Multimeter Review / buyers guide: UNI-T UT71D

[Mr Eastwood]

Think of having a meter rule with divisions of 0.01mm on it, but you are measuring between 2 points which have been marked with a child's crayon. You can read to 0.01mm, but your reading is in reality only going to be correct to 5mm ( the crayon thickness) either way of the correct value.

hehe, I really like that analogy! ;-)

[MJLorton]

Thanks to everyone for posting value information and talking points on this review.

Another little piece to add that was asked about in the comments was the UT61E vs UT71D burden voltage:

I used a 1mA source and measured the burden voltage / voltage drop in both mA and uA ranges on both meters:

UT61E in uA -> 0.62V in mA -> 0.01V

UT71D in uA -> 0.49V in mA -> 0.006V

[SeanB]

I think you must do that with all the future reviews, to show burden voltage on current ranges. As well you can connect another meter and measure open circuit voltage on resistance, continuity and diode test functions. Not often shown and quite important.

Remember high burden voltage is what made Dave develop his popular uCurrent. Very useful comparison point, thugh you might have to do a measurement on each current range, though normally the 10A range will be under 200mV with a similar contribution from the test leads.

[tmm]

<thread dig>

Everyone who watches Martin knows he has certain pet peeves regarding multimeter functionality.

One of mine is overshoot.  When I see a multimeter overshoot dramatically, I automatically distrust it.  So at 31:28 (part 2) when the UT71D shows 455.7V AC on mains, I'm already never going to use that multimeter on anything high voltage.

I don't believe it is actually overshooting. While it autoranges the ADC just spews out live values and the displayed range/decimal point actually lags behind the digits. The digits also seem to be autoranging up while the decimal point is autoranging down. If the display shows 455.7V briefly then it is actually 45.57V, or 4.557v or .4557v. It's a quirk but hardly a deal breaker imo. If you keep it on a manual range, it won't overshoot.

With only a single PTC for input protection and glass M205 fuses, I'm going to agree with not using the UT71D on high power circuits.... Interesting that there is a spot on the PCB for a MOV or spark gap (marked SG1, between the PTC and fuse), and larger fuse holders. I wonder if the UT71E has those parts populated since it's selling point is that it does mains power measurement.

Other things that are surprising are that they have used (what appears to be) a genuine Analog Devices AD636 true rms converter when cyrustek make an equivalent clone, the ES636, it's even mentioned in the datasheet for the cyrustek es51966 that is used in the UT71. They also use a maxim MAX6190a 5ppm/ºC voltage reference and just generally seem to be going out of their way to use reputable brands even when they could probably get away with chinese equivalents.

Things that majorly suck about the UT71 are the annoying beep every time you touch anything and can only disable if you silence all beeps, the hopelessly slow continuity tester (it is actually also reversed biased, try testing a diode with it, wtf?), the 30 second maximum on the backlight, and the resistance mode that seems to only have 4000 counts of resolution.

One other thing to consider is that the UT71 is calibrated with a heap of resistor and capacitor trimmers. You are never going to get long term stability like a Fluke 87 which is trimmed digitally. I'm still happy to use mine as my secondary meter as long as it agrees with my 87V. It sure as hell beats paying for a premium brand datalogging meter with similar accuracy as well...

[Candid]

I know it's an old thread but I think it's interesting to see the changes over the time UNI-T makes to the UT71s.

Today I got my UT-71E and had a look inside. I ordered a CEM DT-9939 (its an Extech EX542) with the UNI-T UT-71E in order to test both and decide which one to keep. It will be the UT-71E.

I was surprised about the fuses I found  (see the photos) and the slightly changed layout so you can add some MOVs directly on the board. So UNI-T seemes to do still board revisions.

Can someone give me an advice which type of MOV I should use for this?

[SeanB]

There is no real input protection aside from the PTC and the resistors. I would add a 275VAV MOV in the RV2 RV3 holes, not using the trio of holes joining the 3 MOV units. Another in the RV1 hole to the lead of RV3.

That might allow it to survive a 300 V spike without blowing up, though with them having used 250V resistors in the input side instead of higher voltage ones does not bode good.  Nice meter, nice ranges but not something for use in high energy or any voltages above 220VAc household mains really.

[Candid]

Thank you. You are right, the UT71 is not the best for high power and high voltage. I had a closer look to the CEM/Extech they did much more for this. My main usage is household voltage so it fits my needs but if I can get a little more security with simple modifications I will do.

Another idea could be to change the two resistors to higher voltage/power ones and add three MOV-07D821K (or better MOV-07D561K http://www.mouser.de/new/bourns/bournsMOV/) e.g. What do you think? There should be enough place in the housing for this.

[retiredcaps]

I can't believe I missed seeing U5 the first time around and not saying about it!