Microamps and Millivolts Measurements on UT61E

[JenniferG]

I like my UT61E I just recently bought, but the Microamps reading mode -- 3rd from last on dial -- is way off compared to the same reading with Milliamps mode -- 2nd from last on dial.  The same goes for the Volts vs Millivolts measurements -- 1st and 2nd on the dial respectively.

Is this normal?   The volts, milliamps and amps modes all seem fine and are within spec compared to my GDM-8251A.

[Kleinstein]

There should be no significant difference between the ranges. However the internal resistance is usually different and this can change the actual voltage / current measurend.
So for the comparison one whould have the voltmeters in parallel and the amp-meters in series.

[Fungus]

Is this normal?

It can be.

Remember that what you're doing is inserting a small resistor in series with your circuit (the meter), and it's a different resistor on each range

[rsjsouza]

What Fungus described above is what is called the "burden voltage", a voltage drop caused by the shunt resistor used to perform the current measurement.

The UT61E has a somewhat high burden voltage when compared with many more advanced meters, but even expensive models suffer from that.
 
This is well covered in the uCurrent project description page from Dave:
https://www.eevblog.com/projects/ucurrent/

[bdunham7]

I like my UT61E I just recently bought, but the Microamps reading mode -- 3rd from last on dial -- is way off compared to the same reading with Milliamps mode -- 2nd from last on dial.  The same goes for the Volts vs Millivolts measurements -- 1st and 2nd on the dial respectively.

Is this normal?   The volts, milliamps and amps modes all seem fine and are within spec compared to my GDM-8251A.

Can you give specifics on how you measure this and what the numerical differences are?

For current, you should have the GDM-8215A and UT61E in series so you can see how much the current actually changes when you shift ranges on the UT61E.  Depending on your current source the current may change quite a bit due to the higher burden resistance of the microamp range.

For voltage, both meters should be in parallel.  I can't think of a good reason for the millivolt range to differ from volts, but we'd need to know your exact test setup and how much variation there is.  Also, AC or DC?

[JenniferG]

I like my UT61E I just recently bought, but the Microamps reading mode -- 3rd from last on dial -- is way off compared to the same reading with Milliamps mode -- 2nd from last on dial.  The same goes for the Volts vs Millivolts measurements -- 1st and 2nd on the dial respectively.

Is this normal?   The volts, milliamps and amps modes all seem fine and are within spec compared to my GDM-8251A.

Can you give specifics on how you measure this and what the numerical differences are?

For current, you should have the GDM-8215A and UT61E in series so you can see how much the current actually changes when you shift ranges on the UT61E.  Depending on your current source the current may change quite a bit due to the higher burden resistance of the microamp range.

For voltage, both meters should be in parallel.  I can't think of a good reason for the millivolt range to differ from volts, but we'd need to know your exact test setup and how much variation there is.  Also, AC or DC?

I just made a video and posted it up on youtube:



I have the UT61E set to Milliamps to start and it's between the Pin 46 output of Arduino Mini Mega and a current limiting resistor to the LED side of  a DIY vactrol -- so DC current.  I wanted to see how much current is drawn with this particular resistor.  At a PWM write setting of 128 (around 2.5v output), I am getting 294 microamps on the UT61E on Milliamps setting.  I then connect the leads over to my GDM-8251A and it reads 299 microamps  (there is a discrepency here and that's why I am looking forward to getting some resistance, amperage and voltage references to test my 8251A).  Anywyas but the real problem is when I switch the UT61E to microamps mode it show only 250 microamps vs 294, or about 85% of the actual reading according to the mA reading on both my UT61E and GDM-8251A.

[bdunham7]

I then connect the leads over to my GDM-8251A

There's the problem--you need to have both meters connected at the same time, in series for current measurements and parallel for voltages.   The meter is always part of the circuit--in this case it is a resistor.  So you are putting three different resistors in your circuit and getting three different current readings, which is exactly what you should expect.

