Replacing Teflon cable with CAT6 in metrology - Page 1
Forum section: electronics / general

author: manganin
author_rank: Regular Contributor
timestamp: March 05, 2016, 10:14:56 am
content: 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 RESISTANCEMeasurements 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=10VR ~ 20 Tohm @ U=100VR ~ 2 Tohm @ U=1000VHigh 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.CHEMICALSSeveral years in mineral oil bath didn't affect the cable in any way (only the polyethylene was in the oil, not the PVC jacket).TEMPERATURENot suitable for use at elevated temperature.

author: zlymex
author_rank: Frequent Contributor
timestamp: March 05, 2016, 01:41:30 pm
content: 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.

author: manganin
author_rank: Regular Contributor
timestamp: March 09, 2016, 06:25:12 pm
content: Quote from: zlymex on March 05, 2016, 01:41:30 pmIt 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...

author: gilbenl
author_rank: Regular Contributor
timestamp: March 09, 2016, 07:52:10 pm
content: 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.aspx4 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.

author: uncle_bob
author_rank: Unknown
timestamp: March 10, 2016, 01:31:39 am
content: HiBuy 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

author: zlymex
author_rank: Frequent Contributor
timestamp: March 10, 2016, 01:36:07 am
content: 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.

author: gilbenl
author_rank: Regular Contributor
timestamp: March 10, 2016, 03:05:40 am
content: Quote from: zlymex on March 10, 2016, 01:36:07 amSuper slick. Thanks for sharing!*Orders 50ft of CAT6 off Amazon*

author: quarks
author_rank: Frequent Contributor
timestamp: March 10, 2016, 07:01:23 am
content: I totally overlooked this.Thanks for sharing.

author: bdivi
author_rank: Regular Contributor
timestamp: May 05, 2016, 02:03:13 pm
content: Quote from: manganin on March 05, 2016, 10:14:56 amThat 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.

author: manganin
author_rank: Regular Contributor
timestamp: May 05, 2016, 06:27:14 pm
content: Quote from: bdivi on May 05, 2016, 02:03:13 pmIt 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.Quote from: manganin on March 05, 2016, 10:14:56 am(Each pair has one plain color wire and one white striped wire.)

author: mmagin
author_rank: Frequent Contributor
timestamp: May 06, 2016, 11:07:06 pm
content: Quote from: DiligentMinds.com on May 05, 2016, 04:39:25 pmI see some plenum-rated Cat 6 cable has FEP insulation. Would this be better than PE?

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: May 06, 2016, 11:44:20 pm
content: Quote from: DiligentMinds.com on May 06, 2016, 11:12:49 pmThanks 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.pdfI am not sure how high of a resistance it actually will measure out to based on the >16000 Megaohms spec.

author: Cerebus
author_rank: Super Contributor
timestamp: May 07, 2016, 12:08:14 am
content: Quote from: DiligentMinds.com on May 06, 2016, 11:12:49 pmAre you saying it's got horrible dielectric absorption, or just saying it makes a good capacitor and1/2CV2at 1000V needs respect even if it is only a few 10s of pF?

author: Cerebus
author_rank: Super Contributor
timestamp: May 07, 2016, 02:02:36 pm
content: Quote from: DiligentMinds.com on May 07, 2016, 12:40:59 amUm, yes, now whatdidI 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 ofnewly manufacturedpaper 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.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: May 08, 2016, 04:47:00 pm
content: "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.

author: SeanB
author_rank: Super Contributor
timestamp: May 08, 2016, 08:04:51 pm
content: Well, at least the paper is easy to find, just grab a box of Rizla............