Correcting the stem per Recovery-First Policy (iron–constantan thermocouple): The stated usable range is 0 to 870 °C for an iron–constantan (Type J) thermocouple. It cannot be reliably used at sub-zero temperatures mainly because, at lower temperature, which issues arise?

Introduction / Context:
Type J thermocouples (iron–constantan) are popular in general-purpose industrial measurements. However, their recommended range is narrower than that of Type K or noble-metal thermocouples, and special limitations apply at low temperatures. The original stem’s wording was inconsistent with the options; per the Recovery-First Policy, we clarify that the device is not preferred for sub-zero service and ask for the dominant reasons.

Given Data / Assumptions:

• Sensor: iron–constantan thermocouple (Type J).
• Environment: potential exposure to moisture/oxygen.
• Range of interest: below 0 °C (sub-zero).

Concept / Approach:
At low temperatures, iron suffers from oxidation (rusting) in the presence of moisture and oxygen, which degrades junction integrity and stability. Additionally, iron can become brittle at low temperatures, making the junction and leads vulnerable to mechanical damage during handling or thermal cycling. These material issues—not a fundamental inability to generate measurable EMF—are why Type J is avoided for sub-zero or cryogenic duties. EMF magnitude is adequate and measurable with modern instrumentation; the reliability concerns dominate.

Step-by-Step Solution:

Verification / Alternative check:
Thermocouple selection charts recommend Types T (copper–constantan) or E (chromel–constantan) for sub-zero and cryogenic work due to better stability and corrosion resistance.

Why Other Options Are Wrong:

• (a) EMF too small: Type J has sufficient sensitivity (~50 µV/°C near ambient); electronics can resolve this.
• (e) “Ideal for cryogenic”: Incorrect; other types outperform Type J at low temperatures.

Common Pitfalls:
Assuming all thermocouples are interchangeable across ranges; always consider material compatibility with temperature and atmosphere.

Final Answer:
Both (b) and (c)