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

✍ Scribed by T.C. Dayton; S.W. Beyerlein; A.R.H. Goodwin

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 236 KB
• Volume: 31
• Category: Article
• ISSN: 0021-9614
• DOI: 10.1006/jcht.1999.0492

✦ Synopsis

The speed of sound u is both formally independent of the amount of substance and can be measured easily with high precision (typically u/u < 1•10 -5 ). Measurement of this quantity offers a favorable route for obtaining density ρ, molar isochoric heat capacity C V,m , and molar isobaric heat capacity C p,m values. This transformation from u to other thermodynamic properties, involves the solution of a second-order differential equation on either isobars, or isochores. In this paper, we compare both equation sets for 1,1,1,2-tetrafluoroethane (HFC-134a) which is a slightly polar fluid. In the absence of reliable experimental speed of sound data over the whole fluid surface, an accurate Helmholtz function equation of state for HFC-134a was used to simulate property data for this study. These data were generated at reduced temperatures T r = T /T c between T r = 0.75 and T r = 1.25 and at reduced pressures p r = p/ p c up to p r = 6. The isochoric equation set was found to be more stable. For data and initial conditions without errors, the fractional uncertainties in ρ and C V,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 conditions, and presence of random errors in the data. When all sources of error are included, the uncertainties in density and isochoric heat capacity were no worse than any of the individual sensitivity studies.