Property Tables for Thermally Perfect Gases at Low Pressure

Tables giving gas properties at low pressure enable the efficient analysis of processes in which the gas is approximated as thermally perfect but not calorically perfect. In addition to specific enthalpy and specific internal energy, thermally perfect gas tables include special functions that depend only on temperature—relative pressure and relative specific volume. These functions may be used to determine pressure, volume, and temperature of thermally perfect gases undergoing hypothetical isentropic processes. However, the definitions of these functions included in widely used thermodynamics textbooks are vague, inconsistent, or incorrect. The intent of this work is to discuss common inaccuracies in the definitions and the tabulated values of relative pressure and relative specific volume. The origins of the tabulated data used in many engineering thermodynamics textbooks are reviewed and consistent definitions are proposed. A table listing thermally perfect gas properties for air at low pressure based on the proposed definitions is presented.