Help on reading calibration data for DMM - Page 1
Forum section: electronics / general

author: spicytofu
author_rank: Contributor
timestamp: November 09, 2022, 01:14:15 pm
content: I just got my DMM calibrated with a data printout. I was looking for something along the lines of before and after, but the only data I see is "AS LEFT", which is probably after. In the test description, they print the name of the test (ranges), the test results, and the upper and lower limits. My question is why does it not tell me the input? Do I assume that the input is 10.0000000 V and that whatever my meter reads, it is the offset to that (my DMM read lower)? Or that the input itself is whatever my meter reads? I am leaning to the former, I expect the the Fluke calibrator is going to be spot on in the sub ppms.The result is that the DMM passes everything, but some values are close to the limits and some are an exact match. I am using this to calibrate my other DMMs that are lower in resolution, (7.5 -> 6.5) but I am unsure if I should adjust the 6.5 meter readings to match the range (for example, add offset+7.5 reading, making this the true absolute value)Does it matter to the measurements? No, but I thought this would be a fun and learning exercise.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 09, 2022, 02:14:31 pm
content: In most cases calibration does not include an adjustment to the DMM. So there would be no seprate before and after readings, just one reading per test point that is good enough to stay inside the limits.You are right, that there should be a set value for the reference used. With a high grade calibration this may be at nominal, or at least very close to it. They may not have a better estimate to the values than spot on. Ideally they should have a guard meter and thus also have a guard reading and maybe an idea on how the test points are of nominal. If they don't give anything, nominal is all you can assume.Where the meter and calibrator disagree, it is still the question on how good the calibrator was / is. With a good 7.5 digit meter there may not be that much TUR and part of the difference could still be the calibrators fault.Also don't get fooled by the accuracies for a shorter time frame - the usual calibrations are testing to the 1 year limits and don't reset the accuracy to the 24 h/30/90 days limits. The shorter limits would abbly relative to the reading and the calibrators error needs to add.

author: ap
author_rank: Frequent Contributor
timestamp: November 09, 2022, 02:42:58 pm
content: I am afraid there is not much you can do with this data yet... Have they calibrated it against the 24h or 90 day limits and their MU is high, leading to the limits they tell you, or have they calibrated it against the one year limits (many companies do, especially with high resolution meters) and their MU is better.In the latter case, since some of the readings are at the limits, you can safely assume that within the next year of use, the meter will get out of spec.So you should find out what they have done before you use it.

author: bdunham7
author_rank: Super Contributor
timestamp: November 09, 2022, 03:01:35 pm
content: Without specific details its hard to say. What models of DMM are involved? Can you post the actual calibration report here?If you just have 'as left' data that doesn't indicate the uncertainty of the source and the test limits are the 1-year spec of your meter, then you really don't have much more than a piece of paper that allows you to use the meter in a place where calibration is required. I certainly wouldn't go calibrating (especially adjusting) other meters with it--this is a problematic method in any case.

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 09, 2022, 05:53:58 pm
content: Quote from: spicytofu on November 09, 2022, 01:14:15 pmThere should be a statement on the certificate saying if there have been any adjustments, if there aren't any then that also is usually stated. If adjusted then the certificate needs to show before adjustment readings unless the unit was sent in with an issue to start with..like no cal data. I know a lab that does a single certificate with before and after readings in the same cert, other do it as two separate certificates.The applied value needs to be clearly stated. If it's 10 V then assume it's with all the zeros you can dream of but don't forget to add on the measurement uncertainty that should also be on the certificate. That could be 4 μV/V + 0.4 μV (4 ppm + 0.4 μV if you prefer).QuoteYour master DMM is in spec, and those readings can go up as well as down. So you can't rely on them being the same. Let us say it was calibrated at 10V and displayed 10.00003 V and its within spec of ±0.00005, they have an expanded uncertainty of 0.00002 V so that reading could have been 10.00005 V or 10.00001 V but you don't know when your meter gets home if its still at the 10.00003 V point its a wandering meter and is now going to show 10V as 9.99997 V (this is why proven stable kit is desired). Now you have gone to use it to adjust your meter under test and it shows 10.0003 V and has a spec of ±0.0005. Do you adjust it? Well, the 1 least significant digit will always want to be ignored 0.0001 V and you would have to calculate your measurement uncertainty first (rough guess 0.00006 V) as you could adjust the meter and make it worse. Adjust only if its outside the spec + your uncertainty. Don't put total faith in my maths, I have been on site for 9hrs today and I have just got in home.QuoteIts what we are here to do. Spec of meter ±0.00005, Imported Unc 0.00002, Resolution of Master 0.00001, Repeatability 0.00001. So 0.00010/SQRT(3) = 0.000058, 0.00002/2 = 0.00001, 0.00001/SQRT(3) = 0.000006, 0.00001/1 = 0.00001, Root Sum Squared = 0.0000597 round that to 2 whole number (yeah I know) and you get 0.000060. You can add thermals and other stuff in there but for now its ok.

