Determination of densities and heat capacities from speed of sound measurements for 1,1,1,2-tetrafluoroethane

The speed of sound u is both formally independent of the amount of substanc e and can be measured easily with high precision (typically Delta u/u < 1.1 0(-5)). Measurement of this quantity offers a favorable route for obtaining density rho, molar isochoric heat capacity C-V,(m), and molar isobaric hea t capacity C-p,C-m values. This transformation from u to other thermodynami c properties, involves the solution of a second-order differential equation on either isobars, or isochores. In this paper, we compare both equation s ets for 1,1,1,2-tetrafluoroethane (HFC-134a) which is a slightly polar flui d. In the absence of reliable experimental speed of sound data over the who le fluid surface, an accurate Helmholtz function equation of state for HFC- 134a was used to simulate property data for this study. These data were gen erated at reduced temperatures T-r = T/T-c between T-r = 0.75 and T-r = 1.2 5 and at reduced pressures p(r) = p/pc up to pr = 6. The isochoric equation set was found to be more stable. For data and initial conditions without e rrors, the fractional uncertainties in rho and C-V,C-m were 0.002 and 0.02, respectively. Sensitivity studies were conducted on the isochoric equation set to document effects of interpolation step size, accuracy of initial co nditions, and presence of random errors in the data. When all sources of er ror are included, the uncertainties in density and isochoric heat capacity were no worse than any of the individual sensitivity studies. (C) 1999 Acad emic Press.