Low-temperature measurement with thermocouples Which thermocouple type in this list is generally unsuitable for measuring sub-zero (below 0 °C) temperatures in practice?

Introduction / Context:
Different thermocouple types are optimized for different temperature ranges and environments. Selection errors can cause poor sensitivity, noise, or drift—especially near the limits of a sensor's practical range. The question asks which listed type is not typically used for sub-zero measurements.

Given Data / Assumptions:

• “Sub-zero” means below 0 °C.
• Chromel–alumel (Type K), Iron–constantan (Type J), and Copper–constantan (Type T) are base-metal thermocouples.
• Platinum–platinum + rhodium (Types S/R) are noble-metal thermocouples intended for high temperatures.

Concept / Approach:
Types S/R (Pt–PtRh) are designed for high-temperature service (furnaces, kilns) with excellent stability at elevated temperatures, but they are expensive and not favored at low temperatures, where their sensitivity and practicality are poor. By contrast, Types K, J, and especially T perform well at sub-zero temperatures and are commonly used in cryogenic and refrigeration applications.

Step-by-Step Solution:

Verification / Alternative check:
Instrument catalogs and standards list S/R lower limits typically at or above 0 °C, with far better performance at high temperatures.

Why Other Options Are Wrong:

• Chromel–alumel (K), Iron–constantan (J), Copper–constantan (T): Frequently used at sub-zero temperatures in industry.

Common Pitfalls:
Choosing noble-metal thermocouples for low-temperature duties leads to unnecessary cost and inferior sensitivity compared to Type T or K.

Final Answer:
Platinum–platinum + rhodium