Colour Coding and Specification
Main points to note include the following. Firstly, there is no colour differentiation between extension and compensating cable. The letter `X’ after the thermocouple Type indicates extension cable, while `C’ denotes compensating cable. Also, colour does not distinguish between classes of conductor in extension cable. Instead, details like JX Class 1 indicates the tighter tolerance material for a Type J thermocouple, for example, while JX Class 2 designates the basic tolerance materials.
With compensating cable, the different alloys used are distinguished by KCA and KCB, for example, indicate Type K thermocouple compensating cable using version A and version B alloys respectively. KCB is the old `VX’ copper vs constantan combination; KCA is the earlier `WX’ iron vs constantan. Clearly, care needs to be taken here. Additional information, like numbers of pairs, conductor cross section, temperature range, manufacturer, etc may be embossed or printed on cables and cable drums.
A table showing colour identification schemes is shown on Page 5 of our Guide to Thermocouple and Resistance Thermometry. All negative legs are white; insulation of the positive legs is as per the chart; and the sheath (if any) is the same colour as the positive leg - except where intrinsically safe circuits are concerned, where it is blue irrespective of type.
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