all this time, they are still within 1 year specifications...barely.Too bad you aren't local, otherwise I could help you import the 'volt' into your lab...kinda, I think I'm only good for ~7.5ppm uncertainty(95%) now.

author: SharpEars
author_rank: Unknown
timestamp: September 27, 2015, 05:13:40 pm
content: Quote from: deadlylover on September 27, 2015, 02:50:31 pmYeah, too bad you aren't local. I live in the Chicago suburbs! I have four 6 1/2s and two 5 1/2s and they all for the most part agree to very few ppm, Three are very close to each other, I wish I knew someone around here with a calibrated 7 or 8 1/2 reference point to see where I'm at, at least at 10 V. I can work it from there...

author: sarepairman2
author_rank: Frequent Contributor
timestamp: September 27, 2015, 10:19:04 pm
content: AHHH 26 PPM ERROR WHATEVER SHALL I DO THE SKY IS FALLING

author: SharpEars
author_rank: Unknown
timestamp: September 29, 2015, 12:57:26 am
content: Quote from: sarepairman2 on September 27, 2015, 10:19:04 pmAt 26 PPM error, what is the point of 6 1/2 digits? You might as well have a 5 1/2 meter, it much cheaper and no less accurate at this level of error...

author: HighVoltage
author_rank: Super Contributor
timestamp: September 29, 2015, 10:24:14 am
content: To check linearity from one 34401A to another 34401A, I start with the 10.0000 V Geller Labs Reference and have all the values for the different meters in a spreadsheet. In addition, I am using the Time Electronics TE1017 reference for other values. This makes the checking of the 6 1/2 digit meter calibration kind of reliable, especially since I am always getting one meter officially calibrated.Here is a setup that has been running for several days.Only the last digit of the 34401A is changing and only by +/- 1 digit over a few daysUsually it shows 9.00003 VDC at NPL100 constantly

author: macboy
author_rank: Super Contributor
timestamp: September 29, 2015, 02:45:38 pm
content: Quote from: SharpEars on September 29, 2015, 12:57:26 amYou are making the very common mistake of equatingprecisionwithaccuracy.Most 6.5 digit meters are accurate (after 1 year) to 30 to 50 ppm of the reading +- a few more digits.Most 5.5 digit meters are in the 50 to 100 ppm range.Even a $5000 Keithley 2001 7.5 digit meter is in the 20 to 40 ppm range. Only a very few meters like the $10000 8.5 digit 3458A get below 10 ppm (it is 8 ppm standard, or 4 ppm with the high stability option).If you think that all meters should be accurate to the last digit give or take a few counts, then you are fooling yourself. Most meters have a precision that isat least10x greater than their specified absolute accuracy. This doesn't mean that the last digit (or two) is not useful, but you need to understand what it is and isn't.

author: SharpEars
author_rank: Unknown
timestamp: September 29, 2015, 05:26:45 pm
content: Quote from: macboy on September 29, 2015, 02:45:38 pmNope, no confusion here. I am totally about the accuracy and keeping things within 20 ppm. The specified yearly drifts are worst case. A lot of older HP stuff is decades old and doesn't drift or drifts very little - way below the yearly specs...

author: saturation
author_rank: Super Contributor
timestamp: September 29, 2015, 07:01:39 pm
content: I actual do this for my home lab but exactly what the ppm is in the end is difficult to say, I explained in here in an older post:https://www.eevblog.com/forum/testgear/when-do-you-really-need-6-12-dmm-i-mean-the-res-andor-the-accuracy/msg235251/#msg235251If you wish it to be just for home use its explained as above. If you wish it for professional use, you'd have to follow standard metrology procedures, whatever it is for the instrument you wish calibrated.Quote from: SharpEars on September 26, 2015, 10:08:19 pm

author: eas
author_rank: Frequent Contributor
timestamp: September 30, 2015, 01:30:02 am
content: Quote from: SharpEars on September 29, 2015, 05:26:45 pmYes, but why?You said:QuoteIf absolute accuracy is really important to you, then you should appreciate that a 6.5 digit DMM gives you a whole 'nother order of magnitude for tracking drift, and adjusting for it, if necessary.

author: bitseeker
author_rank: Super Contributor
timestamp: September 30, 2015, 04:59:52 am
content: Quote from: deadlylover on September 27, 2015, 02:50:31 pmI believe the vintage of your meters are 1996, 1996 and 1991, respectively, hence the last one having drifted further than the first two.I have two from '91 and they measure pretty close to each other as well as to my Greenlee 820A (up to its lower resolution, of course).

author: Dubbie
author_rank: Unknown
timestamp: October 01, 2015, 09:36:05 am
content: I bought a 10v standard off eBay that was guaranteed to be within 2ppm. It arrived with a cal certificate stating that it is within 0.4ppm according to the fluke they used to set it. Anyway my 6.5 digit keysight reads 10.000 00v when I have the temp in my lab as per the cal cert and after a 30 min warmup. From my limited testing it seems to drift up and down by about 10uV per 3 degrees. Not sure if that is the standard, the meter or both.My meter has a 6 month old cal. So I am pretty happy with that. It gives me the confidence to set everything else to that.