The ITS (International Temperature Scale) uses 17 temperature reference points based on the physical phenomenon of the triple point, the vapour pressure, the freezing point, melting point and solidification point of different substances. These distinct points occur at a fixed, reproducible temperature, determined by nature. They do not require the use of a reference sensor in order to measure them. For example, gallium always melts (goes from solid state to liquid state) at 29.7646°C. A distinct fixed point is the triple point. And a distinct triple point is the triple point of water (0.01°C under 1 atm).
The triple point is the point at which the vaporisation curve, the sublimation curve and the fusion curve of a substance meet. It was Josiah W. Gibbs (1839-1903) who recognised and proposed the phase rule: the three states, solid, liquid, gas, only coexist at one unique point within the thermodynamic parameters, the triple point. Vapour, liquid and solid are found together at a unique defined temperature and pressure.
Figure 1. Change in pressure with temperature for the triple point of water
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