Temperature Measurement

Temperature is measured by observing some property of the system which varies with temperature. Such a property is called thermometric property , e.g:

   - The volumes of most liquids increase with temperature.     - The length of a metal rod increases as the temperature increases.     - The pressure of a constant volume of gas increases with temperature 

It is useful to establish a temperature scale so that a cardinal relationship can be established between various systems at different temperatures. This is done by defining the temperature t as a function of a thermometric property X, such that the temperature is a linear function of X, i.e., equal changes in the property X give rise to equal changes in the temperature. Such a linear function is t = a + b X, for which one needs to assign arbitrary temperatures to two values of X to find the values of the constants a and b.

For example, in the case of the Celsius scale, the measurements are based on properties of water at the boiling point and melting point. Suppose the value of the thermometric property is X_b for the normal boiling point and X_m for the normal melting point. Then the temperature is given by t = 100 (X - X_m)/(X_b - X_m), where X is the thermometric property at temperature t, and we have chosen t_m = 0°C and t_b = 100°C. Thenormal melting and boiling points are the temperatures of melting and boiling at 1 atmosphere pressure.

The major temperature scales are the Celsius scale (°C), the Fahrenheit (°F) and the Kelvin (K) scale. Note the absence of the ° sign for kelvin--it is not degrees Kelvin, but kelvins, not capitalized when spelled out, and with the normal English plural which was added to the degree when Kelvin was an adjective modifying that unit.

Different thermometers are used for different temperature ranges. As the reader might have guessed by now, this means that the different thermometers will only agree on the fixed points. However, a set of thermometers have been carefully selected and calibrated so that this is not a big issue in practice.

The standard in this case is the International Temperature Scale, which was introduced in 1927, and revised in 1948, 1968, and 1990. The latest scale is denoted by T₉₀ for the Kelvin scale and is defined from 0.65 K upwards. For instance, between 0.65 K and 5.0 K T₉₀ is defined in terms of the vapor-pressure temperature relations of ³He and ⁴He. The ranges for different materials overlap and any of the valid materials can be used as a standard in the overlapping region.