Thermocouple selection for high temperatures: which pair is suitable for measuring about 1400°C in industrial practice, considering stability and usable range?

Introduction / Context:
Thermocouples are chosen for temperature range, chemical compatibility, and stability. Around 1400°C, only noble-metal or refractory-metal thermocouples reliably operate. The question tests recognition of the appropriate thermocouple family for such high-temperature service.

Given Data / Assumptions:

• Target temperature: approximately 1400°C.
• Industrial measurement, continuous service implied.
• Standard thermocouple pairings and their typical ranges are considered.

Concept / Approach:
Copper–constantan (Type T) is limited roughly to sub-400°C service. Base-metal thermocouples like chromel–alumel (Type K) reach ~1100–1200°C in air but suffer drift at very high temperatures. Noble-metal systems such as platinum–platinum+rhodium (Types S or R) provide stable electromotive force and excellent oxidation resistance up to ~1600°C with appropriate protection. Thus, they are the preferred choice near 1400°C.

Step-by-Step Solution:

Verification / Alternative check:
Standards and vendor datasheets specify Types S/R for continuous use to ~1600°C in oxidizing environments, validating the selection.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing Type K capability in inert environments with long-term stability at 1400°C; noble metals are preferred.

Final Answer:
Platinum–platinum+rhodium