where cr is a reduced speed of sound, c/sf, in which sf is a scaling factor; f 0 and
f 1 are universal functions of Tr and Pr to be determined from data on two reference compounds. The scaling factor was used in lieu of critical data for the speed of sound. It was calculated using Eq.(1), assuming a constant value for γ and using PR EOS for calculation of Vm ð Þ2T c ð∂P=∂VmÞTc term at critical temperature. To have standard values for sf of heavy hydrocarbons the following relation was developed based on calculated data for n-alkanes from C1 to C20: