Electronics > Metrology

Replacing Teflon cable with CAT6 in metrology

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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:

--- Quote from: zlymex on March 05, 2016, 01:41:30 pm ---I use CAT5 cable that is overall screened(not individual-pair screened) and with single copper wires which more durable for me.

--- End quote ---

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

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