Low-temperature thermocouple selection: which of the following thermocouples is well suited for measuring about −50 °C with good sensitivity and stability?

Introduction / Context:
Thermocouples are classified into noble-metal (high-temperature) and base-metal (general purpose, including low-temperature) types. Selecting a sensor for −50 °C requires attention to usable range, sensitivity (Seebeck coefficient), and stability at low temperatures.

Given Data / Assumptions:

• Target temperature near −50 °C.
• Ambient industrial/lab environment without strong electromagnetic noise.
• Standard thermocouple types and common usage ranges.

Concept / Approach:
Type T (copper–constantan) thermocouples are specifically recommended for low temperatures (about −200 to +350 °C) with good stability and relatively high Seebeck coefficient in the sub-zero region, making them excellent for −50 °C. Type J (iron–constantan) typically specifies −40 °C as a lower bound in many datasheets, making −50 °C marginal. Type K (chromel–alumel) can reach −50 °C but has lower sensitivity and higher error at low temperatures compared with Type T. Noble-metal thermocouples (e.g., platinum–platinum+rhodium, Type S/R) are intended for high temperatures and are not preferred for −50 °C measurements.

Step-by-Step Solution:

Verification / Alternative check:
Calibration tables show Type T with strong, linear Seebeck response near −50 °C and good repeatability.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing “capable of” with “optimal”; Type T is preferred for accuracy and stability at −50 °C.

Final Answer:
Copper–constantan