Low emf cable assembly techniques - Page 1
Forum section: electronics / general

author: nikonoid
author_rank: Regular Contributor
timestamp: November 28, 2017, 07:29:14 am
content: I recently purchased this kit for a low emf cable assembly:https://www.ebay.com/itm/263325578490I 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.

author: ap
author_rank: Frequent Contributor
timestamp: November 28, 2017, 08:39:24 am
content: 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 either6) 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.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 28, 2017, 04:32:30 pm
content: 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!

author: HalFET
author_rank: Frequent Contributor
timestamp: November 28, 2017, 05:28:30 pm
content: 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

author: quarks
author_rank: Frequent Contributor
timestamp: November 29, 2017, 10:40:58 am
content: maybe you find some usefulll information herehttps://www.eevblog.com/forum/metrology/diy-low-emf-cable-and-connectors/msg190302/#msg190302https://www.eevblog.com/forum/reviews/what-crimping-tool-do-you-use-for-quick-connect-terminals/msg261108/#msg261108

author: Dubbie
author_rank: Unknown
timestamp: November 29, 2017, 10:52:26 am
content: 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?

author: HalFET
author_rank: Frequent Contributor
timestamp: November 29, 2017, 10:59:11 am
content: 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.

author: nikonoid
author_rank: Regular Contributor
timestamp: December 01, 2017, 05:06:01 am
content: 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.

author: quarks
author_rank: Frequent Contributor
timestamp: December 02, 2017, 09:12:05 am
content: just in case, please make sure you have the necessary die profileThe one in the picture is not what is needed for crimping the spade lug connectors

author: ap
author_rank: Frequent Contributor
timestamp: December 02, 2017, 10:30:04 am
content: Quote from: quarks on December 02, 2017, 09:12:05 amYes, 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.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: December 02, 2017, 01:05:30 pm
content: 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

author: quarks
author_rank: Frequent Contributor
timestamp: December 02, 2017, 02:03:05 pm
content: yes, the Panduit has the matching diea similar Knipex die is the one on the right sideor this one from AMP

author: nikonoid
author_rank: Regular Contributor
timestamp: December 02, 2017, 05:25:28 pm
content: I cannot believe how expensive these are! It is about this specific shape or quality and parallelism of actuator?Are there cheaper alternatives?Thanks.

author: quarks
author_rank: Frequent Contributor
timestamp: December 03, 2017, 08:18:09 am
content: 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 typeIf 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.

author: 2N3055
author_rank: Super Contributor
timestamp: December 03, 2017, 02:49:23 pm
content: 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.

author: 0.01C
author_rank: Regular Contributor
timestamp: December 03, 2017, 06:38:17 pm
content: I use this one.

author: nikonoid
author_rank: Regular Contributor
timestamp: December 12, 2017, 05:02:55 pm
content: 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=1The 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.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: November 05, 2021, 09:49:33 pm
content: Hi. I recently purchased the Knipex 97 33 01 (3-die crimp set) instead of the 97 33 02 (5-die crimp set) because I was unaware that there were two options.What die should I use for the following? I am not sure that the folded crimp is the same as a 'ferrule', which is a continuous metal tube.22-16 AWG hook terminal22-18 AWG ring terminal1.25-6L spade terminalNone of the dies seem appropriate for these types. This is also the case for the 5-die crimp set.https://www.knipex.com/sites/default/files/Product%20data%20sheet%20EN%2097%2033%2002.pdf

author: maat
author_rank: Regular Contributor
timestamp: November 05, 2021, 11:59:04 pm
content: I use a Pressmaster MCT with a Mobile die set/4300-3142/AAA for all 3 types. Might not be the answer you want to hear thoughI can also highly recommend the Pressmaster MCT, because there die sets for almost all connectors (probably, because Pressmaster is the OEM for most crimp tools).

author: nikonoid
author_rank: Regular Contributor
timestamp: November 06, 2021, 03:16:06 am
content: leighcorrigall,The ferrule crimp from Knipex 3 dies crimp set does not look right to me.I frequently use Pressmaster. I have a set with 5 interchangeable dies. I feel that Panduit CT-1570 is even better, but less versatile.Few weeks ago I made a mistake crimping a low thermal fork with Pressmaster before attaching heat shrink. With the rest of the cable complete, I decided to "undo" the mistaken crimp. The weld was so good, it would not unbend for anything. I ended up cutting away the fork one bit at the time under microscope.I would highly recommend one of the solutions above, despite the high cost.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 06, 2021, 08:30:08 am
content: Quote from: leighcorrigall on November 05, 2021, 09:49:33 pmNone of them looks correct to me

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: November 06, 2021, 12:16:21 pm
content: The recommendations do look appealing to me. I want to invest in a die set rather than another crimping tool. Unfortunately, the Knipex website is not well designed and it is difficult to search for parts compared to PressMaster.This is the die I think I need for the job:https://www.misterworker.com/en/knipex/crimping-dies-for-non-insulated-crimp-terminals-and-non-insulated-crimp-51429f/37745.htmlThe picture quarks uploaded is interesting. The dies are slightly different and share the same "13" mark:Quote from: quarks on December 02, 2017, 02:03:05 pmToo bad I cannot find either die set on the Knipex website.

