BM786 or 121GW - Page 1
Forum section: electronics / testgear

author: awesomechapro
author_rank: Contributor
timestamp: December 02, 2024, 02:35:11 pm
content: Looking to buy myself a new multimeter, should I get a BM786 or a 121GW? Which multimeter do you prefer and why?

author: J-R
author_rank: Super Contributor
timestamp: December 04, 2024, 03:59:18 am
content: There probably is not a lot of interest in this topic since it's something that comes up a lot. You should do more research on each model, and provide us with some reasons why you selected those two for consideration. What do you need the DMM for??They are quite different products. So there are good reasons to get both.Some talking points where they trade blows:121GW - long battery life, Bluetooth, many unique features, somewhat slow, accuracy is not amazingBM786 - mediocre battery life, no connectivity option, typical feature set (BM789 is better), reasonably fast, good accuracy

author: BILLPOD
author_rank: Frequent Contributor
timestamp: December 04, 2024, 08:44:48 pm
content: Good Morning Awesomechapro, I've got quite a collection of DMMs, but my 121GW has moreprogrammable features than any of the others. My favorite is the one I call: 'inverse continuity beep', it will beep when continuity is broken. I've found many bad cables by attaching the leads to each end of the suspected cable and then manipulating/massaging the entire length. If the meter beeps, there is a break in continuity. I also love the ability to record data quickly, without having to go through a lengthy set up routine. The Micro SD card is a pain to get to, but it works very well. Also the Bluetooth is very handy. There's lots to love with the 121GW, and you won't regret purchasing it.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 05, 2024, 02:00:01 am
content: 121GW has by far the most functions. If the official firmware would have been open sourced I would have bought one. For example to put in some custom algorithms for sensors or even to fix bugs (It was quite new when I thought of buying it)But a few years ago I thought a lot about what was the most important for me, and I decided it was accuracy, reliability and stability, and I bought the BM869s.Data output was also a must for me, but the optical output of the Brymen is adequate. I would have liked a benchtop meter, but those are out of my price range, and they are also to big (Especially in depth, big empty boxes these days).So as a result, I can build small and cheap specialized meters, calibrate them with my Brymen and then start measuring and logging.But overall, the feature set of the 121GW puts it pretty much in a class of it's own, if it's feature set appeals to you, then buy it. If it does not, then get another meter.

author: EEVblog
author_rank: Unknown
timestamp: December 05, 2024, 05:34:18 am
content: Added some extra in bold:Quote from: J-R on December 04, 2024, 03:59:18 amIMO the 786 is better for field use, or if you simply value a larger display.Maybe also consider the new BM2257 coming soon.

author: Micah Lindstrom
author_rank: Contributor
timestamp: March 02, 2025, 05:31:48 am
content: Am I missing something? The 121GW does have lower burden voltage than the BM785, BM786, and BM789, but not by much and only below 500µA, right? And the Fluke 87V matches or beats the low burden voltage?Moreover, the lower burden voltage seems a little pointless for anything over 50µA in light of the significantly worse accuracy.First consider burden voltages:The 121GW specifies burden voltages of 100, 2, and 0.03 V/A, whereas the BM78x meters specify 200, 2, and 0.03 V/A, and the Fluke specifies 100, 1.8, and 0.03 V/A. Even ignoring the Fluke, the 121GW and BM78x meters still have different range breakpoints, so from 50-60mA and 0.5-0.6A, the BM78x is actually better in terms of burden voltage. And even where the 121GW is better, below 50mA, it's just half the burden voltage. This doesn't seem like a very big difference to me.It certainly is valuable that the 121GW allows directly measuring the burden voltage and displaying it simultaneously.But if burden voltage is a concern, I'd think you need a product like the µCurrent current adapter device which has 0.02 V/A burden voltage under 1250mA, and 10 and 10,000 V/A burden voltages for µA and nA ranges.https://eevblog.store/products/ucurrent-gold-multimeter-adapterI don't see how/why the 121GW product page and manual and branding make such a big deal about low burden voltage.Next consider accuracy:The 121GW does have better resolution and accuracy than the BM78x below 13µA. They are 50k and 60k-count resolution meters, so that's roughly equivalent. The 121GW does have an extra digit displayed below 50µA from its 1nA resolution compared to the BM78x's 10nA. However the extra digit doesn't compensate for the order of magnitude worse accuracy until the current is less than 13µA. The specified accuracy under 50µA for 122GW is ±1.5% + 15nA (15 digits) compared to ±0.075% + 200nA (20 digits) for BM79x.For example consider a current of 35µA. The 121GW reads 35.000µA with accuracy of 1.5%=525nA along with 15 digits = 15nA. So the 121GW total from 525+15 is ±540nA. The BM78x reads 35.00µA with accuracy of 0.075%=26.25nA along with 20 digits = 200nA. That total from 26+200 is ±226nA. BM78x has twice the accuracy here.At 13µA they have the same accuracy.At 5µA, the 121GW and BM78x accuracies are ±90nA and ±204nA, respectively. 121GW has twice the accuracy here.At 200nA, it's ±18nA and ±200nA, respectively.Disclaimer:This is my first time thinking about burden voltage so I could certainly be thinking about something wrong. But I believe I understand the concepts and math well enough based on reading Dave's articlehttp://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf"that was published in Silicon Chip magazine." Note I found that on the 121GW Kickstarter pagehttps://www.kickstarter.com/projects/eevblog/eevblog-121gw-multimeterI also foundhttps://youtu.be/U4JFeU-o2kcwas a good reference on the accuracy calculations.Attached are screenshots of the current specs for EEVblog 121GW, Brymen BM78x, and Fluke 80 Series V.

