Author Topic: Low emf cable assembly techniques  (Read 1253 times)

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Offline nikonoid

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Low emf cable assembly techniques
« on: November 28, 2017, 06:29:14 PM »
I recently purchased this kit for a low emf cable assembly: https://www.ebay.com/itm/263325578490

I am planning to use it for high precision voltage and resistance measurements. (0.1V - 100V; and 1Ohm to 10M Ohm)

I am looking for advice on the best method to assemble the kit, especially from folks who have done something similar before.

Here they come:
1) The kit comes with gold over copper forks. Would I need some specialty tools for crimping them? What tool/size would you recommend?

2) Is it ok to solder the joint after it was crimped? I am looking to add mechanical stability and protect connection from oxidation. Would emf be worse?

3) I believe the cable has Teflon insulation. Any recommendations for striping Teflon insulation? What tool/method would you recommend?

4) Will wire require any additional preparation after striping and before crimping, like using deoxit, sandpaper, acetone etc.

5) Same question as above for preparation of the fork.

6) Anything else to watch out for while assembling/using these?

Thank you very much.
 

Offline ap

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Re: Low emf cable assembly techniques
« Reply #1 on: November 28, 2017, 07:39:24 PM »
Few comments here:
1) There are general purpose chrimp tools available, e.g. with a crimp range of AWG 18 to 22. It is strongly recommended to use a tool that has a mechanism that prevents opening the tool before the full force is applied. This will prevent to apply insufficient forces and bad contacts. Thats what I use for the prefabricated cables.
2) Standard practice for crimped contacts is to not solder therafter. The crimp joint, if made properly, is a gas tight cold welded contact and no soldering is needed. If your tool is not working sufficiently, solder could be applied to fix the issue afterwards as a last resort. The risk to generate additional EMF is pretty low, given the small area and the resultant temperature delta accross it. Or you could even do that as a preventive means if you do not really trust your joints. See also 6) below.
3) These cables have inner PTFE insulation. A special stripping tool is recommended that uses stripping knifes, such as e.g. the Knipex 121202 or similar, normal wire strippers that adjust themselves to the wire thickness are usually not practical for PTFE stripping, the insulation may be damaged (especially with very thin wires).
4) Do not use any Deoxit or similar and do not drill the bare wires before crimping. These wires are silver plated, and the cold-weld process does the rest.
5) No preparation either
6) As always, apply the shrink tubes to the cables before you crimp :-). You could even just solder, avoiding the need for a crimp tool. In practical tests I have not seen differences in EMF, especially as the 3458A e.g. is never exactly at 0, so MATH NULL may have to be applied anyhow depending on what is being measured. The temperature gradient is just too small over the small size for the solder to generate a considerable EMF. Critical is e.g. brass, that can be seen. Tested e.g. with brass versus copper shorts. Key in the end is stability, not necessarily zero EMF from the start. So avoid air drafts arround the contacts.

Some general remarks:
-PTFE cables are recommended in all cases where the insulation resistance is critical (low current or ohms measurements). ETFE, is an option, slightly worse, but mechanically stronger.
-The 3458A is sensitive to dielectric absorption in the cable used while doing ohms measurements (Dr. Frank e.g. wrote a nice article here, but Keysight talks about it as well in its docs), keep that in mind (use DELAY).
-There is obviously a need for combined plug/spade wire sets, e.g. for the K2001/2002. Is in the loop.
-MIL STD 2000, although outdated, is a very good guide for solder techniques, free download. (The equivalent IPC standard is not free)
-The cable has a shield/guard for noise/disturbance reduction. You can use this if you e.g. measure a resistance standard. But take care if you measure a device connected to earth, you can easily generate a ground loop. Avoid that.
-The wires in that cable are twisted shielded pairs. This further reduces induced noise. There may also be common mode noise. In such case, a ferrite core may help, with the wire winded arround.
-Keithley e.g. has a book about low voltage measurements, free download available. Other literature is available too.
Metrology and test gear and other stuff: www.ab-precision.com
 
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Offline HighVoltage

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Re: Low emf cable assembly techniques
« Reply #2 on: November 29, 2017, 03:32:30 AM »
I am using an IDEAL Stripmaster tool for stripping the teflon insulation.
They offer many different blades. (You can start a collection)
I find this one better and more precise than the Knipex stripper.

It is really important, that you cut through the PTFE completely but at the same time, do not scratch the coating of the wire.

