Why Is Compensating Cable Selection Critical for Thermocouple Accuracy?

The connection between the thermocouple and the temperature controller is not a trivial matter. Using incorrect extension wire is one of the most common yet frequently overlooked errors in hot runner temperature measurement. The Fundamental Principle Compensating cables must be made of the same material as the thermocouple elements-or have identical thermoelectric properties-to maintain measurement accuracy. The thermocouple and pyrometer are electrically conducted by thermocouple extension cables or thermocouple compensating cables. Copper wire cannot be used for thermocouple extension leads under any circumstances, yet it is frequently employed by operators unfamiliar with thermocouple principles or seeking convenient solutions. Extension Type vs. Compensating Type Cables are categorized by core parameters into extension types and compensating types. Those with a "X" suffix are extension types, with much higher accuracy than compensating types (suffix "C"). Extension types are preferred for all hot runner applications. The most commonly used wire diameter is 0.5mm². The Error Mechanism When copper extension wire is used in place of proper compensating cable, an additional thermocouple junction is created at the connection point between the thermocouple wire and the copper cable. This unintended junction produces its own thermoelectric voltage, creating an unaccounted error that the temperature controller cannot compensate for through reference junction compensation. The result is a displayed temperature that differs from the actual melt temperature by an amount that varies with the temperature at the connection point. Matching Requirements The compensating wire must be strictly matched to the thermocouple model-K-type thermocouples should be paired with K-type compensating wires, and J-type thermocouples with J-type compensating wires. Substitution is prohibited. This requirement is frequently violated in practice, leading to significant measurement errors that often lead operators to make incorrect setpoint adjustments causing parts defects, while the root cause remains undiscovered. Cable Construction for Hot Runner Applications Modern cable solutions are manufactured with 2 up to 8 thermocouples and up to 16 power cores. The cable should feature screened construction for temperature control to replace the cabling with two different cables. PVC covering and shielding inside around the thermocouple wiring are designed to prevent electromagnetic interference and also make these cables long-lasting for reliable production.