UNI-T UT620B MICRO-OHM METER EVALUATION

[ocw]

The combination of recently obtaining a UNI-T UT620B micro-ohm meter at a good price and seeing Dave's recent video  "EEVblog #1061 - Data IO Programmer REPAIR" prompted me to review the meter.  While the UT620B's 1 micro-ohm resolution wasn't needed for Dave's repair, how else I am going to be able to evaluate my 100 micro-ohm resistors?

The UT620B measures up to 60 mohm  with 1 uohm resolution and a pulsed test current of 10 amps with 0.25% accuracy, up to 600 mohm with a pulsed test current of 1 amp with 0.01% accuracy, up to 6 ohm with 92 kHz AC signal having a DC offset with 100 mA and 0.01% accuracy, up to 60 ohm with the same AC/DC signal with 10 mA and 0.01% accuracy, up to 600 ohm with 1 mA AC/DC with 0.1% accuracy and 6,000 ohm with 100 uA AC/DC and 0.25% accuracy.  There's a higher than usual number of least significant digits added to that which limits the full scale accuracies to 0.043% instead of 0.01%.  But, given the 60,000 display count it is not as significant of a degradation as you might think.  While you lose some resolution, you can obviously take measurements with a lower test current by using a higher range.  On the lower ranges, the test pulses come at about a 1 Hz rate.

The meter is also able to determine the length of a wire based on its resistance, set limits for acceptable readings and document reading changes via an included USB cable and some basic software.  The meter can save up to 1,000 readings and store them in spreadsheet form if desired.  The meter's large display and lighting options makes the readings easy to see.

The meter comes with a lithium battery, charger, carrying case, and two four wire probe options are supplied.  It comes with Kelvin test clip cables and two dual point test probes.  Pictures of the test probes are attached.  While I don't yet know how long the battery will last is actual use, it has lasted for hours without requiring a recharge.  The fat Kelvin test probes are nice for high current connections to larger components and points, I'll be using some of my smaller Kelvin test clips to the meter's banana jacks frequently.

While user features and taking the meter apart are nice, I'm more concerned about the accuracy of the meter.  If it doesn't have that, everything else is meaningless.

Evaluating the accuracy of milli- and micro-ohm measurements is difficult.  If I wanted to make it look as accurate as possible, I could move the meter's connection a milllimeter or two.  I instead tried to make the connections to the locations where I expected to find the correct resistance.  My measurement validity depends on those connections.  I tried to connect all of the meters tested to the same point.  And the accuracy verification is obviously also limited by the precision of the resistors which I used as my standards.  While I can't confirm the precise accuracy of the UT620B, all of my readings point to respectable performance from it given its lower price.

Two other meters are in about the same price range as the UT620B.  These are the Instek GOM-804 and the B&K 2840.  They both have some tempting features, but my I didn't find any specials for nearly as low as what I paid for the UT620B.  There was only one available at that special price, so I'll leave it at that.

Attached are scope pictures of the UT620B's test signal, a look at the included software while measuring a 25 mohm resistor as well as pictures of the meter and its included probes.  You can see the stability of the low resistance measurements on the OAR3R025FLF attachment.  The resistance drifting slowly down was probably due to the heat created by the current.  The meter's test signal for the 60ohm and higher ranges look the same as the scope picture showing the test signal used to measure the 5 ohm resistor while on the 6 ohm range.

Considering its price and performance, it felt like a good purchase.

[ocw]

I am adding some pictures inside of the UT620B.  That is an Entitech 7.4V 4400 mAh li-ion battery.  The T+ and T- inputs are the RCA phono jacks for the included Kelvin clip cables.  The C2, P2, P1 and C1 shrouded banana jacks are for the included spring loaded dual tip test probes (or user supplied cables).

The main board has an ADS1248 24 bit 4 channel ADC and LT1097 and LT1413 op amps.  All three circuit boards are labeled UT620A.  That may translate to mainly a software change between the A and shown B models to achieve the extra resolution and accuracy in the B version.  The A model has 10 micro-ohm resolution and generally 0.25%+25dgs accuracy with a maximum 5 pulsed amps of test current.  The B model has 1 micro-ohm resolution and generally 0.01%+20dgs accuracy with a maximum 10 pulsed amps of test current.

