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DOP Testing:

HEPA filters are normally used only where an extremely high level of cleanliness or purity is required. The requirement may be due to problems caused by the presence of particulates or physiological problems caused by viable airborne organisms. In any event, the efficiency of every filter is of paramount importance and must be measured in an appropriate way.

Mil-Std-282 is recognized as the standard for "hot" DOP efficiency testing and is used for compliance with many HEPA filter specifications. It is also recognized as being "monodisperse 0.3 micron particles" as referenced in EPA and OSHA definitions for HEPA filters.

DOP (dioctylpthalate) is an oil commonly used with vinyl resins to make soft vinyl plastics. It is also used by air filter manufacturers and various testing agencies to make an aerosol to test the effectiveness of air filters. Other oil-like materials, like DOS, can be substituted with similar results.

The DOP aerosol used to challenge HEPA filters to test for efficiency by this standard is known as "hot" or "thermally generated" DOP because it is derived from heated dioctylpthalate oil. Sophisticated equipment is used for carefully controlling oil and air temperatures, air flow rates and mixing conditions. This "hot" DOP aerosol has a very narrow particle size distribution (monodisperse). Because the only way to determine the efficiency of a filter on a specific particle size (fractional efficiency) is to test with particles of that size, DOP is used to produce a high concentration of 0.3 micron particles - that which theory indicates and has historically been considered to be the most penetrating of filter media.

For each test, the average aerosol concentration is measured both upstream and downstream of the filter with a photometer. The inefficiency or penetration in percent can therefore be determined and recorded on the filter label. For example, a filter with a penetration of .008% would mean it was 99.992% efficient, well above the minimum of 99.97% for HEPA efficiency.

DOP aerosol can be generated in the field but the equipment used, while relatively simple and portable, cannot produce truly "hot" DOP that is monodisperse. The DOP generated by such equipment is "cold" DOP which having a broad particle size distribution is polydisperse. Such an aerosol is useful for field testing for leaks and ensuring the integrity of an installation, however, without the ability to particle count the 0.3 micron size particles, "cold" DOP does not provide the ultimate test of filter efficiency.

The penetration or efficiency of a filter is strongly affected by the particle size of the challenge aerosol. A small change in particle size can have a significant effect on penetration. The smaller the particle, the lower the efficiency until the maximum penetrating particle size is reached.

As indicated earlier, "cold" DOP has a broad particle size with larger average size than "hot" DOP. Efficiencies are, therefore, higher with "cold" DOP than with "hot" DOP. The control of temperatures and flow rates with the equipment is critical to maintaining a consistently tight particle distribution which allows for consistent and reproducible efficiency measurements.

Where "cold" DOP can be useful in determining HEPA filter efficiency is when testing in accordance with IEST-RP-CC007.1. For each test, particle counters are calibrated to simulaneously count the number of 0.3 micron particles both upstream and downstream of the filter. Providing the "cold" DOP challange aerosol contains a statistically significant number of 0.3 particles, the inefficiency or penetration in percent can determined. In this test, the polydisperse nature of "cold" DOP is irrelavent because the other particle sizes are not measured.

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