Meter Accuracy Problem: What Would be 0.01% FS of +/-20VDC? - Page 1
Forum section: electronics / projects

author: aw_rw1
author_rank: Contributor
timestamp: October 28, 2022, 08:46:31 pm
content: Meter Accuracy Problem:What is the Full Scale of +/-20VDC? Is it 20V or 40V?What would be 0.01% FS of +/-20VDC?

author: bdunham7
author_rank: Super Contributor
timestamp: October 28, 2022, 09:16:05 pm
content: 20V and 2mV by convention.

author: aw_rw1
author_rank: Contributor
timestamp: October 28, 2022, 10:33:03 pm
content: I was hoping FS was 40V. The instrument that has this 0.01% is off by 1mV.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 31, 2022, 11:27:54 am
content: Quote from: aw_rw1 on October 28, 2022, 10:33:03 pm0.1% would be a 20mV difference of 19mV.

author: CaptDon
author_rank: Super Contributor
timestamp: October 31, 2022, 02:21:24 pm
content: ?? +/- 20 volts Is the full scale. It can go 20 volts either direction so full scale is +20 or -20 volts. The RANGE is 40 volts. I can't comprehend your statement "The one that has this is .1% and off by 1 millivolt". If it is 40 volts full scale at .1% accuracy it could give a reading withing the range of 39.96 to 40.04 and meet its specs. What are you looking for exactly? If you want .01% accuracy it can be had but then you better have a cal lab to verify the maintained accuracy about once every six months. At 20 volts 1% would be .2 volts, .1% would be .02 (20 millivolts) and .01% would then be .002 volts or 2 millivolts 'at full scale' and less at lower readings.

author: aw_rw1
author_rank: Contributor
timestamp: October 31, 2022, 03:25:17 pm
content: CaptDon,Sorry about the typo. The accuracy is 0.01%, not 0.1%.So 40V -> .004Vbut 20 -> .002VThe error at 20V measurement is 5mV.

author: sokoloff
author_rank: Super Contributor
timestamp: October 31, 2022, 03:28:58 pm
content: Quote from: aw_rw1 on October 31, 2022, 03:25:17 pmThat's the sound of a second can of worms opening.How do you know that error? What's the uncertainty on the source you're measuring against?

author: bdunham7
author_rank: Super Contributor
timestamp: October 31, 2022, 04:23:17 pm
content: Quote from: aw_rw1 on October 31, 2022, 03:25:17 pmCould you be more specific about what the actual complete specifications are and perhaps what model it is? A typical tolerance specification will be [%/ppm of reading] + [%/ppm of FS/range or counts]. I don't know of any meters that give only a %/ppm of the FS or range, although I suppose that's possible. Also, the terms 'range' and 'Full Scale' sometimes mean the same thing and sometimes range isless(a 10V range with 20% over ranging has a FS of 12V), but I've never, ever seen a manufacturer imply that the range is twice the FS because the positive and negative scales add up. For meters with a published calibration procedure, you can verify what they mean by looking at the limits for each specified test and doing the math to see what that implies.Say I have a meter with specified tolerances on a stated 10V range of [0.01% of reading] + [0.01% of range]. If this meter is 10,000 counts + 20% over ranging, then the FS is 12V. If you test it at 10.000V, the specified tolerance is 10.000 x .0001 + 10.000 x .0001 = .002V, or 2 counts. The range is 10V not 20 and the FS is 12V, not 24. This is an actual meter, so I can look at the calibration procedure and I see that indeed the test limits are +/- 2 counts.What I'm thinking is that you may be misquoting the specs. How many digits does this meter have?

author: aw_rw1
author_rank: Contributor
timestamp: November 01, 2022, 02:53:10 pm
content: Sokoloff,Yes, that can of worms might be the problem. I am measuring with an Agilent 34401a. This DMM has error at 100.0000V range of +/-0.002% of reading and +/-0.0006% of range. So the error of the DMM is +/-0.001mV (20*0.00002+100*.000006).Bdunham7,The instrument that I concerned about is a Laurel DC meter.https://www.laurels.com/dcmeter.phpIts spec is 0.01% FS ± 2 cts. So the error of the instrument is +/-0.004mV.The total error in the system is +/-0.005mV. This corresponds to the -0.005mV on a 20V reading from the Laurel.

author: sokoloff
author_rank: Super Contributor
timestamp: November 01, 2022, 03:18:27 pm
content: That’s the 24-hour uncertainty; the 1-year is not that much higher (0.0045% + the same 0.0006%), but gives you around 1.5mV of uncertainty (assuming the 1 year period is appropriate to your situation).

author: bdunham7
author_rank: Super Contributor
timestamp: November 01, 2022, 03:55:51 pm
content: As stated, your spec on the 34401A is a little off. Also, I'd recommend making sure your room temp is as close to 23C/73F as you can and giving the Agilent a 3-hour warmup for minimum error.As for your meter, I'm not sure what to say because I don't know anything about that manufacturer nor how their meters are constructed. However, those are very, very aggressive specifications for what is effectively a 20,000 count meter. In addition, they are stated in a very strange manner. You don't typically see a separate %/ppm of FS + counts, simply because those boil down to the same thing. For a 20k count meter, 0.01% = 2 counts. And no %/ppm of reading is unusual but not ambiguous. What they don't state, however, is a confidence interval or temperature window or anything else regarding conditions needed to obtain the stated accuracy. The accuracy specification may well be a 'typical result' statement meaning they don't actually guarantee it, at least not in the same way as your Agilent.For your 34401A, I believe they use a 95% confidence interval, also known as k=2. This basically means that if you look at 100 meters, at least 95 of them will be within the specified tolerances. The k=2 means that if you look at the standard deviation of a large sample of meters, the specified tolerance is set at twice that amount or more. Those two statements are approximately the same to each other mathematically for a standard distribution. Note that this is a very basic explanation and while it is incorrect if you look at it in detail, it gives you an idea what the specs may mean. If your meter's specs are 'typical', then that may mean a ~66%/k=1 or even 50%/k`0.7. If it was a low-quality Aliexpress type meter, the specs may be what I call 'aspirational' or even total BS, but it doesn't look that way in this case.A 5mV error at 20V from a 4.5-digit panel meter is actually remarkably good. How expensive are these things?

author: aw_rw1
author_rank: Contributor
timestamp: November 01, 2022, 07:47:15 pm
content: They cost about $300.

author: CaptDon
author_rank: Super Contributor
timestamp: November 02, 2022, 01:45:49 pm
content: Or they would state F.S. = 20vdc Range = +/- 20vdc. Except for tuning meters and phase meters and maybe automotive amp gauges you really don't see many meters with the rest position at mid scale these days. Used to see them a lot in military, scientific and radio broadcast equipment.