Brymen BM789

[Neutrion]

Hi everyone, finally I had to register myself to clarify something, but was reading the forum for a
while, so not a huge loss...
I am starting a new one for the BM789 as it does not seems to have an own topic yet.

So I got the bm789 with FW. version 08. (The Brymen multimeter, not the other bm789 equipment one can find on youtube,of which I was hoping to see an in depth review, done by the lady presenting it, after accidentally clicking on it, and watching the whole presentation.)

So I found two problems:

1.If I use the mV Scale and feed in a squarewave with 50% duty cycle without(or very minimal like 100-200mV) zero crossing (though it behaves almost the same way with less or more than 50%) something above 1 V p-p (so above the specs for the mV scale) it reads around
650mv on the AC scale, and it stays so even if I raise the voltage to around 30 V .(Can't get further)
 DC or AC+DC overranges properly.
If I change the duty % it will be a bit less than 650mV, around 500mV AC. So it peaks at 50%

The strange thing is, that if I chane the polarity of the leads, it behaves correctly and overranges around 650 mV AC which it should. So with one way the leads connected it behaves as expected, but not the other way around. I assume it depends on which polarity the squarevawe has.

Do you thing this is normal or acceptable? Before anyone mentions it, I know that I should use the higher ranges for signals like this, but when someone measures a lower level signal, and it raises up
above the limit, there is no way of knowing it, you just stop seeing anything above this level, while the voltagemight be skyhigh. I can not test it with higher voltages unfortunately, but would be nice if someone could do that.

So I am not vorrying about accuracy above the specified limit, but about the fact that it does not show overrange on the AC scale on the mV scale.

How do other meters behaves in this scenario?


2. Thats a minor issue, but still annoying: When measuring milli or micro amps, if on auto range, the measured currentis above the displayed range, so the meter has to switch range, on AC+DC mode first it displays "OL" for a second or more and only if you keep the leads connected it displays the proper values.
I thought, it is just a question of a timeout value in the FW, but Brymen says it is not possible to change it.
Should I keep it or not?


Ps:
Thanks to Dave for the inspiring Videos!

And also thanks to Joe Smith for your extensive work, wish we could see this level of testing with some more consumer goods as well, it would mean much less garbage be produced!

I would be especially interested in your oppinion on this, and whether it is a firmware bug or maybe hardware related.
I contacted Brymen through the seller, but they say it is all good and fine with the meter.



 

[2N3055]

mV range on these meters is NOT AC coupled. It is DC coupled.
So what you see is because of that.
You can check it with another meter, just measure resistance of input on BM789.

On my BM869S (that is same in that regard) I usually keep it in DC mV mode, and put AC and DC dual display most of the time.
That enables me to see more at the same time. But BM869S has dual display.

You could also take a 1uF block capacitor and put it in series with meter probe. That would AC couple the meter on mV range.
V AC, OTOH, is AC coupled...

[Neutrion]

But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

[2N3055]

But with the dual display you see at least that it overranged. But with the 789 if I don't check it all the time in dc or ac+dc it could mean the voltage is sometimes all over the place and I don't even see it.

How does this capacitor trick affect the accuracy in dc or ac+dc modes? Did you check the whole bandwith with it?

How do other meters cope with this?

Dual display is the point. That is why I insist that single display meters are less capable than dual display.
Simple is not simpler all the time.

To me simplest meter would have big screen with simultaneous DC component, AC component (RMS), AC+DC (True RMS combined), frequency, duty cycle and accuracy limits. All at the same time. That would be simplest meter to use. You would have position for V, A, Ohms, Diode test. In Ohms, it would show resistance, voltage over resistor, current used and also accuracy limits.

Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. Only AC will be passed, and that one will be pretty much perfect in a frequency range of the meter. It will have high pass effect. 1 uF will have impedance of 3180 Ohm at 50 Hz. That will make a divider with input impedance of meter (10MOhm + 600 pF for mV range, that is 3,466 MOhm combined at 50 Hz.). That makes it 1/1000 of influence, 0.1 %. That is one and half order of magnitude less then meter specs in that range. So simply ignore it. At 100KHz, parasitics will be negligible, so high end won't have any influence.

Draw a circuit in Ltspice and play with it. That is a good way to a get feeling for how it works.

