THE BASICS OF HEPA FILTRATION:

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HEPA Efficiency:

By definition, a HEPA filter must be capable of removing a minimum of 99.97 percent of all airborne particulate matter 0.3 micron (micrometer) in diameter and larger. To ensure this fractional efficiency, manufacturers individually test each filter with a generated aerosol which allows the efficiency on 0.3 microns to be determined. One test known as Mil-Std-282 or the "hot" or "thermal" DOP test, is performed with sophisticated laboratory equipment requiring constant monitoring and calibration to maintain aerosol size, concentration and efficiency measurements. Mil-Std-282 is one of the most widely accepted industry standards for testing 0.3 micron efficiency and is required for compliance with most nuclear, military and U.L. specifications. IEST-RP-CC-007 is another efficiency test which uses sophisticated particle counters which are calibrated to count a specific particle size such as the 0.3 microns required for HEPA filters. Efficiency tests must not be confused with the "cold" DOP test or leak tests which are not intended for measuring overall filter efficiency (for more information go to Testing).

Recently, much confusion has been created with the proliferation of filters labeled "HEPA-type", HEPA-like", HEPA-media" or "hospital-grade HEPA". These filters typically range between only 90 to 99 percent efficient on 0.3 micron. While being an increase in efficiency over regular filters, they are not individually tested and do not meet the HEPA specification.

Similarly, efficiency phrases such as "as small as" or "at" mean that if all particles were that small, the filter would still have that efficiency and should not be confused with the phrase "down to" which may mean a mixture of particle sizes for the stated efficiency.

ULPA Efficiency:

The efficiency of an ULPA filter (ultra low penetration air) is 99.999% on 0.12 microns. The IEST-RP-CC-007 test standard mentioned above is also used for ULPA efficiency testing.

 


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History / Efficiency / Construction / DOP Testing / Principles / Particle Sizes