Dow dielectric compound
How you measure resistance of:
* Dow compound.
* BNC-Banana adapter.
* BNC-Wire adapter.
?
It increase measurement uncertainty.
Good way - remove BNC's, use separated PTFE insulated wire.
If you want coaxial - use only high quality shielded cable with strong and fat PTFE insulation, to remove EMI effects and keep wire-to-wire resistance always high. Try not use any BNC connectors(or use connectors only with PTFE).
Lots of tests and improvements since we last talked. I changed the connectors on the 990M standard resistor (DIY of 10 99M 0.1% HV units)n to MHV. The resistor stack is inside a Hammond aluminum box, lined with HV silicone tape.
Testing with short single conductor silicone wires shows are reading of 989.80M Ohms, which is just right where I wanted (selected lower value resistors for the stack so I could trim it later).
This lets me test all my coaxial and other test cables by using an MHV TEE right at the resistor box, and using one of the ports to attach the cable being tested (far end non terminated).
Any change from 989.8 indicates a leakage path.
Come to find out every one of my right angle BNC to BNC male cables were bad. These were about 12 inches long and had high quality PTFE coaxial cable. I cut them up and tested them on a HP-Yokogawa High Resistance meter at 1000V . Their leakage came from the BNC connectors not the cable. Cut up about 5 of the "cleaner looking" ones before testing the one black covered one, which had no great leakage at all. Evidently somewhere along the line the clean ones were also black but cleaned by some method by a technician, which left some sort of residue inside.
Now I've set up a desk just for resistor testing with a Guarded Wheatstone bridge, an HP-4329 Hi Resistance, HP-4328 milliOhm meter, an ESI 232A high resistance guarded Wheatstone bridge (12 G Ohm capability), the usual Megger etc. Teflon sheeting, shielded boxes in the works, and so on as I learn more.
Now I can test all sorts of cables, actual resistors, wires, etc. The question now becomes, what is considered a low loss for a test cable? The Pomona BNC to clip leads adaptor came in at above 85 or more X 10^ 12 Ohms. (HP
4328A4329A)
At this time I am making new pairs of test cables from Belden RG179 TFE cable. 2 x 6", 12" 18" 36". Even the 6" one shows a slightly different reading when used on the test jig made from my 990M resistor (KS 34470A)- the difference is from 989.8 down to 989.47 which seems to indicate a shunt resistance in the cable.
Next to try to calculate that shunt resistance in G Ohms.
George Dowell