Kelvin clips and cables maximum length ? & Other tips by ITTSB Blog

[Kiriakos GR]

Within 2017 I did completed one large scale investigation regarding maximum length of cables for Kelvin clips.
I did visit several websites of test and measurement brands, and also I did have a good read of several product specifications documents.

According to most Kelvin clips cables manufacturers their choice this is 80 centimeters.

Most to all manufacturers they use coaxial cables and some they use special wiring method for higher performance regarding shielding to ground.
HIOKI 9140-10 cables this is such an example of special wiring method.

   
Regarding Kelvin clips selection this is also one major tricky topic.
a) Kelvin Clips with not fully exposed jaws (inner & outer) surface they are not helping for Zero-out task.
b) Kelvin Clips made of bare copper, they are soft and will oxidize due their contact with oxygen.
c) Kelvin Clips life cycle this is relative to test current of the measuring equipment that those will be connected to.

HIOKI 9140-10 Kelvin clips harness this is special for many reasons.
a) BNC connectors of highest quality  ( closest match = Amphenol B1121A1-ND3G-1-50)
Warning Note: Poor quality and with errors (regarding dimensions) BNC plugs will cause damage at your measuring equipment BNC plugs.

b) HIOKI 9140-10 Kelvin clips jaws they look as chrome plated and oxidation risk this is minimized.

c) Downside of HIOKI 9140-10 Kelvin clips jaws,  metal jaws they are not externally exposed and also these clips they are not practical to use at other applications than components testing. 

In the question of why some brands require a premium price for Kelvin clips harness when this is a product to use on their own equipment ?, the answer this is somewhat simple.
Specific Kelvin clips harness made for specific equipment, this translates to operation of the instrument at default settings.
Test leads Zero-Out does not required, and if specific equipment this has many users, then its one will work with it also at default settings.
Specific planning or concept goal this is minimizing user caused errors due fiddling of equipment setup settings.   
Good practice this is Equipment Power on ...  30 minutes warm-up time ... and start working with it right away.

Other considerations about DIY Kelvin clips.
In electrical applications it might be risky to use an 3500 Euro worth resistance meter because there is always a high risk of damaging it.
The same high risk exists also when using portable LCR meters, such precision equipment's does not come with inputs protection against other voltage sources.

There is a fine line between proper use of Kelvin Clips in to several applications, and if you do not educate your self and develop your own level of wisdom regarding: do and do not ... then you are in danger to end up with a Kelvin harness souvenir at your hands.

Major reason to use Kelvin Clips harness this is for you making comparisons by a measurement of several identical non-powered electrical or electronics components.
Pass / Fail inspection and components characterization this is what production sites will use Kelvin Clips for.

Note for DMM Kelvin clips
Owners of digital multimeter they can use larger in dimensions Kelvin clips, this extending usability to several more applications.
But think of them as alternative to regular alligator clip, because four wires measurement capability this does not translate to micro ohm measurements capability at the product group of digital multimeter.

In the end of the day it might not worth the price about you getting quality clips if you are not a professional whom using them frequently.
Clip Jaws life cycle this is limited, Clips mechanical structure this is a good point to consider, but not major one if you are going to use a Kelvin Clips harness at most of 200 times in your entire life.

Professional made resistance meters as is HIOKI RM3542A, this includes self-calibration mode this activates every 10 minutes, this is a special feature that regular DMM does not have.
Fancy words as Microresistivity they used mostly to confuse people minds (marketing tool about fishing new customers).

Therefore make your buying choices with caution, use facts, think of your equipment capabilities, every measuring equipment has limits and finest Kelvin Clips will not make it better in any way. 

For LCR and micro Ohm meters, you better think as safest choice of clips, the ones with insulation over and under the jaws.

ETA3105 Kelvin clip product specifications
http://www.ittsb.eu/forum/index.php?topic=1314.0

[Kiriakos GR]

Kelvin probe for LCR Meter this will mainly transfer ACV signal at the capacitor under test.
This translates that DC component caused by Low thermal EMF (dissimilar metals touching its other) this is not measured and it is ignored.
Because of above detail most Asian factories they can mix wires and banana plugs and all sorts of Kelvin clips so to produce a LCR Kelvin probe harness product which will work in most cases, because neither cheap LCR meters they do have any high precision specification.

Now a Kelvin probe for 4-wire Resistance measurements’ this is a totally different animal.
A benchtop multimeter will use DC current, Low thermal EMF (voltage) this can be minimized by the use of similar metals (similar metals touching its other) at entire chain of parts in use for such harness construction.

Generally speaking production of quality cables and banana plug and Kelvin clips in mainland China this does not exist.
There is mass production by the use of cheap grade of copper and lots of gold painting instead of plating.

