Brymen multimeters - fault?

[CustomEngineerer]

Just to add some more data points. I have a Greenlee DM860A, which is a Brymen BM869S, and feeding it a signal from Siglent SDG2042X. I also get accurate results in both windows and get OL for each when the voltage gets too high. On all waveforms frequency is 60Hz.

Sorry for the large, craptacular pictures

Amplitude 500 mVpp (which would be about 176.5 mVrms)
Offset 500 mVDC


Amplitude 1000 mVpp (which would be about 353.5 mVrms) - Even here theres no OL on the AC portion because the rms value is still no where near 500
Offset 500 mVDC


Now switching to having the Sig Gen output mVrms values for the amplitude instead of mVpp.

Amplitude 550 mVrms
Offset 500 mVDC
As I bumped up the amplitude, once I hit 550 mVrms the top AC measurement showed OL


Amplitude 500 mVrms
Offset 580 mVDC
I brought the amplitude back down, then slowly increased the DC offset until the DC measurement showed OL at around 580 mVDC

[evava]

Gentlemen, thank you all for your efforts!
But, see please in original post www.eevblog.com/forum/testgear/a-look-at-the-uni-t-ut210e/msg1058296/#msg1058296 on picture headed with:

@520 mV we lose both the UT210E and the Brymen BM869s.
The UT210E behaves here as a 4000 count meter!
The Brymen BM869s stays always below 440mV (50.000 count?) and even worse it does not display any warning!

and you will see, that user hgg has had dial on mV~, not as you on mV= !

On this range mV~ he demonstrates the issue!
(false value is displayed on primary display)

[IanB]

I checked the AC voltage and it seems to be correct. Only the DC voltage seems to have the error, and only when the AC voltage is close to or over 500 mV. In the pictures below the AC voltage is below 500 mV and therefore not over range.

[CustomEngineerer]

I switched it to the other mv AC position and did 2 more measurements. First at 530mVrms, which was accurately measured. Then I bumped it up to 540mVrms and the meter showed OL for the AC measurement. The secondary window in both pictures contains the correct frequency.

[joeqsmith]

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?

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.

Yes, I can replicate this no problem and get the same results.  Continue to push the offset will result in both AC and DC being wrong in both modes.  Again, not a problem for me as I would be looking at the AC DC volt range on it before down switching to at least do a sanity check but there is no reason the meter could not display it was out of range for these cases.

[evava]

Thank you for your honesty!

But, why the user hgg has had displayed 0,000Hz then on secondary display? Another fault when overloaded?

[IanB]

Gentlemen, thank you all for your efforts!
But, see please in original post www.eevblog.com/forum/testgear/a-look-at-the-uni-t-ut210e/msg1058296/#msg1058296 on picture headed with:

@520 mV we lose both the UT210E and the Brymen BM869s.
The UT210E behaves here as a 4000 count meter!
The Brymen BM869s stays always below 440mV (50.000 count?) and even worse it does not display any warning!

and you will see, that user hgg has had dial on mV~, not as you on mV= !

On this range mV~ he demonstrates the issue!
(false value is displayed on primary display)

I have tested the AC mV range and I cannot reproduce the problem noted there. Either the meter reads accurately or it reads OL.

[IanB]

Yes, I can replicate this no problem and get the same results.  Continue to push the offset will result in both AC and DC being wrong in both modes.  Again, not a problem for me as I would be looking at the AC DC volt range on it before down switching to at least do a sanity check but there is no reason the meter could not display it was out of range for these cases.

I think that in my post #28 above the meter is not technically over range. It is odd that the meter reads incorrectly in the mV range, but it does seem to read correctly in the V range. Is it a hardware problem or a firmware problem, I wonder?

[joeqsmith]

May need to go through the zero cross to get the frequency.  Fairly common.  Meters never seem to make good counters.  Switch it to mV DC then enable the counter would have read correct even if it was over driven. 

Different attenuators for the modes.  Makes sense the mV would be wrong and V would be right.  Easier to ask Brymen than try and sort it out.  I'm just to lazy. 

[IanB]

Different attenuators for the modes.  Makes sense the mV would be wrong and V would be right.  Easier to ask Brymen than try and sort it out.  I'm just to lazy.

