shielded twisted pair solid bare copper wire

[guenthert]

preferably 18 AWG -- where would I buy such (small amount, say 30ft, not a whole 1000ft spool)?  Perhaps even a place in the S.F. Bay Area?

[thought about putting this into Buy/Sell, but the answers might be of wider interest -- at least among the voltnuts  ]

[TimFox]

"solid bare copper wire" usually means uninsulated ("bare").
You want to specify twisted insulated solid (not stranded) copper wire, which is probably plastic insulated rather than enameled (as in magnet wire).
18 AWG is kinda large for twisted solid wire, would be quite stiff.

[guenthert]

Well, I was looking for pure copper, not the tinned cables one easily finds (and I suspect are used in networking, at least the smaller diameter ones).  I think I saw one seller on eBay referring to such as 'bare', but he might be as clueless as I am.

Stiff is fine by me, but a little smaller would suit as well.  Not too small though, as then screw terminals  can't be used anymore easily.

[TimFox]

Solid bare copper wire is readily available, and insulated unplated copper wire is also readily available.
The latter is what is normally supplied for power wiring inside buildings, but usually 14 AWG and larger.
I checked my usual source for uninsulated bare copper wire,  https://www.remingtonindustries.com/  but all of their insulated wire seems to be tinned.
They do have 14 AWG THHN solid insulated wire in reasonable spools, which I believe is untinned copper.
You should be able to get that in arbitrary lengths at Home Depot, or in 25 and 50 foot spools in a few colors.
Much smaller wire is used for Cat 5 and Cat 6 cables, some of which should be solid wire but at roughly 24 AWG.

[Conrad Hoffman]

Search for bell wire and thermostat wire. The problem is going to be the shielding and the twist. If the length is short you can twist wires and put a braid over them. What are you trying to accomplish? That could make a difference in what people recommend.

[guenthert]

Search for bell wire and thermostat wire. The problem is going to be the shielding and the twist. If the length is short you can twist wires and put a braid over them. What are you trying to accomplish? That could make a difference in what people recommend.

Yeah, Home Depot did find me thermostat wire, which I am unfamiliar with.  Is that twisted?  Shielded?  From the pictures it didn't look like it would be.  Elsewhere I found 'Fire Alarm' cable, which looked promising (actually, it isn't -- those alarm cables aren't twisted), but I found only large, expensive spools.

Home Depot has https://www.homedepot.com/p/Micro-Connectors-Inc-250-ft-CAT-6-Solid-STP-Outdoor-23AWG-Bulk-Ethernet-Cable-Blue-TR4-560BLOU-250/312137589 .  A bit smaller diameter than I'd desire and still too large a spool, but almost there.

I want to connect a Keithley Nanovoltmeter (181) (I'm planning to connect it directly to the amplifier inside, bypassing the weird Amphenol connector) to a Keithley 7064 low thermal EMF relays card.

[TimFox]

I have made my own shielded twisted pairs with stranded untinned copper wire by adding shield braid around appropriate test lead wire, or test leads with banana plugs.  Untinned stranded wire is easier to find than untinned solid wire (insulated) in these sizes.
I usually put heat-shrink tubing at each end of the shield to hold it to the twisted wires.

[Overspeed]

Hello

Be aware of one detail : industrial copper is not the top specification low oxygen copper , and some '' chinese import '' are made in copper

Regards
OS

[TimFox]

Yes, normal copper wire is usually ETP copper, which can be mechanically harder than oxygen-free copper, but is still a good choice when you worry about thermal EMF.
OFHC and OFE vacuum-melted copper, which are much more expensive, are very useful when you need to flex or bend the wires a lot, and will handle much more abuse before work-hardening and failing due to metal fatigue.
They are also useful in vacuum, avoiding outgassing from absorbed oxygen, etc.
There are also silly uses in over-advertised audio applications.

[guenthert]

   Yeah, I'm sure Belden happily sells the right cable, for a price.  I'm an old miser though.

While Home Depot has a promising candidate, I didn't want a whole spool.  CentralComputers sells Cat7 network cables, which are shielded (STP).  But are they unplated copper?  Meanwhile I learned that copper cable is tinned not (just) for ease of soldering, but to make it resistant to humidity.  So I guess cables meant for outdoor applications will be tinned and patch cables not necessarily.  One way to find out ...

   Turns out, CentralComputers also have spools of network cables, but they were not interested in selling me a few feet (gosh, I miss WeirdStuff, Fry's and Halted/HSC now ...).  They suggested that I get a patch cable and cut of the connectors.  So I did, you know, for science.
   It's nicely shielded (each pair is shielded and all of them once more), I give them that.  And it was unplated copper, sadly however tiny (AWG 26, I'd say) and stranded.  Guess, the latter I could have figured by the ease of bending it.  Oh, well, I needed a cable for a RS-485 connection anyhow ...

[TimFox]

The only thing I can suggest, if you want AWG 18 untinned solid copper wire formed into a shielded twisted pair is the following:
1.  Buy a small roll of AWG 18 solid bare copper wire from Remington (see link above) or other reputable vendor.  Remington stocks AWG 32 through AWG 14;  25 ft of AWG 18 costs $12.06.
2.  Cut two lengths of that wire appropriate to your requirement.
3.  Insulate each length with polyolefin heat shrink tubing, and shrink to fit.
4.  Grab one end of both wires in a vise and twist them together, using a hand-operated drill (not power).
5.  Shove that assembly into a length of shield braid.
6.  Add a short length of "regular" insulated stranded wire at each end of the braid:  strip off about an inch, wrap around the braid, then cover with larger heat-shrink tubing, to give the shield connection.
Leave a reasonable length of unshielded wire at each end, to fit your requirement.

