In a stirred tank chemical reactor the mean age of the reactor contents affects a number of properties of the product, including for example the homogeneity of the product. The mean average age of the reactor contents can be determined by constructing a transparent model of the reactor and filling it with a fluid containing a fluorescent dye and having flow properties comparable to those of the reactor in use. A light is shone on the fluid as it is stirred under reaction conditions and a clear fluid flow into the model. Pictures are taken of the reactor contents and the mean fluid age of the contents of the model are determined relative to the exit age of the contents. This approach can be applied to determine for example which reactor ports to use, what agitator to use, what flow rates to use to improve reactor function.