Talk of Uncertainty  - Page 1
Forum section: electronics / general

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 15, 2023, 06:09:47 pm
content: Starting a new thread to bring it out into the open.Quote from: alm on February 10, 2023, 08:10:23 pmI am currently using my phone to reply to this in a pub as I am working away this week. Wonderful Dartford.GUM documents are quite good but they do take a little time to get your head into them especially if you are not used to it. The example you give with measuring current through a resistor is missing a whole bunch of things and I will delve into that once I get a little more time.Quote from: Rax on February 15, 2023, 04:33:32 amThink of it of accounting for most the possible influences of error in your measurments. Specification of the meter is one, how stable it is, does the temperature of the room effect it, your ability to read the display, does the part change while being tested, EMF etc etc.QuoteMetrology and electronics does suffer from basic concepts going from easy to understand to WTF in a matter Of moments and you loose the will to continue.UoM is one I have started using to just save myself the headache of typing it out each time.In your case you are measuring a 10V source with a 6.5digit meter. Your meter is in spec, it has a cal cert with a UoM to import. So you have some data to work with. Part of the thinking behind knowing about your uncertainty is it allows you to better understand your readings and whether or not you can make a bold statement of my 10V reference is 10V with all the zeros. Another thing about it is that if someone using your 10V ref to calibrate thier meter they have a uncertainty to import into thier budget and shows that you are further down the traceability chain.Imagine you measure your 10V ref and lend it to friend A to calibrate his meter then they use that to calibrate thier 10V ref, that ref then goes onto friend B and they use it etc. For each person the uncertainty would grow and then at some point it will be so far away from the traceability chain that the uncertainty will be into the 1V range and then using that 10V source to calibrate a 6.5digit meter is pointless.I may not be explaining myself the best here at the moment but if I get a chance I will try to generate some graphs to give you an idea of some of the reasons it could be worth keeping an eye on.

author: guenthert
author_rank: Frequent Contributor
timestamp: February 15, 2023, 11:21:38 pm
content: Quote from: mendip_discovery on February 15, 2023, 06:09:47 pmIs it? I'd say the sticker on it only claims that it was at the time of calibration. His buddy might have dropped it since and failed to tell him about it.The meter'snextcalibration will tell, whether it was in spec now (assuming it doesn't wander back and forth -- ideally you calibrate it often until you gather confidence that it won't do such).

author: Rax
author_rank: Super Contributor
timestamp: February 16, 2023, 02:20:44 am
content: Quote from: mendip_discovery on February 15, 2023, 06:09:47 pmThis one sentence completely turned on the light bulb for me. I read it before, but given a bunch of readings this week, it accrued in just getting it. I think I do.And also, genuine congrats, I don't think anyone has gotten such a well documented and elaborate post on a phone, ever.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 16, 2023, 06:58:12 am
content: Quote from: guenthert on February 15, 2023, 11:21:38 pmYes that can happen but sudden changes in kit can happen at any time that is why it's worth having something in your lab that allows you to do a regular cross check.I have a new meter to me that has just come back from calibration. I now need to do some repeatability testing on it and also work on getting the best readings I can out of it. Then in 6 months time I will send it out again to see how its performing. Then maybe get it done this time next year before my 17025 audit so I can bring it onto our schedule and really start to drop our schedule down.

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 16, 2023, 07:00:22 am
content: Quote from: Rax on February 16, 2023, 02:20:44 amI must point out a caveat on that, part of it will be how far down the line they are but also the quality of the gear/method they are using to do the testing.

author: Rax
author_rank: Super Contributor
timestamp: February 17, 2023, 04:58:36 pm
content: Quote from: mendip_discovery on February 16, 2023, 07:00:22 amOf course, it's part of the calculation and statistics of this.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 17, 2023, 05:47:32 pm
content: One fortunate aspect of comparison is that we can usually do a comparison to a far higher degree than the accuracy of the standards themselves. Without that, it would be impossible to build up standards from multiple near-identical values. Of course, it becomes more difficult the further below a ppm we go. So you have the basic uncertainty of the standards, plus the uncertainty of the comparisons.