Brymen multimeters - fault?

[Dwaine]

 My Agilent U1272a seems off with its values. 

Generator
Frequency: 60hz
Amplitude: 330.0mVrms
Offset DC: 315.0mVdc

U1272a
325.40mv AC
320.50mv DC

Also, maybe someone can share the secret to posting photos.

[evava]

I thought you just wanted to know if there was any modes where a meter could give inaccurate data without some sort of warning to the user. 

Yes, I am happy that you took it in that way. Thank you.

[cs.dk]

Also, maybe someone can share the secret to posting photos.

[Lightages]

Here is the response from Brymen:

Thank you for sending us the problem feedback. It is not until the recent years that we realized such a phenomenon.
 
It was caused by signal levels that were beyond the working dynamic range of our A-D converter, and thus the ADC was saturated. We might be able to make it better but we just followed the same old rules on our mV functions for some reasons. Our apology. Most DMMs having a combined AC and DC voltage function path exhibit such bad characteristic to some extents. Using the autoranging ACV and DCV functions to check the unknown signal before using the mV functions is not a bad idea in such a case. We would recommend to always set the instrument to the highest range and work downward for an unknown value.

I can't say that I am happy with this bug, but nothing is perfect I guess. Seeing as it is not the only one to have this problem it seems that it is not easy to get right. It is always a good idea to know the limitations of your tools. It would be nice if these limitations were made known up front.

[joeqsmith]

Here is the response from Brymen:

Thank you for sending us the problem feedback. It is not until the recent years that we realized such a phenomenon.
 
It was caused by signal levels that were beyond the working dynamic range of our A-D converter, and thus the ADC was saturated. We might be able to make it better but we just followed the same old rules on our mV functions for some reasons. Our apology. Most DMMs having a combined AC and DC voltage function path exhibit such bad characteristic to some extents. Using the autoranging ACV and DCV functions to check the unknown signal before using the mV functions is not a bad idea in such a case. We would recommend to always set the instrument to the highest range and work downward for an unknown value.

I can't say that I am happy with this bug, but nothing is perfect I guess. Seeing as it is not the only one to have this problem it seems that it is not easy to get right. It is always a good idea to know the limitations of your tools. It would be nice if these limitations were made known up front.

Thanks for taking the time to check into this.  Can't say I was expecting much different. 

I was talking with a friend of mine today and I brought this subject up.  Of course the question of starting high and working down came up but I explained that new players may not understand this.   

After my data dump, I found out he has a Fluke 87 IV and he will run it for us.    The best part is this led to a story about how he decides to buy a brand new meter.  He has owned the 87 IV for at least 20 years and really likes it.  He calls Fluke and they no longer sell it so he decides to get an 87 V.  Assuming they are both an 87 so they must be the same thing but the V being newer than IV is all.  I then get an ear full of how bad the 87V is (in a little more colorful language) because they don't have all the features he uses with his 87 IV.  Why would Fluke call it an 87?  Why not a whole new number? After he calms down, he does say the 87V was alright for a basic meter but that he ended up getting a couple of Extechs instead that had the features he uses.   Funny stuff.

Maybe in a day or two we can add an older Fluke to the mix.   

[Wytnucls]

What data for the Fluke 185 did you not understand?

Fluke 185 50,000 count
500mV manual range
test waveform: Sine 480mVAC 9VDC offset 60Hz

Selection:
AC: 487mV 60Hz
DC: OL
AC DC:  OL   - mVAC
AC+DC: OL   - Hz (60Hz with < 1VDC offset)

Auto range mode has no discrepancies:
AC: 0.489mV 60Hz
DC:  09.07V
AC DC: 09.04V  00.49V
AC+DC: 09.06V 00.00Hz ( 60Hz <4VDC offset)

I was not sure based on your data that you looked at the DCmV and increased the ACV.  You need to check both cases.

Here are the results for a sine wave of 600mVAC with 9V offset on the 500mV manual range of the Fluke 185:

Selection:
AC        OL     60Hz
DC        OL
AC DC    - mVdc     -  mVac
AC+DC  OL      00.00Hz ( 60.00Hz with <1V offset)

No false readings with DC and AC overload on 500mV manual range.

With auto ranging, everything is fine:
AC        0.611V     60.00Hz
DC        09.07V
AC DC   09.05V    00.61V
AC+DC  09.06V    00.00Hz (60.00Hz with <4V offset)

The meter was conceived by Tektronix. It sure looks like they knew what they were doing!

https://www.eevblog.com/forum/testgear/fluke-185-review/

[joeqsmith]

What data for the Fluke 185 did you not understand?

Fluke 185 50,000 count
500mV manual range
test waveform: Sine 480mVAC 9VDC offset 60Hz

Selection:
AC: 487mV 60Hz
DC: OL
AC DC:  OL   - mVAC
AC+DC: OL   - Hz (60Hz with < 1VDC offset)

Auto range mode has no discrepancies:
AC: 0.489mV 60Hz
DC:  09.07V
AC DC: 09.04V  00.49V
AC+DC: 09.06V 00.00Hz ( 60Hz <4VDC offset)

I was not sure based on your data that you looked at the DCmV and increased the ACV.  You need to check both cases.

