Brymen multimeters - fault?

[evava]

I wonder if that "discovery" by user hgg in thread "A-look-at-the-uni-t-ut210e" regarding Brymen meters is truth, can someone confirm that (Lightages ?)
I dont want cross-link, but I am talking about "Reply #304 on: October 28, 2016, 08:30:20 PM" and following posts (where he shows that mV AC range does not indicate an overload at specific AC signal with DC component).

I was sort of before buying BM869s for my Christmas gift  , but now I hesitate a little.
Would it be possible to fix that eventually by firmware upgrade at home?

Or is it not problem at all?

[IanB]

Here is a link to the post I think you are referring to:

https://www.eevblog.com/forum/testgear/a-look-at-the-uni-t-ut210e/msg1058296/#msg1058296

Every instrument will have some kind of bug or limitation somewhere. If you wait for the perfect meter you will wait forever.

In this case the problem seems to be specifically with the AC mV range, on the AC+DC function, with a very particular kind of wave form.

Is this a measurement you already make with other meters? If you are now aware of this limitation and so can work around it, would that help you to avoid any misleading measurement results?

[evava]

https://www.eevblog.com/forum/testgear/a-look-at-the-uni-t-ut210e/msg1058296/#msg1058296

[2N3055]

Just buy brymen.. That post is faulty.. Poster seems to not understand correctly TRMS AC and AC+DC .. All is fine...

[evava]

Just buy brymen.. That post is faulty.. Poster seems to not understand correctly TRMS AC and AC+DC .. All is fine...

Can you elaborate on this?
Poster maybe understand/ maybe do not understand, but he measures some AC+DC signal, and his two Brymens shows value that is not correct. It would be helpful if he showed screen of his scope with the waveform that he measures by the meters, yes.
I have tried this with two mine multimeters that have got AC mA range, and they both showed when out of range. But I have not got any Brymen yet.
Otherwise, obviously I see Brymen as fine brand, especially with regards to safety.

[The Soulman]

Not going to comment on this is a complete fail on the brymens to be able to read the one fine
and screwing up the other or a limitation in it's capabilities.
In real life this shouldn't be a problem as there isn't much ac with dc-offset around, I can only think
 of audio signals that might have some dc-offset?

[2N3055]

I'm at work now will recreate experiment and explain later..

[rstofer]

In real life this shouldn't be a problem as there isn't much ac with dc-offset around, I can only think
 of audio signals that might have some dc-offset?

I would think AC+DC would be very common in single rail op amp circuits.  It may be possible to measure voltages relative to the virtual ground and this will probably remove the DC component but it should also be possible to measure to the power supply ground.

[Lightages]

I am sorry that I didn't follow up on the other thread. I think it best to not pollute the other thread with off subject posts anyway. It is unclear which meters are not reading correctly. I will try to recreate the test conditions again when I have a moment but I am sure that there is no error.

[joeqsmith]

I have not tried to see if the meter detects the over range but assume what the OP wrote is correct in that it does not in the AC only mode.  If the DC were enabled, you should see over range on the DC part, telling you to switch it out of the mV range.  For me, this isn't a big deal but I could see where for some people this could be a problem. 

[IanB]

I assume most of the time when using a meter you would know what kind of voltage to expect before measuring it since you would be checking measurement against expectation.

If you didn't know, you would most likely put the meter on autorange and measure AC and DC separately to find out what you are dealing with. That would tell you whether to select the mV or V range for subsequent measurements.

[evava]

I have not tried to see if the meter detects the over range but assume what the OP wrote is correct in that it does not in the AC only mode.  If the DC were enabled, you should see over range on the DC part, telling you to switch it out of the mV range.  For me, this isn't a big deal but I could see where for some people this could be a problem.

Yes, I realize that it is always wise to check displayed value with other ranges and even switch between AC and DC, and in case inexplicable inconsistencies/discrepancies appears, then is best to check with scope or at least use another device.

It is interesting, that e.g. Fluke 87V does not have AC mV range at all (on dial), maybe there is some hitch in implementation that, and this is safe bet ?

