Defining Points
Between 0.65 K and 5.0 K ITS-90 is defined by the vapor-pressure temperature relationship of 3He and 4He.
Between 3.0 K and 24.5561 K (triple point of neon) ITS-90 is defined by means of a helium gas thermometer calibrated at three fixed points in this range.
Between 13.8033 K (triple point of equilibrium hydrogen) and 1234.93 K (freezing point of silver) ITS-90 is defined by means of a Standard Platinum resistance thermometer (RTD) calibrated at the defining fixed points and using specified interpolation procedures.
Above 1234.93 K (freezing point of silver) ITS-90 is defined in terms of a defining fixed point and the Planck radiation law.
The table below lists the defining fixed points of ITS-90.
| Substance and its state | Defining point in kelvins (range) |
Defining point in degrees Celsius (range) |
|---|---|---|
| Vapor-pressure / temperature
relation of helium-3 (by equation) |
(0.65 to 3.2) | (−272.50 to −269.95) |
| Vapor-pressure / temperature
relation of helium-4 below its lambda point (by equation) |
(1.25 to 2.1768) | (−271.90 to −270.9732) |
| Vapor-pressure / temperature
relation of helium-4 above its lambda point (by equation) |
(2.1768 to 5.0) | (−270.9732 to −268.15) |
| Vapor-pressure / temperature
relation of helium (by equation) |
(3 to 5) | (−270.15 to −268.15) |
| Triple point of hydrogen | 13.8033 | −259.3467 |
| Triple point of neon | 24.5561 | −248.5939 |
| Triple point of oxygen | 54.3584 | −218.7916 |
| Triple point of argon | 83.8058 | −189.3442 |
| Triple point of mercury | 234.3156 | −38.8344 |
| Triple point of water1 | 273.16 | 0.01 |
| Melting point2 of gallium | 302.9146 | 29.7646 |
| Freezing point2 of indium | 429.7485 | 156.5985 |
| Freezing point2 of tin | 505.078 | 231.928 |
| Freezing point2 of zinc | 692.677 | 419.527 |
| Freezing point2 of aluminum | 933.473 | 660.323 |
| Freezing point2 of silver | 1234.93 | 961.78 |
| Freezing point2 of gold | 1337.33 | 1064.18 |
| Freezing point2 of copper | 1357.77 | 1084.62 |
1 The triple point of water is frequently approximated by the using the melting point of water at Standard conditions for temperature and pressure
2 Melting and freezing points are distinguished by whether heat is entering or leaving the sample when its temperature is measured. See melting point for more information.
Read more about this topic: International Temperature Scale Of 1990
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