# Temperature Measurement

Author: Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser

back to Climatic Measurements

## Introduction

In addition to properties such as mass, volume, each body also has thermal properties that give information about their thermal state.

Temperature is the physical and objective term that classifies the subjective feeling of cold and warm into an absolute scale.

Temperatures are measured with thermometers. Thermometers take advantage of the specific temperature-dependent properties of materials (e.g. volume). For laying down scales and for the calibration of thermometers, temperature fixed points are necessary. The most frequently used two fixed points are:

1. Melting point of ice =273.15 K (0ºC, 32ºF) at standard pressure (1.013 bar)
2. Boiling point of water at standard pressure

If these fixed points are assigned to specific temperature values and the difference is divided, different temperature scales are obtained. The following table shows the correlation between the most important scales:

 Scale Celsius Kelvin Fahrenheit Reaumur Symbol ${\displaystyle \vartheta }$ T tF tR Unit ºC K ºF ºR 1. FP*2. FPDivision 0100100 273.15373.15100 32212180 08080 Use Europe Science USA France(Outdated)

• FP – Fixed Point

The subdivision of the individual scales and their reciprocal conversion occurs linearly. The Kelvin scale is based on the increase in pressure of an ideal gas, the other scales are based on the increase in volume of mercury. Celsius and Kelvin are equal for temperature differences but not for absolute values.

All temperature scales were replaced in 1990 by the international temperature scale ITS-90. This has resulted in the Celsius scale not being based on the boiling point of water at 1013.25 mbar= 100ºC. The consequence is, that the boiling point of water is now 99.974ºC at standard pressure.

The various temperature measuring methods can be roughly divided into mechanical and electronic ones. The best known mechanical methods are based on the expansion of the sensor i.e. bimetallic strips, liquid thermometers (e.g. mercury, alcohol inside a capillary tube) and gas thermometers.

Today, electronic and optical temperature sensors are used most widely. In electronic sensors mostly the change in electric resistance is evaluated through temperature (resistance thermometers such as PT-100, NTC- elements). Optical sensors are widely used for the remote determination of temperatures and for moving hot components (IR thermography).