Question

Which one of the following is the most suitable to measure a temperature of 2000 \(^\circ\)C?

• A. Ordinary mercury-in-glass thermometer
• B. Platinum resistance thermometer
• C. Radiation pyrometer
• D. Constant-volume hydrogen thermometer

Hint:

Remember the contact vs. non-contact rule of thermometry: Whenever a process question specifies a temperature far above the melting thresholds of common metals (\(> 1500 ^\circ\text{C}\)), look immediately for a pyrometer option.

Answer 1

The Correct Option is C

Concept: Different temperature sensors have distinct operating limits based on their materials and physical working principles. When measuring extreme temperatures exceeding 1500 \(^\circ\)C, physical contact sensors melt or degrade rapidly. Non-contact measurement instruments based on thermal radiation theory must be utilized.

Step 1: Analyzing the limits of each instrument.

• Mercury-in-glass thermometer: Limited by the boiling point of mercury and the softening point of glass. Max operational limit is roughly 350 \(^\circ\)C to 500 \(^\circ\)C (if pressurized with nitrogen gas).
• Platinum resistance thermometer (RTD): Extremely accurate but structurally limited by the melting point and stability of platinum wires, functioning safely up to a maximum of around 850 \(^\circ\)C to 1000 \(^\circ\)C.
• Constant-volume hydrogen thermometer: A primary gas thermometer standard, generally useful only for lower to moderate temperature calibrations up to approximately 500 \(^\circ\)C.
• Radiation pyrometer: Operates via the Stefan-Boltzmann law by capturing the electromagnetic radiation emitted from a hot body. Because it requires zero physical contact with the high-temperature zone, it can seamlessly measure temperatures from 1000 \(^\circ\)C up to 3000 \(^\circ\)C and beyond.

Step 2: Conclusion.
For a demanding process environment at 2000 \(^\circ\)C, only a non-contact optical or radiation method like the radiation pyrometer can function safely and effectively.