UT71E Current Measurement Error - Page 1
Forum section: electronics / testgear

author: eCloud
author_rank: Newbie
timestamp: February 06, 2015, 08:12:27 am
content: Ladies & Gentlemen,I recently bought a UT71E to make AC power and current measurements. Unfortunately the meter does not work when switched to 10A AC current measurement.The fuses are OK and the 10A DC range works as expected. But when measuring about 100mA (50 HZ-Sinwave, 30 Volts) in the 10A range of the multimeter the displays shows zero.If I am using the mA range of the multimeter the display shows the correct value. I have attached pictures with a FLUKE 179 as reference.Furthermore there the multimeter beeps. These beeps are - VERY STRANGE - depended on the POSITION of the multimeter. Beeping stops if the multimeter is horizontal and starts if the multimeter lies vertically on the bench.I have attached two pictures illustrating the problemHas anybody made similar experiences? Is this a broken unit, or is this crap? I have seen posts from 2012 in this forum about similar issues (https://www.eevblog.com/forum/reviews/voltcraft-vc940-(alias-unitrend-ut71e)-problem/?nowap).Thanks

author: retiredcaps
author_rank: Super Contributor
timestamp: February 06, 2015, 08:39:56 am
content: In your first picture, the UT71E is reporting that Hz is 96kHz? On the Fluke 179, you might want to hit the yellow button to get to Hz and verify that it is 50Hz.In the second picture, the UT71E is correctly reporting that Hz is 0.050kHz.If your UT71E is beeping depending on its physical orientation, you should open it up and look for obvious signs of damage/dirt/debris or dodgy internal wiring or solder cracks?

author: eCloud
author_rank: Newbie
timestamp: February 06, 2015, 08:45:27 am
content: Hi retiredcaps,thanks for your reply. The signal fed into both multimeters is a clean 50Hz sin wave with about 30V RMS (verified with a Tectonics scope). I have noticed the frequency reading on the UT71E, which must be wrong. Since the UT71 is new, I am cautious opening it. I filed a customer service request with unitrend. Lets see what they say. I will post results here as soon as I get feedback.ThanksNorbert

author: Wytnucls
author_rank: Super Contributor
timestamp: February 06, 2015, 08:52:11 am
content: According to the manual, true RMS readings are only valid for signals of at least 10% of the range. In this case 1A.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 06, 2015, 08:52:50 am
content: Quote from: eCloud on February 06, 2015, 08:45:27 amI can understand that. However, you need to open it to change the battery or replace a fuse. So opening a meter for these two purposes should not void any warranty.BTW, I don't have any Uni-T meters so I don't know their specifications and capabilities especially in the AC current area.edit: Just saw Wytnucls reply there. As I said, I know nothing about Uni-T's AC current capabilities and it shows!

author: Wytnucls
author_rank: Super Contributor
timestamp: February 06, 2015, 09:16:57 am
content: Notwithstanding the RMS limitations, the meter should display something, so is probably faulty. I would try to increase the current to at least 1A, to see if it displays any signal properly. Might be worth trying Peak mode to see if it displays Vpp.If Lightages doesn't comment before I do, I will give you more info when I get home, to try it out on my 71D.

author: tszaboo
author_rank: Super Contributor
timestamp: February 06, 2015, 10:29:24 am
content: Quote from: Wytnucls on February 06, 2015, 08:52:11 amAnd just for the completeness, it also tells you that 10 seconds max, every 15 minutes (which is probably an overkill).

author: Wytnucls
author_rank: Super Contributor
timestamp: February 06, 2015, 11:13:41 am
content: Quote from: NANDBlog on February 06, 2015, 10:29:24 amOnly for currents above 5A.

author: eCloud
author_rank: Newbie
timestamp: February 06, 2015, 12:49:16 pm
content: Quote from: Wytnucls on February 06, 2015, 09:16:57 amthanks for your note. I agree: it should display something, at least - out of range -Lets see if I get an answer from Unit-tech

author: wraper
author_rank: Unknown
timestamp: February 06, 2015, 01:55:14 pm
content: Quote from: eCloud on February 06, 2015, 12:49:16 pmWhy should it display out of the range? This is not overload. What it displays if measuring 1 to 4 mA AC current on the 400mA range selected manually (same percentage of the range)? Probably it just cuts off low readings as cannot display them correctly. True RMS AC usually is specified only above 5% of the range.

author: wraper
author_rank: Unknown
timestamp: February 06, 2015, 02:00:35 pm
content: Quote from: eCloud on February 06, 2015, 08:45:27 amObviously you cannot get 30V into the multimeter on the current range. Fuse or current shunt would just blow if you would do so. Therefore I assume this is voltage before some current limiting resistor?

author: eCloud
author_rank: Newbie
timestamp: February 06, 2015, 02:17:59 pm
content: Obviously you cannot get 30V into the multimeter on the current range. Fuse or current shunt would just blow if you would do so. Therefore I assume this is voltage before some current limiting resistor?30 V is not at the multimeter, but on the load (a lamp, by the way)Thanks

author: Wytnucls
author_rank: Super Contributor
timestamp: February 08, 2015, 07:48:54 pm
content: Testing with a 50Hz sine source of 12VAC (No scary mains here), it takes a current of about 300mA to start to register on the 71D Amps range, otherwise the display shows 0.0 amps. So it looks like your meter behaves correctly. Accuracy is not there though and it is probably better to respect the 10% of range limitation (1A). At 500mA, it still overreads by 20mA, but frequency is correct.The Fluke and Gossen agree with each other on the Amp range.No funny beeps on the 71D.The Gossen's accuracy is also only valid for 10% of range or more, but it has a 3A range.The Fluke 185 has the same 10% of range limitation on its 5A/10A ranges.Probably all to do with the true RMS converter.Ideally, the 71 series should have had a 4A AC range, to prevent a wide gap between 400mA and 1A.

author: eCloud
author_rank: Newbie
timestamp: February 09, 2015, 06:45:40 am
content: Thanks for your efforts. I will use the mA range. In that Range my results are consistent with my Fluke. I bought the 71E for its capability to measure power and Cos Phi. But these also seem to need at least 300 mA current to work.