I. Why Use Compensating Cables?
When measuring temperature with thermocouples, the temperature of the thermocouple reference junction must be kept constant. Otherwise, measurement errors introduced during temperature reading will become a major variable and affect measuring accuracy.
In field applications, the thermocouple reference junction is often located near a high-temperature heat source and cannot maintain a stable temperature. Therefore, the reference junction must be moved to an area with more stable temperature.
Compensating cables are made of the same metal materials as the thermocouple, or low-cost readily available metals with the same thermoelectric characteristics (thermo-EMF) as the matched thermocouple within a specified temperature range (usually 0–100°C). They can extend the thermocouple or serve as connecting wires between the thermocouple and instruments (such as electronic potentiometers).
For this reason, thermocouples must be used with compensating cables.However, compensating cables cannot eliminate the influence of non-zero reference junction temperature, so the reference junction should still be corrected to 0°C in actual use.
II. Key Points for Using Compensating Cables
Each type of compensating cable can only be used with the corresponding type of thermocouple. That is, the thermo-EMF of the thermocouple and its matching compensating cable must be consistent within the specified temperature range (e.g., 0–100°C).
When connecting compensating cables to thermocouples and instruments, the two pairs of connection points must be at the same temperature, and positive and negative poles must not be reversed.A compensating cable acts like a thermocouple within 0–100°C. Current flows from the positive pole through the reference junction to the negative pole. Therefore, the positive pole of the compensating cable must connect to the positive pole of the thermocouple, and the negative pole to the negative pole.Reversed polarity will not only disable compensation but also offset part of the thermocouple's thermo-EMF, resulting in lower indicated temperature.
The temperature at the connection point between the compensating cable and thermocouple must not exceed the rated operating temperature range.
Select the wire gauge of the compensating cable according to the requirements of the matched instrument.For example, thicker gauge can be used for moving-coil instruments to reduce resistance and avoid affecting the instrument's external resistance.
For long-distance thermocouple wiring, stranded and thicker compensating cables are recommended for easier installation and laying.
III. Classification of Compensating Cables by Core Alloy Material
Compensating cables are divided into two types: Extension Type and Compensation Type.
Extension TypeNX (Nicrosil–Nisil), KX (NiCr-10–NiSi-3), EX (NiCr-10–CuNi-45), JX (Fe–CuNi-45), TX (Cu–CuNi-45)
Compensation TypeSC, RC (Cu–CuNi-0.6), KC (Cu–CuNi-40), NC (Fe–CuNi), etc.
IV. When to Use Extension or Compensation Type Cables
Extension TypeWhen the ambient temperature at the cold end of the compensating cable exceeds 100°C or is below 0°C, extension-type cables must be used to ensure measuring accuracy, though they are relatively expensive.
Compensation TypeEconomical and low-cost, but the cold-end temperature must be in a relatively stable environment.
V. Essential Characteristics of Compensating Cables
Stable thermoelectric characteristics, good electrical insulation, long service life.
Flexible, excellent bending performance, easy to install and use.
Stable and reliable sheath material with certain heat resistance and cold resistance.