If you choose a DIY path, then all parts used for the assembly of a Kelvin probe for 4-wire Resistance measurements’, these should come from branded sources or at least from Taiwanese factories which they respect the customer by delivering quality this worth to be exported to EU and the USA.

For example many people get fooled by shopping silicone cables about using them at test leads, when these cables are made for RC toys powered by lithium batteries.
Such wires are not made by pure copper and plating this is made by unknown methods, the end product this is a sum of dissimilar metals touching its other.

[Kiriakos GR]

In the past weeks I am still investigating several aspects regarding Kelvin probe for 4-wire Resistance measurements’.

Is cables length important ?
Is wire diameter important ?

My Fluke 8846A benchtop DMM this is fully capable to explore down to 10 Milli-Ohm with four digits resolution ( example 10.42 Milli-Ohm).
In comparison with HIOKI dedicated resistance meter, I did succeed to confirm that FLUKE 8846A actual potential this is a trusted digits resolution down to 10.4 Milli-Ohm, this translates that last digit at 8846A this is dust in the wind.
In other words I can trust 8846A when clips are perfectly zeroed down to 0.1 MilliOhm = 100000 nano-ohm.

8846A this inject low current for resistance measurement starting from high as 5mA (10 Ohms range) and by stepping up ranges this become 1mA and also even lower at 0.1mA etc etc.
Therefore the selection of wire diameter regarding copper this is mostly at serving in favor of mechanical strengths than assistance for a better and a more accurate measurement.

The question of cable lengths this were answered to me by its self,  in order to measure injected test current I had to double my cables length by using extension cables and to use GDS-320 as milliampere meter.
As soon 8846A when zeroed (test leads), measurement accuracy was as expected. 

What I am trying to make you realize by all these details, this is that it is more of highest importance the true capability of your measuring device than a Kelvin clips harness (no matter of what is made from).

   
   
       

[Kiriakos GR]

By searching to discover how many Kelvin clips ODM sources they are out there? 
I came across to a schematic this pointing out THE proper way for Kelvin clips Zero calibration.
I did try this recommendation at my Fluke 8846A and it did not work at all, this recommendation comes from a manufacturer of milliohm meter from the ones using high current up to 1A at 100 milliohm range.

Kelvin Clips this is an accessory that it serves three different types of equipment.
a)  Kelvin Clips for LCR (ACV test signal)
b)  Kelvin Clips for Resistance - Multimeter circuit – DCV with 5mA Low current source.
c)  Kelvin Clips for milliohm meter dedicated to measure wire resistance and not electronic components.
The meters can use 1A or 10A as source so these to measure at their lowest range of 2 or 20 milliohm.

In summary, they are three different electronic circuit architectures’ all of them using Kelvin clip accessories.
Now let’s leave aside the LCR meters in which Zero calibration this is more foolproof.

For precision resistance measurements’ it is important Kelvin clips to be manufactured in a way that bottom and top copper surface of a single contact point to not be flat.
Most Kelvin clips have a slippery top surface (sanded – polished metal) in which when we try to Zero our Kelvin clips they can not stay stable and unmovable.
My work around this is laying them flat over my workbench, holding them and their cables too for 20 seconds (EMF elimination) and then to use Zero Math at my multimeter.

Another detail which Kelvin clip manufacturers do not pay much of attention, this is mechanical tension force when clips are closed.
When a Kelvin clip has low mechanical tension force when closed, this force will be unsatisfactory also when we perform Zero Calibration, and this translates to unseen issue which can negatively influence our measurement.
Identical problems can occur when Kelvin clip copper surface this is thin and flexible.
Copper it self this is soft, usually thin layer of below 1.00mm of thickness this is brass alloy which this will also flex under some pressure.
Now I am thinking than 3.0 mm of thickness of brass alloy, this is the only one that we can trust for using Kelvin clip at large amount of applications other than measuring a resistor component.

[Kiriakos GR]

In the sector of low-resistance (dedicated resistance meters) I am still more a student than a teacher.

I did try to replace HIOKI kelvin clips by replacing them with the known Chinese version, and I got in to problems.
While I did solder the cables close at the jaws, the HIOKI meter now detects slight increase of probes resistance, which this does not agree with it factory calibration, and the outcome is higher values at 100 milliohm range by 0.6 milliohm.

Now it is obvious that quality of metal in use at the Chinese Kelvin clips, this is very inferior regarding electrical conductivity, and therefore they are a poor choice for dedicated resistance meters.

Theoretically these clips they might shown a better compatibility with regular LCR meters, but this is because regular LCR meters they have poor display resolution.   

In the positive side of thinks I have kept HIOKI clips original jaws, and I am looking for a new plastic body for them.

It is not the first time that Chinese test leads using worst quality of copper and that they are equivalent to garbage.