Two-phase quality (Q)
Learn what vapor quality Q means in the two-phase region, why PT can be ambiguous on the saturation line, and how to use Q in calculations.
In the two-phase region (liquid + vapor), a fluid state is often described by pressure and vapor quality Q. Quality is a convenient way to indicate how much of the mixture is vapor versus liquid.
Definition
Vapor quality Q is typically defined as the mass fraction of vapor in a saturated mixture:
Q = 0→ saturated liquidQ = 1→ saturated vapor
Why PT can be ambiguous on the saturation line
For a pure fluid (and many pseudo-pure refrigerants), at a given saturation pressure, the saturation temperature is fixed (and vice versa). That means many distinct two-phase states share the same P and T but have different qualities, enthalpy, density, and other properties. To make the state unique, you need a third variable—often quality.
Important note on refrigerant blends
Many refrigerants are blends. For zeotropic blends, “saturation” can span a temperature range at a given pressure (bubble point vs dew point), also known as temperature glide. In those cases, pressure + temperature can be ambiguous in a different way, and “quality” may not map as cleanly as it does for a pure fluid. When working with blends, follow the application’s measurement conventions and use the input pair that best matches what you actually know (for example, P–h or P–s).
Practical guidance
- Use P + Q (or T + Q) when you know the state is saturated or intentionally two-phase.
- Use superheat/subcooling (or a single-phase input pair like P–h) when you want a single-phase state away from saturation.