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Brymen BM789

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2N3055:

--- Quote from: bdunham7 on September 14, 2021, 09:54:40 pm ---
--- Quote from: 2N3055 on September 14, 2021, 09:25:06 pm ---So here is a brain teaser :  in your own words it will survive in any range including Ohms, right.  And what will meter set on ohms show when you connect it to say 400V?
It will keep showing Open on the screen. Whoops....

--- End quote ---

Actually in that case they say 'OVERLOAD' or OL.  Which is exactly what they are seeing--voltage above their range limit.  I don't expect them to do more than tell me that they can't get a reading, what I don't want to see is a normal-looking but incorrect reading.


--- Quote ---That is exactly why meters for this "I have no clue what to expect" thing do exist. On my BM525 you have auto mode. It will automatically detect AC, DC or Ohms/continuity.
In a single switch position.

--- End quote ---

I'm not after that.  I'm only asking that the meter do one of two things--give me a reasonably correct answer or say "I don't know".  The only fail is when it gives me a plausible but completely wrong answer.  I'm sure that you could probably make any meter fail this at some point, so it just comes down to what one thinks is reasonable and what other similar meters are capable of.


--- Quote ---And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

--- End quote ---

That's not true, the F87V has a 600 mVAC range, it is just doesn't have a separate switch position for it. 


--- Quote ---When you guys find perfect meter let me know. I am interested, but in meantime I'll work with what I have.

--- End quote ---

OK, it's the Fluke 189.  Which is why they go on eBay for three times the price of a new BM789!   :)

--- End quote ---

It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

Without that kind of Auto select mode it cannot know that what you connected to it is wrong. Except in current mode where you have fuse as an indicator.

As for F87V I apologize, I meant mV DC. My bad. And you know, despite that what I meant, but never mind. I said at the beginning, this happens only if you mix AC and DC.
If you feed it only AC (AC coupled like F87V) there is no problem and will nicely show OVL.

I don't trust used old stuff that much. Not even Fluke, especially when I saw with my own eyes what people do to them.

In my book both BM869 and MTX3293 come pretty close to fantastic... Each in it's own right.

By the way MTX 3293 cannot have that problem because it is single input, single function, and is AC coupled from uV to kV. In AC+DC is DC coupled but uses both to autorange.

2N3055:

--- Quote from: Caliaxy on September 14, 2021, 08:48:26 pm ---
I did read that post (and all the other posts in this thread). I find it excellent and educative (like most 2N3055's posts) but a little bit exaggerated (the part with 10000V on mV scale...). I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.


--- End quote ---
Thank you for the nice words and sorry for the drama.. I tend overexaggerate with comparisons to prove the point works at any scale, my bad.

bdunham7:

--- Quote from: 2N3055 on September 14, 2021, 10:10:48 pm ---It will show whatever it shows when resistance is too high. Like with infinite resistance. Or in a case of BM869, it will start blinking trying to autorange.
Not at any moment it will give indication that you connected VOLTAGE to the inputs. That is my point.

--- End quote ---

OK, true--but that would be an expectation and feature that go beyond what is required and what both the OP and I would expect, which is that the meter simply not display a plausible but grossly wrong number.  It's not just a safety issue, although we like to talk about those dramatically.  Suppose I'm measuring a single-ended square wave like the OP at 300mVAC and it jumps up to 900mVAC?  I expect to see 'OL', not 640mVAC.  I don't think that's unreasonable.

As for not trusting old stuff, I'm in the opposite camp, although I may agree if it is old abused junk.  To me 'new' means 'experimental'.  I start to trust things after a decade or so of flawless service.  And I've been proven right more than once...

Caliaxy:

--- Quote from: 2N3055 on September 14, 2021, 09:25:06 pm ---And no AC mV range.. That is how Fluke solved that problem with F87V.
I prefer to have it, even if it requires thinking when using..

--- End quote ---

You got it wrong. Like BM789, Fluke 87V does have an AC 600mV range (and a DC 600mV range), but they are not on the same rotary dial position. The AC 600mV is on the AC V position, which starts auto-ranging from 600mV up, whereas the DC 600mV range has its own rotary dial position, all for itself. The DC V mode starts auto-ranging from 6V up. All AC V ranges (including 600mV) are AC coupled, so the DC offset is irrelevant, unlike in BM789.

What 87V doesn't have is a combined AC+DC mV mode, hardly useful for many but plagued by the DC offset issue (which I'm starting to believe is a ubiquitous problem across most brands).

AndrewBCN:

--- Quote from: Caliaxy on September 14, 2021, 08:48:26 pm ---...
I didn't say it's a bug or an oversight, I said it is a design fail, which does not infer that Brymen are (or are not) aware of it.

Maybe "design fail" is too harsh. What would this be then?
...

--- End quote ---
Seems to me it's quite simply this:
1. Don't read the BM789 User Manual (or pretend that you didn't read it).
2. Select the ACmV 600mV range even though you know full well you are going to exceed the maximum measurable voltage for that range
3. Carefully select a large DC offset that will saturate the DC coupled ACmV input circuitry of the DMM, thus preventing the detection of an overload condition.
4. Bitch about it on the EEVblog.  :rant:

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