9. Compensation and Extension Cables:

In accordance with DIN 43 722, compensation cables are marked with the letter "C", which is placed after the code letter of the corresponding thermocouple (e.g. "KC"). The electrical conductors (i.e. the flexible strands or solid wires) consist of so-called substitute materials that have a different chemical composition from the materials of the corresponding thermocouple (see the table at the end of this section). Various substitute materials may be used for a thermocouple type, with an additional letter "A" or "B" (e.g. "KCA" and KCB") being added to the designation of the cable in this case. The substitute materials have the same thermoelectric properties as the corresponding thermocouple in the application temperature range permissible for compensation cables. For the temperature range permissible according to cable type, see DIN 43 722. 
Compensation cables are used for thermocouple types K, N, R, S and B, as the substitute materials (particularly in the case of noble metal thermocouple types R, S and B) are much better priced than the (original) materials of the corresponding material. Extension cables for thermocouples (T/C extension cables) are designated, in accordance with DIN 43 722, with the letter "X", which is placed after the code letter of the corresponding thermocouple (e.g. "KX"). The electrical conductors consist of materials that nominally have the same chemical composition as the (original) materials of the corresponding thermocouple. The thermoelectric voltage of the conductor pairs, however, only needs to match the standardized basic values of the corresponding thermocouple to a temperature of 200°C (only up to 100°C for type TX).
T/C extension cables are available for thermocouple types K, N, J, T and E (all types in accordance with DIN IEC 584) as well as for types L and U (in accordance with DIN 43710). The limit deviations (tolerances) of the compensation and T/C extension cables are defined in DIN 43 722 (or in DIN 43 710 for cable types LX and UX) and apply to the cable’s condition at the time it is supplied. The limit deviation is the maximum additional deviation in microvolts that is produced in a measuring circuit if a compensation or T/C extension cable is inserted into it and the cable is inserted within its permissible temperature range.  
The loop resistance of compensation and T/C extension cables is of secondary importance in many cases, as modern electronic measuring devices have high input resistance, which means that even cable resistance of several kiloohms fails to cause significant measurement error.    
   
The following table provides an overview of the materials and colour codes of compensation and T/C extension cables and the materials of the corresponding thermocouples

   
   
Go to the Practice page        (Installing Temperature Sensors)

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