Would the BM789 be a good choice, & why isn't it still produced like the BM786? - Page 1
Forum section: electronics / beginners

author: cedd
author_rank: Newbie
timestamp: October 11, 2024, 04:11:36 pm
content: I want a general purpose DMM for home maintenance and basic circuit board work. I also want a meter with a LoZ function which the 786 doesn't have. Looking at the offerings, the BM789 seems perfect for me. I do have some hesitation though because I will likely need to buy it used since it's no longer in production, and it seems it had some kind of resistor reading bug according to its megathread on this forum. (https://www.eevblog.com/forum/testgear/brymen-bm789/325/)Would I be able to buy one used and then update it to fix the bug without too much trouble? Also, what's the reason for not continuing its production like the 786?I considered the BM235 but the low True RMS response frequency dissuaded me. I'd rather invest in a more performant meter now and grow into it rather than buying a nicer meter down the road.I'm also considering waiting for the 121GW to be restocked though I'm concerned its greater weight & bulk might be annoying. The bluetooth connectivity stuff doesn't appeal to me much but I'm open to it since it's all open source. The way Fluke is going with their closed garden app ecosystem I absolutely hate and would never buy.Any advice on which meter might serve me best would be much appreciated!Thanks

author: J-R
author_rank: Super Contributor
timestamp: October 12, 2024, 04:10:05 am
content: Why do you say that the BM789 is out of production? Welectron is probably the cheapest place to get one for US buyers and they also carry a nice selection of accessories:https://www.welectron.com/Brymen-BM789-Multimeter_1The resistance bug was supposedly fixed in firmware version 11 back in 2022 as mentioned in the thread you linked, so I wouldn't expect new units to have the issue. Also, only some units had the issue even if the firmware wasn't updated, and it requires a specific resistance range which means you probably would never see it in normal use.In addition to the BM789, I typically suggest having a look at the BM869s, which has the dual display and optional PC interface, although missing some features like non-contact voltage detection (EF).Ultimately, you probably should buy a couple DMMs since there is always going to be a reason to pick up one over another...

author: cedd
author_rank: Newbie
timestamp: October 12, 2024, 04:26:12 pm
content: Thanks for the info! My biggest concern with the 789 is it's LoZ function. In its manual it says it has an abrupt scale up to hundreds of Kohms and that seems different than, for instance, Fluke's implementation. But maybe it isn't different since Fluke's is also an AutoV which probably has an abrupt ramp up as well and it could just be they don't mention it in their manual.I'm considering getting a max of two meters, one for electrical and another for circuitry instead of a general purpose. It's a hard choice for me because I think ~$200 is a significant investment and relying on manuals and reviews is a difficult way to be confident the right meter was chosen. It would be nice if the community had settled on specific meters as standards, but it appears as if the one settled on for residential electrical is Fluke and they seem to be price gouging based on a perceived reputation they've built up along with incestuous government supplier relationships.

author: J-R
author_rank: Super Contributor
timestamp: October 13, 2024, 03:13:54 am
content: First, it's not ideal to have two threads with the same topic being discussed (BM789 LoZ). Second, as mentioned by electr_peter in the other thread, I agree that the input impedance of the LoZ AutoV shouldn't matter to the user. Per the manual, the resistance changes with the input voltage from 2.1k to 670k, but the power draw is not drastically different: 0.83 Watts to 1.5 Watts. But if this is an important thing to you, then building your own LoZ circuit is trivial and the better path.If $200 seems like a lot, then it might be better to start with some low-cost DMMs until you get a better handle on what you are needing. The cheapies can end up in your glove box when you eventually upgrade. And if you're doing a lot of work on the bench, you'll really want a bench meter at some point anyway.There are plenty of YouTube personalities who have done DMM shootouts and reviews, and they attract comments from viewers that you can also consider (as well as forum threads). The "community" is never going to agree on a single DMM to buy, because there are just too many variables. You will just have to do your homework and formulate a few options that have what you want/need/like.The Fluke pricing conversation always seems to rehash those same tropes, but it ultimately comes down to a few things:- Fluke has a REAL reputation, not just perceived, and that is why a lot of people still buy Fluke DMMs. (You're telling me the people crying about the price don't actually want to buy one? Makes no sense.)- Being a business primarily based out of the US, there are additional costs involved, such as regulations compliance and employees that require a specific level of compensation to support their quality of life.- Fluke also offers a high level of warranty and support coverage. You can see an example of what this cost adds by looking at Brymen rebrands from Greenlee. It almost doubles the sales price.

