Error bars represent 95% confidence interval calculated using t-statistic. Mean values seen in Figure (top) remain fairly constant across the range of U/U mf being considered. There is a slight drop in ∆P 3 compared to ∆P 2 while ∆P 4 is greater. Mean differential pressure is indicative of solids hold-up in the region across which it is measured. There is a greater concentration of solids in the region corresponding to ∆P 4 compared to ∆P 2 and ∆P 3 .