Do NOT use universal (self adjusting) stripper for PTFE insulation.
It will not work correctly!

For crimping, it is not easy to find the correct crimping tool. I have a drawer full of them and each terminal requires great attention, to have the correct crimping tool and crimping force. Good quality crimping tools are very expensive!

« Last Edit: November 29, 2017, 03:34:07 AM by HighVoltage »
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Online HalFET

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Re: Low emf cable assembly techniques
« Reply #3 on: November 29, 2017, 04:28:30 AM »
If you want to go all the way you can also use cadmium solder. Nasty stuff, but lower thermal emf and it's difficult to source due to RoHS regulations. But there is one easy place where you can still get it easily from: The solder sleeves popular for avionics still use a CdSn or CdPb alloy in the solder ring sometimes, so some carefull sidecutter action can get it out. Farnell still sells them, for example this one has CdPbSn solder: http://be.farnell.com/raychem-te-connectivity/cwt-9001/terminal-sleeve-1-7mm-clear/dp/2101891
« Last Edit: November 29, 2017, 07:59:30 AM by HalFET »
 

Offline quarks

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« Last Edit: November 29, 2017, 09:44:12 PM by quarks »
 

Offline Dubbie

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Re: Low emf cable assembly techniques
« Reply #5 on: November 29, 2017, 09:52:26 PM »
I have always wondered, does adding extra solder cause any thermocouple effect at all? Or because there is some copper to copper contact, this doesn’t happen?
 

Online HalFET

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Re: Low emf cable assembly techniques
« Reply #6 on: November 29, 2017, 09:59:11 PM »
Simply put, any metal-metal interface with dissimilar alloys will cause a thermal EMF of some degree. But copper is problematic simply because the interface copper-copper oxide does a fun 1 mV/°C, which has the potential to screw up less than precise measurements under some conditions. Honestly the best mitigation strategy is still keeping everything at the same temperature and designing your system correctly. Think it's fairly well explained in the Keithley low level handbook.
 
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Offline nikonoid

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Re: Low emf cable assembly techniques
« Reply #7 on: December 01, 2017, 04:06:01 PM »
Thanks for recommendations. I ordered IDEAL stripper #45-092.

I do have a ratcheting crimper. Hopefully it is good enough for the job:


I will post some photos once I assemble the leads.
 

Offline quarks

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Re: Low emf cable assembly techniques
« Reply #8 on: December 02, 2017, 08:12:05 PM »
just in case, please make sure you have the necessary die profile
The one in the picture is not what is needed for crimping the spade lug connectors
 

Offline ap

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Re: Low emf cable assembly techniques
« Reply #9 on: December 02, 2017, 09:30:04 PM »
just in case, please make sure you have the necessary die profile
The one in the picture is not what is needed for crimping the spade lug connectors

Yes, this rather looks like a crimp tool for crimping wire end sleeves, so the diameter of the crimp area in your tool looks smaller. But you can exchange the inserts on this tool as it looks.
Metrology and test gear and other stuff: www.ab-precision.com
 

Offline ManateeMafia

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Re: Low emf cable assembly techniques
« Reply #10 on: December 03, 2017, 12:05:30 AM »
When I bought my first set of low thermal spades, I asked the seller for the recommended crimp tool. One of his customers recommended the Panduit CT-1570. I bought it and have had good results from it.

http://www1.panduit.com/heiler/InstallInstructions/N-TMPA22153A05--Rev6--ENG.pdf
 

Offline quarks

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Re: Low emf cable assembly techniques
« Reply #11 on: December 03, 2017, 01:03:05 AM »
yes, the Panduit has the matching die

a similar Knipex die is the one on the right side


or this one from AMP


« Last Edit: December 03, 2017, 01:19:05 AM by quarks »
 

Offline nikonoid

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Re: Low emf cable assembly techniques
« Reply #12 on: December 03, 2017, 04:25:28 AM »
I cannot believe  how expensive these are! It is about this specific shape or quality and parallelism of actuator?

Are there cheaper alternatives?

Thanks.
 

Offline quarks

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Re: Low emf cable assembly techniques
« Reply #13 on: December 03, 2017, 07:18:09 PM »
For a good quality crimp you need to have a perfect match of wire size, crimp connector, die profile (especially made for each connector type) and optimal crimping force. This unfortunately only comes with good quality tools and therefore is not cheap. 50€ to 80€ will most likely be the lowest ballpark. If you already have a quality crimper look for the specific die matching the connector. A good quality die alone will probably be around 40€.

