UNI-T UT61E does not show zero in AC Voltage range with shorted test leads

[Fungus]

I assume its 10 counts max as well based on that note.

I'm not sure if "will not affect stated" is the same as "max allowed", but that's just me.

[joeqsmith]

I'm not sure if "will not affect stated" is the same as "max allowed", but that's just me.

If it could handle a higher number of residual digits with the leads shorted with no affect, they would have stated that higher number but instead they placed it at 10.    If it made no difference, they wouldn't have made the point of adding the note to their spec sheet.   

Again, this is my interpretation of their document.   The OP could try and reach out to UNI-T.   They may be willing to expand on what is written.   Of course, you could do that as well.   

[maginnovision]

I'm not sure but I'm with fungus here. It's in the notes of specs it sounds like they're just saying that 10 digits doesn't affect anything. It's also not true rms under 10% of range so it may see some DC offset. Also doesn't specify under 45hz and you're looking at (ideally) 0. If it was 100 counts I'd see an issue but 25? I don't know. Ask uni-t maybe they'll tell you if that's ok or not.

[rsjsouza]

On my unmodified UT61E (non-GS version) it took 2 min to zero the meter in VAC and 2-1/2 min to zero it in mVAC.

Just for comparison, the U1273A settles in 46 counts in VAC (4.6mV) and 30 counts in mVAC (30ųV).

After 2 min the Brymen BM857 settles in 8 counts in VAC (800ųV) and 7 counts in mVAC (70ųV).

After 2 min the U1282A settles in 4 counts in VAC (400ųV) and 1 count in mVAC (1ųV).

After 2min the Fluke 87V zeros in VAC.

After 2min the Fluke 189 settles in 31 counts in VAC (3.1mV) and 12 counts in mVAC (12ųV)

All measurements made at 18°C.

[Shock]

That's so far 3 Uni-T 61E models showing the same settling to zero. I tested a good and flat battery on my Fluke 112 and found battery capacity does actually affect it's ability to zero properly.

Fluke 87V settles to zero
Fluke 112 settles to zero
Fluke 117 cheats and zeros automatically
Fluke 87 (old and crusty) got to about 9
and various Fluke 70 series (none of which are true rms) all zero

[Per Hansson]

It takes my TUV / GS rated UT61E  ca 2 minutes to settle in VAC mode to 2 counts with probes connected together, if I give it another minute it goes down to 1 count.
In mVAC it takes 1 minute to 5 counts and 1.5 minutes to 0 counts.
I have just yesterday modded it with a LT1790BCS6-1.25 voltage reference, but I do not think it makes any difference.
Also input protection transistors Q1 & Q8 are still missing, but likewise I do not think that makes any difference either for this test.

[Fungus]

On my unmodified UT61E (non-GS version) it took 2 min to zero the meter in VAC and 2-1/2 min to zero it in mVAC.
After 2 min the Brymen BM857...
After 2 min the U1282A...
After 2min the Fluke 87V...
After 2min the Fluke 189 settles in 31 counts

It takes my TUV / GS rated UT61E  ca 2 minutes to settle in VAC mode to 2 counts with probes connected together, if I give it another minute it goes down to 1 count.

The elephant in the room is that nobody waits two (or three) minutes for the reading to "settle" when measuring AC voltage.

In real life we just connect the probes and look at the number. 

Edit: Also ... this Uni-T is only specified at 0.8% accuracy so an extra digit or two down there is a red herring.

[rsjsouza]

Yes. The absolute zero is a non-issue for the most part of our measurements. We usually work in approximations.

[Fungus]

Yes. The absolute zero is a non-issue for the most part of our measurements. We usually work in approximations.

We also buy 5% resistors and use "5V" USB power supplies that measure 5.1V. 

[joeqsmith]

The elephant in the room is that nobody waits two (or three) minutes for the reading to "settle" when measuring AC voltage.

In real life we just connect the probes and look at the number. 

The note seems to be a specific test case to determine if the meter is working properly.   I'm not sure how long my 61E takes to settle within 10 counts but the datasheet makes no reference to time.

[Fungus]

The note seems to be a specific test case to determine if the meter is working properly.   I'm not sure how long my 61E takes to settle within 10 counts but the datasheet makes no reference to time.

That, too. The leads may be shorted but there's obviously a high impedance somewhere in the circuit if a capacitor is taking a couple of minutes to get close to zero. It may be that the capacitor will fully discharge if you left it overnight.

Is that impedance there during actual measurements?

I just did an informal test where I shorted my Brymen BM857s leads together and waited for the display to go to 0.0001. I connected it to an AC source and let it settle for a minute then repeated both steps.