[Neutrion]

Well yes dual display is alway better so I was also thinking a lot about the other Brymens. But than finally this became the choosen one. Was waiting on Joes channel for it to give me a final go, but he also not tested this thing.

Connecting through the capacitor is not a trick. This is what you do when you want to AC couple signal (strip it from DC component). DC component will be affected, of course, i.e. it won't be measured at all. .

Oh yes, I read parallel first, but that might wouldn't make any sense But thanks for the calculation, though this way I ended up having an ac only range.

But anyway, there are obviously ways around, but do you have exact the same issue with the 869s?
Also the second issue with the ac+dc mA measurement? Cause that one would be easier to solve in FW I suppose.

[bdunham7]

How do other meters behaves in this scenario?

OK, so I'm not clear on your exact signal.  You didn't specify the frequency, so I'm trying 100Hz and 1kHz.  As for the squarewave, am I correct in understanding that you have a 'DC' squarewave, IOW, the low is ~0V and the high is ~1V (or whatever).  So on a Fluke 115 116, with a 1-volt top squarewave I get OL on the 600mV AC range and ~500mV on DC, which seems correct.  With 500mV top voltage, I get ~350mV on AC and ~250mV on DC, which is also correct.  Reversing the polarity did not change the results. 

I can try other meters if you like.  If I misinterpreted your signal characteristics, let me know and I'll try whatever you actually used.

I didn't fully understand your second issue.  Is it just that the meter displays 'OL' for a time before settling on a correct reading, or is it something else?

[Neutrion]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

But thanks for the test! 

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

[bdunham7]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

The first does seem to be a bug to me.  I'm not sure what causes it internally, but it must have something to do with the way the chipset looks at AC.  External AC coupling may 'fix' it, but you shouldn't need to.  Many meters are 'DC-coupled' at the input stages even on AC ranges--the F115 116 I just used for example is ~10M on all ranges AC or DC, except 6V which is ~11M.

The second is just life with autoranging and doesn't seem to me to be an issue.

[2N3055]

Oh yes the frequency... so depending on the duty % up to about 500Hz (from 40) the problem is there, but above that it seems to disappear, and it overranges. And as I said, I could not sweep it through with higher voltages. And yes, all others as you just wrote.

Regarding the mA scale, yes, there is only the short "OL" which is annoying, because you would have to keep the probes on for 2-3 seconds longer to make sure it is settled.

The first does seem to be a bug to me.  I'm not sure what causes it internally, but it must have something to do with the way the chipset looks at AC.  External AC coupling may 'fix' it, but you shouldn't need to.  Many meters are 'DC-coupled' at the input stages even on AC ranges--the F115 I just used for example is ~10M on all ranges AC or DC, except 6V which is ~11M.

The second is just life with autoranging and doesn't seem to me to be an issue.

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

[bdunham7]

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

The OP may be able to clear this up, but I didn't see that it was specifically an autoranging issue, more that the meter didn't read OVERLOAD when it was clearly overloaded.

Now the DC vs AC input is an interesting characteristic. There is the external impedance and then there is how things are coupled and measured internally.  For the Fluke 116, for example, VAC and mVAC are both DC-coupled inputs, but internally it appears that VAC is AC-coupled whereas mVAC is not.  So given a 0-1V square wave, the 600mVAC range displays 'OL' but the VAC range correctly displays ~0.500V.  And given a 0-500mV square wave, the 600mVAC range displays ~350mV but the VAC range displays ~0.250VAC.  So mVAC is TRMS AC+DC.  And a closer look at the front panel reveals that I should have known that, but I didn't.   



Now the OPs meter gives the option of DC, AC and DC+AC.  It functions properly on DC and DC+AC, but fails to uprange or indicate overrange on the AC scale, and then only in one direction.  That seems wrong to me.  Thinking about it, perhaps there is some clamping bit that is breaking down too soon and actually keeping the sensed voltage from reaching the overrange threshold.  If some other EEVBlogger has a 786 or 789, perhaps they could help the OP out by running this test and seeing if it is a bug/feature or if the OPs unit has a defect.

[2N3055]

It's not a bug. If you have DC signal with AC superimposed on top, you might get to the point where signal is DC pulsating between two levels, one of which goes over ADC range. But still might be too fast switching for meter to up range... Autoranging is not magical.. There are scenarios where you can confuse it.