For example my other meter Minipa ET997TR http://www.goodluckbuy.com/minipa-et-997tr-et997tr-3-0-lcd-handheld-digital-multimeter-with-temperature-test.html does not suffer with that(on its separate AC mV range), as well as UT-61e - there is something about meters for $50 range    (and they again have theirs "properties"/quirks/odds  )

[2N3055]

RMS is not a real value..  It is a imaginary DC value that would give same thermal equivalent power like the complex waveform you are measuring, if you were to connect them to the same pure resistor (no parasitic inductivity or capacitance).

So if you have same sinusoidal waveform with and without DC component, it will greatly differ... Real TRMS is AC+DC.. AC only is not really RMS, except for signals that have no DC component, in which case it is the same... Many TRMS voltmeters don't have AC+DC, only AC RMS, which is not really true RMS measurement.. It is ok if you only want to know AC component...

If you are measuring with the scope, and use DC coupling you are measuring AC+DC RMS, with AC coupling, also only AC RMS... So it is easy to have mix of instruments that are not measuring the same thing..

Also, you have to keep signal within instruments AD converter range.. so 200mV on top of 10V you measure on 20V range if you are doing AC+DC, or you can measure at 400mV on AC only...

You have to understand what are you measuring... That is why we have oscilloscopes, voltmeters alone are not enough to characterize unknown signal..

[evava]

RMS is not a real value..  It is a imaginary DC value that would give same thermal equivalent power like the complex waveform you are measuring, if you were to connect them to the same pure resistor (no parasitic inductivity or capacitance).

So if you have same sinusoidal waveform with and without DC component, it will greatly differ... Real TRMS is AC+DC.. AC only is not really RMS, except for signals that have no DC component, in which case it is the same... Many TRMS voltmeters don't have AC+DC, only AC RMS, which is not really true RMS measurement.. It is ok if you only want to know AC component...

If you are measuring with the scope, and use DC coupling you are measuring AC+DC RMS, with AC coupling, also only AC RMS... So it is easy to have mix of instruments that are not measuring the same thing..

Also, you have to keep signal within instruments AD converter range.. so 200mV on top of 10V you measure on 20V range if you are doing AC+DC, or you can measure at 400mV on AC only...

Thank you.
I'm trying to understand what you're saying.

It remains only briefly say why some multimeters (ut61e) show OverLoad, and others not? What is wrong with them, what is so wrong with the signal? Why some cheap multimeters (ut61e) are not confused with the signal?
That I do not understand.

[2N3055]

Back in the old days instruments were analog and manual. If you connected it wrong..... puuffff... So we where taught propper practice.. If you didn't know what are you measuring start with 1000 V range. And then you lower the range to get better precision if needed.. And also you would try both AC and DC.. Just in case.. The effect could be because meter was put to fixed range, or maybe because it wasn't .. Bottom line, know your instrument, read manual thoroughly,and you'll be fine. Pick a book or two about electrical measurement and start reading.. It's journey not a destination....

[joeqsmith]

That wasn't very useful.  Not sure what happened to my post so we will try again...

It remains only briefly say why some multimeters (ut61e) show OverLoad, and others not? What is wrong with them, what is so wrong with the signal? Why some cheap multimeters (ut61e) are not confused with the signal?
That I do not understand.

I don't think there is anything wrong with your signal.   I also do not see why ANY meter could not detect if it's ADC is running near it's limit under any of the settings.     

With the 87V, it does not support the AC+DC mode that I get with the Brymen.  You have to read the AC and DC then carry a calculator.   I recently picked up a couple of old Fluke 97 scopemeters that can read both the AC and DC simultaneously but it does not have the ability to calculate the AC+DC.  I am guessing the square and square root took too much processing.   This was part of the reason I have the BM869s.  If I ever buy a new Fluke it would be something like the their graphing meter but it would have to be better than they offer now.  All beside your point about the over range. 

Other equipment (scope, SAs) can have similar problems being saturated with no indication.  For me, this has never been much of a problem. 

I have not looked at the Brymen but assume that it can't detect the OR as you mention.   I could repeat your test along with checking a few other meters I have but I really am not sure what we would learn.  Replicating your results may be good, but I don't doubt what you are seeing.   Maybe it would give you a more general idea on if there are other meters that don't always detect when they are out of range.   It would be no problem for me to put a 500mV signal on top of what ever bias we want within the limits of the meters. 

