Abstract The experimental evaluation of the residence time distribution is a common method to characterise the hydraulic behaviour of a plant. It can be used to provide input/output models for reactive systems when overlapping the experimental hydraulics with a reaction kinetics through numerical convolution. A CFD study of different geometries both injection module and sensor module enable us to visualise the expected flow patterns in the manifold. Injection is critical. The standard T-piece approach exhibits a rather unfavourable behaviour, whilst injecting against the stream provides a rather effective plume of tracer that then is passing through sensor and plant.