UNI-T UT71D review

[Wytnucls]

1.   Introduction


 
Here is a review of one the UNI-T flagship multimeter series, the 40,000 count UT71D, which is the only one with a massive 9,999 registers for internal logging.
It arrives as a reasonably priced comprehensive package, providing everything one needs to utilize all its features, at no extra cost.
Most high-end manufacturers will charge extra for logging software and RS232 PC communication hardware.
On some meters, the internal logged data is not even accessible to the user, unless the meter has a PC connection.
Four other models are available in the series:
Two meters with a 20,000 count, the base unit 71A with reduced accuracy, no internal logging and no temperature measurement and the 71B with 100 memory registers.
Two more meters complete the line-up with a 40,000 count, the 71C, otherwise identical to the 71B and the 71E, also with 100 memory registers, but capable of dual measurements, voltage and current to display power data (W, VAr, VA, PF)
I have owned the 71B and the 71D for a couple of years now, both bought in China from electronics shops, with a one-year warranty.
Most people will be familiar with these DMMs already, as Martin Lorton has released a couple of videos about the 71D. They will be linked at the end of this review. Dave has a 71A in his multimeter trunk, but hasn't reviewed it yet.
Martin did a pretty good job with his review, so I will expand on some of the most important features from an owner’s perspective and mostly avoid a repeat of what he has covered already.

2.   What’s in the box?


 
The soft nylon case is pretty useful, sturdy and large enough to accommodate a few extra items, like screw-on probe hooks and SMD tweezers.
The meter itself is quite light for its size (395g), as it lacks a beefy rubber holster, but has enough heft to not feel cheap at all. At first sight, the LCD appears surprisingly and pleasantly very large.
The first set of probes I got with the 71B were great, very flexible, made from silicone, whereas the later batch came with stiff PVC leads and, while adequate, are best replaced by something more pliable.
The short lead croc clips are very useful and I reach for them all the time.
The logging software is on a CD, installs easily on a windows machine and provides enough features to satisfy most users.
The USB optical module fits snuggly on the top of the meter, looks well-made and has a useful cable length of about 1.5 meter.
The K-type thermocouple works fine but looks rather flimsy and fragile.
The slip-on probe croc clips are ok, but I rarely use them.
The Chinglish manual explains most of the features, but has a few mistakes and some omissions. Still, it's pretty good by Chinese standards.

3.   Ergonomics
The meter has a pleasant sleek narrow body with very little padding, which doesn't take much room on the bench, and a big ergonomic range switch which emits a mildly annoying beep at each selection.
The meter feels surprisingly sturdy, with no creaking detected when trying to twist the body.
It is probably not the ideal meter for someone who throws stuff in a heavy toolbox, but is quite at ease on an electronic bench.
The front plate has 8 switches, with 6 of them providing multiple functions. Getting on top of the menu system and features requires some practice, but is well worth the effort for someone who wants to use all the features they paid for.
The meter has separate DC and AC voltage positions, DC mV, C/F temperature, uA, mA , 4-20mA  percentage loop, Hz and duty cycle.
The 9V battery compartment is a bit tight but manageable and the lid is held in place with a machined screw fitting into a metal insert.
Unfortunately, there is no direct access to the fuses and 4 self-tapping screws need to come out, to split the case open.
Some people complained about the short bale which has a shallow angle, but it never bothered me and I fixed the slight rattle with a small rubber damper.
One of the best features of the meter is the large LCD display which has good contrast and a 2-step bright backlight. The curved glass increases reflections, so possibly not the best for use in a lab with multiple light sources.


