Some problems with my Brymen BM869S - Page 1
Forum section: electronics / testgear

author: chronos42
author_rank: Regular Contributor
timestamp: May 25, 2017, 09:29:05 am
content: Hi together,there are a lot of tests, reviews and teardowns of the Brymen BM869S and all the reviews agrees that the BM869S is a very good meter.I also agree largely and I also have a BM869S.But after some testing and using I found two weak points and I am wondering that nobody found this before. (Beside of the complete useless hold function.)First the biggest weak point: The mV AC range.What is wrong with this? At a first glance there is nothing wrong. But if you take a closer look there is a big problem. The meter cannot handle a mV AC together with a high DC. In the V range there are no problems, it can handle AC, DC, AC+DC in any combination without any Problems, this was shown in several reviews.But if I set the meter to mV AC and connect it to a DC source I can see a problem. The meter must show 0V AC, Independent from the DC. Every meter I had tested did this in the AC mV range, even with high DC offsets.But the BM869S failes this test:Switch the meter to mV AC, connect it to a DC source and increase the DC slowly from 0 to 0.8V DC.Everything is as expected, the Brymen shows 0mV AC.But:Increase the DC over 0.8VAt 1V DC the Brymen shows a constant AC of 7.5mV.At 1.2V DC it shows 25mV AC.At 5V DC it shows 42mV AC. Keep in mind, it must show 0.00mV AC!At 6V DC ist shows 117mV AC.At 7 V DC it shows 298mV AC!!At 9V DC it overranges.It is not possible to measure a small AC voltage together with a DC voltage of several Volts. That means e.g. if you will measure a AC voltage in the mV range at a point that is biased with 5..6V DC you will get totally wrong results.I really wonder that nobody found this issue. Do I have a defective BM869s or is this a design error?Next is the DC 10A Current mode. I compared the BM 869S with my Gossen Metrahit Energy. At a first glance there are no problems.I connected the two meters in series and loaded it with exactly 10A DC. The Metrahit energy monitores the temerature of the shunt and until it had reached 70°C i stopped the test. The Metrahit goes back to zero within seconds. But the Brymen seems to have a problem with thermo voltage. The meter shows -15mA DC current and it took 15 minutes going back to 0 A. Ok, not a big deal, but no other meter I had tested has such a big offset after warming up.Can someone please confirm this behaviour or do I have a defective meter?

author: Fungus
author_rank: Super Contributor
timestamp: May 25, 2017, 10:24:50 am
content: a) What does the manual say about DC offset?b) Did you press the REL button?

author: Wytnucls
author_rank: Super Contributor
timestamp: May 25, 2017, 10:36:45 am
content: TRMS converters are not very good at measuring AC below 5% of the range.The Brymen seems to be quite sensitive.None of my meters show more than 2mVAC at 3VDC.My best one is the Gossen 26S showing 1.6mVAC with 15VDC offset.

author: chronos42
author_rank: Regular Contributor
timestamp: May 25, 2017, 10:42:03 am
content: Quote from: Fungus on May 25, 2017, 10:24:50 amHi,The manual says nothing about the max. usable DC voltage in the AC mV range, only the max overload DC is defined to max. 1000V DC.Actual I can zero it with the REL button, but I think that will not help, because it also Zeros the AC voltage.

author: Fungus
author_rank: Super Contributor
timestamp: May 25, 2017, 10:53:22 am
content: Quote from: chronos42 on May 25, 2017, 10:42:03 amIt was worth a try...DC offset is one of the things joe tests in his 'robustness' thread. It's not in his spreadsheet though so you'd have to watch the videos. Most meters fail quickly, your Brymen doesn't seem especially bad.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2017, 10:55:31 am
content: There was a fairly large thread running on this site about it. High AC on DC in DC mode, High DC on AC in AC mode and such. All there for those who dare use the search feature.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 25, 2017, 11:00:15 am
content: There are thermal limits where a shunt will no longer operate correctly. At 80 °C thermal drift begins to occur, at 120 °C thermal drift is a significant problem where error, depending on the design of the shunt, can be several percent and at 140 °C the manganin alloy becomes permanently damaged due to annealing resulting in the resistance value drifting up or down.Did you measure the temperature of the shunt in the Brymen? It may have reached high temperatures with 10A passing through it.

