Author Topic: just got my new UNI-T UT61E, and i think it has a problem  (Read 9538 times)

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Offline benoved

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just got my new UNI-T UT61E, and i think it has a problem
« on: June 17, 2013, 01:14:31 pm »
Hi
I just got my new UNI-T UT61E in the mail. when turning it on to measure volts, with no probe attached, it measures 0.07 volts and then going down after few seconds to 0.02 volts.
when i turn the dial to the mV , it is always OL - over limit !
when i connect the probe and short them together, it goes to 0 in both case, but when probes are disconnected it returns to the previous values above 0

on the resistance it measure around 0.43 ohm when the probes are shorted together !

is there a problem here or is it its normal behavior for this unit, or should i ask to replace it ?
« Last Edit: June 18, 2013, 06:26:06 am by benoved »
 

Offline grumpydoc

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #1 on: June 17, 2013, 01:23:11 pm »
Fairly normal.

The mV range has a very large input resistance - stray electrical fields can charge up the probe leads to a few hunderd mV very easily.

The 61E is also well know for having fairly high resistance probes - 0.43 ohm with the probes shorted is about right. I think I get 0.35 ohm with the probes cleaned but I don't tend to use the supplied probes for resistance measurements (in fact I don't tend to use the 61E for resistance per se - just continuity and diode check).
 

Offline mariush

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #2 on: June 17, 2013, 02:18:07 pm »
My probes (the UT61E came with) dirty hover between 0.19ohm and 0.33 ohm, with jumps to 1-2 ohm depending where the tips make contact.

After cleaning them right now by rubbing them in a ball of cotton dabbed in isopropyl alcohol, they show a consistent 0.13 ohm

Clean them, they're OK if you clean the tips constantly.

The mV is fine as grumpy said.
 

Offline benoved

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #3 on: June 17, 2013, 04:02:01 pm »
I agree with what you say about the resistance, but im still concern about the non zero voltage (i also noticed that its always measure negative votage in this scenario), and the OL in the milivolt section.

I dont see such behavior with my older uni-t multimeter UT33d. when probes disconnected it shows 0.

I added a video which might make it clear.



compare it to the video review i saw , start looking about 40 sec after the start of the video and there the multimeter act much nicer.

.

what do you think ?
and is there a test i can do to verify my multimeter has no real problem ?
« Last Edit: June 17, 2013, 04:06:15 pm by benoved »
 

alm

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #4 on: June 17, 2013, 04:27:09 pm »
Read the spec sheet for both meters, and compare the input impedance on the lowest ranges. You will see the reading drop to close to 0 V if you put a 10 MOhm resistor across the inputs.
 

Offline Monkeh

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #5 on: June 17, 2013, 04:27:36 pm »
Try testing a known voltage? ::)
 

Offline PedroDaGr8

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #6 on: June 17, 2013, 04:43:13 pm »
You are freaking out about nothing mate, I promise.

Remember on the mV setting the input impedence is around 10Mohm IIRC. This means that 0.2nA of current can induce 2mV of signal. So its very easy for stray radiation to cause this to occur. It's a non-issue.

As for the high probe resistance, this is also well documented. Clean the tips well, the mold release for the tips is very sticky and it causes issues.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin
 

Offline TJM

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #7 on: June 17, 2013, 04:49:16 pm »
Hello, I'm new here  ;D

Actually the manual says >3000M on the mV DC range for UT61E, the rest of the ranges are 10M. With that kind of input impedance it immediately picks up all sort of RF noise.

UT33D got ~10M on all ranges.
Open Source, Open Hardware radioactivity monitoring project http://radioactiveathome.org/
 

Offline PedroDaGr8

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #8 on: June 17, 2013, 06:12:50 pm »
Hello, I'm new here  ;D

Actually the manual says >3000M on the mV DC range for UT61E, the rest of the ranges are 10M. With that kind of input impedance it immediately picks up all sort of RF noise.