 

[Wytnucls]

4.   Features
The ADC has a true RMS 40,000 count and a faster 4000 count option on start-up, if required.
The meter window displays 3 parameters at once, a feature which is very useful for Max/Min measurements, for instance. All the display options can be seen by pressing ‘Hold’ while turning the meter on.
The yellow button provides an AC+DC measurement for voltage and current. The AC bandwidth is reduced on current from 100KHz to 10kHz. AC+DC is also available on current ranges.
Here is a bandwidth comparison with a Gossen 26s, which has a bandwidth of 20kHz:



The Max/Min feature takes a reading every 2 seconds, with both extremes displayed on the top row 4,000 count places. Further presses of the button toggles the readings to the main window for a higher resolution.
Peak Hold can capture 10uS transients on fluctuating voltages and currents. It is much faster than the standard 250uS. It is also useful to display peak voltages of a sine waveform.
Internal storage has 9,999 registers and all can be recalled and viewed directly on the meter (I can't do that on my Gossen 30M). Stored data includes measurement type and units.
The bar graph can be removed through the menu options and can also be modified with the zero in the middle of the scale.

There is a Pass/Fail feature, providing an aural warning if the entered top (fast beeps) and bottom (slow beeps) limits are exceeded by the new measurement. Procedure: SET UP LOW- SET (HOLD to move to next digit +/- to alter digit - Exit) SET UP HIGH (same procedure) Press STORE to set limits to OFF

The caps are discharged automatically through the 1.1K PTC before measurement, if they carry excess voltage. Capacitance measurement covers a wide range from 1pF up to 40mF.
Here is a comparison with 2 LCR meters for capacitance measurements and a picture showing temperature readings:

 

Frequency can be measured accurately, up to 40MHz.
Thermocouple temperature is displayed with a 0.1C resolution.
The meter has auto ranging and the current range is usually displayed in the top left window, with the frequency, when relevant, displayed in the top right place.
The ‘REL’ button is useful to get rid of test leads resistance or capacitance for accurate small caps and low value resistor measurements (<400 Ohms). The top right display shows the reading at the time the REL button was pressed. The top left display shows the current reading and the main display shows the difference between the two, staying in the original range. Caution: The meter does not auto range anymore in that mode, showing OL above the current range.



Less useful for most casual users is the 4-20mA loop feature, which displays mA current in a percentage format.

5.   Safety
This is probably the most contentious aspect of the meter.
It does have one 1.1K PTC and two current limiting resistors (1K and 100K)
Less conspicuous are the crowbar transistor circuits protecting the meter up to 1000V.
It also has aural warnings on the current jacks, to prevent dangerous selections.
The current ranges are protected by 2 fast acting glass fuses of 10A and 500mA respectively.
The uA and mA ranges also have a diode bridge, including a Zener diode, to clamp high voltages.
There is a low battery voltage warning, during which measurements are not to be relied upon.
That configuration with glass fuses doesn’t comply with the latest IEC regulations and a new PCB version was released with extra space on the board for HRC fuses and wider creepage distances.
The meter was never tested by independent safety organizations, so its rating of CAT III 1000V/ CAT IV 600V is still under question.
It is best to avoid using this meter on high power circuits until proper testing has been published.



6.   Accuracy
General accuracy is respectable, at 0.025%+5 on the mV DC range and 0.05%+5 on Volt DC.
The DC current accuracy is 0.1%+15 on the mA/uA ranges.
The burden voltage is a little high at 5mV/mA on the 40mA range and 500uV/uA on the 400uA range (1.8mV/mA and 100uV/uA on the Fluke meters).
The resistance tolerance is 0.3%+8 up to 40kOhm and on caps, 1.2%+20 up to 400uF.
Frequency readings are usually spot on, at 0.01%+8 up to 40MHz.
AC current measurement, because of the true RMS converter limitation of 10% of range, is tricky. There is a zone between 400mA and 1A, where AC measurements are not to be trusted. Ideally, the meter should have had a 4A extra range, to cover the dead zone. DC current measurements are not affected.
The cold junction temperature is tapped on a transistor PN junction on the middle of the PCB and with the provided K-type probe, measurements are good up to 230C. Accuracy is quoted at 1%+30 above 40C.
Peak Hold accuracy is 1.2% of scale+25digits for voltage and current.
Over the last 2 years, my UT71 meters have experienced very little drift, and stayed within the original 12 month guaranteed accuracy deviations.
The 2ppm/C Maxim external voltage reference has a maximum drift of 50ppm/1000 hours and usually settles down after that period.
Here is a recent comparison of least significant digits on DC Volt and resistance between my most accurate Gossen 30M as a reference and the Gossen 26S, 71B and 71D (All the meters have their original calibration):

