Vapor quality

In thermodynamics, vapour quality is the mass fraction in a saturated mixture that is vapour; i.e. saturated vapour has a "quality" of 100%, and saturated liquid has a "quality" of 0%. Vapour quality is an intensive property which can be used in conjunction with other independent intensive properties to specify the thermodynamic state of the working fluid of a thermodynamic system. It has no meaning for substances which are not saturated mixtures (i.e., compressed liquids or superheated fluids).

Quality ${\displaystyle \chi }$ can be calculated by dividing the mass of the vapour by the mass of the total mixture:

where ${\displaystyle m}$ indicates mass.

Another definition used by chemical engineers defines quality (q) of a fluid as the fraction that is saturated liquid. By this definition, a saturated liquid has q = 1. A saturated vapour has q = 0.

An alternative definition is the 'equilibrium thermodynamic quality'. It can be used only for single-component mixtures (e.g. water with steam), and can take values <0 (for sub-cooled fluids) and >1 (for super-saturated vapours):

${\displaystyle \chi _{eq}={\frac {h-h_{f}}{h_{fg}}}}$; where h is the mixture specific enthalpy, defined as:

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