The following graph, developed using the Thermodynamic Properties for Excel add-in, is a Temperature/Volume Diagram for steam. This diagram shows the temperature versus specific volume for lines of constant pressure. This is typically one of the first diagrams that will be introduced to students in their Introduction to thermodynamics textbook. Today using computer programs, the use of such diagrams is no longer necessary but are still of use for educational purposes to give students the big picture concept as to how these parameters relate to one another. Note that a log scale is used for the x axis (specific volume), otherwise the saturated liquid and vapor lines will appear to lay on top of one another and the shape of the steam dome will not be apparent. 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.

Although the specific volume of the compressed liquid appears to be identical in the graph above, as shown in the graph below, there is in fact a difference in the specific volumes at various pressures. But essentially, liquid water can be considered to be incompressible. The close up view of the saturated liquid portion of the graph shows this difference in specific volume for various pressures.