The following graph, developed using the H2oProperties for Excel add-in, is a Temperature/Entropy Diagram for steam. This diagram shows the enthalpy versus entropy at various steam qualities and for lines of constant pressure and constant specific volume. Historically engineers would use such diagrams for various engineering process design purposes. Of course, today using computer programs, the use of such diagrams is no longer necessary. Temperature/entropy diagrams are still of use for educational purposes to give students the big picture concept as to how these parameters relate to one another. The Thermodynamic Properties for Excel add-in is based upon the IAPWS (International Association for the Properties of Water and Steam) IF97 Industrial Formulation for the Thermodynamic Properties of Water and Steam. Note the red 220.9 bara constant pressure line that just touches the top of the saturated steam dome. This is approximately the critical point where the density of the saturated liquid and saturated vapor are identical. At pressures above the critical point (22.064 MPa) there is no change of phase from liquid to steam.