Models are the starting point for any computation-based engineering activity and thus errors in the construction - instantiation - coding of models are beyond the most expensive ones. Any improvement of this situation requires a systematic approach that enables automatic generation of coded models. We discuss on a simple example the systematic construction of process models based on system theoretical concepts that gives us the possibility to guarantee the completness and structural correctness of the resulting equations. Whilst we explain the basic ideas on a very simple example, the method expands seamless to very large scale systems. As part of the discussion we shall also explore the link to the behaviour theory as it was defined in the mathematical system theory community. This then provides a clear basis for the mathematical analysis in the abstract signal space whilst having a clear physical interpretation in the thermodynamic and fluid-dynamic state space.