Short answer

An RTD changes resistance with temperature. A thermocouple produces a small voltage based on a junction of dissimilar metals. That difference determines the wiring, the input hardware, and the error modes.

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What to look for

Two-wire RTD wiring includes the lead resistance in the measurement. Three-wire arrangements compensate for much of that lead effect when the leads are matched; four-wire measurement separates current and voltage paths for higher accuracy.

Thermocouple systems require the correct thermocouple type and cold-junction compensation. A stable but implausible temperature can be a configuration mismatch rather than a failed probe.

FIELD CASE / REASONING PATH

When the symptom appears in the plant

An industrial oven was consistently overbaking products. The control system used a Type J thermocouple, but a technician had inadvertently used Type K extension wire when repairing a broken connection near the heating zone. The junction of the dissimilar metals created a secondary cold junction, introducing a significant offset error in the temperature reading. Replacing the extension wire with correct Type J wire fixed the issue.

DIAGNOSTIC SEQUENCE

Five checks before replacement

  1. Identify the sensor type: RTDs typically have 2, 3, or 4 identical looking wires (often red/white), while thermocouples use specific color-coded pairs (e.g., red/white for J, red/yellow for K in US standards).
  2. Measure the sensor resistance: A Pt100 RTD should read near 100 ohms at 0°C, while a thermocouple should show a near short circuit (a few ohms).
  3. Verify the input card configuration in the PLC or temperature controller matches the connected sensor type.
  4. Check the entire cable run for thermocouples to ensure the correct extension wire type was used and polarity is maintained.
  5. For 3-wire RTDs, ensure all three wires have identical resistance and the compensation lead is wired to the correct terminal.
PRACTICAL NOTES

Use RTDs for high accuracy at lower temperatures (<600°C); use thermocouples for high temperatures, vibration, or fast response times.

Never splice thermocouple wire with standard copper wire or standard terminal blocks; it creates unwanted parasitic junctions.

RTD extension wires must be equal length and identical gauge to prevent lead wire resistance errors.

BEFORE YOU REPLACE THE PART

Bench checklist

  • Confirm the supply at the device terminals.
  • Confirm the meter is in the correct mode and position.
  • Confirm the input range matches the signal type.
  • Confirm the reference/common is intentional.
  • Record the reading before changing the wiring.

Reference families

  • ASTM E230 (Thermocouple Standards)
  • IEC 60751 (RTD Standards)

Always verify the current edition and the exact device manual for the installation.

READ THE PRIMARY MATERIAL

Official references

These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.

This note is an educational reference, not a substitute for the equipment manual, site procedures, or qualified electrical work.