Thermocouple selection in process instrumentation: Which of the following thermocouple pairs offers the widest practical temperature measurement range for industrial use (covering very high temperatures as well as a useful lower range)?

Introduction / Context:
Thermocouples are widely used temperature sensors in chemical and process industries because they are rugged, self-powered, and cover broad ranges. Different alloy combinations define different usable spans. This question tests whether you can identify the thermocouple family that spans the widest useful temperature range, especially toward the high end needed in furnaces, kilns, and reactors.

Given Data / Assumptions:

• No special protective atmospheres are assumed beyond normal industrial practice.
• “Widest range” emphasizes the upper limit achievable with reliable performance.
• Comparisons are among standard, commonly referenced types.

Concept / Approach:
Base-metal thermocouples like Type J (iron–constantan), Type K (chromel–alumel), and Type T (copper–constantan) are economical and cover moderate ranges. Noble-metal combinations using platinum and platinum/rhodium (Types S, R, and B) tolerate much higher temperatures with stability, enabling accurate measurement in the 0 to 1600+ °C region depending on type. Therefore, when the criterion is the widest practical measurement span with high maximum temperature capability, the platinum–platinum/rhodium family is preferred.

Step-by-Step Solution:

Verification / Alternative check:
Vendor data sheets and standards consistently show Types S, R, and B reaching temperatures beyond base-metal limits while maintaining calibration stability in oxidizing environments.

Why Other Options Are Wrong:

• Iron–constantan: Useful but limited at high temperatures and vulnerable at low temperatures due to iron oxidation.
• Chromel–alumel: Very versatile but still capped below noble-metal upper limits.
• Copper–constantan: Excellent at low temperatures; low upper limit.
• Type N: Improved stability vs K, but upper limit lower than S/R/B.

Common Pitfalls:
Confusing accuracy with range; some base-metal types can be more sensitive at moderate temperatures but cannot withstand extreme heat like platinum-based types.

Final Answer:
Platinum–platinum/rhodium (Types S/R/B)