Thermometry

Thermometry is the science and practice of temperature measurement. Any device used to measure temperature is known as a thermometer. In this in-depth content, we will explore the principles, types, and applications of thermometry, as well as the differences between various types of thermometers.

Principles of Thermometry

The principle behind thermometry is based on the physical properties of materials that change in a predictable way with temperature. These properties include:

Volume: Many substances expand with an increase in temperature.
Electrical resistance: The resistance of conductors changes with temperature.
Radiation: Objects emit radiation whose intensity and wavelength distribution depend on their temperature.
Color: Certain materials change color based on their temperature.

Types of Thermometers

There are several types of thermometers, each based on different physical principles:

Liquid-in-glass thermometers: These thermometers use the expansion of a liquid (usually mercury or alcohol) in a glass tube to measure temperature.
Bimetallic strip thermometers: These consist of two metals with different expansion rates that are bonded together. Temperature changes cause the strip to bend.
Resistance temperature detectors (RTDs): These use the change in electrical resistance of metals with temperature to measure it.
Thermocouples: These are made from two different metals joined at one end. A temperature difference between the joined end and the other ends generates a voltage.
Infrared thermometers: These measure the thermal radiation emitted by an object to determine its temperature.
Semiconductor thermometers: These use the temperature-dependent properties of semiconductors to measure temperature.

Temperature Scales

Temperature is measured in degrees using various scales, the most common being Celsius (°C), Fahrenheit (°F), and Kelvin (K). The Kelvin scale is the SI unit for temperature and is based on the properties of an ideal gas.

Formulas in Thermometry

The relationship between different temperature scales can be expressed with formulas:

Celsius to Fahrenheit: $ F = \frac{9}{5}C + 32 $
Fahrenheit to Celsius: $ C = \frac{5}{9}(F - 32) $
Celsius to Kelvin: $ K = C + 273.15 $
Kelvin to Celsius: $ C = K - 273.15 $

Examples

Liquid-in-glass thermometer: If a mercury thermometer reads 20°C, what is the temperature in Fahrenheit?

Using the formula: $ F = \frac{9}{5}C + 32 $

$ F = \frac{9}{5} \times 20 + 32 = 68°F $

Bimetallic strip thermometer: A bimetallic strip is made of brass and steel. Brass expands more than steel for the same increase in temperature, causing the strip to bend towards the steel side when heated.
Thermocouple: If a thermocouple generates a voltage that corresponds to a reference table value of 500°C, the temperature at the measurement junction is 500°C.

Differences Between Thermometer Types

Feature	Liquid-in-glass	Bimetallic Strip	RTD	Thermocouple	Infrared	Semiconductor
Principle	Liquid expansion	Metal expansion	Resistance change	Thermoelectric effect	Radiation	Semiconductor properties
Temperature Range	Narrow	Wide	Wide	Very wide	Wide	Wide
Accuracy	Moderate	Low	High	Moderate	Moderate	High
Response Time	Slow	Moderate	Fast	Fast	Very fast	Fast
Durability	Fragile	Durable	Durable	Durable	Durable	Durable
Cost	Low	Low	High	Moderate	High	Moderate

Applications of Thermometry

Medical: Measuring body temperature to detect fever.
Industrial: Monitoring processes that require precise temperature control.
Meteorological: Recording weather temperature data.
Research: Conducting experiments that involve temperature-dependent phenomena.

Conclusion

Thermometry is a crucial aspect of scientific measurement, with a wide range of applications across various fields. Understanding the principles, types, and differences between thermometers is essential for selecting the right thermometer for a specific application and for interpreting temperature measurements accurately.