This information is part of the Modelica Standard Library maintained by the Modelica Association.

Note that the inner energy neglects the pressure dependence, which is only true for an incompressible medium with d = constant. The neglected term is (p-reference_p)/rho*(T/rho)*(∂rho /∂T). This is very small for liquids due to proportionality to 1/d^2, but can be problematic for gases that are modeled incompressible.

It should be noted that incompressible media only have 1 state per control volume (usually T), but have both T and p as inputs for fully correct properties. The error of using only T-dependent properties is small, therefore a Boolean flag enthalpyOfT exists. If it is true, the enumeration Choices.IndependentVariables is set to Choices.IndependentVariables.T otherwise it is set to Choices.IndependentVariables.pT.

Enthalpy is never a function of T only (h = h(T) + (p-reference_p)/d), but the error is also small and non-linear systems can be avoided. In particular, non-linear systems are small and local as opposed to large and over all volumes.

Entropy is calculated as

s = s0 + integral(Cp(T)/T,dt)

which is only exactly true for a fluid with constant density d=d0.