The International System of Units holds seven "base units." And over time, each of these has - in a sense - left the physical world. Temperature has been defined by the so-called triple point in a sealed glass cell of water. The triple point is "the temperature at which water, ice and water vapor exist in equilibrium," according to NIST. That, you might think, was constant enough. But the water can contain chemical impurities, and those impurities can shift the triple point to create inaccuracies.
It's just like tap water in a pot on a stove. Common knowledge says the water will boil at 100 degrees Celsius (212 Fahrenheit; 373.15 Kelvin). But as soon as you add solute, like salt, the boiling point goes up. The salt is an impurity.
But soon we will have new constants that include an exact numerical value for the Boltzmann constant to define temperature. Then there are the Planck and Avogadro constants, which will also improve our sense of accuracy.
Getting these measurements, and keeping them stable with exact numerical values, is becoming increasingly important in our hyper-connected digital world - it all has to align somehow. And just think of the scale over which things are measured these days. Mass, for instance, is measured over an almost unimaginable scale - from that of an atom to that of planets. Precision is more than a plus.
All this has to go through a final stage of approval at the General Conference on Weights and Measures in November 2018.
http://www.dw.com/en/our-vanishing-world-of-physical-units/a-43839842
