The vast differences that can exist between materials with respect to their conveying
potential are illustrated in Figures 8.1 and 8.2.
For just these two plots a third material type has been added to complete the picture
with regard to material types. This is material type C which covers materials that have
very good permeability. To have such properties the material has to have a mean particle
size above about 1 mm and for the particle size to be almost zero, so that it is
effectively mono-sized. This group includes materials such as nylon and polyethylene
pellets, and seeds and grains, such as peanuts. In terms of numbers of materials, this
group is in a minority, compared with the other two, and so as not to confuse the issue