The OP may be able to clear this up, but I didn't see that it was specifically an autoranging issue, more that the meter didn't read OVERLOAD when it was clearly overloaded.

Now the DC vs AC input is an interesting characteristic. There is the external impedance and then there is how things are coupled and measured internally.  For the Fluke 116, for example, VAC and mVAC are both DC-coupled inputs, but internally it appears that VAC is AC-coupled whereas mVAC is not.  So given a 0-1V square wave, the 600mVAC range displays 'OL' but the VAC range correctly displays ~0.500V.  And given a 0-500mV square wave, the 600mVAC range displays ~350mV but the VAC range displays ~0.250VAC.  So mVAC is TRMS AC+DC.  And a closer look at the front panel reveals that I should have known that, but I didn't.   



Now the OPs meter gives the option of DC, AC and DC+AC.  It functions properly on DC and DC+AC, but fails to uprange or indicate overrange on the AC scale, and then only in one direction.  That seems wrong to me.  Thinking about it, perhaps there is some clamping bit that is breaking down too soon and actually keeping the sensed voltage from reaching the overrange threshold.  If some other EEVBlogger has a 786 or 789, perhaps they could help the OP out by running this test and seeing if it is a bug/feature or if the OPs unit has a defect.

OP is pushing signal that is 1 V P-P, with offset such that signal dips only 200mV below zero. He is doing it on a 600mV range.
That range CANNOT range up. It is mV position on the switch. So meter being DC coupled it is overloaded and shows, pretty much crap...
 "......something above 1 V p-p (so above the specs for the mV scale) it reads around
650mv on the AC scale, and it stays so even if I raise the voltage to around 30 V .(Can't get further) ....."

He pushed 30V in 600mV input, that is NOT AC coupled.  So ADC reads full scale and that's it.   As I said there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

[bdunham7]

That range CANNOT range up... there is NO ranging in that switch position. It is fixed 600mV position. I do agree that OVRLD indicator (together with backlight flashing) should be shown instead.

OK, then it definitely isn't an autoranging issue! 

Yes, the lack of an overrange indicator in one direction seems to me to indicate a specific defect.

OP:  Send your meter back and if you get a replacement, tell us what it does.

Edit:  After thinking further, I'm still not a fan of the meter not reading a 1V_p-p signal in mVAC mode because that does seem to be explicitly within range--0.5VAC.  A true AC mode would imply blocking a reasonable DC component at some point in the signal chain.  That's what my 189 that has all three modes does.

[Neutrion]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

And the question is, will it be possible at all to change this in FW.

By the way 2N3055 did you check the 869s whether it does the same?

[2N3055]

It doesn't behave the same but it is the same problem:

mV is same, only 600mV (500mV on BM689S) DC range.
For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload.
Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC.

If you have mV level AC signal, you need to decouple it from DC by using capacitor as I said. Then you disconnect capacitor and measure DC volts separately.

There is  this misunderstanding that meter can simultaneously measure 10 mV AC  and 40V DC , or vice versa, because it has dual display.
It cannot.  Meter has ONE input attenuator and one ADC.  If you set it for 100V it cannot be in position for 10mV. And vice versa.

Meter that could measure 1 mV AC riding on top of 300V DC would need 2 separate  front end paths, with 2 separate attenuators and front end amplifiers. ADC could be multiplexed, but it would be nice to have two of those too.

Then you could simultaneously measure both AC and DC, completely independently (and at different scales)  at the same time and calculate AC+DC RMS and have that too. You could also measure frequency and duty cycle from AC path with good sensitivity. Also you could optimize AC and DC paths, AC for frequency response, and DC for best AC rejection.

I personally don't know of any meters that do that. I wish it existed, I would gladly buy one.
People that  work on tube amplifiers would love it.

It is up to the operator to know what to do or not.

Good practice requires that you start with meter in higher ranges anyways. Start with measuring AC  and DC in volts range that will also autorange where it needs to be. If there is ANY mention of volts then and there, you CANNOT measure in mV.
If you do have 10mV AC  riding on top of 10V DC, you can use capacitor. That will work well.

If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times...
Funny enough, meter might  have more suppression than that at 50Hz.. But input might start smoking too...
It probably won't, because Brymens have fantastic protection circuits.

But don't you ever confuse what meter can SURVIVE with what is expected for meter to measure.
You can only expect it to measure what is inside range of peak voltage of that range.
Everything else is abuse and survival of user error..

