Geometric or Absolute Filter Fineness
The geometric filter fineness indicates the diameter of a sphere that just cannot pass through the largest opening of the filter medium. This information only takes into account the structure and composition of the filter medium. The particle shape and flow conditions are not taken into account.
Nominal Filter Fineness:
In practical operation, the actual size of the retained particles depends on many factors. Solids concentration and the width of the particle size distribution have a great influence, but the particle shape also plays a decisive role.
An elongated cylinder may pass through a circular opening whose diameter is greater than the outer diameter of the cylinder. The length of the cylinder is largely irrelevant here. However, if the opening in the filter medium is a curved tube of the same diameter, the length of the cylinder has a significant influence.
The nominal filter fineness thus always describes the interaction of a filter medium with a specific product. It can usually only be determined by tests.