Subatmospheric vapor pressures for fluoromethane (R41), 1,1-difluoroethane(R152a), and 1,1,1-trifluoroethane (R143a) evaluated from internal-energy measurements

Vapor pressures were evaluated from measured internal-energy changes Delta U-(2) in the vapor + liquid two-phase region. The method employed a thermod ynamic relationship between the derivative quantity (partial derivative U-( 2)/partial derivative V)(T), the vapor pressure p(sigma), and its temperatu re derivative (partial derivative p/partial derivative T)(sigma). This meth od was applied at temperatures between the triple point and the normal boil ing point of three substances: fluoromethane (R41), 1,1-difluoroethane (R15 2a), and 1,1,1-trifluoroethane (R143a). In the case of R41, vapor pressures up to 1 MPa were calculated to validate the technique at higher pressures. For R152a, the calculated vapor pressure at the triple-point temperature d iffered from a direct experimental measurement by less than the claimed unc ertainty (5 Pa) of the measurement. The calculated vapor pressures for R41 helped to resolve discrepancies in several published vapor pressure sources . Agreement with experimentally measured vapor pressures for R152a and for R143a near the normal boiling point (101.325 kPa) was within the experiment al uncertainty of approximately 0.04 kPa (0.04%) for the published measurem ents.