[SOLVED] Brymen BM867 multimeter Crest bug? - Page 1
Forum section: electronics / testgear

author: Deckert
author_rank: Regular Contributor
timestamp: August 30, 2013, 12:09:00 pm
content: Hi,Edit: See feedback from Brymen at the bottom of this post.I recently purchased a BM867 based on the EEVBlog video overview, comments on the forum and some other online reviews. It's a fantastic meter and I get a lot of joy using it.However, I may have discovered a bug on it and would like to know if others could replicate it or if it is a problem specific to my unit. It has to do with Crest detect Mode in the AMP range.The test is simple:- short the common and 10A input jacks with a test lead- switch to the A scale- now press CREST button- does yours also measure at least 0.65A as a max? (without having any current flow through it)Can owners of the BM869 perhaps try the same test?The above problem does not allow me to take current in-rush measurements!Update from Brymen:Quote--deckert

author: rastro
author_rank: Frequent Contributor
timestamp: August 30, 2013, 12:22:17 pm
content: I wonder if the Brymen BM869 also has this issue. I'm thinking about buying one as a second meter.I suspect that the BM867 and BM869 may use the same main board. I recall seeing a tear down picture of the BM867 and it looked like there was a temperature stencil on the pcb for the mode selector knob where the BM869 has this mode setting. Temperature measurement is not a feature on the BM867 as I understand.So maybe BM869 owners can also check for this issue.

author: Carrington
author_rank: Super Contributor
timestamp: August 30, 2013, 02:50:49 pm
content: Quote from: Deckert on August 30, 2013, 12:09:00 pmTest BM869:- Short the common and 10A input jacks with a test lead.- Switch to the A scale.- Now press CREST button. -> The same, I get 00.83 Amp. (DC), and without test lead, 003.6 mA (AC).

author: macboy
author_rank: Super Contributor
timestamp: August 30, 2013, 03:14:22 pm
content: Why short the inputs? You would shorts the V input to common to ensure zero input voltage, but it you want to absolutely ensure zero current, leave the inputs open. What do you see with the test repeated with open inputs?(That said, I am struggling to imagine why shorting the input would cause this...)

author: Carrington
author_rank: Super Contributor
timestamp: August 30, 2013, 03:31:59 pm
content: Quote from: macboy on August 30, 2013, 03:14:22 pmWith open probes: I get 00.79 Amp. (DC), and without test lead, 003.6 mA (AC).

author: rsjsouza
author_rank: Super Contributor
timestamp: August 30, 2013, 03:42:08 pm
content: Same thing on my BM857... With shorted/open inputs.

author: Deckert
author_rank: Regular Contributor
timestamp: August 30, 2013, 04:20:09 pm
content: Quote from: Carrington on August 30, 2013, 03:31:59 pmYup, I get the exact same values with open probes on A and mA scale. Seems that the crest sampler might be starting up too soon, without first waiting for the A/D to settle (or something).Now that I've confirmed this is not only my unit, I've contacted Brymen with a full explanation and step-by-step instructions to replicate it. But I wonder if this can be fixed at all - does the meter support firmware upgrades?--deckert

author: Tepe
author_rank: Frequent Contributor
timestamp: August 30, 2013, 04:25:26 pm
content: My BM869 shows 0.65 A DC with COM and A shorted with a test lead, and 3.4 mA DC open circuit. It also shows 3.4 mA AC open circuit.

author: Fsck
author_rank: Super Contributor
timestamp: August 30, 2013, 04:39:06 pm
content: same thing on my 869. .94A shorted and ~9mA open for crests.

author: Lightages
author_rank: Unknown
timestamp: August 30, 2013, 08:35:33 pm
content: This bug is confirmed on my BM869. I will write an email to Brymen about this.P.S.If there is no fix for this, then it will certainly take Brymen down a notch in reputation. This does not happen on my BM257 BTW, at least not to this extent. It does pick up some noise though.P.P.S. Interestingly enough this also happens on my UEi DM397. Perhaps it is a particularly difficult thing to design correctly.

author: nanofrog
author_rank: Super Contributor
timestamp: August 30, 2013, 08:46:20 pm
content: Confirmed on BM857.Open: 7.4mAShorted: 0.74A

author: Marco
author_rank: Super Contributor
timestamp: August 30, 2013, 09:12:57 pm
content: Quote from: Deckert on August 30, 2013, 04:20:09 pmWouldn't it be more likely they use precision rectification and a cap?Although it would be better if they used the ADC, that could be fixed in firmware.

