Particle mixing depends not only on the mixer design, but also on particle properties such as size, shape, density and surface characteristics. Experiments should therefore be performed with particles that have the same properties as the particles to be processed in the industrial mixer. A simple method which uses thermal tracer particles can accurately measure the speed and quality of mixing in fast, vertical mixers. This method could be easily implemented in full-scale industrial mixers. Since the thermal tracer can consist of the actual, unmodified additive particles, it can give more accurate results than methods which rely on particle modifications. Experiments with polyvinyl chloride (PVC) resin showed that a deflector improves the speed and quality of mixing over the whole range of impeller rotation speeds. A counter blade has a detrimental effect at low impeller rotation speeds, but gives the best mixing speed and quality at high rotation speeds.