[EEVblog]

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

[2N3055]

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

They still have same input attenuator..

BM869 has DC blocked AC path on AC volts..
But it is not blocked on mV range. Despite having two positions on switch, both AC mV and DC mV is DC coupled.
So injecting 10V DC into it will confuse meter, despite being in AC mV mode.
AC volt ranges are fine.
I don't understand why they didn't DC block it on a dedicate AC mV position either..

On BM78X series they simply dispensed with a confusion and have single mV switch position that is DC coupled.

[bdunham7]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

Yes, the Fluke 189 has AC, AC+DC and DC.  It's a little different because the AC range is a separate switch position.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

Well first of all, if it is overrange, it should say so, not just give you a wrong number.  Second, it happens in one direction, leading me to think there's a specific defect.  Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range.  So why does it read 650mV??

[2N3055]

But do you have a meter which also has the same AC,  AC+DC, DC range? Because AC+DC is OK and it overranges properly.

Yes, the Fluke 189 has AC, AC+DC and DC.  It's a little different because the AC range is a separate switch position.

I already contacted Brymen through the seller as mentioned, and their ansver was, that this is OK because the meter works well WITHIN the specs. So I suppose all the BM789 should work like this, and possibly the 786 as well. I hope some other guys with 789s will show up.

Well first of all, if it is overrange, it should say so, not just give you a wrong number.  Second, it happens in one direction, leading me to think there's a specific defect.  Third, what you are asking it to do is not unreasonable and the correct result if it is properly measuring AC of a 0-1V square wave is 500mV--which is within the range.  So why does it read 650mV??

He was measuring "more than 1V" with excursions below zero of 100-200 mV. That makes it 1.2-1.3V P-P and right at the saturation point.
It had to simply show "OVL" and that is it.

When I do the same to BM869S it will show "OVL". But because of DC path, I can inject DC into it and make it go crazy too, with specific values.
But it behaves much better.

[bdunham7]

For instance, if I feed 300mV RMS 400Hz signal with 700mV offset I get propper 300mV AC reading. If I go higher offset than that, it starts showing bad AC amplitude, less than it is. Until DC offset reaches 8V when it shows overload.
Any do you know why? Because you are (me in this case) severely overloading input amplifier with DC.

That seems to me to be a deficiency in design.  Or at least one not meeting normal expectations.

There is  this misunderstanding that meter can simultaneously measure 10 mV AC  and 40V DC , or vice versa, because it has dual display.
It cannot.  Meter has ONE input attenuator and one ADC.  If you set it for 100V it cannot be in position for 10mV. And vice versa.
I personally don't know of any meters that do that. I wish it existed, I would gladly buy one.
People that  work on tube amplifiers would love it.

The old Fluke 867 Graphing Multimeter could do this, but I can't think of any others.  Other than that, the answer to simultaneous DC and ripple measurement is two meters or a 6.5 digit DMM which can measure 10mV on the 100V scale.  Or you can just switch back and forth.

If you do have 10mV AC  riding on top of 10V DC, you can use capacitor. That will work well.

If you have 150V AC riding on top of 1 mV DC, you're out of luck. You are asking meter to suppress difference in amplitude of 150000 times...
Funny enough, meter might  have more suppression than that at 50Hz.. But input might start smoking too...

There's a difference between normal mode rejection of AC in the DC mode, which seems an unavoidable limitation to me, and blocking of DC in a true AC mode (not AC+DC) which to me seems a normal and expected function of a meter.  I can read the ripple (only) of a PSU by simply selecting mVAC and connecting the leads.  Of course, the F116 fails this test, but it also tells me that it isn't true AC on the front panel and it indicates overload.  I then have the option of moving the selector switch and using the 6V true AC mode, which fully blocks DC.  I would be interested to see what happens on the other ranges of the Brymen because they appear to have adopted the same strategy of using a single switch position for all 3 functions and then selecting the mode with the button.

[Caliaxy]

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

[2N3055]

The old Fluke 867 Graphing Multimeter could do this, but I can't think of any others.  Other than that, the answer to simultaneous DC and ripple measurement is two meters or a 6.5 digit DMM which can measure 10mV on the 100V scale.  Or you can just switch back and forth.

