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=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.
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.