[Wytnucls]

7.   Calibration
Calibration is best done with the closed-case procedure, with the aid of an accurate voltage/resistance standard or calibrator for all ranges.
To enter closed-case calibration, push ‘Exit’ and the blue button together on start up. Press ‘Hold’ to capture correct inputs (multiples of 19).
Otherwise, pots can be tweaked internally:
VR1 AC current (1A 60Hz)
VR2 DC volt multi-turn pot
VR3 AC volt
VR4 Temperature cold junction with probes shorted for ambient temp
VR5 AC+DC volt <50 digits with probes shorted.





8.   PCB Architecture



The meter has a double-sided board with most components on one side.
The switches, the ADC, the LCD, one of the LCD controllers and the backlight LEDs are on the reverse side.
The piezo speaker is anchored on the back case with spring contacts to the main board.
A small PCB placed on top of the current jacks provides the detection for the aural warning.
The whole meter can easily be disassembled without de-soldering any parts.
Metal cans shield the microcontroller and the precision resistor network.
The soldering quality is generally fine, with flux residue in some places where a few components where hand-soldered.
   


9.   Operation
Here are some of the most important aspects of the meter that can affect your measurements.
Settling times for capacitance are for 400uF 1.5 seconds, for 4mF 3.75seconds and for 40mF 7.5 seconds. The bar graph movement gives an indication of the time left for big caps readings.
The auto discharge can take some time, depending on the initial voltage
The adjustable auto power off (APO) can be disabled or set to 10, 20 or 30 minutes.
The backlight has 2 levels of brightness and is adjustable to 10, 20 or 30 seconds.
The ‘Peak Hold’ can capture fast transients of at least 10uS duration.
The bar graph updates 10 times per second and has 40 segments.
The Low/High aural warning triggers stay in memory, even after meter shutdown, so the values need to be removed from the set up screen, after each use.
The voltage impedance is 10MOhm, except for mV, which is 2.5GOhm.
Continuity beep is triggered below 50Ohm and the testing voltage is set at -1.2V.
The diode open voltage is 2.8V. Most LEDs light up (YGRW) and the forward voltage is displayed, except for the white LED.
The storage interval default to 1 second, but can be increased up to 255 seconds.
A new series of data can be appended to the existing ones in memory or set to overwrite them and start anew from register 0001.
In ‘Recall’ mode, it is possible to page through the previous recordings with the ‘+’ and ‘–‘ keys. Each data point was stored with its unit and type of measurement.
Press ‘Hold’ while in ‘Recall’ mode to send all readings in memory to the PC software.
When measuring AC voltage the true RMS reading is valid from 10% of the range upwards and cannot exceed the max crest factor of 3.0. The frequency should be kept below 100 kHz for an accurate measurement.
In AC+DC mode, add 1%+35d to the AC accuracy specifications.
5A is the limit for continuous measurement.
Current draw is an average of 7mA and the 9V battery should last about 100 hours.

Here is a digest of procedures for proper memory storage operation:

There is no memory delete prompt as such.
All one can do is append a new batch of data to the existing data in memory or choose to overwrite the existing data from the beginning.

To store new data in memory:
Press STORE key
Press HOLD key if you want to overwrite old data, otherwise skip this step
Press STORE key to set recording interval (then + key repeatedly to set from 0 to 255 seconds) (Hold key continuously for fast scroll)
Press STORE key to start recording
Press EXIT key to stop recording

To review stored data:
Push STORE key for 1 second (RECALL)
Select registry number with the + or - keys
Press HOLD key to send all stored data in memory to computer via USB

At any time, to send a single measurement reading to the computer via USB, push MAXMIN key for 1 second

[Wytnucls]

