How are digit values interpreted in an accuracy specification? i.e. "0.5% + 30d" - Page 1
Forum section: electronics / beginners

author: cedd
author_rank: Newbie
timestamp: October 16, 2024, 09:58:07 pm
content: I'm comparing the accuracy specifications of various types of electrical readings for the Fluke 117 and the BM789. Fluke uses "counts" for its constant term while Brymen uses "digits". The specs are in the form of "Accuracy = ± ([% of Reading] + [Counts OR Digits])"For example, when measuring millivolts in the 600mV range @ 45-500Hz, Fluke claims a "1% + 3 (counts)" accuracy. I interpret this to mean that for a voltage of 500mV, Fluke will show a reading of 494.7-505.3mV. Brymen on the other hand, when measuring 600mV @ 50-60Hz claims an accuracy of "0.5% + 30d". I don't know how to interpret the "30d" part and I haven't been able to find an explanation.The specifications used in these examples can be found here:Fluke 117 manual, page 20:https://www.instrumart.com/assets/fluke115_116_117_manual.pdfBrymen BM789 manual, page 26:https://static.eleshop.nl/mage/media/downloads/BM789-5-manual-print1-r7.pdf

author: PGPG
author_rank: Frequent Contributor
timestamp: October 16, 2024, 10:47:51 pm
content: For me digits and counts means here the same.Fluke has 4 digits display while Brymen has 5 digits. So Brymen 30d practically means the same as Fluke 3 counts.

author: cedd
author_rank: Newbie
timestamp: October 16, 2024, 11:33:22 pm
content: Quote from: PGPG on October 16, 2024, 10:47:51 pmLet's plug it into the example. Do you mean that a 500mV @ 50-60Hz input on the BM789, with +-(0.5% + 30d) accuracy, would give a reading of 494.5-505.5mV?This is possible but what are you basing this opinion on that "counts" and "digits" are the same in this context?

author: xrunner
author_rank: Super Contributor
timestamp: October 17, 2024, 12:03:03 am
content: Quote from: PGPG on October 16, 2024, 10:47:51 pmAt the top of the Brymen specs they say this -General SpecificationDisplay: 4-5/6 digits 60,000 counts.So they use the term "counts" purposely. Not sure why they would later switch to using "d" in the accuracy specs.

author: showman
author_rank: Contributor
timestamp: October 17, 2024, 01:43:12 am
content: Quote from: cedd on October 16, 2024, 11:33:22 pmNot an opinion. This is how all multimeter accuracy specs work. But if you don't believe us, here's an app note straight from the horse's mouthhttps://s3.amazonaws.com/download.flukecal.com/pub/literature/2547797_6203_ENG_B_W.PDF

author: PGPG
author_rank: Frequent Contributor
timestamp: October 17, 2024, 11:28:36 am
content: Quote from: cedd on October 16, 2024, 11:33:22 pmNo!You calculated it like specification would be saying 300d while it says only 30d.Did you noticed in linked by you datasheet and in what I have written that Brymen has 5 digits (while Fuke has 4)?Did you noticed what I have written that from that (5 digits display compared to 4 digits display) 30d is like 3c for Fluke (means is practically the same error value)?So for 500mV:- ideal reading is 500.00- with 0.5% is 502.50- with 30d is 502.80Quote from: cedd on October 16, 2024, 11:33:22 pmI just can't imagine different interpretation so I use the only one that fits in my head