author: Carrington
author_rank: Super Contributor
timestamp: August 30, 2013, 09:55:12 pm
content: I will contact with Brymen tomorrow, but I don't see how we can upgrade the firmware. Maybe through the (infrared) serial port?

author: Deckert
author_rank: Regular Contributor
timestamp: August 30, 2013, 09:59:10 pm
content: Quote from: Marco on August 30, 2013, 09:12:57 pmAgreed - I don't know how DMMs work nowadays, but the ICL7106 that I studied years ago uses a dual-slope converter.Edit:maybe the crest-mode uses an A/D converter, since it samples at 1000 times per second (and reduces resolution at the same time).QuoteWell, I think this can easily be solved in firmware - just reset to zero after a short delay. The problem is that I don't know if one can upgrade the firmware without sending it back to the factory.--deckert

author: Deckert
author_rank: Regular Contributor
timestamp: August 30, 2013, 10:08:51 pm
content: Quote from: Lightages on August 30, 2013, 08:35:33 pmAye - the more people that write to them about this, the better the chances are that they will start to react to it.Quote from: Lightages on August 30, 2013, 08:35:33 pmIt really shouldn't be that difficult. As a work-around, I used a 0.2 ohm resistor and measured the voltage spike over the resistor using Crest mode on the voltage scale. That works like a charm.--deckert

author: Lightages
author_rank: Unknown
timestamp: August 30, 2013, 10:22:23 pm
content: Oh yes there are work arounds, but we should not need to do this. Also if your peak is above the baseline erroneous reading then you still get the correct reading. I wonder how many other meters with a peak detect or "crest mode" have the same problem.

author: Carrington
author_rank: Super Contributor
timestamp: August 30, 2013, 10:29:51 pm
content: Quote from: Lightages on August 30, 2013, 10:22:23 pmWhat I'm wondering is if they will do the same as OWON, i.e. fuck you, not my problem.

author: alrj
author_rank: Newbie
timestamp: August 30, 2013, 11:33:43 pm
content: Just did the test on my BM869, didn't get the same result.- Shorted COM and A- Turned on to A scale, reading is about 0.0001A DC with 0.000A AC- Pressed crest: OL (reading is DC only) on both MAX and MIN.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: August 31, 2013, 12:45:30 am
content: Hold on folks, I am getting very conflicting behaviour, It is looking like a PEBKAC situation to me. Testing a BM869 I initially couldNOTconfirm any failure, my unit was reading around 0.4 mA on the amp and mili-Amp range and then all of a sudden I got it to read .8 then .9 Amp on MAX and Min only but it is very inconsistent, as if it was sampling noise (on a large loop antenna). I could not duplicate on my BM857 at all but haven't played with it as much. When on AC amps the display takes a while to stabilize and wind down to negligible numbers as if a high impedance input stage is discharging.edit:Ok I had more time to play around and I can replicate now consistently. Only on the Amp range with a short present A<-->com. Open socket works fine and the mu-Amp range also works as expected.BM857a reads .33 to .45 amps plus minus on crest max,minBM869 reads .62 then .81 max, .94 min on crestAC +DC crest is a ridiculous 6.021 amps max and 4.130 minaveraging seems to work fine on all ranges. I guess it confirms a multimeter doesn't make a good O'scope substitute. The crest feature is a bit of a dubious and gimmicky thing.2nd Edit: more experimentationI can get the BM869 to mess up byonlyplugging a lead into the 10 Amp socket and leaving COM open. InpErr not always detected if I then shift to mu-amp range. It appears that the lead detect circuit on the 10Amp socket interacting with crest measurement.

author: Deckert
author_rank: Regular Contributor
timestamp: August 31, 2013, 11:56:40 am
content: Quote from: chickenHeadKnob on August 31, 2013, 12:45:30 amI get the problem always, no matter what configuration I use. I found this issue while using it to measure actual DC in-rush current to my circuit. To test this, simply put it in series with any DC circuit, switch the circuit off and press CREST mode. Itshouldgo to zero, then measure in-rush as you switch on the circuit.Mine consistently measures 0.65A the moment I press the CREST button, sometimes going to 0.85A on a second "beep". This happens open-circuit or closed. My jack-detection works 100% of the time though.By the way, seems you can check the firmware (or model number) by pressing and holding the Delta-button, then turning it on. Mine reads "869-P9". I wonder what that number represents.--deckert

author: Tepe
author_rank: Frequent Contributor
timestamp: August 31, 2013, 12:52:47 pm
content: Mine also says 869-P9

author: Carrington
author_rank: Super Contributor
timestamp: August 31, 2013, 02:14:07 pm
content: Quote from: Tepe on August 31, 2013, 12:52:47 pmPressing and holding the Delta-button, while turning it on, mine reads "869-P9" too.

