Technical Guide
Thermocouple Extension vs Compensating Wire
Both carry the thermocouple signal from the sensor to the instrument. The difference is the conductor alloy — and that decides accuracy, temperature range and cost.
| Property | Extension cable | Compensating cable |
|---|---|---|
| Conductor alloy | Same alloy as the thermocouple (e.g. KX: Chromel/Alumel) | Different, cheaper alloy matching EMF over a limited range (e.g. SX: copper/copper-nickel) |
| Typical types | KX, JX, TX, EX, NX | SX / RX (also KCA, KCB) |
| Accuracy | Higher — tighter tolerance | Lower — acceptable within its range |
| Usable cable temperature | Wider (insulation dependent) | Narrow, typically 0–100°C / 0–200°C |
| Cost | Higher for base-metal, moderate | Much lower than platinum for R/S types |
| Typical use | Base-metal thermocouples K, J, T, E, N | Noble-metal thermocouples R and S |
How to choose
- Match the cable type to the sensor: Type K sensor needs KX (or KC) cable — never copper wire.
- For R and S platinum thermocouples, compensating cable (SX/RX) is the practical choice.
- Keep the cable within its rated temperature; compensating cable drifts outside its range.
- Use shielded or twisted pairs near motors and VFDs to cut electrical noise.
Avoiding measurement errors
- Reversed polarity at the head or panel gives wildly wrong readings — check the color code.
- Mixing ANSI (USA) and IEC (Europe/India) color codes is a frequent cause of reversed wiring.
- Every junction of dissimilar metals is a new thermocouple — use matched connectors.
Color codes
Under IEC 60584-3 the positive leg colour identifies the type (K green, J black, T brown) with white negative. Under ANSI MC96.1 the negative leg is always red. Use our engineering tools for the full color-code table and loop resistance.
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