1. Missing compaction. The magnesium bed is subject to changes occurring due to the reacting magnesium. The decrease in magnesium mass during one measurement run is 2.4 % and 2 % for the coarse and the fine magnesium turnings. Though magnesium turnings are replenished after every measurement run, the compaction of the magnesium bed by means of the jogging motor might be less effective as in a dry magnesium bed. There might be a small pile up of replenished magnesium turnings at the top of the bed right above the product outlet, too small to be visually noticeable by looking through the viewing window, but significant enough to form an inhomogeneous bed with voids and thus less magnesium in the reactive zone of the reactor cartridge. Under dry conditions and independent of the size of the magnesium turnings, 10 % less magnesium mass fits into the reactor cartridge when the jogging motor is omitted, showing the importance of the jogging motor for the compaction of the bed. 2. Channelling (short-circuit flow). The formation of channels within the bed due to the reacting magnesium would also cause an increase in residence time, if the velocity within such a channel is increased (less pressure drop) but the velocity within the remaining homogeneous bed is reduced. Though the recorded measurement data do not clearly exhibit a strong channelling behaviour (exemplary data in figure ), the risk for channelling does remain for packed beds. Most likely there is not only one channel formed but a few channels with different characteristics, so that channelling is not quite visible within the recorded data. Within the present flow behaviour, most likely both reasons exist at the same time, but differentiation between both effects is out of scope of this study.