UT33D got ~10M on all ranges.

Thanks, I was too lazy to check so just made an assumption. :-D Yeah that means , basically three orders of magnitude higher so you are looking at femtoamp currents causing mV readings.
The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, "You know, I want to set those people over there on fire, but I'm just not close enough to get the job done." -George Carlin
 

Offline benoved

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #9 on: June 17, 2013, 07:44:02 pm »
Thank you all for all the information and the advice's.
i tried to put it in my microwave with the door almost close to see if it will help against rf, no big change.
i made few measurements of voltage and it seem to work ok, i don't have anything with better accuracy so i cannot check it for accuracy.
its interesting that after i did some measuements the voltage was reduced to below 0.01v.
after few hours i turned it on again, and now volatge is above 0.02v, but when trying the millivolt sections its not OL anymore, rather than 50mv....

it seem very random.
i did try to measure frequency too and it was accurate ...
 

Offline benoved

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #10 on: June 17, 2013, 09:45:43 pm »
OMG !, i just try to look at the frequency of this "phantom" volts and i see 50Hz stable. this is the AC 220v wall power in here. wow its so sensitive.
 

Offline JimmyMz

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #11 on: June 18, 2013, 01:43:45 am »
Yeah that means , basically three orders of magnitude higher so you are looking at femtoAmp currents, to cause a mV reading.
I just did the mathematics, 3 Gohms multiplied by 666 fA (R * I = V) results in an ~2mV reading.

The meter looks fine. I pulled mine out to compare with the video. I'd say that your measurement numbers would have been lower, if you would have had the use of both hands, and touched the probes together at the "notches." I suggest you use a real "fine" grit sandpaper (wet sandpaper, if you have any) on your probe tips to obtain a lower probe resistance value; it worked for me (and only took 5 minutes). I'd also like to point out, making a voltage adjustment on the 61E is as simple as turning the dial of a pot labeled (IIRC) V1 or V2, but you'd need a voltage standard to make a meaningful adjustment. By the way, it's only something to consider, and I'm not implying that your meter is out of specification.
« Last Edit: June 18, 2013, 01:59:06 am by JimmyMz »
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Offline 807

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #12 on: June 18, 2013, 03:14:26 pm »
...compare it to the video review i saw , start looking about 40 sec after the start of the video and there the multimeter act much nicer...
...but you were comparing your readings with one which didn't have the probes connected.
 

Offline benoved

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #13 on: June 18, 2013, 06:50:15 pm »
...compare it to the video review i saw , start looking about 40 sec after the start of the video and there the multimeter act much nicer...
...but you were comparing your readings with one which didn't have the probes connected.
i did test with and without the probes and there was no difference in the numbers i saw.
 

Offline benoved

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #14 on: June 18, 2013, 06:53:59 pm »
today i see different numbers for the voltage reading, its around -0.0030 (was around -0.0700 yesterday at the peak).

but the information you gave here make sense, i was convinced this multimeter works fine.

thank you all !
 

Online Rick Law

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Re: just got my new UNI-T UT61E, and i think it has a problem
« Reply #15 on: June 18, 2013, 11:36:59 pm »
today i see different numbers for the voltage reading, its around -0.0030 (was around -0.0700 yesterday at the peak).

but the information you gave here make sense, i was convinced this multimeter works fine.

thank you all !

Try this for fun - connect a low value resistor (I use a 1ohm) across the probe.  Use an alligator clips so you can free your hands.

Sit still - until you get a stable reading.  Now move (wave) your hands across around where the blue/yellow buttons are and about 1/2-1 inch to the meter.  The meter now is a motion detector.  Each time you move your hand across, your meter reading will change.  It is more sensitive right to left than the other direction (if memory serves, it changes about 0.5 ohms).

Now wrap the bottom 2/3 (from just below the LCD to bottom) with kitchen aluminum foil - double layer.  That (almost) stops the number flexing.
 


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