Although the Kelvin and Celsius scales are defined using absolute zero (0 K) and the triple point of water (273.16 K and 0.01 °C), it is impractical to use this definition at temperatures that are very different from the triple point of water. Accordingly, ITS–90 uses numerous defined points, all of which are based on various thermodynamic equilibrium states of fourteen pure chemical elements and one compound (water). Most of the defined points are based on a phase transition; specifically the melting/freezing point of a pure chemical element. However, the deepest cryogenic points are based exclusively on the vapor pressure/temperature relationship of helium and its isotopes whereas the remainder of its cold points (those less than room temperature) are based on triple points. Examples of other defining points are the triple point of hydrogen (−259.3467 °C) and the freezing point of aluminum (660.323 °C).
Thermometers calibrated per ITS–90 use complex mathematical formulas to interpolate between its defined points. ITS–90 specifies rigorous control over variables to ensure reproducibility from lab to lab. For instance, the small effect that atmospheric pressure has upon the various melting points is compensated for (an effect that typically amounts to no more than half a millikelivin across the different altitudes and barometric pressures likely to be encountered). The standard even compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample. ITS–90 also draws a distinction between “freezing” and “melting” points. The distinction depends on whether heat is going into (melting) or out of (freezing) the sample when the measurement is made. Only gallium is measured while melting, all the other metals are measured while the samples are freezing.
A practical effect of ITS–90 is the triple points and the freezing/melting points of its thirteen chemical elements are precisely known for all temperature measurements calibrated per ITS–90 since these thirteen values are fixed by its definition. Only the triple point of Vienna Standard Mean Ocean Water (VSMOW) is known with absolute precision—regardless of the calibration standard employed—because the very definitions of both the Kelvin and Celsius scales are fixed by international agreement based, in part, on this point.
Although “International Temperature Scale of 1990” has the word “scale” in its title, this is a misnomer that can be misleading. ITS–90 is not a scale; it is an equipment calibration standard. Temperatures measured with equipment calibrated per ITS–90 may be expressed using any temperature scale such as Celsius, Kelvin, Fahrenheit, or Rankine. For example, a temperature can be measured using equipment calibrated to the kelvin-based ITS–90 standard, and that value may then be converted to, and expressed as, a value on the Fahrenheit scale (e.g. 211.953 °F).
ITS–90 does not address the highly specialized equipment and procedures used for measuring temperatures extremely close to absolute zero. For instance, to measure temperatures in the nanokelvin range (billionths of a kelvin), scientists using optical lattice laser equipment to adiabatically cool atoms, turn off the entrapment lasers and simply measure how far the atoms drift over time to measure their temperature. A cesium atom with a velocity of 7 mm per second is equivalent to temperature of about 700 nK (which was a record cold temperature achieved by the NIST in 1994).
|Substance and its state|| Defining point in kelvins|
| Defining point in degrees Celsius|
|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 water||273.16||0.01|
|Melting point1 of gallium||302.9146||29.7646|
|Freezing point1 of indium||429.7485||156.5985|
|Freezing point of tin||505.078||231.928|
|Freezing point of zinc||692.677||419.527|
|Freezing point of aluminum||933.473||660.323|
|Freezing point of silver||1234.93||961.78|
|Freezing point of gold||1337.33||1064.18|
|Freezing point of copper||1357.77||1084.62|
1 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.