Which of the following thermocouples is capable of measuring a temperature of -50°C?

Solution (By Examveda Team)

1. Temperature Range of Each Thermocouple:
• Platinum–Platinum Rhodium (Type R or S) → Typically used for high temperatures, around 0°C to 1600°C. These thermocouples are designed for furnaces, metallurgy, and very high-temperature environments. Below 0°C, they become extremely insensitive and inaccurate.

• Chromel–Alumel (Type K) → Has a wide range of about −200°C to 1250°C, but its accuracy at low temperatures is poor. It becomes unstable and exhibits non-linearity below −50°C, so it is not preferred for precise low-temperature measurement.

• Iron–Constantan (Type J) → Range is around −40°C to 750°C. It cannot reliably measure below −40°C because iron begins to undergo changes (oxidation and magnetic effects). Measuring −50°C is already outside its optimal working range.

• Copper–Constantan (Type T) → Range is −200°C to +350°C. This thermocouple is specifically known for its excellent accuracy and stability at low temperatures. This makes it ideal for refrigeration, cryogenic applications, and any measurement involving temperatures below 0°C.

2. Why Copper–Constantan is Best for −50°C:
Copper–constantan provides:
• High sensitivity at low temperatures
• Very stable EMF output
• Minimal drift in the sub-zero range
• Accurate readings in the −50°C region

This is why industries such as food processing, environmental monitoring, and cryogenics commonly use Type T thermocouples for temperatures like −50°C.

3. Final Conclusion:
Only Copper–constantan (Type T) is specifically designed to measure −50°C accurately and reliably.

✔ Correct Answer: Option D – Copper–constantan