If you start with a DC voltage of 450 mV and then slowly wind up the AC voltage towards 500 mV, the DC reading remains accurate all the way up to about 490 mV or so, then suddenly drops with a fairly sharp transition. It makes me think of a calculation bug in the firmware.

You are right, we have to ask Brymen to give their analysis of this problem. However, until this evening I did not know there was something to ask about.

[omgfire]

I switched it to the other mv AC position and did 2 more measurements. First at 530mVrms, which was accurately measured. Then I bumped it up to 540mVrms and the meter showed OL for the AC measurement. The secondary window in both pictures contains the correct frequency.

Sorry, I forgot to mention that the 60Hz AC waveform is above the 0V line and the lowest troughs touch
the 0V line.    Its a positive AC waveform. 

[evava]

And what about Daves BM235?
(if someone dares?)   

@722mV we also lose the Brymen BM235 as its a 6000 count meter, but again no warning
of the error!

Same firmware bug?

[IanB]

Thank you for your honesty!

But, why the user hgg has had displayed 0,000Hz then on secondary display? Another fault when overloaded?

I cannot reproduce the problem reported by hgg. Here is the meter displaying 504 mV AC (and the correct frequency) when superimposed on 380 mV DC. This keeps the entire cycle positive with no zero crossing.

[Wytnucls]

I cannot reproduce the problem reported by hgg. Here is the meter displaying 504 mV AC (and the correct frequency) when superimposed on 380 mV DC. This keeps the entire cycle positive with no zero crossing.

Vpeak = VRMS × SQRT2 = 504mV x SQRT2 = 712mV
-712mV+380mV= -332mV

So, there is zero crossing with your figures.

[evava]

We now know that we have to have displayed 0,000Hz on secondary display. Then we are sure that there is not any zero crossing.

[joeqsmith]

Put the meter into DCmV and run it up with a DC only source.  You will find it reaches the limit at +/-530mVish.  Switch it to ACmv and put in a sinewave with no bias and you will find it reaches the limit at 1.5Vp-p or again 530mV RMS.   I'm sure there is some headroom in where they set the over range compared with the actual limit of the hardware.  If you put the meter into DCmv and show both the AC and DC with 520mVACRMS and raise the bias the meter even to 400mV the DC will already have a small error.  Continue to raise the bias to 500mV and both the AC and DC will be off.   Continue to raise the bias even higher (about 583mV) and the meter will read roughly 529mVDC.  Once you hit that magic 530, it flags the out of range (of course none of the readings the meter is putting out are right).

I don't know what it would take to add additional protection to flag it. I guess contact Brymen and get their thoughts. 

Not sure their threshold for the counter and if they have hysteresis or not.  Doubt it is a perfect zero volts.  You can bias it to where the meter will not read it in both AC and DCmV settings.  Switch to volts and it should pick it up.

[IanB]

Vpeak = VRMS × SQRT2 = 504mV x SQRT2 = 712mV
-712mV+380mV= -332mV

So, there is zero crossing with your figures.

Oops. I was thinking of the half wave voltage 

[IanB]

We now know that we have to have displayed 0,000Hz on secondary display. Then we are sure that there is not any zero crossing.

Not quite so, it can display a low AC voltage and 0.000 Hz even when there is a zero crossing, if the DC bias is large enough.

[joeqsmith]

IanB is awake... 

I don't think it's a question of dare to run Dave's BM235.  Just didn't know you were interested.

With the BM235, again set to DCmV setting with a DC voltage looks like it over ranges at +/-660mV.  When set to ACmV, it over ranges at 1.875Vp-p (again, this is a 60Hz sinewave with no bias).  Of course, this is 660mV RMS.  No surprise.

Applying 659mV RMS with 450mV DC bias, both the DCmV and ACmV appear correct.
Increasing the bias to 460mV, the DCmV appears correct and the ACmV is out of range.
Now we push it to 656mV of bias and again the DCmV is correct, however the ACmV is no longer showing out of range and the value is incorrect.

So, yes it appears to have similar problems.  Again, I'm not too surprised as it sounded like someone else had already done the test. 

[joeqsmith]

Pulling out everyone's favorite meter, the UT61E.  Keep in mind that mine is now slightly modified.   