author: chronos42
author_rank: Regular Contributor
timestamp: May 25, 2017, 11:04:20 am
content: Quote from: Wytnucls on May 25, 2017, 10:36:45 amHi,Yes, but the TRMS converter should not see any DC in the AC only mode. And also this is not a problem of the accurecy of the TRMS converter.I also checked a Gossen Metrahit Energy and a (older revision) Benning 7-1 in the AC mV mode at a function generator. Not many generators can generate +10V DC together with 20mVss AC because of limitations from the output attenauator.My old hp 8165A can and both meters had no problems to read the 20mV AC. The 20mV is the lowest AC that the 8165a can generate under this conditions.The Brymen failed this test.For shure at 2mV every meter would fail this test, but this is not the point, they will not overrange like the Brymen..

author: chronos42
author_rank: Regular Contributor
timestamp: May 25, 2017, 11:14:56 am
content: Quote from: joeqsmith on May 25, 2017, 10:55:31 amHi,I had read this thread, but mayby I have overlooked the mV related issues. It was clear for the higher V ranges, but I have never seen the combination AC mV together with higher DC V. Also in your really great videos I only have seen mV DC together with high AC. But as I said, mayby I have overlooked this.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 25, 2017, 11:23:18 am
content: Most of the testing was done in the mV ranges. Actually, I think the original question started with the mV range. You would need to read through it. Fairly common problem on the meters I looked at and I looked at a lot of meters. Even my latest meter has problems always coming up with the correct values in every mode with every combo.

author: chronos42
author_rank: Regular Contributor
timestamp: May 25, 2017, 11:43:12 am
content: Quote from: joeqsmith on May 25, 2017, 11:23:18 amHi,You have much more meters than I, so if you have time maybe you can do a quick test with 8..10V DC in the AC only mV range. I am pretty sure the most meters will read 0V AC.I have checked a Metrahit Energy, Benning M7-1 and all of them have passed this simple test. Passed means, that they settled down to 0 V AC after short time as expected. Only the Brymen failed this simple test. The only thing I would to know is if I have a broken unit or if this is a desingproblem of this meter.And as I wrote before, it is possible to read a 20mVpp 1kHz Signal together with a 10V DC in the AC (AC only) mV range. The Benning and the Gossen had no Problems at all.

author: evava
author_rank: Regular Contributor
timestamp: May 26, 2017, 11:06:45 am
content: the thread ishttps://www.eevblog.com/forum/testgear/brymen-multimeters-fault/(mV AC range does not indicate an overload at specific AC signal with DC component)

author: chronos42
author_rank: Regular Contributor
timestamp: May 26, 2017, 12:38:17 pm
content: Quote from: evava on May 26, 2017, 11:06:45 amHi,thank you for this link.Ok, I see now this is a known issue, but i am wondering that this seems to be accepted from the most people here.I am sure the most user of this and other instruments are not aware that the Instrument shown only bullshit without any warning when they try to measure a small (only) AC signal together with a DC bias of some Volts. Everybody would cry if a oszillosscope could not handle this Situation, but it seems to be not a problem with a multimeter.At least there has to be a hint in the manual.Some will now say : "Use a oscilloscope for this measurments"Yes, that's right, but on the other side:The Manual says: You can measure up to 100kHz in the AC mV range an there is no word about limitations from additional overlayed DC voltage. This is simply not true. And the user trusts the wrong results.

author: joeqsmith
author_rank: Super Contributor
timestamp: May 26, 2017, 04:54:23 pm
content: Any time I have had to look at small AC signals with a multimeter riding on a large DC bias, I AC couple it. Yes a scope can be a very handy tool to have.Like I said, even that Metrahit I am looking at has this problem. You say yours does not but I suspect that you just have not tried enough test cases.

