An empirical equation for the gas compressibility factor, Z

An empirical equation is presented for the gas compressibility factor, Z, as a function of reduced temperature and reduced pressure. The average error of the equation is 2.5%, median error is 1.3%, and the coefficient of determination (R²) is 0.98. The equation is presented in a manner ready for use in software code. Comparison is made to the traditional generalized compressibility factor chart. The equation was found using genetic programming for symbolic regression coupled with the Levenburg-Marquardt algorithm for fitting-parameter optimization.