Identifying thermocouple type from EMF at high temperature If a thermocouple develops about 31 mV at 800°C (hot junction), which common thermocouple type is most consistent with this EMF?

Introduction / Context:
Each thermocouple pair exhibits a characteristic electromotive force (EMF) versus temperature. By comparing a measured EMF at a specified temperature with standard tables, one can infer the thermocouple type. This is useful for maintenance and troubleshooting when markings are missing.

Given Data / Assumptions:

• Measured EMF: ≈ 31 mV at 800°C.
• Standard cold junction assumption: near 0°C (typical for tabulated values).
• Common industrial types considered: K (chromel–alumel), J (iron–constantan), S/R (platinum–platinum rhodium).

Concept / Approach:
Type K yields roughly 33 mV at 800°C; Type J is typically higher (around the low 40 mV range), while noble-metal Types S/R provide much smaller EMFs (roughly 9–12 mV in this region). A value near 31 mV aligns best with Type K, considering typical tolerances and reference junction conditions.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check with manufacturer tables: Type K around 33 mV at 800°C; Type J significantly higher; Type S/R much lower.

Why Other Options Are Wrong:

Common Pitfalls:
Ignoring cold-junction temperature compensation; small differences are expected if the actual reference is not at 0°C, but the order-of-magnitude differences still distinguish types.

Final Answer:
chromel–alumel (Type K)