Polymer melt flow instabilities limit the optimum throughput conditions in the plastic processing industry with high economic and environmental consequences. A new set-up able to measure on-line pressure fluctuations inside a slit-die arising from polymer melt flow instabilities is introduced by using a capillary rheometer. This set-up consists of three high sensitive piezoelectric pressure transducers located inside a slit-die together with the application of advanced mathematical tools to process the acquired time dependent pressure signals. Our novel tool improves the time resolution by a factor of ∼10 3 and under optimum conditions pressure fluctuations about 10-5 bar can be detected. Examples of these pressure oscillations arising from the main melt flow instabilities typically observed during extrusion based processes of commercial polyethylenes are presented.