In thermodynamics, the reduced properties of a fluid are a set of state variables scaled by the fluid's state properties at its critical point.
These dimensionless thermodynamic coordinates, taken together with a substance's compressibility factor, provide the basis for the simplest form of the theorem of corresponding states.
[1] Reduced properties are also used to define the Peng–Robinson equation of state, a model designed to provide reasonable accuracy near the critical point.
[2] They are also used to critical exponents, which describe the behaviour of physical quantities near continuous phase transitions.
The reduced specific volume (or "pseudo-reduced specific volume") of a fluid is computed from the ideal gas law at the substance's critical pressure and temperature:[1] This property is useful when the specific volume and either temperature or pressure are known, in which case the missing third property can be computed directly.