Theory:

Normally, a physical quantity that expresses the degree of hotness or coldness of a substance is known as temperature.
Temperature is a measure of the average kinetic energy of the particles in a system.
Thermometer
A thermometer is used to measure temperature directly.

Thermometer

Thermometers are calibrated (measuring devices) with standard scales. The scales mostly used to measure the temperature are:
• Celsius
• Fahrenheit
• Kelvin
Celsius: In thermometers, the melting point of pure ice $$(0°C)$$ is taken as Lower Fixed Point (LFP), and the boiling point of water $$(100°C)$$ is taken as Upper Fixed Point (UFP).

Fahrenheit: In thermometers, the melting point of pure ice $$(32°F)$$ is taken as Lower Fixed Point (LFP), and the boiling point of water $$(212°F)$$ is taken as Upper Fixed Point (UFP).

Kelvin: In thermometers, the melting point of pure ice $$(273 K)$$ is taken as Lower Fixed Point (LFP), and the boiling point of water $$(373 K)$$ is taken as Upper Fixed Point (UFP).
Conversion of the scales of temperature
The conversion of the scales of temperature is mainly used to change one scale to another scale.
The general formula for scale conversion is $\frac{C-0}{100}=\frac{F-32}{180}=\frac{K-273}{100}$.
 Conversion of Celsius to Kelvin: We convert the Celsius to Kelvin by adding $$(273)$$ to the Celsius of temperature.Formula:  For example: Convert $$90° C$$ into Kelvin.  $\begin{array}{l}K\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}C\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}273\\ \\ K\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}90\phantom{\rule{0.147em}{0ex}}+273\\ \\ K=363\phantom{\rule{0.147em}{0ex}}\mathit{Kelvin}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}363\phantom{\rule{0.147em}{0ex}}K\end{array}$ Conversion of Kelvin to Celsius:  We convert the Kelvin to Celsius by subtracting $$(273)$$ from the Kelvin of temperature. Formula:  For example: Convert $$323 K$$ into Celsius.  $\begin{array}{l}C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}K\phantom{\rule{0.147em}{0ex}}-\phantom{\rule{0.147em}{0ex}}273\phantom{\rule{0.147em}{0ex}}\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}323\phantom{\rule{0.147em}{0ex}}-\phantom{\rule{0.147em}{0ex}}273\phantom{\rule{0.147em}{0ex}}\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\mathit{50}\mathrm{°}\mathit{Celsius}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}50\mathrm{°}C\end{array}$ Conversion of Celsius to Fahrenheit: The temperature $$T$$ in degrees Fahrenheit ($$F$$) is equal to the temperature ($$T$$) in degrees Celsius ($$C$$) multiplied by $$9/5$$ and plus $$32$$.Formula:  For example: Convert $$20 °C$$ to degrees Fahrenheit. $\begin{array}{l}F\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}C\phantom{\rule{0.147em}{0ex}}×\phantom{\rule{0.147em}{0ex}}9/5\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}32\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\\ \\ F\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}20\phantom{\rule{0.147em}{0ex}}×\phantom{\rule{0.147em}{0ex}}9/5\phantom{\rule{0.147em}{0ex}}+\phantom{\rule{0.147em}{0ex}}32\phantom{\rule{0.147em}{0ex}}\\ \\ F\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}180/5\phantom{\rule{0.147em}{0ex}}+32\\ \\ F\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}36\phantom{\rule{0.147em}{0ex}}+32\\ \\ F\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}68\mathrm{°}\mathit{Farenheit}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}68\mathrm{°}F\phantom{\rule{0.147em}{0ex}}\end{array}$ Conversion of Fahrenheit to Celsius: The temperature $$T$$ in degrees Celsius ($$C$$) is equal to the temperature ($$T$$) in degrees Fahrenheit ($$F$$) minus $$32$$,and multiplied by $$5/9$$.Formula: $C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\left(F\phantom{\rule{0.147em}{0ex}}-\phantom{\rule{0.147em}{0ex}}32\right)×\phantom{\rule{0.147em}{0ex}}5/9\phantom{\rule{0.147em}{0ex}}$ For example: Convert $$68 °F$$ to degrees Celsius: $\begin{array}{l}C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\left(F\phantom{\rule{0.147em}{0ex}}-\phantom{\rule{0.147em}{0ex}}32\right)×\phantom{\rule{0.147em}{0ex}}5/9\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\left(68-32\right)\phantom{\rule{0.147em}{0ex}}×\phantom{\rule{0.147em}{0ex}}5/9\phantom{\rule{0.147em}{0ex}}\phantom{\rule{0.147em}{0ex}}\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}\frac{36×5}{9}\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}4×5\\ \\ C\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}20\mathrm{°}\mathit{Celsius}\phantom{\rule{0.147em}{0ex}}\mathit{or}\phantom{\rule{0.147em}{0ex}}20\mathrm{°}C\end{array}$
Application of various thermometric scales
Fahrenheit scale: The Fahrenheit scale is used by physicians for clinical thermometers.

Kelvin scale: The Kelvin scale is used by scientists in laboratories.

Celsius scale: The Celsius scale is used for weather forecasting and detecting fever for patients in the hospital.