An equation of state is a mathematical relation between pressure, molar volume, and temperature of a system. In the graph below, van der Waals equation of state isotherms are calculated. The axes are labeled with the reduced pressure, p / p_c, and reduced volume, V / V_c, where p_c = a / 27b² and V_c = 3b. The individual isotherms are calculated with the reduced temperature, T / T_c, where T_c = 8a / 27Rb.

The formula is

Where the isotherms are plotted using reduced variables p_r = p / p_c, V_r = V / V_c, and T_r = T / T_c.

To graph p_r enter the parameter (i.e., T_r) and hit New Plot. Up to 5 plots can be displayed at one time. The Clear button will remove all plots. To see the parameters for each plot hit the Legend on/off button. The Redraw button will refresh the graph. This is useful when the function domain (i.e., V_r ) has been changed. To see the value of each plot at a given point, move your cursor to the desired location then click and hold. Note: The V_r axis is plotted on a base 10 logarithmic scale.