Porosity and Bed Volume

Porosity is the volume of air in the three-dimensional membrane structure and is typically given as a percentage of the membrane's total volume. For example, a piece of membrane that measures 1 cm x 1 cm x 0.014 cm (i.e., 140 µm thick) has a volume of 0.014 cm3 (14 µL). If its porosity is 70%, the volume of air comprising the pores is 9.8 µL (0.7 x 0.014 L). The volume of air is also referred to as the membrane's bed volume. The bed volume of a membrane or other porous material can be used to calculate the total volume of liquid reagent required to wet it out.

Porosity is typically not related to, nor controlled by, pore size. These two parameters are essentially independent. Porosity is also unrelated to thickness. Rather, it is a function of the polymer and casting process used in the manufacture of the filter.

In contrast, bed volume is directly proportional to both porosity and thickness. Bed volume increases with porosity as there is less polymer occupying the membrane’s structure. It also increases as thickness increases because there is more total volume in the structure. For large scale filtration, membrane bed volume is of little importance because the total volume being filtered is normally orders of magnitude greater than the total bed volume of the filter. This parameter can be important in analytical and diagnostic applications, where very small volumes of liquid are being processed.

Example:

A piece of membrane that measures 1 cm x 1 cm x 0.014 cm (i.e., 140 µm thick) has a volume of 0.014 cm3 (14 µL). If its porosity is 70%, the volume of air comprising the pores is 9.8 µL (0.7 x 0.014 L).