10.   PC software



The meter comes standard with a USB RS232 cable with an optical connection module.
The software may seem pretty contrived at first, but its functionality is quite acceptable, after some experimentation.
Toggle the ‘USB connect’ soft button to stop and start your logging session.
Values can be entered in the Min/Max windows for aural exceedance warning.
Tick the ‘Repeat’ function to suppress non-changing measurements.
Enter your required sampling interval in seconds in the next window.
Afterwards, the log can be saved as an Excel worksheet or sent to a printer.
Use the ‘Recall data’ feature when you want to retrieve data from the meter’s internal memory. You can set a start time to match the original capture if you wish and also match the logging interval of the original data capture .
The graph is the most frustrating. You have to use your mouse to select the area of interest. Drag right with the left mouse button pushed in to select a small window and when you’re done, drag left to zoom out to the original graph.
Hold the right mouse button in and drag to reposition your viewing window.
Tick ‘Fill’ for a blue bar graph instead of a continuous line.
Choose a number of points per page if required and press ‘+’ to enlarge the graph window.
Multiple meter logging is possible, in two separate windows.
Fast logging (8 samples per second) in real time is possible, wen the meter is in 4,000 count mode.

11.   Components
U1 ES51966P Cyrustek dual-slope ADC
U2 M430F149 micro controller TI
U3 U9 HT1621B Holtek LCD controller x2
U4 Opamp STEZ832 062C (thermocouple cold junction circuit)
U5 ES636 (AD636J) TRMS to DC converter 1% error, crest 6 (Cav=10uF)
U6 Dual Opamp STEZ832 062C (frequency circuit)
U7 Voltage regulator 5.5V
U8 Voltage regulator 3.5V
U10 24LC515H Memory microchip CMOS I2C Serial EEPROM 512K
U11 HCF4011 Multiplex NAND gates AESA
U12 Maxim MAX6190A bandgap voltage reference 2ppm/C 1.25V 50ppm/1000hrs
U13 Dual Opamp ST7444 72C (TRMS circuit)
U14 Precision Opamp OP1177 AO47 (V Ref circuit)

1 PTC (WMZ11-75HV 1.1K)
2 Crystal Oscillators (4 MHz for the ADC and 32 kHz for the TI uC)
8 Precision metal film resistors 1W TCA5C6 (4x2.5M and 1M11, 101K, 10K1, 1K01) (0.05% resistance tolerance and 10ppm/C). More recent boards (from REV6) have SMD precision resistors instead, replacing the through hole resistor network.
 




12.   Value
Nowadays, the UT71D meter sells for about 135.00 USD, including shipping costs on eBay, from Hong Kong and China sellers.
I still haven’t found another meter that comes close to the specifications and features of this UNI-T, at this price point.

13.   Variants
They were a few rebranded meters available in Europe and the US, but I think that most have been discontinued by now.
It is still possible to find a Tenma, Voltcraft and a few others on numerous internet sites, but not necessarily with the same set of features as the originals.
 


14.   Modifications
There usually is space for HRC fuses and an MOV/spark gap on the board, but while probably made safer, doesn’t necessarily meet the latest safety regulations.
There have been several board revisions over the years, so this may not be always feasible, depending on the PCB configuration.
I didn't bother modifying the 71D and I would advise against it if you only work with low power circuits, like I am. I soldered a few extra bits in the 71B, but it was more for the fun of the exercise rather that a quest for total safety.
Here is a picture of the modifications on my 71B:


 
15.   Criticisms
Here are most of the perceived flaws mentioned by some owners. While some are justified, others might just be nit-picking.
The reflective LCD curved acrylic cover was mentioned a few times as a nuisance in rooms with multiple light sources.
The lack of a blank insert for the USB plug recess was also criticized. The one provided with the UT61 is a perfect fit though.
It has an irritatingly slow continuity, but at least the signal is latched.
I did find a way to achieve a faster continuity test which doesn't involve any modifications:
1. Turn the range switch to Ohm and select the diode mode by pressing the blue button twice. You are now in the 4V range.
2. Press the RANGE/SETUP button until the meter beeps and displays OFF in the 'low' setting.
3. Press the HOLD button once to access the count display.
4. Press the HOLD button repeatedly to select each number in turn.
5. Change each number with the + and - buttons until your display reads 27400 or slightly lower and press EXIT to store it.
6. You are now in a fast mode for your continuity testing, staying in the diode range.
7. When you are done with the continuity testing, press RANGE/SETUP until it beeps again, then press STORE to display OFF in the low mode.
8. Press EXIT to return the meter to its usual state.
It is a bit of a convoluted procedure, but at least the function is available if you don't have another meter at hand.

