Question

Which of the following is most suitable to measure a temperature of \(2500^\circ\text{C}\)?

• A. platinum resistance thermometer
• B. mercury in glass thermometer
• C. constant volume hydrogen thermometer
• D. radiation pyrometer

Hint:

For any temperature measurement question in competitive exams, if the temperature is specified above \(1500^\circ\text{C}\), contact devices are ruled out.
Always select a pyrometer (either optical or radiation) for these ultra-high temperature applications.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
This question asks for the most suitable instrument to measure an extremely high temperature of \(2500^\circ\text{C}\).
Measuring high temperatures is common in industrial processes such as metal smelting, glass manufacturing, and refractories.
At such high temperatures, physical contact sensors will melt, degrade, or undergo rapid chemical reactions, making non-contact optical/radiation-based measurement necessary.

Step 2: Key Formula or Approach:
We must evaluate the operational temperature limits of the given measuring instruments:
1. Mercury-in-glass thermometer: works based on thermal expansion of mercury. Limits: \(-39^\circ\text{C}\) to about \(350^\circ\text{C}\) (up to \(500^\circ\text{C}\) if pressurized with nitrogen).
2. Platinum resistance thermometer (PRT): works based on the variation of electrical resistance of platinum with temperature. Limits: \(-200^\circ\text{C}\) to about \(1000^\circ\text{C}\) (maximum up to \(1200^\circ\text{C}\) under special conditions).
3. Constant volume gas (hydrogen) thermometer: works on the principle of pressure variation of a gas at constant volume. Limits: up to about \(1500^\circ\text{C}\).
4. Radiation pyrometer: works on the principle of thermal radiation emitted by a hot body, governed by the Stefan-Boltzmann law:
\[ E = \epsilon \sigma T^4 \] where $E$ is emissive power, $\epsilon$ is emissivity, $\sigma$ is Stefan-Boltzmann constant, and $T$ is absolute temperature.
Because it is a non-contact method, it has no theoretical upper temperature limit, making it ideal for temperatures above \(1500^\circ\text{C}\) up to \(3000^\circ\text{C}\) and higher.

Step 3: Detailed Explanation:
The temperature of \(2500^\circ\text{C}\) is far beyond the melting point or stable operating range of materials used in contact thermometry.
Mercury boils at \(357^\circ\text{C}\), and even with high-pressure nitrogen gas filling, a mercury-in-glass thermometer cannot exceed \(500^\circ\text{C}\).
Platinum has a melting point of \(1768^\circ\text{C}\), so a platinum resistance thermometer would melt long before reaching \(2500^\circ\text{C}\).
Gas thermometers face containment material limitations (such as silica or platinum vessels) which soften or become permeable to gases at temperatures well below \(2000^\circ\text{C}\).
A radiation pyrometer detects the intensity of thermal electromagnetic radiation emitted by the hot target.
Since it does not need to be in physical contact with the hot object, it is completely immune to thermal degradation and is the standard device for measuring extreme temperatures like \(2500^\circ\text{C}\).

Step 4: Final Answer
Therefore, the radiation pyrometer is the only suitable instrument, which corresponds to option (D).