Pore rating or NMWL

Pore size relates to the filter’s ability to filter out particles of a certain size. For example, a .05 micron (µm) membrane will filter out particles with a diameter of .05 microns or larger from a filtration stream.

With ultrafiltration (UF) and reverse osmosis (RO), filter pore size is irrelevant because the pores are so small. These filters are rated according to their nominal molecular weight limit (NMWL) or their molecular weight cutoff (MWCO). For example, a UF membrane rated at 30,000 will exclude a test protein with a molecular weight of 30,000 Daltons. Ninety percent of that test protein will be retained on the upstream side and 10% will pass through into the filtrate, resulting in concentration of the protein.

Although all membranes are rated for a particular pore size, pore size ratings alone are an unreliable measure of filter effectiveness because these ratings vary from manufacturer to manufacturer and from product to product. The function of the filter – is it a prefiltration, clarification, or sterilization filter, for example - and how well the filter performs that function have to be considered.