author: bdunham7
author_rank: Super Contributor
timestamp: March 02, 2025, 06:29:27 am
content: Quote from: Micah Lindstrom on March 02, 2025, 05:31:48 amAll three meters appear to use 100R, 1R and 0.01R shunts so the burden voltage will be the current times whatever shunt is in use plus the resistance of the relevant fuse and related circuitry (leads, etc). However, what you're missing is that the 121GW moves to a lower resistance shunt earlier than the others. So for example, compare the 5mA range on the 121GW with the 6000µA range on the 786--the former is using the 1R shunt for 2V/A total while the latter is still using the 100R shunt for 200V/A (as listed--IDK why it would be that high unless I'm wrong about the 100R shunt). The same thing for the respective 500mA and 600mA ranges where the full scale burden voltage will be only 15mV on the 121GW but 1.2V on the 786 (again, going by the spec). If you go through the specs, you'll see that the 121GW will not ever exceed 100mV burden until you go over 3A while the 786 will reach 1.2V at 6mA and 600mA.

author: J-R
author_rank: Super Contributor
timestamp: March 02, 2025, 12:20:26 pm
content: I think this is another case where published specs don't follow to the real world. Sorry to say, but UEi (121GW) can't compete with Brymen (BM78x) in the accuracy department. And Fluke (87V) beats them both. What I mean is that the Fluke might be 100 times better than expected, the Brymen might be 10 times better but the UEi will just barely meet spec. Also, the 121GW does suffer a bit from noise and interference when leveraging the amplified current ranges, so an almost unreasonable amount of extra care is required.Does it actually matter in your scenario whether you measure 1mA as 1.001mA or 1.1mA? Consider a bench DMM if this is a serious requirement, along with calibrations and references for sanity checking. For low burden applications, check out the Keithley 480/485. For example, the 485 is 2mA full scale with an input resistance of only 0.1 Ohms.

author: bdunham7
author_rank: Super Contributor
timestamp: March 02, 2025, 04:00:19 pm
content: Quote from: J-R on March 02, 2025, 12:20:26 pmThis brings up the argument against low burden designs, at least noisy, amplified ones. If you have a more accurate meter with sufficient counts, you can obtain low burden voltages by simply selecting the next higher range. You don't need a high-count meter to make this work. I just tried measuring 5.00000mA with a Fluke 289 (50.000mA range, 50k counts, 1R shunt+fuse) and I got 4.999mA or 5.000 depending on tongue angle. With a Fluke 27 (32.00mA range, 3200 counts, 1R shunt + fuse) it was 5.00mA. In both cases the burden voltage would be the same or less as the 121GW using the 5mA range.

author: Micah Lindstrom
author_rank: Contributor
timestamp: March 03, 2025, 01:38:03 am
content: Quote from: bdunham7 on March 02, 2025, 06:29:27 amAh yes, I was definitely missing that thank you! So the 121GW combines the resolution of the Brymen BM78x with the low burden of the Fluke 80 Series V, even beating them in certain ranges, and especially beating them below ~10µA. The generally lower accuracy of the 121GW is still a major limitation though.The Fluke 287/289 still has similar burden voltage as the 121GW along with far better accuracy, but those are a different class of meter with significantly more bulk and expense.I'm a very visual person so I put together this graph to help me compare them (and I've attached Excel source file if you care, too, although it's a bit of a mess).