[coppice]

Can you show some pictures of what your standard resistors of less than one milli-ohm look like? Its really hard to terminate resistances in a very predictable way when they get that small. I'm used to using 4 terminal resistors, and imprecise resistors which get calibrated in circuit for such small values.

[ocw]

Can you show some pictures of what your standard resistors of less than one milli-ohm look like?

I forgot about some resistors which I should have included in my accuracy tests:   some Vishay WSLP2512L5000FEA  0.5 milli-ohm 1% SMD resistors.  A picture of that test is attached.

As I said in my first message, confirming the 0.25%+25dgts accuracy on its lowest resistance range is difficult.  I didn't want to pay more than the cost of the meter for the appropriate precision to confirm that specification.  Plus, it is tough to connect the meter in an appropriate manner to not influence the reading.  Even connection to my four wire 0.1 ohm Y09300R10000B0L resistor needed to be precise to not influence the reading.  But, it is hard to go too far wrong in the connection to the shown SMD resistor.  I was surprised to get so close of a reading match.  And, with the clip connection being stationary and solid, the reading did not change at all during my 10 second connection.  All of my UT620B tests have been using four wire Kelvin clip leads.

I have some 0.2 milli-ohm Isabellenhütte BVR 1% resistors ordered to serve as another reference.  I am going to settle for that much accuracy for tests below 1.0 milli-ohm.

[ocw]

I continued my weekend evaluation of the UT620B micro-ohmmeter.  I have not yet commented on the buttons.  They need to be hit just right for an entry to be recognized.  And, once that you have perfected the angle, you might get contact bounce.  When you are pressing the Start/Stop button, a double press means that you stay in the same mode without a change.

Further tests and familiarization with the meter have taught me the best location to connect to a resistor for the most accurate readings.  Different test leads with different clips have been found to be better for different resistors.  The fat Kelvin clip leads are best for larger resistors.  My thinner Kelvin clip leads are better for mid size resistors.  For measuring four lead components and those where connecting four different clips is possible, I have found that the most accurate results are obtained by using four clip test leads.  When measuring SMD components I have found that the test leads which I made using small AIE 502008C clips are the best.  I can attach those to resistors down to 2512in/6332mm components.  Single clip Kelvin clips are required for smaller resistors (which are not as common for resistors having very small values).

With the above knowledge I remeasured the values of many of the small value resistors which I had on hand.  Most were never used resistors obtained from Mouser/Digikey.  A few were mounted on test fixtures which may have influenced their measurements.  Four were Vishay VPR221 eBay specials obtained from China.  Almost all of the resistors measured had values from the UT620B which met the meter's plus resistor's specifications.  For the lowest value resistors tested (100 micro-ohm) the 0.25% +25 digits meter accuracy plus 5% resistor accuracy translated to the meter having a low standard of around +/-30% to meet the meter's accuracy rating.  However, the three 100 micro-ohm resistors tested were all measured to have values between 96 to 101 micro-ohms.  The first time that I tested the large Vishay WSBM8518L1000 resistor it measured 101 micro-ohms.  I took the test leads off, reconnected and remeasured--again 101 micro-ohms.  Its 36 watt power rating translates to a 600 amp limit.

The data on the resistors which I measured is shown on the attachment.  I include the model number of the resistors.  The ones with measured values outside of the UT620B's ratings are highlighted in either orange (just outside of the ratings) or red.  Those six resistors with improper measurements include the four VPR211 resistors which were sent from China--surprise, surprise, surprise.  Those resistors are highlighted in yellow.  The 10 and 100 ohm VPR211 resistors obtained from Mouser were fine. The two others with readings outside of the meter's limits were ones mounted on my test fixtures.  The readings which were intended to measure the UT620B's accuracy instead proved more valuable in measuring the accuracy of the resistors which I have.

Other than the button problem, I have been happy with the meter.  Only one resistor failed to survive the tests.  I forgot about its power/current limits and fried it during testing with a pulsed 10 amps.  While the pulsed current means that the average resistor dissipation won't be as high as you first think, I far exceeded the 1/2 amp limit of that one.