New standard standard
As of January 1, 1990 the boiling temperature of water in 1 atm will not be exactly 100ºC. For many people this will be the most spectacular conclusion of the decision recently adopted by the International Committee on Weights and Measures. Change of international scale. This change is due to the advances experienced in basic thermometry since 1968.
Since 1954, the triple point of water (i.e., the only equilibrium point between liquid, gaseous and solid water) to define Kelvin has been valued at 273.16 K. The value of Kelvin for the definition of Celsius temperatures has been reduced by 273,15. To set the value of other temperatures, including boiling water, according to these definitions, thermodynamic instruments such as the constant volume gas thermometer will be used. The use of this type of thermometers is a laborious work.
On the other hand, there are other tools that measure changes in concrete and repetitively, even if they are not strictly precise from the thermodynamic point of view. The objective of the international scale has been to unite precision and repeatability.
The first International Scale was established in 1927 and was revised in 1948 and 1968. The new ITS-90 scale is the next step in this process.
In addition to the thermometry of gases, other techniques have been used, such as the pyrometry of spectral radiation, the calorimeter of radiation and the measurement of the electrical noises of the resistors.
The new boiling point of water according to the new scale will be 99,975ºC.
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