Filtration is proceeding simultaneously during the extraction process. Filters with defined pore size are used according to the particle sizes of interest specified in the test standard.

Filtermembranes are in general subdevided into foamed and woven filters.

In order to preserve the most precise gravmetric results as possible the influence of the filter itself on the result has to be minmized. Moisture absorptin of polyamid (PA) filters has to be considered significantly higher compared to filters out of polyesters which are more suitable.

To reduce the particle loss during filtration, RIO GmbH uses (if possible) a perforated aluminium shell in which the filter is inserted and kept for the entire test procedure.

Additionally, a metallic suction filter with sharp angles was self-fabricated to surpress the occuring capillary forces.