Inverted forms of the new helium vapour pressure equations

Equations relating the vapour pressures of He³ and He⁴ to temperature on the ‘Échelle Provisoire de Température de 1976’ have been approved by the Comité International des Poids et Mesures. Using these definitive equations temperatures can only be calculated from measured pressures by iteration. Moreover, the equations are not amenable for use at less than full precision. Alternative equations expressing temperature as a function of pressure over somewhat reduced ranges are now presented in forms which are more easily solved for temperature and are capable of further simplification if reduced accuracy is acceptable.