Of course, the glass fuses elicited a lot of scorn, but, at the time of production, high voltage HRC fuses were not a safety requirement. On the positive side, if you blow them regularly, they are dirt cheap to replace.
The bale angle is sometimes considered too shallow. 30 degree from horizontal, which is fine for me.
It has a too slow LCD update rate for some, with one refresh per 2-3 seconds. (My Gossen 26S behaves the same way)
The battery compartment is too tight for comfort. It is a tight fit, but no enough to provoke lasting frustration, in my view.
The meter is fragile and may not survive a drop test. Probably true, but for electronic work, if kept on the bench, it should be relatively safe from any damage.

16.   Conclusion
I never regretted buying these two meters and although I have acquired a few more ‘respectable’ ones since then, I still reach for the 71s on a regular basis.
I think they have tremendous value for money and are ideal for hobbyists who don’t want to spend a fortune on top-end meters and their optional logging accessories.
Of course, they are not perfect by any means, but UNI-T managed to produce a dependable full-featured meter, at an affordable price.
The inaccuracies on low ACA are a concern, especially on the 71E, so keep that in mind if you're considering buying one of those meters.
Hopefully, the next version will be even better and will comply fully with the latest safety regulations.

Martin Lorton's review:

[Wytnucls]

I have attached here the most relevant datasheets and any other useful information:

1.AD636 TRMS to DC converter: http://www.farnell.com/datasheets/1780748.pdf
2.Maxim voltage reference MAX6190: http://datasheets.maximintegrated.com/en/ds/MAX6190-MAX6198.pdf
3.Calibration procedure: https://www.eevblog.com/forum/testgear/uni-t-ut71d-review/?action=dlattach;attach=93453
4.EEPROM memory chip: https://www.eevblog.com/forum/testgear/uni-t-ut71d-review/?action=dlattach;attach=93454
5.Holtek LCD controller: https://www.eevblog.com/forum/testgear/uni-t-ut71d-review/?action=dlattach;attach=93455
6.TI microcontroller MSP430F: http://html.alldatasheet.com/html-pdf/82008/TI/MSP430F147/24/1/MSP430F147.html

[retiredcaps]

Thanks for a thorough review and follow up after 2 years of operation.  Most people just do the initial review when it is new and don't bother to update afterwards in terms of accuracy, reliability, usage, etc. 

[miguelvp]

Yeah, thank you, I was reading it as you were updating the images, since I have the 71B a lot of it pertains my DMM.

So again, Thank you very much and thanks for maintaining the DMM spreadsheet!

[Wytnucls]

Glad someone appreciates the effort and, hopefully, the review will help owners master their new shiny equipment and dispel some preconceived ideas about the UT71.

[Lightages]

Good review! I agree on all the points and the UT71X series does offer quite a bit for the money for electronics use. All your opinions are on the mark IMHO. Generally, Uni-T seems to offer good value for the money within the limitations you have mentioned.

I had a bit of a bad experience with my UT71E being out of spec but it is alright now.

[Wytnucls]

Thank you. Feel free to add some of your comments over here about your personal experience with the 71E and its extra features, when you find some spare time. 

[Vgkid]

Thanks for the good write up, I was considering of grabbing a c-specced one to replace my Fluke 87-1.

[Lightages]

Just remember that the UT71X series comes nowhere close to the safety and build quality of the Fluke. Keep the UT71X s away from electrical work and high energy circuits. They are in no way a replacement for the 87 series, more like a compliment for electronics work.

[Vgkid]

I will mainly use it for lower voltage stuff, along with fixing my 87.

