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ISO 31-4 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to heat. It is superseded by ISO 80000-5.
Its definitions include:
| Quantity | Unit | Remarks | |||
|---|---|---|---|---|---|
| Name | Symbol | Name | Symbol | Definition | |
| thermodynamic temperature | T, (Θ) | kelvin | K | The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. | For practical measurements, the International Temperature Scale of 1990 defines several fixed points and interpolation procedures. |
| Celsius temperature | t, φ | degree Celsius | °C | The degree Celsius is a special name for the unit kelvin, for use in stating values of Celsius temperature. | t = T − T0, where T0 = 273.15 K |
| linear expansion coefficient | αl | reciprocal kelvin, kelvin to the power negative 1 | K | ||
| ... | |||||
Annex A of ISO 31-4 lists units of heat based on the foot, pound and second and some other units, including the degree Rankine, degree Fahrenheit, British thermal unit and others. Annex B lists conversion factors for three versions of the calorie.
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