This information is part of the Modelica Standard Library maintained by the Modelica Association.
The linear compressibility fluid models contained in this package are based on the assumptions that:
This results in models that are only valid for small temperature ranges, but sufficient to model compressibility and e.g., the "water hammer" effect. Another advantage id that only 3 values need to be measured to have an initial model. Hydraulic fluids can often be approximated by this type of model.
That means that the density is a linear function in temperature and in pressure. In order to define the complete model, a number of constant reference values are needed which are computed at the reference values of the states pressure p and temperature T. The model can be interpreted as a linearization of a full non-linear fluid model (but it is not linear in all thermodynamic coordinates). Reference values are needed for
Apart from that, a user needs to define the molar mass, MM_const. Note that it is possible to define a fluid by computing the reference values from a full non-linear fluid model by computing the package constants using the standard functions defined in a fluid package (see example in Common, LinearWater_pT).