The temperature of tempering oil baths maintained at 400°C during heat treatment of steel is measured by a/an __________ thermocouple.

The correct answer is: A. Chromel-alumel

✅ Explanation:
Tempering oil baths at 400°C require a thermocouple that is:

Accurate

Durable

Cost-effective

Suited for mid-to-high temperatures

Option Analysis:
🔹 A. Chromel-Alumel (Type K) ✅
Temperature range: –200°C to +1260°C

Most widely used industrial thermocouple

Stable and reliable at 400°C

Resistant to oxidation, making it suitable for oil bath environments
✅ Best choice for this application

🔹 B. Iron-Constantan (Type J)
Max temperature: ~750°C

Prone to oxidation above 400°C
❌ Less suitable for continuous use at 400°C in oil baths

🔹 C. Platinum-Platinum/Rhodium (Types R, S, or B)
High-accuracy, but designed for much higher temperatures (up to 1700°C)

Very expensive
❌ Unnecessary for 400°C, unless extreme precision is required (e.g., in labs)

🔹 D. None of these
Incorrect because A is correct

✅ Final Answer: A. Chromel-alumel