author: cedd
author_rank: Newbie
timestamp: October 13, 2024, 08:00:31 pm
content: You're right I could have been more careful to better separate out these two topics instead of combining them here. My mistake!Quote from: J-R on October 13, 2024, 03:13:54 amI don't see how you determined the watts would only have a range of 0.83 - 1.5. There's an inverse-proportional relationship between amps and resistance, and the resistance range is >300x while your calculated watt range is only 2x. Also, my concern with this implementation isn't the power draw but its ability to troubleshoot current leakage and compromised voltage sources, not just ghost voltage. Here's the link to the video that demonstrates these other uses for the LoZ function in case you overlooked it:https://techcircuit.org/how-lowz-works-and-why-its-useful/#google_vignette

author: J-R
author_rank: Super Contributor
timestamp: October 13, 2024, 11:04:45 pm
content: Quote from: cedd on October 13, 2024, 08:00:31 pmMy vote for topic starting is that ideally you'd have one revolving around getting help with what DMM you should buy, and it could have covered the LoZ and and the other topics all in context. I think it's better for the conversation to wander than to start another completely new topic in another area, such as Test Equipment or Metrology.Quote from: cedd on October 13, 2024, 08:00:31 pmP=V^2/R, based on the four values provided in the BM78x datasheet.The "other uses" of LoZ aren't really any different from each other. You are applying a small load (around 1W) and measuring the voltage. That is all. The resulting voltage being displayed still requires some knowledge to decipher, and ideally you would measure at least with HiZ and with LoZ (and perhaps even with a larger load). Specifically, you might measure ~30VAC with LoZ mode as seen in the video, but you also get ~30VAC with HiZ and that would tell you you're probably checking a 24VAC doorbell or HVAC transformer circuit and nothing to do with "ghost/leakage/compromised" voltages.There are also other scenarios that can require some thought to figure out, such as a circuit with an open neutral at the breaker panel, or the fact that some lighting circuits switch the neutral and not the hot.Side note, unfortunately the 121GW Low Z mode really only works around 12V and above. A VERY handy use case for LoZ is for measuring suspect batteries and the BM789 works well for that. Alkaline chemistries rarely require it, but NiCd, lithium (primary), and dry cells (zinc-carbon, etc.) are somewhat notorious in my opinion.

author: cedd
author_rank: Newbie
timestamp: October 14, 2024, 07:54:03 pm
content: QuotePower is exactly as inversely proportional to resistance as current, so this doesn't satisfy my confusion in trying to reconcile a ~300x difference in resistance affecting an only ~2x difference in that power calculation.QuoteThat's fair, they are essentially the same, though I think the circuit resistances tend to be different. With leakage we might expect to see more loads >50K(from stray currents through low-conductivity materials) while with compromised voltage we might see more <10Kloads from poor connections.I don't know when the 789 "abruptly switches to 670K" but assuming this switch occurs at some point for all readings, these are the readings we might expect when measuring a:3K leak: ~60V before ramp up, and ~120V after ramp up. (HiZ will measure ~120V)100K leak: ~0.4V before ramp up, and ~16V after ramp up. (HiZ will measure ~119V)As you can see, it doesn't make much of a difference for the 100K leak but it would be largely indistinguishable from HiZ for the 3K leak assuming the 670K ramp up happens in most readings (which maybe it doesn't, the description in the manual is sparse and I honestly don't know when the switch occurs)

author: J-R
author_rank: Super Contributor
timestamp: October 14, 2024, 10:21:20 pm
content: Not sure how to respond to your power confusion. What I posted is the formula given the known variables (voltage and resistance). Run the numbers as listed by Brymen and you'll see? The power drawn from the circuit must be the same power dissipated by the DMM.The total resistance of the BM789 in LoZ mode is not a step increase, it's a curve due to the PTC heating up. Hotter PTC, higher resistance. The specific one used by DMMs is going to be fast acting, of course.

author: cedd
author_rank: Newbie
timestamp: October 14, 2024, 11:43:27 pm
content: I trust you got those numbers from the manual but they still don't make sense to me as they seem to defy the inverse proportional relationship between power/current and resistance. Also, I read in the manual that the step-up is abrupt at higher voltages (which I assume 120V would fall under). This is where my concern is rooted.Here is the description of it's AutoV mode from the manual:QuoteAs I'm writing this now and after having mulled all of this over for the past few days, I see that it's explaining the behavior at its highest voltage range: ~1000V. At this extreme, the PTC transition will be abrupt, happening in a fraction of a second. This means we could expect a slower ramp-up at 120 volts, maybe taking a few seconds, or even 10s of seconds, to reach its max impedance of ~670K. I suspect this is how Fluke implements it as well.Thanks for helping me toward this understanding, I appreciate it!