author: Wytnucls
author_rank: Super Contributor
timestamp: September 01, 2013, 02:57:24 am
content: Although some multimeters can measuring inrush current, few have that capability advertised, as it can be a hit and miss affair. The best instrument for the job would be a clampmeter, with a special triggering circuit implemented.The other way to measure that short duration current would be with a storage oscilloscope, using a current probe or a current shunt.As far as I can tell, Brymen doesn't list inrush current measurement as a feature for their multimeters.

author: Lightages
author_rank: Unknown
timestamp: September 01, 2013, 04:22:49 am
content: Brymen does list the "Crest" function as being intended to be used in the current modes. This is a failure of the intended use, or at the very least a limitation.

author: Carrington
author_rank: Super Contributor
timestamp: September 01, 2013, 01:05:26 pm
content: Anyone have any news from Brymen?

author: Wytnucls
author_rank: Super Contributor
timestamp: September 01, 2013, 01:42:50 pm
content: The 'Crest' or 'Peak Hold' function probably works fine if it is selectedafterthe circuit is energized. It should register transient durations of 1mS or longer. Inrush current measurement, with the 'Crest' selection, obviously was not considered by Brymen at meter conception .The Max/Min feature might detect inrush current, if it lasts for longer than 200mS. Has anyone tried it yet?

author: Lightages
author_rank: Unknown
timestamp: September 01, 2013, 04:05:11 pm
content: The point is if you want to measure a pulse of current that doesn't exceed 0.8 amps, it won't be visible as the Crest function causes the meter to immediately read 0.83 amps. The meter is blind to anything under 0.83 amps when using Crest mode. Yes I have tested this and it gets worse if there is actually a current flowing before you push the Crest button. With one amp, the meter displays 1.79 amps in crest mode at start up.

author: Deckert
author_rank: Regular Contributor
timestamp: September 01, 2013, 07:26:35 pm
content: Quote from: Wytnucls on September 01, 2013, 01:42:50 pmHm? Not sure what you are referring to here - the meter jumps to 0.85A the moment the CREST button is pressed, regardless of whether the meter is shorted, open-circuit or in-circuit (although I've seen it jump higher if in circuit). The whole point is that it should start at zero before the circuit is energised, but it doesn't. It starts off at 0.85A, rendering any in-rush measurement useless.Quote from: Wytnucls on September 01, 2013, 01:42:50 pmThe manual specifically refers to "... or current signal duration as short as 1ms ... available to display 5000uA, 500mA and 10A ranges". I tested my circuit's inrush current using a 0.2 ohm shunt resistor and measured it on the voltage scale. The circuit spikes to 900mA on switch-on. Confirmed this with a DSO over the same shunt resistor and figures are within 5% of each other, which is good enough for me.So the meter is certainly capable of measuring inrush - the A range must just start at zero.Quote from: Wytnucls on September 01, 2013, 01:42:50 pmYah - tried it a few times, but not really suitable in this scenario - it only detects between 180mA and 250mA (depending on how I catch it) using my test circuit.--deckert

author: Deckert
author_rank: Regular Contributor
timestamp: September 01, 2013, 07:28:37 pm
content: Quote from: Carrington on September 01, 2013, 01:05:26 pmNo yet, but I first reported it to Brymen late on Friday, which was already after 5pm in Taiwan, so I'll cut'em some slack till Monday or so--deckert

author: Lightages
author_rank: Unknown
timestamp: September 02, 2013, 04:14:17 am
content: Here is the reply from Brymen:QuoteYes this specification is in the manual, I somehow missed it. These meters are operating within specification. It is not that useful a function with these very loose specs but it does have its uses.

author: Deckert
author_rank: Regular Contributor
timestamp: September 02, 2013, 09:12:02 am
content: Quote from: Lightages on September 02, 2013, 04:14:17 amAgreed, as per spec, the meter is within limits. I will admit that it makes it less useful on the 10A scale, so using a shunt resistor seems to be the only practical way of measuring current in-rush.I got the same response, but I'm still very pleased with the response from Brymen (from their marketing manager, no less). No real delay in getting an answer from them.--deckert

author: Carrington
author_rank: Super Contributor
timestamp: September 02, 2013, 11:01:24 am
content: Ok, then how can this be explained:Quote from: alrj on August 30, 2013, 11:33:43 pmP.S.: Brymen gave me the same answer.

