Metrology Question - Page 1
Forum section: electronics / general

author: LloydofDSS
author_rank: Newbie
timestamp: March 21, 2022, 11:21:52 pm
content: If I have 10 technicians all making only one measurement like 1 volt AC. This is the only measurement they make. Can I send out only 1 DVM for outside calibration and then perform a comparison to that meter at 1 volt AC to verify the other 9 are calibrated for our purposes. So long as I document everything?

author: bdunham7
author_rank: Super Contributor
timestamp: March 21, 2022, 11:33:14 pm
content: That depends on what rules you are required to follow. Is this an ISO9000/9001 issue? Under most rules that are likely to apply to you this would either be unapproved or explicitly discouraged. There might be workarounds or other ways to reduce your calibration costs. How accurately do you need to measure the 1 VAC?

author: mendip_discovery
author_rank: Super Contributor
timestamp: March 22, 2022, 12:49:17 pm
content: Another option is to send the meters out but request only the 1 V AC range to be calibrated. This should be cheaper.The way you are thinking about doing should be ok but you might end up opening a can of worms with regards to procedures, training, competency etc. Then the customers/auditor etc still might not like it. Would be better to use a voltage source and have that calibrated but again it does open you up to more work.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 22, 2022, 02:17:22 pm
content: This is actually just what calibration labs do. They relate their equipment to higher standards and then use it to calibrate other instruments.The devil is in the details. Uncertainty grows in each step of the process. As long as you appropriately document this, and the resulting uncertainty meets your needs there is nothing wrong with the concept. It may or may not save money. Often though the reason for doing this is to save time which can have indirect and sometimes large savings.

author: james_s
author_rank: Super Contributor
timestamp: March 22, 2022, 05:46:41 pm
content: I used to work for a ISO9000 certified company and this sort of thing would NOT have been permitted. It would work fine of course but that's beside the point, ISO is all about documenting the process and following what the documentation says.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: March 22, 2022, 06:00:28 pm
content: But that's not the fault of ISO9000. You get to define your procedures and what is and is not acceptable for your application.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 22, 2022, 09:02:46 pm
content: It is just as Conrad says. ISO 9000 requires that you have procedures and follow them. If your procedures say that calibration of a voltmeter requires soaking in cow dung at a temperature of 32 +/- 3 deg C for a minimum of two weeks and nothing else, an ISO auditor will verify that those procedures are compliant with your organizations processes for writing and controlling procedures, and verify that you are actually doing the cow dung soak, monitoring the temperature and duration and that is it. If you did you are ISO 9000 compliant.

author: LloydofDSS
author_rank: Newbie
timestamp: March 22, 2022, 09:43:05 pm
content: Thanks for the kind responses. I have been in this field for over 40 years and this has not come up for me until now. It seems so long as the calibration is traceable back to a known NIST recognized source and is properly documented, you can use a calibrated instrument to calibrate others in turn. It only becomes practical if you only make one or two specific measurements to a known accuracy. The accuracy I am required to achieve is on the order of 1%. When a meter is used for all kinds of measurements, sending it to a cal lab is critical and cost effective.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2022, 09:59:45 pm
content: Quote from: CatalinaWOW on March 22, 2022, 09:02:46 pmI agree there is some latitude of that sort in the ISO9000 process and I'm not any sort of expert, but I believe cow dung would be a problem because although there are probably all sorts of funny anecdotes where a not-so-good process has been accepted by an auditor, what is missing is that in general you have to demonstrate that your documented processes actually accomplish what you say they do. If your process requires a 1% or less uncertainty in a reading, you'd need to demonstrate or explain how the cow dung procedure accomplishes that.For something like a voltmeter, the default procedure is to obtain a calibration certificate from an accredited calibration laboratory. For process control and QC applications like the OPs, the ANSI Z540 w/ OOT data only (so you can verify that your meter was not out of tolerance right before the calibration) is the good enough default. You can use a non-accredited lab if you have a reason and explain why you need to do that and how your process will still achieve the stated goal, or you can write your own calibration procedure and follow that, subject to the same requirements. However, you still have to pay someone to write up all that stuff and your ISO audit may cost a bit more because of the additional time and procedures covered. I can almost positively guarantee that the cheapest, easiest way to deal with the OPs issue is to just find a calibration lab that emphasizes low cost and quick turnaround.

