Question

For the measurements of very high temperature say around ${5000}^{o}C$ (of sun) , one can use :

• A. Gas thermometer
• B. Platinum resistance thermometer
• C. Vapour pressure thermometer
• D. Pyrometer (Radiation themometer)

Answer 1

The correct option is D Pyrometer (Radiation themometer)

Pyrometer:
A pyrometer is a type of thermometer used to measure high temperatures. Various forms of pyrometers have historically existed. In the modern usage, it is a non-contacting device that measures thermal radiation.

The thermal radiation can be used to determine the temperature of an object's surface. A modern pyrometer has an optical system and a detector. The optical system focuses the thermal radiation onto the detector.

The output signal of the detector (temperature T) is related to the thermal radiation or irradiance (j )of the target object through the $Stefan-Boltzmannlaw$, the constant of proportionality , called the Stefan-Boltzmann constant and the emissivity of the object.
$j=ϵ\sigma T^4$

This output is used to infer the object's temperature. Thus, there is no need for direct contact between the pyrometer and the object, as there is with thermocouples and resistance temperature detectors (RTDs).