[Conrad Hoffman]

If you're connecting to a nanovolt meter, there's almost no current flowing, so why the need for any particular gauge? Why the need for solid wire? If you've ever stripped back very old untinned wire, it's not pretty, in fact it can react with the insulation. Look at old audio cables with high strand count in PVC; the wire is often green! I'd be making this out of 26 AWG silver plated wire with Teflon insulation, built as described above. Or, medium fine enameled magnet wire. Bare when you scrape it!

[guenthert]

If you're connecting to a nanovolt meter, there's almost no current flowing, so why the need for any particular gauge?

To quote the operations manual of the 7064 relays card:
"2.3 Wiring
[..]
With a contact potential of less than 1uV from input to output, #18 AWG solid copper wires are recommended to minimize the effect of thermal EMFs."

    The twisting and shielding I deem necessary as there is just a lot of EMI here around.  I could try w/o shielding, but w/o twisting it's unlikely to be fruitful.

Why the need for solid wire? If you've ever stripped back very old untinned wire, it's not pretty, in fact it can react with the insulation. Look at old audio cables with high strand count in PVC; the wire is often green! I'd be making this out of 26 AWG silver plated wire with Teflon insulation, built as described above. Or, medium fine enameled magnet wire. Bare when you scrape it!

   Connection to the 7064 is made at the barrier strip terminals.  The amplifier within the 181 uses screw terminals.  For such solid wire is imho *much* easier to use.  I don't care how the cable looks in several decades.

[TimFox]

I have also seen really crappy surfaces on fine stranded bare copper wires, such as in old power cords.
Fine strands means lots of surface area, and the rubber in old power cords is far from inert.
Is your EMI problem 60 Hz or RF?

[Overspeed]

Hello

Telephone wire are 0.5mm2 and copper quality is average good , that available in staright or twisted pair in cable , usually that available from scrap company , recycling company or telephone company which have refuse in the bin

I use bit of them to make jumper on PCB

Regards
OS

[iMo]

CAT patch cable would work fine, imho. There are 4+ pairs in the cable, use a pair as a single wire for your purpose. Those are shielded as well, stranded or solid.
Be aware some solid CAT patch cables are made of copper-clad aluminum wire..

[Wallace Gasiewicz]

I have access to old good coax, usually Belden RG 58, which may be kinked or otherwise unfit for RF.
If you have a radio or CB shop nearby, they will probably have some as "junk" usually free.
I think that 75 ohm TV cable is also good copper, lots of that stuff around.
You can remove the shield or leave it on if it will fit.
Some sensitive voltmeter measurements call for shielded wire

[Overspeed]

Hello

I agree and a major concern is corrosion for any copper or brass connector and connection . uncoated bare copper need a particular care and a dry storage up to a inert gaz storage .

Contact resistance is also concern

https://materion.com/-/media/files/alloy/newsletters/technical-tidbits/issue-no-06---the-importance-of-contact-force.pdf

Regards
OS

[guenthert]

[..]
Is your EMI problem 60 Hz or RF?

60Hz is dominant (particularly when it's cold and my upstairs neighbor turns on his space heater), but most meters filter that out quite well.  Problem are switch-mode power supplies with switching frequencies some 40kHz to 1MHz (does that qualify as RF?) which weren't so common when those old meters were designed.  I'm sure there is high frequency stuff as well, but I cannot measure that and it's probably of low field strength (with the exception of cell phone perhaps, but I can at least keep my own away).  Worst offender by a large margin is PowerLAN, which I have to turn off for the time I make measurements.

[Overspeed]

Hello

In a lab that possible to built a '' non AC '' aera and using a faraday cage , no a size of a football stadium I have one of the size of a british telephone box I have resuse a surplus french telephone box which are in stainless steel

https://hollandshielding.com/Mu-copper-faraday-cages

Regards
OS

[Conrad Hoffman]

FWIW, I use solid twisted pairs from phone wire for voltage metrology applications and it works very well. Easy enough to add shielding if you need it. I do clean the ends of the wires as needed.

[Overspeed]

Hello

This cable is a nice option as that possible to open it and recover two twisted pairs with shield or even use only one pair and let the cable as it

https://www.belden.com/products/cable/electronic-wire-cable/multi-conductor-cable/83321e

The cable OD is under 5 mm so 0.2 inch

Regards
OS

[Wallace Gasiewicz]

Anytime you have two metals in contact there is a Seebeck effect, which may affect your measurements.
A little gold plating probably does not cause much effect though
Very sensitive measurements should be with connections of same metal, in this case copper

Here is a Quote:

Thermal EMF

When two, dissimilar metals are joined a voltage is created. This voltage is known as the thermal electromotive force (EMF) or the Seebeck voltage. The Seebeck voltage is dependent on the temperature of the junction and the composition of the metals joined. The specific metal-to-metal junctions result in specific temperature coefficients (µV/°C), also known as Seebeck coefficients. The following table lists the most common metals and their respective Seebeck coefficients.
Junction    µV/°C
Copper-Copper   <0.3
Copper-Gold       0.5
Copper-Silver            0.5
Copper-Brass    3
Copper-Nickel    0.5
Copper-Lead-Tin Solder    1-3
Copper-Aluminum    5
Copper-Kovar           40
Copper-Copper Oxide    >500

[TimFox]

Note that copper-gold and copper-silver junctions have quite low EMFs, but copper-copper oxide is huge.
Teflon-insulated silver-plated solid copper wire is readily available.

[1audio]

I think the best available cable would be some long obsolete IBM Twinax:  BELDEN | 9860 0101000  Before the surplus stors all vanished they probably had some. Solid conductors in a shielded cable will be really stiff so not to desirable usually. Search eBay for "IBM Twinax" to find some. Most examples will be stranded.