author: Marco
author_rank: Super Contributor
timestamp: September 02, 2013, 11:34:17 am
content: They have a 45 mOhm shunt, the shot noise difference compared to the voltage range measurement with an external 0.2 Ohm shunt is (sqrt(0.045)/0.045)/((sqrt(0.2)/0.2))~= 2 times worse (relatively). It's nice that the meter works in specifications, but their explanation doesn't hold water unless they run the measurement a whole lot slower in the voltage range.

author: ablacon64
author_rank: Frequent Contributor
timestamp: September 02, 2013, 02:14:24 pm
content: Not a bug, it's a feature...

author: BennyBoy
author_rank: Contributor
timestamp: February 26, 2014, 06:56:29 pm
content: I see this post is a bit old, but I wanted to tell of my results with my 869. In A mode with Crest engaged I get 37 ma in Min. If I charge a cap an whip the lead from an LED(in series with the meter), I get a max of 507 ma. Yes it would be nice if it read zero for min, but it works great as a max peak, measuring as fast as I can whip a lead past a charged cap.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 24, 2017, 09:29:16 pm
content: Seems to be having for the brymen 869s as well. Bummer should have known it earlier before I bought this month ago.Anybody has any solution to it ?

author: Specmaster
author_rank: Super Contributor
timestamp: July 26, 2018, 11:24:52 am
content: I have just purchased a new BM867 but I did not know about this thread until just now. I conducted the same tests on my new meter and I confirm the followingFirmware is 869-P9DCA Max +0.69ADCA Min -0.64AACA Max -0.16AACA Min -1.50AMy serial number is 183270401

author: evava
author_rank: Regular Contributor
timestamp: July 27, 2018, 07:18:49 am
content: Perhaps it is answer to the question: "If Brymen BM869s is cheaper ...., why people would still buy Fluke?"....to paraphrase title of older thread.

author: Specmaster
author_rank: Super Contributor
timestamp: July 27, 2018, 07:34:35 am
content: Quote from: evava on July 27, 2018, 07:18:49 amLet's face it most meters are cheaper than Flukes, I have plenty of Flukes but the Brymen is still my go to meter.

author: evava
author_rank: Regular Contributor
timestamp: July 27, 2018, 09:16:21 am
content: Still I can not understand why Voltage measurement Crest is regardless OK..and can not understand why this "feature" is present for five years..e.g.Quote from: Marco on September 02, 2013, 11:34:17 am

author: rsjsouza
author_rank: Super Contributor
timestamp: July 27, 2018, 09:23:42 am
content: Quote from: anishkgt on April 24, 2017, 09:29:16 pmIf you read Brymen's nice explanation n this thread, you will see the "bug" is, in fact, a design choice: they chose to make a crest feature available but correctly reported the loss of accuracy in the manual. This is a typical physical limitation of analog to digital converters. Any ADC running at a faster speed will reduce the SNR and consequently its ENOB.

author: exe
author_rank: Unknown
timestamp: July 27, 2018, 10:07:32 am
content: Quote from: rsjsouza on July 27, 2018, 09:23:42 amBut other meters don't suffer from it that much... Anyway, if first 100 counts don't make sense, then why showing them at all? This is just misleading.I tend to think there is a design error somewhere (but it's good it is documented).

author: dirtcooker
author_rank: Regular Contributor
timestamp: May 27, 2019, 04:49:18 am
content: I bought a BM869 a couple weeks ago, and I like the meter very much. However, the Crest mode is unreliable. On the V setting, measuring a 4.1 v battery in Crest mode for example, I can get voltage readings as high as 4.7v, or -4.7 when switching the probes around. Usually, the bad reading is the one not currently showing, e.g., MIN when the measurement is made with MAX showing, and vice-versa. Thus, I avoid Crest mode and use REC mode instead. I cannot reproduce these erroneous readings consistently. Repeating the measurement several times in a row usually does the trick though. If you connect the meter to the circuit BEFORE pressing Crest, then it is useable for catching fast transients and less likely to be spooked by noise.I would much rather have a Fluke-style Hold function that works normally. I don't care about catching fast ms pulses. I use a scope for that sort of thing.

author: dirtcooker
author_rank: Regular Contributor
timestamp: May 27, 2019, 12:10:13 pm
content: Quote from: rastro on August 30, 2013, 12:22:17 pmI just bought a BM869 (5/2019) and yes, the BM869 has the Crest bug. It is otherwise an excellent meter.