author: ScoobyDoo
author_rank: Regular Contributor
timestamp: November 06, 2021, 12:33:24 pm
content: Hello leighcorrigal - I am using a Rennsteig (acquired by KNIPEX) crimping tool:RENN-6240203 Crimp. Pliers with die set PEW12.20 for non-insulated terminals Capacity (mm²) 0,25-0,75 / 1,0-1,5 / 2,5I bought this tool in 2018 (151€) and Rennsteig asked me to send sample lugs and PTFE cable so they could recommend the correct die set (claws).Their service was top notch - the crimping quality is excellent - top quality - really huge difference with cheap crimping tools you order from AliExpress.Your case might need same or slightly different die set (claws) you need to ask advise from manufacturer.Best regardsScoobyDoo

author: coppercone2
author_rank: Super Contributor
timestamp: November 06, 2021, 04:43:53 pm
content: the statement about lubricating terminals prior to crimping is debatable. This is a document that shows what happens when grease gets inside the crimp during assembly. Its not necessarily a bad thing.https://www.te.com/usa-en/about-te/capabilities/engineering-innovation/influence-of-terminal-lubrication-on-crimp-reliability.html

author: mendip_discovery
author_rank: Super Contributor
timestamp: November 07, 2021, 10:30:37 am
content: By the looks of it, you have done well to do what you have done. It's all a learning experience.We could argue over what is the best crimp tool and wire strippers all year. You can spend thousands on tools and use them only a few times on one specific task. So don't beat yourself up about it.With regards to the ideal strippers there are lots of cutting jaws available for them and some are rather specific to a make/type/model of wire and even then they are not perfect but we are sometimes talking on a 10 μm between a nice clean cut and either not cutting or damaging the wire. Using a sharp knife is sometimes the only way to go.I have some pdf docs here (Def-Stan 59-71 and some BS standards) that go into really heavy info about crimping and to be honest for your use its overkill but all you need to ensure is that the wire isn't going to just pull out, its crimped enough that the wires are nicely bunched to avoid oxidation and that there is a good connection you could run a few amps through it and check the voltage drop (22awg which is 0.34mm2I think, which should have a min force of 50N and with 4A should not exceed 8.4mV for 105°C wire, see attached images for more).Treat this as your first attempt, once you get better at measuring stuff you will look to improve on what you have done with better stuff.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 07, 2021, 10:52:31 am
content: When I started to build my own Low EMF cables, I tried many different crimp tools and it got better and better. I probably wasted lots of expensive LowThermal, Inc. terminals and lugs but at the end I got really good results from the looks of the crimping and stability and repeatability.Here are some pictures of a few different cables and lugs I crimped.The most expensive crimp tools are not always the best for the job.Also shown is the connection of the shield to the ground cable.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 07, 2021, 10:53:36 am
content: And a few more pictures

author: mzzj
author_rank: Super Contributor
timestamp: November 07, 2021, 11:28:41 am
content: Quote from: Dubbie on November 29, 2017, 10:52:26 amFor thermal EMF you need temperature differential over dissimilar metals. In this case it any temperature differential over the joint is extremely small and the generated thermovoltage is shunted by direct copper-to-copper connection.Effect is probably REALLY small and you would need nanovoltmeter to worry about it (and possibly even then need to introduce some unrealistic thermal gradients)I just did some experiments with 3458A, 1mm2 "lamp cord" wire and 40/60 solder:1. "lap solder joint" with wires overlapping each other by about 5mm(overlapping but not twisted together): max EMF about 0.3uV when I introduce large thermal gradient by grabbing one side of the copper wire between my fingers.2. "butt joint" with 1mm gap bridged with solder: 1.7uV max EMF when I introduce large thermal gradient by grabbing one side of the copper wire between my fingers.3. "twisted lap joint" max EMF about 0.3uV when I introduce large thermal gradient by grabbing one side of the copper wire between my fingers.4. "twisted lap joint" with tinned copper ferrule crimped over the joint: 0.03uVAll measured with NCPL=500, Math Null. Short-term stability seemed to be better than 0.01uV during measurements."Crimped and soldered" joint is probably better than number 1. or 0.3uV.Also note that this was under rather extreme thermal gradient over the joint.At some point you would need to start to worry about copper vs copper vs copper, different batches having dissimilar thermal EMF or cold-working introduced stresses changing the thermal EMF.(These are all very real problems with thermocouples)

author: coppercone2
author_rank: Super Contributor
timestamp: November 07, 2021, 03:38:33 pm
content: the crimp tool die pressure curves has a inflection point around the region that is best mechanically and electrically. you can deviate from this region with pressure control to get a lower resistance /stabler? electrical connection at the expense of stressed copper, which will be mechanically weaker, or a mechanically stronger joint that is electrically weaker, if i recall. optimum pressure probobly depends on strand parameters also.so you can adjust crimp pressure/height so long your requirements are met, but it would require large scale testing.