Temperature Scale, International (ITS-90) The more often used temperature scales are the Celsius scale * and the Fahrenheit scale *. They are not suitable for scientific purposes. In SI temperature is measured in Absolute thermodynamic scale (the Kelvin scale). In view of the difficulty of measuring on the thermodynamic scale, an international scale has been adopted by the Comite International des Poids et Mesures. It is called the International Temperature Scale of 1990 (ITS-90), which is designed to conform as closely as possible to thermodynamic temperature and expressed in units of kelvin. The scale is based on fixed points, such as the triple point of Ne, O₂, Ar, Hg, H₂O, melting point of Ga, freezing point of In, Sn, Zn, Al, Ag, Au, Cu. For very low temperatures the scale is fixed by the properties of He³ and He⁴.