Thermocouple fundamentals for process instrumentation Which of the following standard thermocouple pairs typically offers the least overall temperature measurement range in practical industrial use?

Introduction / Context:
Thermocouples are robust, inexpensive temperature sensors widely used across chemical plants, refineries, and laboratories. Each thermocouple type (identified by its dissimilar metal pair) has a characteristic usable temperature span determined by its thermoelectric stability, oxidation resistance, drift, and electromotive force output. This question asks which listed pair usually has the smallest practical measurement range in routine service.

Given Data / Assumptions:

• Compare common industrial pairs: Copper–constantan, Iron–constantan, Chromel–alumel, and Platinum–platinum/rhodium.
• “Least temperature measurement range” refers to the typical recommended usable span in practice, not the absolute laboratory extremes.
• Ambient-to-moderate and high-temperature capabilities are both relevant.

Concept / Approach:
Type T (Copper–constantan) is valued for cryogenic and low-temperature applications (roughly from about −200 °C up to near 350 °C) but loses advantage as temperatures climb due to copper oxidation and stability limits. By contrast, Type J (Iron–constantan) extends higher (to around 700–750 °C). Type K (Chromel–alumel) covers a very broad span (to about 1100–1200 °C). Noble-metal Types S/R (Platinum–platinum/rhodium) are used at very high temperatures (to ~1600 °C), though they are seldom used at sub-zero temperatures. Hence, in typical practice, Copper–constantan offers the smallest overall span.

Step-by-Step Solution:

Verification / Alternative check:
Vendor handbooks and standards show these recommended operating ranges; Type T is optimized for low temperatures with a modest upper limit, confirming the conclusion.

Why Other Options Are Wrong:

• Chromel–alumel: Very broad high-temperature capability.
• Platinum–platinum/rhodium: Used for very high temperatures; wide span.
• Iron–constantan: Extends higher than Type T in practice.

Common Pitfalls:
Confusing “accuracy” or “sensitivity” with “range”; a sensor may be very accurate yet still have a limited temperature span.

Final Answer:
Copper-constantan