Hi everyone, extruder we shipped 18 months ago. The end user rang this morning, barrel zone 3 is running hot and they're scorching product. The zone reads about 15 degrees low, so the loop pushes the heater to make the difference up. Type K thermocouples into a 1769-IT6 on a CompactLogix.
They moved the control panel about 6 m last month and extended the thermocouple wiring to reach it. On my say so they've swapped the zone 3 thermocouple for a new one, same reading, and checked it against a hand probe in the thermowell next to it, still 15 low. Zones 1 and 2 read a bit low as well.
Is the calibration out on that channel, or am I looking at a module swap? Zones 1 and 2 being a bit low as well is what makes me doubt it's the channel, does that fit with the panel move?
Before you touch the 1769-IT6. What did they extend the wiring with, type K extension, or whatever two core was on the shelf?
Asked them. Answer was "the yellow one from stores". Photo attached, they sent it off a phone. It's yellow tri-rated two core, the same reel they use for control wiring. Copper. Not sure they knew there was a difference.
Copper is the problem, not the module. A thermocouple measures the difference between the hot junction and the point where its two metals stop and copper starts. That point is meant to be at the module, where the IT6 has its cold junction sensor and compensates for the terminal temperature. With a copper splice the reference junction moved to the splice, in warm trunking around 40 degrees, and the module still compensates for its own terminals. Low by the difference.
Fix: pull the copper, run type K extension wire, KX, all the way to the module, polarity right. Calibration is fine, leave it. Zones 1 and 2 are the same fault, cooler splice.
The barrel zone wiring is in this one: https://plctr.com/utilizing-plc-in-plastic-and-rubber-processing/
Partly on me. Our wiring drawing says "T/C extension" and nothing about the wire type, so stores picked a colour and I guess nobody questioned it. Drawing gets fixed this week.
Yellow only means type K when it's the right yellow.
End user pulled the copper out and ran KX extension back to the 1769-IT6, polarity checked with a lighter on the tip. Zone 3 within 1 degree of the hand probe now, product fine.
The copper moved the reference junction out to a splice in warm trunking while the module carried on compensating for its own terminals. Type K the whole way back and it reads true.
Zones 1 and 2 were extended the same way and were only 3 low, so probably the splice location like marek says, but I never got them to measure it. Thanks marek, dan.
Splice closer to the module, cooler spot, same fault, less of it.