Thermocouple selection by EMF at 400 °C: a thermocouple develops 22 mV (referenced to 0 °C cold junction). Which standard thermocouple pair most closely matches this EMF at 400 °C?

Introduction / Context:
This item tests practical thermocouple identification from a given electromotive force (EMF) at a specified temperature. In plant instrumentation, recognizing which thermocouple type fits an observed EMF helps troubleshoot mismatches, wiring swaps, or incorrect transmitter ranges.

Given Data / Assumptions:

• Measured temperature: 400 °C.
• Reported EMF: 22 mV with a 0 °C reference (standard tables assume 0 °C cold junction).
• Standard base-metal and noble-metal thermocouples are under consideration.

Concept / Approach:
Each thermocouple has a characteristic Seebeck coefficient and tabulated EMF–temperature curve. At 400 °C (0 °C reference), approximate EMFs are: Type J (iron–constantan) ≈ 21–22 mV; Type K (chromel–alumel) ≈ 16–17 mV; Type T (copper–constantan) ≈ 17 mV but not typically rated to 400 °C; noble-metal types (S, R) are only a few millivolts at 400 °C. Therefore, 22 mV corresponds most closely to Type J.

Step-by-Step Solution:
List representative EMFs at 400 °C from standard tables for candidate types.Compare each to the observed 22 mV.Type J aligns at ~21.8–22 mV; select iron–constantan.

Verification / Alternative check:
A quick rule: Type J produces higher EMF than Type K in mid-ranges; noble-metal types produce much lower EMF at the same temperature. Cross-checking against a calibration card or ISA/IEC tables confirms the match.

Why Other Options Are Wrong:
Chromel–alumel (K): About 16–17 mV at 400 °C, not 22 mV.Platinum–rhodium (S or R): Only a few mV at 400 °C, far below 22 mV.Copper–constantan (T): EMF can be similar at lower temperatures, but it is not suitable for 400 °C and typical tables do not show 22 mV at this point.

Common Pitfalls:
Forgetting the cold-junction reference (0 °C assumed here); mixing °C with °F or relying on linear approximations beyond their valid range; assuming higher EMF always implies a higher-temperature-rated sensor.

Final Answer:
Iron–constantan (Type J)