Filters are available in a variety of materials, pore sizes and classifications. A depth filter with a 0.8µm nominal rating is not the same as a membrane filter with a 0.8µm absolute retention rating.

A depth filter consists of a non-uniform fibrous structure. Its rating is based on the average size of the particles loaded into the filter before breakthrough occurs. This breakthrough can result in loss of sample.

A membrane filter rating is based on 100% of the particles equal to or greater than the predetermined pore size of the filter. The pore size is determined and verified during manufacture. Due to the porous structure of membrane filters smaller particles are also retained, which means they cannot be used as "sieves" to size particles. Membrane filters are recommended in most applications because breakthrough will not occur.

It is also important to note that the pore size rating for both filter types is "liquid" filtration rating. Traditionally this rating means a liquid containing the particles is passed through the filter. All filters have had their pore sizes determined by this method, including Omega's products. This method was developed because most early applications involved liquid filtration. The use of membrane filters in air sampling applications is a more recent development. Filters have been found to perform more efficiently for air sampling. This is due to the reduced drag on the particles in air. Also, the static charge which builds up on the filter enhances the retention capabilities.

Several researchers have written papers to show  the phenomena involved.^{1 2 3} Their findings show the following "rule of thumb". A 0.8µm liquid rated filter will retain 50%+ of particles as small as 0.08µm, which is an order of magnitude smaller than the liquid rating. This phenomenon is generally true for filters with a liquid rating from 0.45µm to 5.0µm.

Several types of membrane filters have been found to be suitable for the collection of aerosols in Industrial Hygiene applications. See the chart below to determine the appropriate filter for your application. You can also consult the appropriate standards or contact Omega for assistance.

¹ Microcontamination March 1985 "Evaluation of Membrane Filters" K. L. Rubow, Benjamin Y. H. Llu

² Air and Water Pollution 1963 Vol. 7 pp. 501-509 "The Efficiency of Membrane Filters" W. J. Megan and R. D. Wiffen

³ Limnology and Oceanography Vol. 14 No. 3 May 1969 "Retention of Particles by Screen and Filters" R. W. Sheldon, W. H. Sutcliffe