Describing the properties of a substance in a wide range around the critical point

It is shown that one can use variables characterizing the distance of a point in the $\rho T$ diagram from the spinodal and the symmetry line to obtain a phase diagram that is the same for different substances. In this diagram, segments of straight lines represent the spinodal, binodal, critical isotherm, and symmetry line. These variables also enable one to formulate a simple equation of state for a wide region around the critical point.