Physical systems theory describes systems which obey the elementary principles of macrophysics (e. g. , thermodynamic 'laws') by networks and inter-related elements. Bond graphs represent these elements by mnemonics of their property (e. g. , C for capacitance) and the interconnections by so-called bonds, which represent power exchange between the elements. The usual bond graph method fails however to describe the convection of momentum and entrophy by a compressible flow of matter in an Eulerian frame of reference. It often happens that a large part of multidisciplinary system can be very well described in terms of bond graphs, while only a small part is a convective system, of which no proper bond graph exists. The frame has to be Eulerian, because the system through which matter flows has to be described, and not an amount of matter which may leave the system (Lagrangian frame). This demonstrates the need for a proper bond graph description of open convective systems.