Replacing Teflon cable with CAT6 in metrology

[manganin]

Multipole PTFE cable is not easily available and even more difficult to find if shielded type is required.

Even though the polyethylene insulation sets some limitations, some people have succesfully used CAT6 cable as a replacement. It is dirt cheap, available from every electronics store and has very good performance if not used in extreme conditions.






INSULATION RESISTANCE

Measurements were made with a HP 4329A High resistance meter.

The setup represents the "worst case". Resistance was measured between the plain color wires of all pairs connected together and the white striped wires connected together. Cable shield braid connected to the meter Guard to avoid noise pickup. The cable length was 15 meters.

R ~ 200 Tohm @ U=10V
R ~ 20 Tohm @ U=100V
R ~ 2 Tohm @ U=1000V

High enough for most metrology applications. For example 2 Tohm in parallel with 10 kohm causes an error of 0.005 ppm.

The voltage dependence was remarkable. The 10V and 100V were stable, but the 1000V seemed to be too much for the insulation and the readings became "noisy". The sample was a cheap no-name shielded CAT6 cable ("worst case" again). It would be interesting to see if others get similar results.

For comparison a same style PVC insulated cable measured only ~3 Gohm. It was connected the same way as the CAT6, but the length was only 2 meters. No visible voltage dependence. The previous 10 kohm example would give 3 ppm error.


CHEMICALS

Several years in mineral oil bath didn't affect the cable in any way (only the polyethylene was in the oil, not the PVC jacket).


TEMPERATURE

Not suitable for use at elevated temperature.

[zlymex]

That's indeed a very good idea.
I use CAT5 cable that is overall screened(not individual-pair screened) and with single copper wires which more durable for me.

[manganin]

I use CAT5 cable that is overall screened(not individual-pair screened) and with single copper wires which more durable for me.

It would be interesting to know if the voltage depencence that I measured is typical for polyethylene. Or if it has something to do with the quality of the cables that I bought...

[gilbenl]

Seen this solution lurking, glad to hear from someone who's had it in practice for some time.

I've using this stuff in ongoing rack build: http://www.skycraftsurplus.com/224braidedshieldnickelplatedptfejacket.aspx

4 strand+shield allows 1 wire/1 meter connection into the switch. Wished it was pure copper though.

@zlymex-I really like the lower cable in the third picture as it nicely separated the 4 pairs into 4 wire (meter-ready) bundles. How did you dress the shield wire for each 4 wire bundle? It almost looks like you split the shield in half and applied heat shrink.

[uncle_bob]

Hi

Buy the "plenum rated" version and get the best of both worlds. It's more expensive that the Home Depot version. It also holds up a *lot* better.

Bob

[zlymex]

Thanks @gilbenl.
Make a hole on the screen, bent, take two pairs out, cut another part of screen, soldered into the joint(be quick when soldering).
If two sets of 4W are not far apart, I just un-braided and split the screen.

[gilbenl]

Thanks @gilbenl.
Make a hole on the screen, bent, take two pairs out, cut another part of screen, soldered into the joint(be quick when soldering).
If two sets of 4W are not far apart, I just un-braided and split the screen.

Super slick. Thanks for sharing!

*Orders 50ft of CAT6 off Amazon*

[quarks]

I totally overlooked this.
Thanks for sharing.

[bdivi]

The setup represents the "worst case". Resistance was measured between the plain color wires of all pairs connected together and the white striped wires connected together. Cable shield braid connected to the meter Guard to avoid noise pickup. The cable length was 15 meters.

That is actually the "best case" scenario. Measuring resistance between different pairs with guarded shield between them is effectively eliminating all leakage. The "worst case" or the real life scenario is to measure the resistance between the conductors in one pair and see what the actual insulation resistance of the material is.

[manganin]

That is actually the "best case" scenario. Measuring resistance between different pairs with guarded shield between them is effectively eliminating all leakage. The "worst case" or the real life scenario is to measure the resistance between the conductors in one pair and see what the actual insulation resistance of the material is.

It was measured between the conductors of a pair, so the guard doesn't affect. And all four pairs connected in parallel making the worst case even worse.

Resistance was measured between the plain color wires of all pairs connected together and the white striped wires connected together.

(Each pair has one plain color wire and one white striped wire.)

[mmagin]

Polyethylene is a very good insulator-- not as good as PTFE, but close, and it does not have some of the bad characteristics of PTFE [dielectric absorption].  Similar to the fluorocarbon family, polyethylene does not absorb water very much [<0.01%], so is relatively unaffected by humidity.

I see some plenum-rated Cat 6 cable has FEP insulation.  Would this be better than PE?

[ManateeMafia]

Polyethylene is a very good insulator-- not as good as PTFE, but close, and it does not have some of the bad characteristics of PTFE [dielectric absorption].  Similar to the fluorocarbon family, polyethylene does not absorb water very much [<0.01%], so is relatively unaffected by humidity.

I see some plenum-rated Cat 6 cable has FEP insulation.  Would this be better than PE?

FEP is in the Teflon family and has very good electrical characteristics.  The only thing to watch out for is to not use the cable at 1000V, and then immediately use it for a low voltage-- (the "charge" will need time to dissipate).  If you are not using it at higher voltages, then this is a non-issue.  FEP handles much higher heat than PE, and like PTFE will not absorb water vapor.

Thanks for the warning. It is good to know before hooking up to a HV source.

Belden makes a FFEP insulated cable model 82688 . I ended up getting a "small" quantity of it. Haven't had time to test it yet. It is not CAT6 but token ring so this is only two pair.
http://www.belden.com/techdatas/metric/82688.pdf

I am not sure how high of a resistance it actually will measure out to based on the >16000 Megaohms spec.

[Cerebus]

FEP is in the Teflon family and has very good electrical characteristics.  The only thing to watch out for is to not use the cable at 1000V, and then immediately use it for a low voltage-- (the "charge" will need time to dissipate).  If you are not using it at higher voltages, then this is a non-issue.  FEP handles much higher heat than PE, and like PTFE will not absorb water vapor.

Are you saying it's got horrible dielectric absorption, or just saying it makes a good capacitor and ¹/₂CV² at 1000V needs respect even if it is only a few 10s of pF?

[Cerebus]

I'm not certain if "dielectric absorption" is the right property-- I'm getting old, and my memory is not so good... 

Um, yes, now what did I come in here to comment about? Oh yes...   

From the rest of what you say, it sounds as if you are talking dielectric absorption - or "soakage" in old money for those of us who remember ten bob notes and threepenny bits.

Talking about terrible soakage, I was searching on ebay the other day for a cheap kit of poly-whatever capacitors and encountered boxes of newly manufactured paper and wax capacitors obviously targeted at audiophool electric guitarists. I conclude the latter because the kit was for a named guitar and consisted of 1 (one) 50 nF cap and 1 (one) 100 nF cap for, drum roll, £51.00 GBP! At 25 sobs a capacitor I fully expect those to come with a photograph of each individual capacitor being hand rolled on the thighs of naked virgins by the ghost of Bob Pease.

[Edwin G. Pettis]

"I fully expect those to come with a photograph of each individual capacitor being hand rolled on the thighs of naked virgins by the ghost of Bob Pease."

Good line, I think Bob might have gotten a good laugh out of that himself and then tore into them for being stupid.

[SeanB]

Well, at least the paper is easy to find, just grab a box of Rizla............