The phases of a substance and the relationships between its properties are most commonly shown on property diagram. A large number of different properties have been defined, and there are some dependencies between properties. For example, at standard atmospheric pressure and temperature above 212F, water exists as steam and not a liquid; it exists as a liquid at temperatures between 32F and 212F; and, it exists as ice at temperatures below 32F. In addition, the properties of ice, water, and steam are related. Saturated steam at 212F and standard atmospheric pressure has a specific volume of 26.8 ft 3/lbm. At any other temperature and pressure, saturated steam has a different specific volume. For example, at 544F and 1000psia pressure, its specific volume is 0.488 ft 3/lbm.
There are five basic properties of a substance that are usually shown on property diagrams.
These are: pressure (P), temperature (T), specific volume (n), specific enthalpy (h), and specific entropy (s). When a mixture of two phases, such as water and steam, is involved, a sixth property, quality (x), is also used.
There are six different types of commonly encountered property diagrams.
These are: Pressure-Temperature (P-T) diagrams, Pressure-Specific Volume (P-n) diagrams, Pressure-Enthalpy (P-h)diagrams, Enthalpy-Temperature (h-T) diagrams, Temperature-entropy (T-s) diagrams, and Enthalpy-Entropy (h-s) or Mollier diagrams.