“…16 The defining point, and the primary and secondary fixed points, involve thermodynamic equilibrium among three phases (gasliquid-solid, triple point) or between two phases (liquidsolid, freezing point, or gas-liquid, boiling point) of a pure substance. 16 The defining point, and the primary and secondary fixed points, involve thermodynamic equilibrium among three phases (gasliquid-solid, triple point) or between two phases (liquidsolid, freezing point, or gas-liquid, boiling point) of a pure substance.…”