DCmV  setting seems to over range around +/-225mV DC
ACmV will over range around 639mVp-p or again 225mV RMS 

With 100mV DC bias and 638mVp-p 60Hz sinewave, both the AC and DCmv readings are correct.
Increasing the bias to 200mV, both readings are still correct.
Increasing the bias to 226mV causes the DCmV to over range but the AC is still correct. 
Now as I continue to push the bias higher, the ACmV appears correct. 
Next I apply 200mV DC if bias and increase the amplitude of the AC signal well outside the range.  We start to see the effects on the DC reading.  Again, they could have a catch that if the AC component was out of range that the DC would flag an error that it was not correct. 
Of course driving the meter with 200mVp-p and a 300mV bias, no frequency counter in any of the modes. 

Guessing other meters will have various problems if I took the time to look at them.  For me, not a big deal. 

[joeqsmith]

Pulled out my favorite meter of all time, the lime green TIP 194 II.  Now keep in mind this meter is currently damaged and has high leakage on one of the controllers port pins that effects the resistance measurements. 

In DCmV the meter over ranges at +/-500mV. 
In ACmV the meter over ranges at 1.417Vp-p or again 500mV RMS (60Hz sinewave)

With 490mVRMS biased with 400mV DC, both the AC and DC  millivolts readings are correct. 
Raising the DC bias to 499.9mV with 490mV RMS applied, the meter provides the correct readings for both.
Next I apply 600mV of bias to our 490mV RMS signal and of course the DCmV reading is now out of range.  But the ACmV reading is now effected again with no indication that there is a problem.   With 1VDC bias the ACmV is all the way down to 374mV.

The TPI 194II does support the AC+DC and is a tri-display.  But sadly this is not supported in mV ranges.  It's too bad I could not get a controller to replace the one on the meter.  I would have then attempted to improve it like I show with that low end UT61E. 

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Looking at the Extech EX540
In DCmV the meter over ranges at +/-412mV. 
In ACmV the meter over ranges at 1.168Vp-p or again 412mV RMS (60Hz sinewave)

With 409mVRMS biased with 400mV DC, both the AC and DC millivolts readings are correct. 
Raising the DC bias to 500mV, the meter over ranges on the DCmV but there is a slight effect on the ACmV.
With 1VDC bias the ACmV is down to 399mV which is better than the TPI 194 II but it's an error and there again is no indication that there is a problem. 

This meter also supports AC+DC calculations and appears to show an over range for any combination that goes above 412mV. 

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And I'm sure everyone wants to know the UNI-Ts top of the line UT181A does.  Again this meter was damaged like the TPI with one hit from the grill starter.  I was able to repair it and made some pretty drastic mods to the PCB.  This meter is not original but I do not believe I changed anything that would effect this test. 

In DCmV the meter over ranges at +/-612mV. 
In ACmV the meter over ranges at 1.731Vp-p or again 4612mV RMS (60Hz sinewave)

With 610mVRMS biased with 600mV DC, both the AC and DC millivolts readings are correct.
Raising the DC bias to 700mV, the meter over ranges on the DCmV but the ACmV is correct. 
With 1VDC bias the ACmV is down to 587mV and again there is no indication that there is a problem. 

This meter will calculate AC+DC and has a tri-display.  In this mode with the same 1VDC bias, the AC+DC and mVDC are both over ranged but the mVAC shows the 587mVAC which is of course not correct.  But if the OL were not enough to sway the user into thinking there may be a problem with their measurement, they go one step further and enable the yellow triangle with the lightning bolt.   Still, really it's not full proof and the ACmV should show an error as well or you just know someone is going to trust that number...

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Scott's going to ask about the Fluke 97.  That's a special snowflake as it auto ranges both the AC and DC values.  You exceed the mV range, it will switch.


I don't disagree with the OP that there is a problem with some meters showing inaccurate results under certain conditions without any warning to the user.  Hopefully running these few meters shows that this is not something unique to one brand. 

[IanB]

I don't disagree with the OP that there is a problem with some meters showing inaccurate results under certain conditions without any warning to the user.  Hopefully running these few meters shows that this is not something unique to one brand.

Since you have broadened the field to several other meters, this begs the question: what would a bench meter do? Does a typical bench meter have a fundamentally different internal architecture that avoids such limitations, or would it fall prey to similar issues?

