Brymen BM867s Crest Factor - Should it read 0A with open circuit leads? - Page 1
Forum section: electronics / testgear

author: Bakaman
author_rank: Contributor
timestamp: September 21, 2021, 04:08:42 pm
content: Hi Folks,My first post, so please be gentle.I've just taken delivery of a 867, and am thinking that it looks pretty good so far.Testing out the Crest mode on an AA NiMH charging Panasonic NiMH smart charger current, I'm thinking that looks pretty useful. Crest peak of 2.8A is detected, whereas the Min/Max mode records 1.05A max and an average of 0.35A. That could save me getting the scope out in some situations for basic stuff like this.But a query arises when I go to Crest mode with no current flowing (leads in place but either open circuit or short circuit tips). Operating in Autorange mode it reads close to 0A, but when I press Crest I get:0.64A/-0.69A with OC in AC mode and 6.36A / 5A in DC mode.That seems a bit strange to me, if not in fact faulty.Can someone else who has got one of these give it a try and see if the same thing happens. Leads are in the 10A and Com receptacles.Thanks.

author: Bakaman
author_rank: Contributor
timestamp: September 21, 2021, 11:08:55 pm
content: If someone could verify their meter's performance then I would be grateful.It only requires a few seconds, and doesn't need anything to be connected. Just loose unterminated leads plugged into 10A and Com terminals and left open circuit, and then the Crest button pressed.Thanks in advance.

author: Caliaxy
author_rank: Frequent Contributor
timestamp: September 22, 2021, 12:54:32 am
content: Don’t worry. The crest factor is the ratio of two numbers (two measurements: the peak value and the RMS value). When the two numbers are 0 or random values around 0 it’s normal to get nonsensical ratios (just imagine you divide 0 by any number or any number by 0 or, say, 0.0001 by 0.1 then the other way around).To answer your question, I’m quite sure my BM869S behaves similarly (it sometimes displays .OL as well).

author: Bakaman
author_rank: Contributor
timestamp: September 22, 2021, 11:41:35 am
content: Thanks, that is logical indeed.It is still a great meter, I just need to remember that just about every piece of test equipment will have its own 'features', and some that only appear in rare or unusual circumstances. In this case, I need to remember not to panic when an unexpected value appears on the display, and when it does, to do other tests to work out if it is a true reading or not. And as a general principle, to make sure that I interpret readings appropriately, doing a sense check as it were.In an ideal world, an intelligent meter could do rules-based checking before displaying the result, e.g. to assess Crest max as zero if the current is zero. It could be included in the processing rules of an intelligent meter, but processing rules can sometimes get things wrong too (like the cat photo being confused by Google's AI system into appearing more like guacamole), and then a good intelligent meter (if such a thing exists) would probably cost a whole lot more than a 867s.

author: HKJ
author_rank: Super Contributor
timestamp: September 22, 2021, 12:00:06 pm
content: On Brymen meters Crest is not "Crest factor" but a peak function and due to the very fast detector circuit it has very wide tolerances.

author: Bakaman
author_rank: Contributor
timestamp: September 22, 2021, 01:37:34 pm
content: Good point, it then moves the question away from a ratio issue, to a 'feature' issue. I'll have to do some more testing to work out at what current the Crest function gives a correct value.So does anyone else's 867s meters work in the same way as mine?

author: Caliaxy
author_rank: Frequent Contributor
timestamp: September 22, 2021, 01:39:33 pm
content: Quote from: HKJ on September 22, 2021, 12:00:06 pmOops, you are correct: it’s a peak hold measurement (which allows you to manually calculate the crest factor, if you wish). I take my explanation back. Mine behaves the same way (and shows much smaller non-sense values on the mA range with the leads in the air). Everything seems fine though if you apply an actual signal.

author: Kean
author_rank: Unknown
timestamp: September 22, 2021, 02:20:51 pm
content: I also get similar nonsense crest mode readings for both AC and DC current on my 869S with open probes

author: HKJ
author_rank: Super Contributor
timestamp: September 22, 2021, 03:05:48 pm
content: Quote from: Bakaman on September 22, 2021, 01:37:34 pmI believe that the value has a tolerance of +/-100 count.

author: Bakaman
author_rank: Contributor
timestamp: September 22, 2021, 03:38:34 pm
content: Thanks everyone, a useful learning experience. I'll relax now.