That is why I usually simply use BM869S in volt mode most of the time. It has enough resolution to show you milivolts even then..
And if I need more detail, I already know what signal looks like and know mV range will give me no problems.

[2N3055]

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

If it is DC coupled on mV range, I bet you it can be made to show crap..
Unless they, for some reason, kept 10 MOhm termination and then AC coupled after that. Which would be good design. I wish Brymen did it that way.

Longer I have MTX3293, better i like it . It has clear AC only (capacitor coupled), AC+DC, DC (both DC coupled, obviously) and Low Z modes right there and single inputs 100-mV-1000V with distinction that 100mV won't autorange.
But BM869S sits right next to it. Fantastic meter, but nothing is perfect.

I have few complaints about MT3293 too:
- Small fonts on big screen
- despite having all the hardware it cannot display AC, AC+DC, DC triple display.

Nothing is perfect.

[bdunham7]

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

If it is DC coupled on mV range, I bet you it can be made to show crap..
Unless they, for some reason, kept 10 MOhm termination and then AC coupled after that. Which would be good design. I wish Brymen did it that way.

I don't have the 87V, but my other models actually do keep the 10M (or a little higher on the low ranges--it's not completely constant) resistance in AC mode.  This is actually an important feature if you use certain HV probes because otherwise if you were measuring HV AC+DC you might accidentally end up with the entire HVDC component across the blocking capacitor.

it cannot display AC, AC+DC, DC triple display.

My F289 also declines to give me all 3 at once and I think it is because they actually don't have the hardware.  I haven't thought about it too hard though....

[Neutrion]

He was measuring "more than 1V" with excursions below zero of 100-200 mV. That makes it 1.2-1.3V P-P and right at the saturation point.
It had to simply show "OVL" and that is it.

When I do the same to BM869S it will show "OVL". But because of DC path, I can inject DC into it and make it go crazy too, with specific values.
But it behaves much better.

Yes that is correct up to the limit it was measuring OK the 650mV came somwhere above the limit.

But does the 869 correctly shows "OL" for AC in this scenario?  Did you try to switch polarity?

With the 869  it makes sense to have everything on the same switch position, but with the single display if there is no way to show overrange they certainly could put in a separate AC mV position.

And the question is, will it be possible at all to change this in FW.

If it's related to the lack of a DC blocking cap in AC mV mode then there is likely nothing you can in firmware.
Dual display meters have a physical DC measurment path a separate AC coupled path, and they get measured separately.

But is also impossible to show somehow that it overranged?

Why is that?

Well, if this is not a bug and it works as intended, it's a design fail.

I replicated the issue on all the Brymen meters within my reach (235, 789 and 689S): AC mV shows nonsensical values in the presence of a high DC offset. The DC offset doesn't have to be huge. I tried 500mV AC on top of a 2V DC offset - something that one would likely encounter in normal everyday life.

Other cheap(er) meters (Fluke 101 and 107) do the same (show nonsensical values rather than OL). Daves GW121 does it too.

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

I guess it's one of those situations of "know your meter's shortcomings" and "use it accordingly". Check for DC bias before measuring AC mV. No meter is perfect.

Thanks for letting us know!

But if the mentioned meters can also display all the different things on the mV scale just like the Brymen, than it should not be fundamentally impossible to show the fact that it overranged. What causes it with the Brymen? The chipset? The input circuit? And as I said it depends on thefrequency.

[Caliaxy]

Other even cheaper ones (UT81E and Aneng 8009) don't, they correctly display OL (kudos to them). Fluke 189 (in AC mV) is not tricked by a high DC offset either. Fluke 87V doesn't have this issue at all because it doesn't have a separate AC mV position on the dial in the first place 

Are you sure about F87V ? Connect other meter in ohms mode and measure input resistance of F87V. Go through modes and ranges. Not only input resistance won't be constant 10 MOhm, but when it goes Hi impedance then it is AC coupled.

I only meant that Fluke 87V doesn't have an "AC mV" mode on the dial at all, only a "DC mV" one (which has a single 600 mV range). It does measures AC mV in "AC V" mode, which goes down to 600 mV range (and all AC ranges are AC coupled). The lowest DC V range in "DC V" mode is 6 V, hence the separate "DC mV" 600 mV DC mode.

Not sure why they did that - maybe it was their way of dodging the bullet. They must have figured the problem and this is how they avoided it. They don't call themselves Fluke for no reason