[joeqsmith]

I don't disagree with the OP that there is a problem with some meters showing inaccurate results under certain conditions without any warning to the user.  Hopefully running these few meters shows that this is not something unique to one brand.

Since you have broadened the field to several other meters, this begs the question: what would a bench meter do? Does a typical bench meter have a fundamentally different internal architecture that avoids such limitations, or would it fall prey to similar issues?

My bench meters are both old from the HP era.  These are auto ranging and there is not a separate mV mode.  So like my Fluke 97, they will switch. 

But we can have a little more fun with Fluke.  Hate to let them off the hook.  The 97 is a little special but let's try 5KY's Fluke 107.   This meter has been abused beyond the levels of any meter I have played with.  Eventually I damaged it and was able to clean up the mess and replace the failed components but the circuit board is no longer perfect.   The meter works fine but I doubt it would handle another 15,000 volt hit.    

This meter has a mV AC mode.   Selecting it and applying our 60Hz sinewave with no bias, it over ranges at about 1.707Vp-p or 600mV RMS. 
With about 1.557Vp-p or 550mV RMS applied, the meter read 550mV as we would expect.  Adding 1VDC of bias, the meter now read 462mV. Again, wrong and there is no indication that it's wrong.  Fluke you failed me once again!      Really you want a small meter that's robust this thing is going to be hard to beat!

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To be fair to Fluke, that 107 is for the Asian market.  But I do have a Fluke 115. Good all around meter, nothing special.  This meter was never damaged and is unmodified.  It has both DC and AC mV ranges. 

In the ACmV range it can handle around 1.866mVp-p 60Hz sinewave or about 660mV RMS.
In the DCmV range it can handle again about 660mV DC.

With a 650mV AC RMS signal applied with no bias it of course reads 650mV.  Now I add 100mV of DC bias.   In DVmV the meter reads 100mV as it should.  It's no where near out of range.  Selecting the ACmV it reads 657mV.  Oops I guess this is wrong!   Now if I take the DC bias up higher the ACmV will over range at some point but there is this area where it is giving wrong reading without any indication. 

[joeqsmith]

Too bad the AM510 and AM530 do not have a separate mV function.  Seems Danaher's other divisions should be in on this.  Maybe AMPROBE did this for a reason?!    

But I do have a Fluke 17B+ which also has both AC and DC mV ranges.   Again, this meter was damaged and I was able to repair it.  There was no damage to the PCB and doubt this test will be effected by anything I have done with it.  Overall a decent meter. If it only had TRMS but we are playing with nice clean waveforms now so no big deal.  So the first average meter is up to bat...

In DCmV the meter over ranges at +/-440mV. 
In ACmV the meter over ranges at 1.242Vp-p or again 440mV RMS (60Hz sinewave)

With 430mVRMS biased with 400mV DC, both the AC and DC millivolts readings are correct. 
Raising the DC bias to 500mV, the meter over ranges on the DCmV but there is no effect on the ACmV.
With 600mV of bias the meter not display over range in the ACmV.  The upper left lightning bolt in the LCD is active along with the red backlit triangle with the lightning bolt. 

Now lets raise the bias even higher.  At 900mVDC (again, still our 430mVRMS 60Hz sinewave applied) the both lightning bolts turn back off and all is fine, except the meter is now reading 421mVAC.  With the bias increased even further to 1.5VDC, the meter displays 185mVAC!!  Shame on you Fluke not telling me something is wrong!!

[joeqsmith]

Got to thinking about it and of course we can force the manual range on the AMPROBE.  So I dusted off 5KY's AMPROBE AM530.  Again, damaged during my testing but repaired.  It's in good shape and don't see a problem with it running this test. 

So, VAC and manual range to the xxx.x mV scale.  Looks like it over ranges at 404mV AC RMS. 

I apply 380mV AC RMS at 60Hz and add about 1.9VDC of bias.  The meter now reads 231mVAC.  Seems like an error.   I deselect the manual range and it shifts up a range and displays the correct value.  I select DC and it displays the correct value.  So if the autoscale mode was smart enough to know to shift up a range, why isn't it smart enough to tell me I am over range in manual mode?  Another BIG FAIL!