Temperature units and temperature scales
To put the measurement of temperature on a quantitative and objective basis, with sufficient accuracy we need an agreed temperature scale, units and reliable thermometers to work with.
Historically, the Fahrenheit scale was based on the melting point of ice at 32 °F and the boiling point of water at 212 °F, both at standard atmospheric pressure. Similarly, the Celsius (centigrade) scale used a ‘fundamental interval’ from 0 °C to 100 °C.
These units and scales are still used, but since 1954 the unit of temperature has been defined following a suggestion originally made by William Thomson, later Lord Kelvin, over 100 years earlier. The Kelvin scale.
What is the Kelvin scale based on?
When a material is cooled, it loses heat, and its temperature decreases. In principle this could continue until a point is reached where there is no more heat left to lose. This temperature is the absolute zero. Kelvin’s scale begins with the value zero at absolute zero and on this scale, water freezes at 273 degrees and boils at 373 degrees.
Since 1954 the unit for thermodynamic (absolute) temperature has been defined by assigning the value 273.16 kelvin to the triple point of water, the unique temperature at which the liquid, solid and vapour phases of pure water are in equilibrium.
Triple point of water
The triple point of water is the melting temperature of ice at the vapour pressure (i.e. in the absence of air).
The three phases, solid, liquid and vapour, are all present together.
The triple-point condition can be set up in practice using a glass cell with a re-entrant tube for insertion of the thermometer.
The cell contains a sample of pure water under vacuum. To prepare the cell for measurement some ice is frozen around the central tube using solid carbon dioxide (dry ice) at about -80 ° C. When enough ice has formed the cell is placed in ice at 0°C.
Inside the cell the temperature settles within 0.0001K of the triple point, 273.16 K; any heat flowing in or out just melts or freezes some ice. The long thin shape is to allow deep immersion of a thermometer for calibration.
All three temperature scales are related to each other.
Temperatures measured in degrees Celsius, use the definition that the temperature t in °C is the temperature T in kelvins minus 273.15:
Thus the triple point of water is both 273.16 K and 0.01 °C exactly.
Relative to Celsius temperatures, Fahrenheit temperatures are defined such that
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