temperature scales - definition (long) - thermodynamics and thermal physics

Definition: A number of empirical scales of temperature have been in use: the Celsius scale is widely used for many purposes and in certain countries the Fahrenheit scale is still used. These scales both rely on the use of fixed points, such as the freezing point and the boiling point of water, and the division of the fundamental interval between these two points into units of temperature (100 degrees in the case of the Celsius scale and 180 degrees in the Fahrenheit scale).

However, for scientific purposes the scale in use is the International Practical Temperature Scale (IPTS), which is designed to conform as closely as possible to thermodynamic temperature and is expressed in the unit of thermodynamic temperature, the kelvin. The 1968 version of the table (known as IPTS-68) had 11 fixed points defined by both Celsius and thermodynamic temperatures. The most recent version (IPTS-90), introduced in 1990, has 16 fixed points with temperatures expressed in kelvins:

Triple point of hydrogen: 13.8033
Boiling point of hydrogen (33 321.1 Pa): 17.035
Boiling point of hydrogen (101 292 Pa): 20.27
Triple point of neon: 24.5561
Triple point of oxygen: 54.3584
Triple point of argon: 83.8058
Triple point of mercury: 234.3156
Triple point of water: 273.16 (0.01°C)
Melting point of gallium: 302.9146
Freezing point of indium: 429.7485
Freezing point of tin: 505.078
Freezing point of zinc: 692.677
Freezing point of aluminium: 933.473
Freezing point of silver: 1234.93
Freezing point of gold: 1337.33
Freezing point of copper: 1357.77

Methods for measuring intermediate temperatures between these fixed points are specified; for example, at low temperatures (0-5K) they are measured by means of vapor-pressure determinations of ³He and ⁴He; at high temperatures (above 1234.93K) a radiation pyrometer is used.