Electronics > Metrology

Precision Resistors - General Thread

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3roomlab:

--- Quote from: manganin on March 05, 2016, 10:14:56 am ---
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.

--- End quote ---

https://www.eevblog.com/forum/metrology/replacing-teflon-cable-with-cat6-in-metrology/msg887437/?topicseen#msg887437

DavidKo:
Look for the oxygen free speaker cable. It is cheap, but check the isolation. I have made a cable with bananas and there is resistance between wires  :), you can always split it to two single wires and use air as insulator.

Alex Nikitin:
I use CAT6 cable where I need a screen and where the wire resistance is not a major issue (voltage measurements) however it is not very flexible and needs careful handling. For mid-low resistance measurements and high currents I build my own test leads from silicone rubber isolated wires and Staubli plugs as on the photo. I've done some isolation tests and silicone rubber is pretty good, leakage current measured in a fixture as on the photo below is <0.5pA at 100V (a plain mains grade PVC isolated wire measured in the same way leaks about 100pA at 100V). The Staubli plugs are made from gold plated "copper alloy" (somewhere I've read "beryllium copper") unlike a bronze used for the majority of banana plugs and test leads like on the photo easily give me offsets below 1uV without special measures when used to connect something like SR1010 and KVD. I used two different makes of silicone rubber isolated wires, one Chinese make from Amazon (1.5mm2 , 400 strands of tinned copper, 600V 20A rated) which is surprisingly good and comes in various colours. 0.9m of this wire is about 10mOhm terminated. Another is a double-insulated wire from RS (1mm2, 250 strands of plain copper, 1000V 20A rated) which comes pre-cut at 1.2m and at that length measures about 25mOhm terminated. Both wires are very flexible and nice to use.

Cheers

Alex

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