[XOIIO]

If possible, could you list the rest of the chips on that model, and maybe dump the EEPROM of yours? I have a feeling this and the UT71C are identical and you could hack the meter to unlock the rest of the storage. I'm also interested in increasing the backlight duration but I don't have what I need to dump ROM's yet, so it would be a chance to look around in there ahead of time.

https://www.eevblog.com/forum/testgear/hacking-the-uni-t-ut71c-(wipneed-help)/

[miguelvp]

You already asked him that and he said he didn't have the equipment to dump the eeprom.

Edit: https://www.eevblog.com/forum/testgear/multimeter-software-that-workes-with-many-brands/msg440279/#msg440279

[XOIIO]

You already asked him that and he said he didn't have the equipment to dump the eeprom.

Edit: https://www.eevblog.com/forum/testgear/multimeter-software-that-workes-with-many-brands/msg440279/#msg440279

derp

[Wytnucls]

If possible, could you list the rest of the chips on that model
https://www.eevblog.com/forum/testgear/hacking-the-uni-t-ut71c-(wipneed-help)/

All the components are listed already.

[miguelvp]

derp 

Fixed it for you

[Wytnucls]

Here is a spreadsheet that may interest some people. It shows the performance one can expect from the 71 series multimeters, while measuring resistance.
The accuracy tolerances aren't as tight as on the Gossen or Fluke machines, but at least the results stay within specifications, even after 2 years without calibration.
As far as I understand, the meter passes a low current through the DUT resistor and then measures the voltage on the mV scale to derive the resistance. The test current will vary, depending on the range selected.
There are fluctuations at times, but generally the multiple test results are consistent and repeatable.
If you need very accurate Ohm measurements though, the more expensive meters will have a distinct advantage.

All meters are compared to the 30M used as a standard.
The light blue columns show the least significant digits deviations and the purple ones, the maximum deviation allowed within that range.
I hope it all makes sense.
(As an aside, the Gossen 26S shows incredible performance, as the original and probably last calibration was done in 2006!)

[XOIIO]

Hmm, I was wondering, do you know where/if there is a trim pot to cal the resistance? That would make adding an MOV easy.

[Wytnucls]

All the potentiometers are listed in post #2.
There is no pot for resistance calibration.
The only calibration available is closed-case, through software, for each range, with a calibrator or accurate resistance source.

[XOIIO]

All the potentiometers are listed in post #2.
There is no pot for resistance calibration.
The only calibration available is closed-case, through software, for each range, with a calibrator or accurate resistance source.

Ah alright, I was hoping you just forgot to list it, that's too bad. Guess I won't be adding any MOV's

[Wytnucls]

Some people complained that the temperature readings were way out on their meters.
Here is a picture of my 2 meters with the Gossen 30M as a reference, each using a different thermocouple wit the tips bunched together in a sealed environment and allowed to settle for about 10 minutes.
All the meters have their original temperature calibration. If you find that yours is way out, calibration is possible by tweaking the temperature potentiometers on the Uni-T PCBs.
According to the meter booklet, the K-type probe is good up to 230C. Above that temperature, a rod contact probe has to be used.
Accuracy is given as 3%+30 digits between -40C and 40C, which I think is very conservative for the middle of that range. Above 40C, it increases to 1%+30, until 400C. Maximum range is 1000C.
Accuracy on the Gossen 30M for the K-type is 0.7%+0.3K, which is about half a degree at 22C.

[Wytnucls]

Here is a screenshot of the waveforms of the 71D and 26S, while measuring 100uF 10V caps of the same batch.
The meters have different measurement methods:
The 71D (blue trace) charges the cap to a low voltage with a constant current, then measures the capacitance through an RC discharge and then displays the reading at the end of the RC curve. The cap is discharged before the next measurement.
The 28S charges and discharges through an RC circuit with a higher voltage than the 71D and doesn't discharge the cap in between measurements.
The LCR meter gives a capacitance of 96uF and 98uF for the 2 caps.
The 71D measured 107.3uF and 110.1uF.
The 28S measured 114.7uF and 115.8F.

[Shock]

I have two new Uni-T 71Ds (same batch) and temp testing show approx 2 degrees C difference when the same thermocouple was used on both meters at room temp.  The thermocouples on both meters themselves are a fairly close match.  I suspect one meter is reading low and is out of cal.