Positron emitting particle tracking (PEPT) can be used to trace the path of a radioactive particle within opaque fluids in pilot scale equipment. Isokinetic tracers can be used to characterize flow fields inside such equipment. A pilot scale heat exchanger was designed and used with PEPT to evaluate the accuracy and applicability of the technique; measurements were performed over a range of flows found in food processing plant, including extruder flows. Velocity distributions were estimated from the measured tracer locations for a range of barrel speeds, flow rates and theological properties. The flow field is 31); barrel speed and flow rate had a more pronounced effect on the velocity distributions than the rheological properties of the fluids. The flow field was simulated using a simple 21) approach and a full 3D simulation. The simulated velocity distributions compared favourably to the experimentally determined profiles: trends were correctly predicted by the 21) model whilst statistical agreement was found between data and the 31) simulation. The agreement between PEPT and CFD reinforces the validity of both methods. (c) 2005 Elsevier Ltd. All rights reserved.