Based on the residence times, it can be concluded that these poorly perfused portions account for up to 3 mL. Here, pump D shows the most distinct deviation from the ideal residence time for both the fine and coarse magnesium turnings. This is related to the comparatively strong pulsation of the pump, which seems to have an effect on the flow conditions leading to lower or higher residence times compared to the hydrodynamic residence time and depending on the introduction and the displacement of the tracer, respectively. It is noticeable that pump C and pump A show the same residence time differences in absolute values between introduction and displacement, but the smallest deviation to the hydrodynamic residence times were obtained with pump A introducing 2 mol/L toluene-in-THF, whereas with pump C using pure THF to displace the tracer toluene. However, compared to the fine turnings, the difference in absolute values between pump C and pump A is less pronounced for the coarse magnesium turnings. This is related to the overall more inhomogeneous magnesium bed, in which the slight differences in the operation of pump C and pump A are less noticeable. In contrast to pump D, pump A experiences only a brief stop in liquid delivery followed by a slower increase in volume flow, when the syringes are switched. Thus, pump A has less impact on the flow behaviour. Since pump C generates a pulsation-free volume flow, the effects of the pump are least pronounced here, so that the results of pump C show the Bodenstein toluene tetrahydrofuran density (25°C) / (g mL numbers and residence times without additional influence of the hydrodynamics imposed by the pumping characteristics.