Here is the "cheapest" good quality tool I have for the connector type


If you only want to make a few crimps and the investment is hard to justify. You should ask if someone nearby has the tool and make the crimps for you.
« Last Edit: December 03, 2017, 10:49:04 PM by quarks »
 

Offline 2N3055

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Re: Low emf cable assembly techniques
« Reply #14 on: December 04, 2017, 01:49:23 AM »
I use modular MCT frame Pressmaster.  Pressmaster is OEM for Amphenol and other manufacturers.. Best price/performance I could find.
You buy few inserts, and you are good for many types of connectors..

Regards.
 

Offline 0.01C

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Re: Low emf cable assembly techniques
« Reply #15 on: December 04, 2017, 05:38:17 AM »
I use this one.
Guildline Datron Fluke L&N ESI HP
0.01C
 

Offline nikonoid

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Re: Low emf cable assembly techniques
« Reply #16 on: December 13, 2017, 04:02:55 AM »
Thank you for all the help and information. I finally have received the kit and necessary tools to assemble it. Let me describe the process I went through. Comments and suggestions are welcome.

The cable I got is definitely "special" in multiple ways, from how it is constructed down to how individual trends are pressed together.

Right from the beginning the usual methods of removing insulation did not work for me at all. In the end I took very sharp x-acto knife and carefully sliced outer Teflon shell under stereo microscope at 10x. The outer shell is so slippery that I had to wear rubber gloves, not to protect the cable, but to be able to have some grip on it. Under Teflon outer shell you can see what looks like a Kapton (polyimide) tape wrapped around it.
 


Again more X-acto knife work to expose the shield. Then you pushed shield towards itself to create a "bubble" in the middle. This way you can easily shorten the shield with side cutters without a risk of damaging internal wires.



To strip wires I got a recommended Ideal Stripmaster wire stripper. In practice I found it to be a good instrument, but less than ideal for this purpose. In fact, quite useless actually.
https://www.amazon.com/gp/product/B000NBA8GM/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
The insulation of inner wires is too slippery and more soft than usual insulation. In this stripper the bar coming down to hold the cable would severely damage insulation. Also gage 16 position of the stripper was too large just scraping insulation without cutting and gage 18 was too small cutting off some strands.

After many unsuccessful experiments I ended up using old an fashion stripper. I would just press onto a wire with 18 gage knife opening, turn stripper 180 degrees back and fourth and pull insulation off. It worked perfectly 9 times out of 10. The 10th time this left a bit of insulation I cleaned up under microscope.

Internal wires seem to have 3 layers on insulation: Teflon, Kapton, plus a third thin layer that I was not able to identify. The wire on the inside is multi-stranded. However it is twisted and pressed into round rigid shape. I used one of the recommended crimpers: Panduit CT-1570. It is very beautiful, german made precision instrument, however it is also very expensive. I would position the crimp in the tool to be perfectly centered and aligned and press on it lightly, so I do not have to hold it anymore. Then put the wire in and press down fully.





The heat shrink included in the kit appeared a little too tight for crimped cables, so I widened it a bit with needle-nose pliers: just stick them in and carefully pull handles apart to stretch the heat shrink.




Soldering on a shield/guard wire:


And finished product (heat shrink on crimps not yet shrunk):


When finished, I realized that I could not put these cable onto 3458a. I did not know that binding posts came in different thicknesses. These forks have about 5.3mm between tines. This was sufficient for 732a, but not for 3458a. Binding posts on ESI resistors were even wider at 6.3mm. There was no possible way I could widen the fork this much by stretching without substantial deformation. I ended up making a simple contraption to help me to file tins down a bit to get 6.3mm between them. I can confirm that forks are bright red copper on the inside. The filed part will surely oxidize. I am not too concerned about that, as this is not the part of the fork that is making the main contact. And even it if touches active part somewhere if should not create thermal voltage. I am considering bending the forks at 90 degree to make it easier to attach them to binding posts.


As a part of my kit, I also received plug in terminals to be used with meters without binding posts, like Keithley 2002. They are beryllium copper, so same EMF coefficient as solder, so I soldered two of them together as a short for testing. They feel kind of tight going into a terminal and I am concerned about scratching internals. I am not sure if I will end up using them.




I am planning to do some testing of the cable and this short and will post results.

 
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