[IanB]

I have not looked at the Brymen but assume that it can't detect the OR as you mention.   I could repeat your test along with checking a few other meters I have but I really am not sure what we would learn.  Replicating your results may be good, but I don't doubt what you are seeing.   Maybe it would give you a more general idea on if there are other meters that don't always detect when they are out of range.   It would be no problem for me to put a 500mV signal on top of what ever bias we want within the limits of the meters.

I think the test to perform would be this:

Set the meter on the mV range and select the DC (primary) + AC (secondary) display function.

Apply a DC signal within the 500 mV range of the meter, then superimpose an AC signal of varying amplitude, either more or less than 500 mV.

Does the meter always display the DC and AC components accurately, or does it give an incorrect reading in either primary or secondary display without indicating OL?

[joeqsmith]

As I understand it, the OP is looking for the meters to give them an indication if it is out of range, no matter the mode the meter is in.  Idea that the meter should tell you if the reading are inaccurate no matter what you are doing.  So AC, AC and DC, AC+DC all should give some indication no matter if it is mv or volts. 

Some of the meters I have are averaging if we want to run them along with the RMS meters and we want to keep the frequency out of it, just stay with the OP's simple clean sinewave at 60Hz.  Not that it's real world...    If we agree on some working ranges, multiple people may be able to run separate tests as well.  More meters, more data.   

Should be no problem to set something up if this makes sense. 

[IanB]

I understand there are many possible variations and combinations, but someone (not the OP) in another thread did a specific test with the BM869s where they used the settings I described. In this test they measured something like a 450 mV AC signal offset by 225 mV (giving a positive waveform varying between 0 mV and 450 mV). In this test the Brymen did not correctly read 450 mV AC in the secondary display and also did not indicate an over range condition.

There has been some dispute over whether the meter actually behaves this way or whether there was some kind of experimental error.

If you are conveniently able to set up that particular test, it would be interesting to know the results.

[joeqsmith]

So 159 AC and 248 DC or  25,281 + 61504 = 86785.  Sqrt = 295.  Brymen seems to be calculating it right.  Seems to agree close enough to sig gen and DSO.  Not sure what this problem would have been.  Maybe you misunderstood what they asked, or I am misunderstanding what you are asking.

[IanB]

Looks good. As it happens I just dragged out some stuff onto my bench and did similar experiments. The Brymen did seem to read accurately, and it did indicate an over range situation in both primary and secondary displays.

Let me emphasize I am not asking the question directly, I am just interpreting a question asked by others, and trying to help the OP in this thread. In the other thread linked from the beginning of this thread, one poster said they observed a problem and another poster said this was highly unlikely (neither of whom are the OP, and neither of whom are me).

One puzzling observation was that after I had been dialing AC and DC voltages up and down and making the meter indicate OL and come back into range again, I did at one point see the meter reading about 100 mV too low on the DC value. I don't know why it was doing that and the problem cleared after I moved the rotary switch to a different range and back again. Maybe a firmware glitch of some kind? It is hard to say unless I can reproduce it.

[joeqsmith]

Sorry, I only ran the settings I thought you were asking for and have not read the other thread.  Are you displaying the DC and AC or AC+DC and AC?  I trust the Brymen does the calculation correctly as I use it but there could be a problem and I just haven't come across it.

So let's push it to just under the limit.  1Vp-p with a 500mV offset.  Meter has no problem. 

[joeqsmith]

Now lets push it over the edge and see if what OP has makes sense...

So 1.2Vp-p with a 600mV offset.   Should do very nicely.   DSO showing 431mV (did not reset stats so ignore) vs 424.  Good enough.
Now with the BM869s set in ACmv, we see the error.   Selecting AC+DC, we see over range.  No surprise there but OP is right that the really we know the answer in the ACmv setting will be wrong and the meter does not give any indication.

Again, not too surprised by this as this is what I understood OP was seeing.  Personally, I don't have a problem with the meter working as it is but I can see this being a trap for some. 

[IanB]

OK, I have identified a problem. In the first picture the meter is reading a DC voltage with no AC component. It apparently reads accurately.

In the second picture there is the same DC voltage, but with an AC voltage added on top. Now the meter is misreading the DC voltage with quite a large error.

Maybe you could try the same values and see if you see the same results?

[IanB]

If we drop the AC voltage down a bit, the DC voltage reads correctly again. There seems to be a problem in calculating the DC component when the AC component gets close to or over 500 mV.