Rheological behavior of plastic melts is rather complex as it depends on various interactions. In single phase systems it depends mainly on macromolecular structures of the polymer. In multi-phase systems it depends on macromolecular properties of polymers, filler properties, interactions between polymers, interaction between fillers and the polymer(s) as well as interaction between the fillers itself. Inline monitoring of rheological characteristics for continuous plastic processes are realized mostly based on relative values – e.g. to monitor pressure values – or with inline viscosity measurement systems. Due to the measuring principle of those inline viscosity measuring systems only shear viscosities are measured. Simplified systems - with a fixed throughput rate and capillary geometry – are also called inline MFI/MFR sensors, whereas some others are able to measure entire viscosity curves over a certain shear rate range. Unfortunately elongational viscosity characterizations of polymers or polymer systems are mainly conducted off-line by using equipment like the Rheotense system [Göttfert et al.]. Beside time delay between production and measuring, also measurements in a molten state or at certain temperatures are therefore somehow difficult. Within the scope of this contribution an online elongational viscosity measuring instrument and its advantages as a smart sensor in continous plastic operation will be presented.