Here are the results for a sine wave of 600mVAC with 9V offset on the 500mV manual range of the Fluke 185:

Selection:
AC        OL     60Hz
DC        OL
AC DC    - mVdc     -  mVac
AC+DC  OL      00.00Hz ( 60.00Hz with <1V offset)

No false readings with DC and AC overload on 500mV manual range.

With auto ranging, everything is fine:
AC        0.611V     60.00Hz
DC        09.07V
AC DC   09.05V    00.61V
AC+DC  09.06V    00.00Hz (60.00Hz with <4V offset)

The meter was conceived by Tektronix. It sure looks like they knew what they were doing!

https://www.eevblog.com/forum/testgear/fluke-185-review/

Right, you have ran the low AC with high DC again.  Select the DCmV range, apply a bias say 50mV lower than where it over ranges.  Now increase the AC level and see of the DC value changes.  IMO, it's not a big deal if it does have a problem.  Just would be good to know is all.   

[Wytnucls]

Right, more testing:

DC bias 460mV

DCmV manual range:

ACmV     DCmV
100        463.9
200        463.9
300        464.1
400        447.0
500        446.0
600        446.3
1000      447.0
2000      OL

[joeqsmith]

Right, more testing:

DC bias 460mV

DCmV manual range:

ACmV     DCmV
100        463.9
200        463.9
300        464.1
400        447.0
500        446.0
600        446.3
1000      447.0
2000      OL

Thanks.  Really, not too bad compared with some of the ones I looked at.  I was hunting for the sweet spot when I ran them. I assume your 447 was the worst or that's the limit of the AC sources resolution.     

[Wytnucls]

It is the worst reading. It happens from 390mV up to 1.3V. Above that last AC voltage, the overload (OL) is displayed.
Function Gen limit is 6VAC RMS.

[Wytnucls]

The 30,000 count Gossen MetraHit 26S behaves more or less in the same way:
With a 280mV offset:
On mV DC manual scale, it reads 280mV until AC RMS is raised to 510mV, displaying then 265mV. Above 1.3V AC, it reads a correct 280mV until 1.4V AC, when OL is shown for any higher AC voltage.

[Wytnucls]

Checked two bench meters (Keithley 2000 and Rigol DM3058E) with the same kind of signals.
Again, no dedicated mV range, but tested with manual range selected.
A rising DC offset (<9V) with a fixed sine wave voltage elicits a small AC error for both (-3%).
An increasing AC sine wave with a fixed DC offset value below the maximum of the mV range causes large increasing DC errors on the Rigol, while the Keithley just goes into overload.

[joeqsmith]

Good work.  Not sure if you are trying to see if they ever under range once they over range.  If you look at the Fluke 17B+ for example, it detects the over range but as I continue to drive it harder the over range warnings turns back off and it starts displaying data again.  Dave's BM235 did the same thing.  You can see these on page 2.   

I never put more than 10V into the meters and never tried to over drive volts ranges except with the UT90A where it does not have a mV range.

[joeqsmith]

My friend with the 87IV sent me his findings.

VACrms   Voffset   AC Reading Vrms   Reading DC (V)      Notes
0.1   0   0.09997   NA      AC mV Scale
0.1   1   0.09999   NA      AC mV Scale
0.1   2   0.09997   NA      AC mV Scale
0.1   3   0.09996   NA      AC mV Scale
0.1   4   0.09998   NA      AC mV Scale
0.1   5   0.09997   NA      AC mV Scale
0.1   6   0.09998   NA      AC mV Scale
0.1   7   0.09995   NA      AC mV Scale
0.1   8   0.09994   NA      AC mV Scale
0.1   9   0.09993   NA      AC mV Scale
0.1   9.859   0.09992   NA      AC mV Scale

0.1   0   0.09987   0      DC or Pulsed mV Scale  AC on Main, DC on 2nd
0.1   0.5   0.1   0.501      
0.1   1   0.1001   1.003      
0.1   1.09   0.1   1.094      
0.1   1.097   OL   OL      

0.1   0   0.0999   0.00003      DC or Pulsed mV Scale  DC on Main, AC on 2nd
0.1   0.5   0.0999   0.50085      
0.1   1   0.1   1.0031      
0.1   1.09   0.1   1.0934      
0.1   1.097   OL   OL   


Very impressive but he did not try reading the DCmV with higher AC levels.   I did get to hear heard again about how crappy the 87V was by comparison.     

[joeqsmith]

Joe, would you be so kind and try overloading this meter with AC volts on DC range, and with DC volts on AC range, like in this thread?
https://www.eevblog.com/forum/testgear/brymen-multimeters-fault/75/

And please, continuity is? (very slow, slow, fast, very fast ?)

Thank you.

I ran this test with the new CEM DT-9939.  In the mVDC mode, it over ranges at 411mV.   In the mVAC mode it ORs at 411.49mV.

I applied a 400mVDC bias on a 200mVp-p 60Hz sine and the meter reads 70.67mVAC.  In mVDC it reads between 397 and 400mV. 
I continued to increase the AC amplitude to 1Vp-p with the 400mVDC bias.  The meter reads 353.28mVAC which is correct.  In the mVDC mode it reads between 378mV and OR.   So there is some effect to the DC. 

I tried it with 400mVACrms applied and then pushed the DC bias to 0.993V.  The meter reads 411.93, then will OR.  So again the DC has some effect on the AC as well.  I am not sure why it is better than the EX450.     

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.