Steam & Water Properties

T-s diagram of water: dew curve and bubble curve

Click the temperature–entropy diagram to read the state of water at that point. Under the saturation dome you get the steam quality and the properties of the liquid–vapour mixture.

Temperature–entropy diagram of water

Click or tap the diagram. Thin grey lines are isobars. Keyboard: arrow keys move the point (Shift for larger steps).

State at the selected point

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Dew curve and bubble curve

On a temperature–entropy diagram the two-phase region of water appears as a dome. Its left edge is the bubble curve: saturated liquid, the state at which the first vapour bubble forms when liquid is heated. Its right edge is the dew curve: saturated vapour, the state at which the first liquid droplet forms when vapour is cooled. The two curves meet at the critical point (373.95 °C, 220.64 bar, s = 4.412 kJ/(kg·K)).

For a pure substance such as water, boiling and condensation happen at one temperature for a given pressure. On a pressure–temperature diagram the dew curve and the bubble curve are therefore the same line: the saturation curve shown on the P-T calculator.

Steam quality

Under the dome, pressure and temperature are tied together, so a third quantity is needed to fix the state. The usual one is the steam quality x (dryness fraction): the mass fraction of vapour in the mixture, from 0 on the bubble curve to 1 on the dew curve. Mixture properties are mass-weighted averages, for example h = hf + x·hfg and s = sf + x·(sg − sf).

Example computed by this page: at 180 °C (10.03 bar) with x = 0.9, h = 2575.8 kJ/kg and s = 6.140 kJ/(kg·K).

You can type a quality directly in the “Steam quality x” field: the point moves inside the dome at the current temperature.

Reading the isobars

The thin grey lines are lines of constant pressure. In the liquid region they run very close to the bubble curve, because pressure has little effect on the entropy of liquid water. Under the dome each isobar is horizontal: boiling takes place at constant temperature. In the vapour region they rise steeply. Isobars above the critical pressure (220.64 bar) pass over the dome without a plateau.

Outside the dome this page finds the pressure numerically from the temperature and entropy you picked, then evaluates the IAPWS-IF97 equations at that pressure. For the full saturation data, see the saturated steam table.