The second time I connected it to the AC source it gave the same reading in less than five seconds. It seems the capacitor charges a lot faster when there's a real signal applied to it.

Another thing to test is touching the leads with your hands when it's in AC mode with nothing connected. I can make the readings go all over the place when I do it. The tiny capacitance between my hands is swamping any error due to residual electrons in the system.

[Shock]

When I tested on my Flukes it was a very logarithmic curve, so if that is the same behavior on the 61E if it doesn't move the last 20 or so counts in a few minutes (but others 61E do reach zero). The meter is probably out of cal, but as I mentioned I'd at test it fully first and put a new battery in, I wouldn't assume I'm correct, but evidence points that way.

In fact if it was me I would zero cal the AC if everything checks out. Mentioned on this website VR4 seems to be a zeroing adjustment specifically for AC. At your own risk with the proper tool! It's a lot easier when you have the gear to ensure you can recover from mistakes.

[joeqsmith]

We certainly can dream up all sorts of tests to run but in this particular case, the designers seem to have made it clear that we short the leads and make sure its 10 counts or less.   

I would imagine if we let it settle for 10 years once the batteries have long leaked out, so no digits are displayed that may constitute less than 10 counts.  I don't think that was the designer's intent.   I guess we could short the supplied leads together without twisting them and have them them routed to an AC source large enough to exceed the 10 counts as well but the may assume the user has enough common sense that it wouldn't be tested that way.   

Is that impedance there during actual measurements?

My guess is yes.  If you have been following the thread where we are looking to measure 100uV,  I was asked to try this with the Fluke 87V using the high res mode.   This clip gives you some idea how long the meter takes to settle.   I wouldn't use this meter for this measurement, not only because of of slow it is but it's just not very accurate.   Obviously at higher levels, the 87V is going to settle faster.   

....  zeroing adjustment specifically for AC. At your own risk with the proper tool! ....

 

[Fungus]

Is that impedance there during actual measurements?

My guess is yes. 

Empirically: My Brymen seems to go towards a stable reading of an AC voltage much faster than it goes towards zero with shorted leads.

(it takes about five minutes to reach a display of 1 count and another four to reach zero)

[Fungus]

In fact if it was me I would zero cal the AC if everything checks out. Mentioned on this website VR4 seems to be a zeroing adjustment specifically for AC. At your own risk with the proper tool! It's a lot easier when you have the gear to ensure you can recover from mistakes.

So it seems like it's just badly calib^W adjusted, not faulty. Maybe it's just an offset voltage on an op-amp that hasn't been trimmed properly or something like that.

Quote from that page: "i always knew the UNI-T stock calibration is rather sucky"

[CDaniel]

The true-rms chip is responsable ( usualy cheap AD737 which is the slowest ) ... if you read the theory . It has a settling time dependent on the component values used , so implementation varies . The averaging capacitor is electrolytic , so it takes time to discharge . Also it has a nulling pot , if not adjusted right this situation will happen , when a number of counts will remain as an offset on oll AC ranges

[Shock]

So it seems like it's just badly calib^W adjusted, not faulty. Maybe it's just an offset voltage on an op-amp that hasn't been trimmed properly or something like that.

VR4 looks like some kind of bias/offset for the True RMS DC converter. I would want to send it back or at least be compensated, a dollar a digit sounds fair enough doesn't it?

https://www.analog.com/media/en/technical-documentation/data-sheets/AD737.pdf

[Fungus]

VR4 looks like some kind of bias/offset for the True RMS DC converter. I would want to send it back or at least be compensated, a dollar a digit sounds fair enough doesn't it?

It definitely looks fixable, it probably has no effect on real-life readings, but yeah... it's not as it should be.

[joeqsmith]

In fact if it was me I would zero cal the AC if everything checks out. Mentioned on this website VR4 seems to be a zeroing adjustment specifically for AC. At your own risk with the proper tool! It's a lot easier when you have the gear to ensure you can recover from mistakes.

So it seems like it's just badly calib^W adjusted, not faulty. Maybe it's just an offset voltage on an op-amp that hasn't been trimmed properly or something like that.

Quote from that page: "i always knew the UNI-T stock calibration is rather sucky"

If I procure a brand new DMM and it is out of alignment or failed to pass all the test cases called out in the manual, by definition its faulty.  I would not expect to have to send a brand new meter in for alignment and calibration before putting it into service.   

We also buy 5% resistors and use "5V" USB power supplies that measure 5.1V.   

In this particular case, I would also be concerned about the general quality.  If one adjustment was off, what else it wrong.  If you really don't need the product to meet it's published specs, you may as well get a lower end meter.   That free HF one may be good enough.  I've seen them float behind a boat before.