The Relationship between Pore Size and Filter Efficiency

Users of membrane filters are often confused by the "pore size" printed in the specifications for a particular catalog number. For filters used for air sampling, pore size has nothing to do with the collection efficiency of a filter. The face velocity governs the efficiency, through the filter and the aerodynamic diameter characteristics of the particles. All membrane filters, with one exception, have collection efficiencies in excess of 99% for particles down to 0.05 µM, diameter. Pore size is an indicator of the pressure drop across a filter, not its collection efficiency. The one exception, are filters which are manufactured by bombarding a solid surface, such as Mylar, with neutrons. An imperfection is created on the surface, which is subsequently acid etched to form a microscopic hole through the membrane. Filters of this type will have significantly reduced efficiencies when a combination of large pore size (3-5 µM) and small diameter (less than 1 µM) is encountered. Etched membranes are not specified for any of the NIOSH or EPA sampling methods.

Historically, filters selected for use in a particular application have had a smaller pore size than was required. This was only a minor problem, prior to the advent of battery operated sampling equipment. Too small a pore size would only result in a too rapid, plugging or blinding, of the filter so that the requisite quantity of air could not be drawn through it. With battery operated equipment, too small a pore size would lead to high power consumption, premature shutdown and with unregulated equipment, and incorrect flow rate.

When selecting a filter for an application, the important points to consider are:
If a regulating body has recommended a type and pore size, use it.
If a range of pore sizes have been recommend, use the largest.
Consult the manufacturer of your sampling equipment for the correct type and pore size to use.
Many manufacturers provide application notes regarding correct filter applications.
Avoid small diameter filters and small pore size membranes.

The physics of the filtration of particulate material from a gas stream is complex. Several mechanisms are acting in concert or opposition. Those desiring an understanding of filtration theory should consult the following source, which also contains a data table.

K. Willeke, Baron, P.A., AEROSOL MEASUREMENT, PRINCIPLES, TECHNIQUES AND APPLICATIONS, Van Nostrand Reinhold, New York, 1993.

See Chapter 10 starting on page 179. Table 10-5 on pages 196-7 is particularly illuminating.

R. Gussman, July 1998

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