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A P-n diagram is another common  Diagram for Watertype of property diagram. Figure10 is the P-n diagram for purewater. A P-n diagram can beconstructed for any pure substance.A P-n diagram is different from aP-T diagram in one particularlyimportant way. There are regionson a P-n diagram in which twophases exist together. In theliquid-vapor region in Figure 10,water and steam exist together.For example, at point A, waterwith a specific volume (nf), givenby point B, exists together withsteam with a specific volume (ng),given by point C. The dotted lineson Figure 10 are lines of constanttemperature. The quality of themixture at any point in the liquid-vapor region can be found because the specific volumes ofwater, steam, and the mixture are all known. The quality can be found using the followingrelationship.