The results of a pulse-injection tracer experiment will depend upon the methods of injection and measurements of the tracer when the fluid velocity is not uniform across the injection and measurement planes. For steady segregated flows (no mixing between elements) general equations can be derived to describe the differences between, e.g., area-average and flow-average measurements of tracer. The treatment is simply extended to cope with mixing in the direction of flow, but cannot cope generally with transverse mixing. Taylor diffusion in pipe flow can, however, be examined further, and the differences between expressions in the literature can be more fully drawn. The differences are small, and may be negligible in practice. © 1971.
Turner, J. C. R. (1971). The interpretation of residence-time measurements in systems with and without mixing. Chemical Engineering Science, 26(4), 549–557. https://doi.org/10.1016/0009-2509(71)83034-8