The Effects of Moisture on the Breakdown - Pulse - Noise of Mineral - Insulated Instrument Cable

Cable resistances (R) and breakdown-pulse-noise (BPN) were measured before, during, and after heating 60-cm portions of a 10-meter mineral-insulated coaxial cable to various temperatures (T), mostly 320°C. A severe BPN-condition and a very low R within about 60 cm of the high-T end of the cable was shown to be due to atmospheric moisture admitted through an end-seal leak inflicted by a previous test. A similar, but much less severe, BPN-condition 90 to 120 cm removed from either end was attributed to a small amount of moisture that had collected at these positions during the same previous test independent of the end-seal leak. These results suggest that the BPN problem might be eliminated by better hygroscopic measures in cable manufacture.