Subatmospheric vapor pressures evaluated from iInternal-energy measurements

Vapor presstires were evaluated from measured iInternal-energy changes in the vapor+liquid two-phase region, ]U ʹ-ʹʹ. The method employed a thermodynamic relationship between the derivative quantity {~Uʹ-"/?I" b and the vapor pressure {p.I and its temperature derivative I~p?Tl,. This method was applied at temperatures between the triple point and the normal boiling point of three substances: I,l.l,2-tetralluoroethane (RI34al. pentalluoroethane {R 125 ). and difluoromethane (R32 I. Agreement with experimentally measured vapor presstires near the normal boiling point (101.325kPa) was within the experimental uncertainty of approximately + 0.04 kPa I +_ 0.04 ~ I. The method was applied to R I34a to test the thermodynamic consistency of a published p p T equation of state with an equation for/)~ Ibr this substance. It was also applied to evaluate published p, data which are m disagreement by more than their claimed uncertainty.