High-temperature measurement: To measure a tempering oil bath maintained near 400 °C during heat treatment of steel, which thermocouple type is most appropriate for reliable industrial service?

Introduction / Context:
Thermocouple selection balances temperature range, environment, stability, and cost. Oil bath tempering of steel around 400 °C is common in heat-treat shops. The sensor must tolerate the temperature, oxidizing potential, and cycling, while providing adequate sensitivity and durability.

Given Data / Assumptions:

• Measurement temperature ≈ 400 °C.
• General industrial environment; cost and ruggedness matter.
• Need for stable, repeatable signal over repeated cycles.

Concept / Approach:

Type K (chromel–alumel) is the general-purpose thermocouple for −200 °C to about 1100–1260 °C, with good oxidation resistance. Type J (iron–constantan) is limited by iron oxidation and is preferred for lower temperatures or reducing environments. Noble-metal Types S/R are accurate at very high temperatures but far more expensive than required for a 400 °C oil bath.

Step-by-Step Solution:

Verification / Alternative check:

Vendor datasheets and standards list Type K as the default for general furnace and bath work up to ~1100 °C. Field practice confirms reliable service in tempering operations.

Why Other Options Are Wrong:

Type J: Iron leg oxidizes; shorter life at elevated temperatures in oxidizing conditions. Type S/R: Technically suitable but uneconomical overkill for 400 °C oil bath tasks.

Common Pitfalls:

Confusing accuracy class with suitability; ignoring sheath selection (Inconel/SS) and junction style (grounded/ungrounded), which also affect performance.

Final Answer:

Chromel–alumel (Type K)