WasserPTDiagramm

WasserPTDiagramm, in German often called Wasser-PT-Diagramm, is a phase diagram that maps the stable phases of water as a function of pressure and temperature. It delineates the equilibrium boundaries between the solid (ice), liquid (water) and gaseous (steam) phases, including the fusion (solid–liquid), vaporization (liquid–gas) and sublimation (solid–gas) lines, and marks the triple point and the critical point.

The diagram is typically drawn with temperature on the x-axis and pressure on the y-axis. The triple point occurs at 0.01 °C and 611.657 Pa (about 0.00604 atm), and the critical point at 374.14 °C and 22.064 MPa. The solid–liquid boundary for water has a negative slope, reflecting that the melting point decreases with increasing pressure because ice is less dense than liquid water. The liquid–gas boundary has a positive slope, so the boiling point rises with pressure. The solid–gas boundary extends from the triple point and defines conditions under which ice can sublimate or deposit.

Applications and use: the diagram helps determine the phase state of water for given pressure and temperature, and supports the design and analysis of equipment such as steam turbines, condensers, refrigeration, drying and freezing processes, and cryogenics. It also underpins the interpretation of steam tables and thermodynamic property data. Limitations include that it represents pure water at equilibrium; mixtures, impurities, non‑equilibrium states, and near-critical nonlinear effects are not captured and may shift boundaries.