[JenniferG]

I then connect the leads over to my GDM-8251A

There's the problem--you need to have both meters connected at the same time, in series for current measurements and parallel for voltages.   The meter is always part of the circuit--in this case it is a resistor.  So you are putting three different resistors in your circuit and getting three different current readings, which is exactly what you should expect.

Confused.  I have it wired in series.  And within the same meter it's off by 85%.  Forget the GDM-8251A. 

[coromonadalix]

loll   I would not trust the Uni-T meter  loll  not the same resolution and precision


Up your game a little,  with this kind of meter  you get some very basic resolution,  get an good 50k counts or 60k counts  instead

IE:  more specialized equipment if you play at low levels like this

[bdunham7]

Confused.  I have it wired in series.  And within the same meter it's off by 85%.  Forget the GDM-8251A.

You can't forget the GDM-8251A.  I meant both meters in series, and of course the pair in series with your circuit.  If you set it up that way, what you should see is both meters reading approximately the same current (within tolerances) and then the current changing on both meters when you switch the UNI-T to the microamps range.

[JenniferG]

Say I only had the UT61 -- I really wish I wouldn't of brought up the GDM-8251A now as it seems to be convoluting things.

So you wire the UT61E in series to find out the current.  You turn the dial between amps, ma, and ua.   For any mode you still have it wired the same.. in series.

What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

[rsjsouza]

Attached are the measurements of the three ranges of the UT61E using a separate meter. You can tell how different their shunt resistance values are.

On the circuit, the shunt resistor will be placed between the power supply and the circuit being tested. Depending on its value, a proportional voltage will be dropped by it (Ohm's law). Now, think the voltage that is powering your circuit is slightly lower as you progress from A --> mA --> µA ranges. That slight lower voltage will cause your circuit to draw a bit less current.


                       shunt resistor
                            on DMM
    |---------------------/\/\/\/---------------------|
    |                                                 |
Vsupply                                            Circuit
    |                                             under load
    |                                                 |
    |-------------------------------------------------|

[joeqsmith]

You're changing ranges, you're selecting 61E's different internal shunts.  Different shunts have a different resistance that you now have in series with your circuit.   Of course the current changes. 

I modified a UT61E to reduce the burden voltage in the uA range.  The video goes over how this circuit works if interested.

[colorado.rob]

What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

Burden voltage. EEVBlog #2. https://youtu.be/u2yRR4G3yTA?t=202
Look how young he is!

[bdunham7]

What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

If you include the GDM-8251A in the circuit, then you will actually see the current change on its display even though you only changed the UT61E dial and you'll realize that the current is actually changing and it isn't a reading error.  Try it!

Lets say you take your original circuit and in addition to everything else, you put a 5 ohm resistor in series.  You take a measurement, then you swap out the 5 ohm resistor for a 500 ohm.  You'd expect less current, right?  So when you turn that dial on your UT61E, that is exactly what you are doing--swapping out shunt resistors that are in series with your circuit.

[JenniferG]

So this is normal? I shouldn't return it? I have 12 days to return it.  The millivolts vs volts is off about the same as well.

I'll have to study burden voltage and shunts and all that in multimeters.. I'm a newb.  I just don't know if I got a lemon and only have aroudn 10 days to return it.  Seems like Uni-T would of picked a better value for shunt resistor which is mroe accurate.

[joeqsmith]

Yes, it's normal for the UT61E and others and basic circuits 101.

Seems like Uni-T would of picked a better value for shunt resistor which is mroe accurate.

Like anything, it's a tradeoff.  If you want a constant burden, use an external shunt and measure the voltage across it.

[rsjsouza]

As others have said and the references that were sent to you also make it clear, it is not a lemon but a normal behaviour that does not indicate any problems with it. Any meter that measures currents with a shunt resistor will have this behaviour.

[bdunham7]

So this is normal? I shouldn't return it? I have 12 days to return it.  The millivolts vs volts is off about the same as well.

Yes, the issue of shunts, resistance and the so called burden voltage (the voltage drop across the meter in the current ranges) is an issue with any meter and does not represent a defect.  You should do what I suggest with your other meter in series just to make sure that the entire difference is actually due to this and there isn't some other problem.  The readings will vary, but the two meters should agree (within tolerances) at all times if they are in series.

