METHOD NOTE 02 · DIAGRAMS & EVIDENCE

Reading P–h and T–s diagrams: four checks

bupt.ai · AI-assisted method note · Illustrative, not measured · Revised2026-11-01

More lines on a thermodynamic diagram do not necessarily mean a more complete model. Before assessing cycle performance, identify the fluid, axis units, curve calculations and model behind the state points. The following checks apply both when reading papers and when using the engineering equation workspace。

1. Align axes, units and reference states

A P–h diagram plots specific enthalpy horizontally and pressure vertically; a T–s diagram usually plots specific entropy horizontally and temperature vertically. Equal spacing on a logarithmic pressure axis does not represent equal pressure increments. Before comparing data, check absolute versus gauge pressure, Pa versus kPa, J/kg versus kJ/kg, and mass versus molar bases.

Enthalpy and entropy values depend on the reference state. Different absolute enthalpy values from two sources do not, by themselves, show that either is wrong. For comparisons across libraries, fix the fluid, composition, units, model version and reference state, then compare results at the same state.

2. A saturation dome requires phase equilibrium

For a pure fluid, saturated liquid and saturated vapor define the two-phase boundary. At saturation, T and P alone do not determine the liquid–vapor mass ratio; additional information such as vapor quality is needed. CoolProp uses mass quality Q = 0 for saturated liquid and Q = 1 for saturated vapor, and notes that T/P inputs near saturation may return errors.

Finding two density roots at the same temperature and pressure is not a complete validation of vapor–liquid equilibrium. Check the saturation algorithm, its range and independent references. A failed connection near the critical point must not be hidden by adding a smooth line. A critical-point marker does not validate the critical conditions.

3. Vapor quality is different from solution concentration

For an equilibrium two-phase mixture of a pure fluid at a fixed pressure, interpolate between the saturation endpoints using mass quality: h = hf + x(hg − hf). Here, x is the vapor mass fraction. Absorption-cycle models often use x for lithium bromide solution concentration, a different quantity that cannot be substituted into the same formula.

Placing LiBr–H₂O solution points of different concentrations inside water’s saturation dome does not make it a solution phase diagram. Distinguish the refrigerant and solution branches, and specify the solution model and composition. Refrigerant mixtures likewise require treatment beyond a pure fluid’s single saturation-temperature interpretation.

4. What do the lines between state points represent?

Solved endpoints, schematic connections between equipment ports and paths calculated point by point are different objects. Equal endpoint enthalpies can support an isenthalpic approximation, but do not establish the absence of heat exchange or other losses. Isothermal, isentropic and isobaric guide lines need property calculations under the corresponding constraints. Legends should state whether arrows show actual flow or simply connect numbered points.

For papers or reports, retain at least the fluid and composition, input and output units, property model and version, reference state, treatment of missing points, and the distinction between endpoints, schematic connections and process paths. Without this information, even an attractive diagram is difficult to reproduce.

References: CoolProp saturation-state interface and reference-state documentation. This is an independently written framework for checking diagrams; it does not claim that every plotted layer on this site has undergone independent property validation.