author: chronos42
author_rank: Regular Contributor
timestamp: May 26, 2017, 06:35:03 pm
content: Quote from: joeqsmith on May 26, 2017, 04:54:23 pmHi,I said my Metrahit Energy does not have a problem with a 20mVpp Signal together with a 10 V DC in the AC mV range. So it seems it can handle this without an external decoubling capacitor. You have a different Metrahit. But I agree with you that it is a good practice to use an external decoubling capacitor. And I also agree that I should try more test cases with the Metrahit Energy. So I will playing around to figure out under which conditions it also failes (or maybe not).I only was just quite surprised how obviously the Brymen failed the DC test in the AV mV range. I did not expect that because I never had seen this issue so clearly in the past with any other of my multimeters.

author: chronos42
author_rank: Regular Contributor
timestamp: May 26, 2017, 07:17:07 pm
content: Hi,After playing around now some pictures:All three meters are in parallel.The Scopemeter shows the DC component. It is clearly to see that the Metrahit Energy has no problem to show the small AC V under all conditions. The Brymen fails over approximate 1 V DC.

author: tautech
author_rank: Super Contributor
timestamp: May 26, 2017, 07:35:19 pm
content: Quote from: chronos42 on May 26, 2017, 07:17:07 pmJoe has said the Brymen and the Gossen he has have the same problem, you need to either check his YT channel or watch the vids in his DMM robustness thread. The first vid on the Gossen is here:https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/msg1207306/#msg1207306Further on (not sure which vid) he shows a few meters that have the same problem as yours.

author: 2N3055
author_rank: Super Contributor
timestamp: May 26, 2017, 09:02:24 pm
content: Know your tools... That is most important. No single instrument is perfect.Also, you measure different things on different meters... Hence different results..Your Metrahit Energy is 800+€ instrument, specialized for certain measurements.. It has measurement of power(W), energy (kWh), DC/AC events recording, disturbance detection and recording, harmonic and distortion analysis... For purposes of powerline analysis, it has specially optimized AC/DC paths.. One of big powerline problems is DC component on powerlines because of asymmetric half periods... So it probably has front end better optimized for simultaneous AC and DC measurement.Metrahit Energy is not so much multimeter as much as a power line analyzer.. And a darn good instrument... For people needing that functionality, worth every penny..In comparison to Brymen 869S, Metrahit Energy also has less resolution, just one temp channel, no capacity measurement.. and 4x the price...Saying that Brymen 869 can't do the same thing as Energy is not significant. It doesn't have half of functions Metrahit has.But BM869S does equally good job as FLUKE 99 on your own photos... FLUKE autoranged to volts as you increased voltage... If you put Brymen to same range as FLUKE it does the same...BM869S is not some magical thing that will dethrone kings of meters from Fluke, Gossen, Keysight etc.. It is excellent price/performance general purpose multimeter that is decent, safe, good quality.... Just few years ago you either had give a lot of money for a big brand, or you bought very cheap utter crap that could kill you... No middle ground.Brymen is better that anything in that price class from big brands.. For that money big brands will give you safe, reliable instruments, but with just few basic functions, low res etc..For the price of Gossen magnetic holder and 2 crocodile clips (63+49 USD) you can buy BM257 that is one hell of meter.... For many people enough for all their needs for a handheld meter..Nobody said it was better that Gossen Metrawatt M246B... Or Gossen Energy.. Which I would like to buy one day... For powergrid measurement...But BM869S is infinite times better than Gossen Metrawatt M102A, and it is better in specs than Gossen Metrawatt M205A... They are what you can get from Gossen for the same money...Best regards

