Transit time measurements with radioactive tracers in a model riser were examined by a nondispersive tracer (Co-60) encapsulated in a sphere. The flow dispersion of a CH3 Br-82 gaseous tracer was measured and compared with a normal distribution. The curve of the tracer concentration C(t) versus time was shown and evaluated for form and peak area. The transit time was measured by means of three methods, i.e., electronic watch recording, maximum point and center of mass. Signals were collected by two gamma detectors coupled to an electronic watch and multichannel analyzer. The measuring arrangement was positioned on the riser, operating under ambient conditions. A better precision was obtained by the center of mass measurements as seen in the comparison of statistical deviations. In applying the RTD method, the mean transit time, variance and asymmetry of the tracer distribution as a function of time were calculated. Experimental uncertainties occurring from tracer injection are avoided by taking the first detector peak as the time reference (t=0). The RTD method allows the further evaluation of the flow pattern. Steady laminar flow and plug-flow can be characterized. (C) 2004 Elsevier B.V. All rights reserved.