The millivolts vs volts would concern me a lot more as there isn't a similar, readily apparent reason for it if the difference is significant.  Can you elaborate on that? 

[JenniferG]

The millivolts vs volts would concern me a lot more as there isn't a similar, readily apparent reason for it if the difference is significant.  Can you elaborate on that?

Doing a PWM write of 7 (out of 255 values -- 8 bit) from pin 46 on Mini Mega 2560 to a resister/LED (same DIY vactrol as above for amperage test).  The red terminal is connected to the resistor leg and the other terminal to ground.   It reads 0.1418V -- as approximately predicted beforehand -- on volts setting and 67.2 mv on millivolts setting.

[joeqsmith]

As others have said and the references that were sent to you also make it clear, it is not a lemon but a normal behaviour that does not indicate any problems with it. Any meter that measures currents with a shunt resistor will have this behaviour.

Certainly meters that are switching in different shunts would,  which is most.   It's been a while since I looked but it seems that Gossen Ultra I have uses a single shunt.   Comes with a high price tag.  It's so worth it!!        

[bdunham7]

Doing a PWM write of 7 (out of 255 values -- 8 bit) from pin 46 on Mini Mega 2560 to a resister/LED (same DIY vactrol as above for amperage test).  The red terminal is connected to the resistor leg and the other terminal to ground.   It reads 0.1418V -- as approximately predicted beforehand -- on volts setting and 67.2 mv on millivolts setting.

OK, so that doesn't mean it's broken either.  What you have is not a steady voltage, but rather 5V about 2.7% of the time.  In the voltage range the meter can accommodate and integrate the 5V peaks properly, but in the mV range, those peaks are clipped by the input circuitry and the meter's ADC gets reduced peaks to do the averaging with and gives you a lower number.  That's not ideal behavior and it would be better if it simply displayed 'OL' or overload in that case, but it is a budget meter from Uni-T and in any case, all meters have their limitations.  Short duty cycles and high crest factors are not easy to deal with and even good meters have their limitations in that regard.

It's worth noting that for many purposes--such as figuring power dissipation--the integrated average DC current or voltage is actually not the reading you need.  TRMS AC+DC is what is needed for that.  IOW, 2 volts half the time is not always the same thing as 1 volt full time.

[Fungus]

What I'd really like to know is why I am getting 250ua for the microamps mode and 299ua for the milliamps mode.

Because when you turn the dial you change the resistance of the meter.

(and the meter is in series in your circuit)

[coromonadalix]

Once again at theses values  the uni-t meter will round them sometimes, IT IS not made for these low currents values

You are trying to do things with aproximatives equipment it seems ...   seeing you make some arduino or mcu projects contradicts the knowledge you need to have for precises  or  low values measurements

Sorry it seems harsh,  but  many members here gave you really solid explanations here and you need to understand them.

The equipment capacities too,  you are using cheap meters to do extensive work / measurements,  it kinda become stupid in a sense

threads like this will go for ever  loll

[JenniferG]

Once again at theses values  the uni-t meter will round them sometimes, IT IS not made for these low currents values

You are trying to do things with aproximatives equipment it seems ...   seeing you make some arduino or mcu projects contradicts the knowledge you need to have for precises  or  low values measurements

Sorry it seems harsh,  but  many members here gave you really solid explanations here and you need to understand them.

The equipment capacities too,  you are using cheap meters to do extensive work / measurements,  it kinda become stupid in a sense

threads like this will go for ever  loll

I agree I need to learn what has been said. I am very appreciative.  I do plan on re-reading this thread to learn from it.  A bit overwhelmed though as I am a newb to electronics.  I've journaled the link to this post already.    It's just one I have to come back to and re-read to try and understand it.  So I guess I'll keep this meter, again I only have 10 days left in return widnow.  That was my main concern right now.   I understand about half of what's being said here.

I agree I am stupid; or more ignorant rather -- I actually have a genius IQ.