author: chronos42
author_rank: Regular Contributor
timestamp: May 27, 2017, 06:03:40 am
content: Quote from: 2N3055 on May 26, 2017, 09:02:24 pmHi,to make it clear: I will not blame the Brymen as a bad meter. It is an excellent meter for this price. I only will point out that very faulty behavior in the AC mV range.And I do not compare the Brymen with the Gossen, this was only an example for the AC problem. Instead of the Metrahit Energy I can do the same thing with my Benning M7-1, a 170EUR meter, with the same result.BTW:The Metrahit Energy has indeed less resolution than the Brymen. But it has the same (or better) accuracy, so the higher Resolution of the Brymen is pretty useless.And of course the Energy has a capacity measurement as you can see at the pictures.And as you also can see in the pictures the Fluke 99 is in the manual range. It was only for showing the DC component. The Brymen and the Gossen are in the AC mV range, there is not autorange function in this mode.Regards

author: joeqsmith
author_rank: Super Contributor
timestamp: May 27, 2017, 12:17:18 pm
content: Quote from: 2N3055 on May 26, 2017, 09:02:24 pmI looked at the Energy but as I went through the documentation, it was just not good enough for what I would want to use it for. IMO, if you are going into the field and need to look at AC, have a look at what HIOKI offers. The ones I normally use cost more than Energy by a fair amount but are worth it. For my home hobby use, the Ultra would better suit my needs. IMO, it's just not a very good product.

author: 2N3055
author_rank: Super Contributor
timestamp: May 27, 2017, 07:26:15 pm
content: Quote from: chronos42 on May 27, 2017, 06:03:40 amHello Chronos!Thank you for a bit of clarification, I better understand what you meant now. And I double checked datasheet, of course you're correct it has capacitance measurement function... My mistake, sorry.It would be interesting to see what kind of multimeter chipset and differences in schematics Benning has so it measures better.And I do not say you are wrong with what you see. What I wanted to say it is not error, or some kind of defect. Here on this forum we discuss engineering, so we have to insist on accuracy of statements.On BM869S mV mode is implemented in such a way that it doesn't decouple input path with capacitor before first stage of amplification. So 500mV range means 500mV of DC, and 500mV AC in combination with it. So instrument is not specified for 10VDC + 10mVAC on mVAC position. That is what it is....Problem is that BM869S doesn't show Err or Overload when it overloads from that reason....That IS a problem, and engineering oversight. So you must be careful.In my checking that range has total dynamic range of +-760mV, roughly 1.5V P-P. If you stay inside that, measurements are perfect.When I first investigated this, I was confused why it was implemented this way.. It seems that both AC and DC mV switch positions are using AC+DC measurements all the time, just displaying different on display. AC mV is not capacitor decoupled, it is still AC+DC just with AC on primary display.It certainly does preclude some measurements (like small AC voltages riding on top of large DC offset) directly, but now that I know how it works, I simply can put a capacitor in series with instrument and do it rightly... It is small annoyance, but gives me good results..If they would change the meter so it does this with proper decoupling I would like that.. It would make meter even better.Funny thing is that if you measure on DCmV switch position with ACmV in secondary display, it WILL show you DC OVL so you know you are pushing it. So I generally use that most of the time. It will show same ACmV value, it is just on a smaller secondary display. And it shows DC component too if there is any.. So I use it all the time because it gives more information at same time.I personally think if they improved this ACmV issue, implemented auto hold (measure hold) function and diode testing similar to Fluke (short beep on good diode) that it would be instrument very hard to beat, price regardless..And accuracy wise, my BM869S is WAAY better than specified on DC specs.. It agrees within 2 last digits with calibrated 6.5 digit DMM.. Also, it is very stable, so increased resolution is actually usable... But I wouldn't claim it better than GOSSEN high end meter.. But as you also agree, it is a small miracle that we can even COMPARE them in same sentence..Thing is, I don't see need for better handheld than BM869S in my work. If I need better precision and resolution, I measure with my benchtop DMMs.Thank you for a nice discussion...All the best,Sinisa

author: evava
author_rank: Regular Contributor
timestamp: May 28, 2017, 07:15:50 am
content: Quote from: 2N3055 on May 27, 2017, 07:26:15 pmVery nice explanation!Quote from: 2N3055 on May 27, 2017, 07:26:15 pmAnd very nice recommendation!Thank you!