Warren Boswell
Member
Slightly off topic. I'm just curious as to whether anyone else ever encountered this. Maybe one of you wizards have an explanation...
I was recently called to a fairly simple job on one of our compressors. The second stage suction thermocouple (type T) was reading O/C. I started checking the loop, and after a little fault-finding, saw the five meter run of cable to the thermocouple was reading 9 ohms. Thinking there might be damage to the cable, since this seemed too much for such a short run, I had the cable replaced. Thermocouple read again, and problem solved. A short while later the same couple started giving trouble again. Turns out that the terminal was slightly loose on the termination card and it would occasionally go O/C for a short period. There never was a problem with the cable, for when I measured another section, it turned out that that type of cable had a high resistance per meter (Silly mistake, I know). But here comes the mystery...
Upon measuring the same piece of cable I'd installed three months earlier, I saw the resistance over the same length had changed from 9 Ohms, to 21 Ohms. There was no damage to the cable, and it was not exposed to any extreme heat or cold which might effect it in any manner. Just an average piece of cable on a cable tray.
Anyone have an explanation why the resistance would change over time?
I was recently called to a fairly simple job on one of our compressors. The second stage suction thermocouple (type T) was reading O/C. I started checking the loop, and after a little fault-finding, saw the five meter run of cable to the thermocouple was reading 9 ohms. Thinking there might be damage to the cable, since this seemed too much for such a short run, I had the cable replaced. Thermocouple read again, and problem solved. A short while later the same couple started giving trouble again. Turns out that the terminal was slightly loose on the termination card and it would occasionally go O/C for a short period. There never was a problem with the cable, for when I measured another section, it turned out that that type of cable had a high resistance per meter (Silly mistake, I know). But here comes the mystery...
Upon measuring the same piece of cable I'd installed three months earlier, I saw the resistance over the same length had changed from 9 Ohms, to 21 Ohms. There was no damage to the cable, and it was not exposed to any extreme heat or cold which might effect it in any manner. Just an average piece of cable on a cable tray.
Anyone have an explanation why the resistance would change over time?