On-line measurement of the rheological properties of polymers is less well established than laboratory based measurements. Instruments tend to operate on the use of capillary or slit dies, with pressure drop measurement along the length of the die. The approach is suitable for near real-time monitoring of production processes that are typically extrusion based. Approximate melt flow rate or shear viscosity values may be obtained, depending on the geometry used. The technique is most suited to quality control applications.