Fluke 8846A / DMM 4050 and Special Pomona Kelvin probes 4W for mOhm resistance

[Kiriakos GR]

This article will enlighten, us interested for Low- milliohm measurements, about these special in design Pomona Kelvin probes 4W.

Specific Fluke 8846A / DMM 4050, they have in common at Ohms range specification, as low as 10 micro Ohm resolution.
This information also translates, that at 4W Sense input, any instability as DCV micro volt or even further down to nano volt , this can and will cause issues.
8846A / DMM 4050, they have in common 100nV resolution.

The idea here is us, using entire digits resolution of 8846A digital display, at 4W resistance measurement. 
Elimination even or tiny EMF quantity this is our goal.
Only then we can trust the measurement down to the last digit.
Another benefit will be last digits not fluctuating, and a faster measurement stabilization.

Pomona electronics USA, since 2003, this introduced two special Kelvin 4W probe harness.
a) Pomona 5940  Kelvin Clip Cable Test Lead Set
b) Pomona 6303  4-Wire Kelvin Probe 2mm tip Set with Double Banana Plug Leads

Later on, a revised Pomona 6303 design, this become this FLUKE TL2X4W PTII.
While this 4-Wire Pomona 5940 Kelvin Clip this never circulate under FLUKE brand name.

Both 4-Wire Pomona products, they were designed for eliminating thermal EMF.
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Fluke 8846A / DMM 4050 they both represent, a cut down version of FLUKE metrology class of largest DMM, the ones in use for verification of FLUKE calibrator output.

Fluke 8846A / DMM 4050 and at front panel (only), they have installed a patented design of gold plated split contacts terminal.
Along of regular 4-Wire inputs also with gold plated contacts (female banana input).

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The information that I had overlooked when I made my own 4-Wire Kelvin clip harness, this is my choice at using Nickel plated banana.
This period of time, I was finally able to get in my hands a FLUKE TL2X4W PTII.
Because of it, I started looking at Pomona electronics USA, and I discovered the entire story about these special products. 

My new goal, this is upgrading my own 4-Wire Kelvin clip harness, and to add Staubli gold plated bananas, which I have all ready in stock.

According to my understanding, you cannot compare side by side, the performance of two special 4W harness, if they are not both be made by identical product design concept. 
Especially when your new goal, this is ultimate stability when measuring lowest values of resistances.

Dedicated resistor meters ( HIOKI RM3542A ), this use it electronics as aid to eliminate EMF, by constantly reversing current source polarity.

In conclusion, if you own FLUKE design of bench-top DMM, you need to also follow the Pomona way, about the construction of your test leads.   

[Kiriakos GR]

Soldering of new banana plugs this is the easy part.
Now I need to verify how noiseless can be the improved harness.

I did start my testing with one 1 mOhm surface mount resistor.
For the shake of correctness, I did record 30 minutes of data logging.
10 first minutes = thermal stabilization.
At next 20 minutes the measurement was very stable.
And so I did started a new 5 minutes data logging session, so to record min/max deviation of a stable measurement.

I am simply attaching in the form of viewable statistics, screenshot of my first testing session. 

[Kiriakos GR]

10 first minutes = thermal stabilization.

Today I feel the need to kick is the ass, entire Fluke Calibration team, this involved at writing 8846A documentation.
At several points of this documentation, they did write ... if you are after precision measurements, then use first Fluke 884X-SHORT

At my first test with my 4W cable harness at 4W Ohms mode, I did record and noticed approximately  6~7 minutes of what I described as thermal stabilization.

Today that I was testing two clones of  Fluke 884X-SHORT, I did confirm for a second time,  that Ohms measurement this requiring again 6~7 minutes, so this to earn maximum stability.

And now my new Eureka find
While 8846A this is warmed up for several hours,  it Current source chip at 4W Ohms mode,  this has it own warm-up time.
This translates that Current source chip, this needs six minutes in operation, and then you may trust the measurement. 

Therefore at 4W Ohms mode,  the job of Fluke 884X-SHORT, this is less relative at causing thermal equilibrium.
It primary job, this is forcing DC Current source chip this stabilizing faster, and the word faster this translates to five minutes.

If you create a similar SHORT by keeping connected as a short, your Kelvin harness and clips, DC Current source chip this will be ready in seven minutes.

This topic, of actual warm-up period in minutes, of this DC Current source chip, per DMM brand and model, this has never be reviewed before.
I did select this 8846A years ago, because of it high resolution and high measuring detail at it supported ranges. 

I would not be able to discover this detail and have proofs about it,  without using TsDMMViewer Data Logger for FLUKE 884xA