author: bdunham7
author_rank: Super Contributor
timestamp: March 22, 2022, 10:13:20 pm
content: Quote from: LloydofDSS on March 22, 2022, 09:43:05 pmNot really. You still have to demonstrate that the process you use to transfer the calibration is sufficiently precise to meet your requirements. Yes at 1VAC at some reasonable frequency (50-400Hz?) and a requirement of 1%, perhaps that isn't actually all that hard todo(AWG + bench meter) butdocumenting and provingthat takes a bit more effort and persuasion. However, I have no practical experience regarding what you might be able to get away with. What type of meters are your techs using?You might be able to extend your calibration intervals, but I suspect that even that process will be more cumbersome and expensive than just finding a local cal lab that can turn your stuff around quickly and cheaply.

author: dietert1
author_rank: Super Contributor
timestamp: March 23, 2022, 03:49:25 pm
content: Sometimes i am wondering how people obtain a calibration certificate by a remote lab and then get the device shipped back by mail. Is there a special mail service that protects the validity of the certificate?I mean in this special case a sensible answer might be in between. Like get two of the ten meters calibrated externally and then use those two to check success after return and to implement the internal calibration procedure for the other eight instruments. Anyway there should be a procedure for regular internal calibration checks. With a 1 % requirement and ten working instruments that should pretty much guarantee valid cal.Regards, Dieter

author: bdunham7
author_rank: Super Contributor
timestamp: March 23, 2022, 04:20:59 pm
content: Quote from: dietert1 on March 23, 2022, 03:49:25 pmWhy would mailing it invalidate the certificate? Obviously damaging an instrument after calibration may affect calibration, but that could happen any number of ways. Certain instruments may have special handling or environmental requirements, but for the most part I think that as long as they are packed well enough to avoid damage, nobody even thinks about these things, even if they should. I had a voltage standard that got slightly 'shifted' by exposure to cold during shipping, so it's not so far fetched and idea--but every cal lab and manufacturer I know routinely ships instruments by mail or common carrier after calibration without a second thought.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 23, 2022, 04:31:42 pm
content: This comes back to understanding what your needs for calibration are. If you are doing something that requires ten digit accuracy then shipping induced drift is something to worry about. If the need is for 5% accuracy almost anything is good enough.ISO 9000 is not about maintaining precision, it is about maintaining quality of delivered product. The relationship of precision to quality of delivered product varies widely.In something related to the OP's problem I once worked on production of an item which had multiple parallel lines. It was much more important that the lines matched each other than that the absolute value was correct. We did periodic calibration to standards, but then supplemented this with cross calibration/adjustment across the lines.

author: dutch66
author_rank: Contributor
timestamp: March 23, 2022, 11:52:00 pm
content: As was mentioned by others ISO9000 does not force you to do the calibration in a certain way. It only forces you to do the calibration in the way described in your calibration procedures. A company is free to write those procedures as they please. When they are written you must adhere to them. Whether the practice you select is accepted by your clients is a completely different question.At my previous employer we went through the process of becoming ISO9000 certified. We opted to have two categories of test equipment in the company. One for general use that would not be calibrated annually and one for critical test purposes which would be annually calibrated. The meters were marked accordingly and we had no issues with the ISO audit.Importing calibration and spreading it inside the company may be perfectly acceptable as long as it is properly documented. The maintenance of that documentation can quickly become an issue.

author: guenthert
author_rank: Frequent Contributor
timestamp: March 24, 2022, 12:05:47 am
content: Quote from: bdunham7 on March 23, 2022, 04:20:59 pmI'm not so sure about the no second thought part. It is well documented practice to compare a device to be calibrated against an artifact or local calibrator before and after calibration to verify that it didn't unduly deteriorate during transport.