Viscosity correlation for isobutane over wide ranges of the fluid region

A new representation of the viscosity of isobutane has been developed. The representative equations include zero-density and initial-density dependenc e correlations. The higher density contributions to the residual viscosity are formed by a combination of double polynomials in density and reciprocal temperature and of a free-volume term with a temperature-dependent close-p acked density. The new full surface correlation is based on primary experim ental data sets selected as a result of a critical assessment of the availa ble information. The validity of the representation extends from the triple point to 600 K and 35 MPa in accordance with the modified Benedict-Webb-Ru bin equation of state by Younglove and Ely (1987). The uncertainty of the r epresentation varies from +/- 0.4% in the dilute gas phase between room tem perature and 600 K to +/- 3% in the thermodynamic ranges in which the equat ion of state is valid as well as where primary experimental data are availa ble.