author: spicytofu
author_rank: Contributor
timestamp: November 09, 2022, 06:50:56 pm
content: Great advice. The DMM is a Keithley 2010, old model but very capable. It is way overkill for our use, but company had money to splurge. Never calibrated so I am glad to see things did not drift much (other than those values near the limits). I cant post anything since this meter is use for secure work, in fact, the calibration report is now behind the "vault" and I cannot bring it out.My goal was, say a 100 ohm range was reading 99 ohms (fake numbers). I assume the calibrator (Fluke 5520A), would be bang on 100 ohms and if my meter read 99 ohms, then the true value of a measure resistor would be 99 + 1. I have some precision resistors in the lab I can grab, get the value, then add in +1 to calibrate my 34401As.

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 09, 2022, 07:13:16 pm
content: Secure work, lol. I can't see why people knowing the meters calibration report would interfere with security. But you lot across the pond are a strange bunch. I have to be escorted onsite due to the American owners, for the fear of me seeing an actuator for a fuel system in an aircraft, the fact I am there to test the test unit that tests the actuator so I get to see them up close. Then I go to UK company making high end command and control systems with cameras for tracking objects at the curvature of the earth and no escorting because well they just don't leave sensitive info around for me to steal, computers with any drawings are locked, if there is a printed document it doesn't show anything that would cause a stir. Meanwhile, the American firm just sells the parts to the Chinese. I go to places making parts for the MOD, Rolls Royce etc and they never get too faffy with secrets. The only place that was a bit security minded was G4S (they do cash collections at shops etc) and the company that prints credit cards.Anyway, yup you can do that. When I do resistances I tend to quote the currently stored value, eg 100.00040 Ω because I have fixed resisters and they are not adjusted to perfect 100 Ω but the stored values in the calibrator are just updated as they drift.Might be cheaper to just find a good lab in your area that does a half decent job of calibrating your meter for a good price, but only do that if you suspect it of being outside of spec.

author: bdunham7
author_rank: Super Contributor
timestamp: November 09, 2022, 08:03:16 pm
content: Quote from: spicytofu on November 09, 2022, 06:50:56 pmIf you want to try and informally verify that your 34401A(s) are in spec, then I suppose you could try that sort of thing. However, actually making an adjustment to the 34401A on that basis would be sheer madness, IMO. Without actually seeing your calibration report I can't say much more about your case specifically. However, consider the specs for each unit on their 100 ohm ranges:The 1-year spec for the 34401A is 0.010% rdg + 0.004% range, or 140ppm total, so the limits would be +/- 14 milliohms.The 1-year spec for the 2010 is 52ppm rdg + 9ppm range, or 61ppm total, limits are +/- 6.1mRThe 1-year spec for the 5520A is 28ppm rdg + 1.4mR, so the total tolerance is +/- 4.2mR.If you accept the old rule-of-thumb that you need guardbanding for any TUR <4.0, then the 5520A is not even quite adequate to calibrate the 34401A directly, let alone the way you are proposing. And unless it is specially characterized and used along with a much more precise (8.5 digit) DMM, the 5520A is nowhere near good enough to be calibrating the 2010 on any of its ranges. I'm afraid your calibration report does no more than verify that your meter is probably not badly broken.