Influence of the hydraulic radius on a) permeability and b) formation factor. The diagrams use the same symbol and line conventions as in Figure 1. Three horizontal error bars (dark purple) help visualize the relative uncertainty δr_H/r_H determined for the various materials (i.e., ±10, ±15 and ±20% for the unconsolidated sands, sintered glass beads and Fontainebleau sandstone, respectively).