The efficiency of any filter becomes better with every trapped particle. Each particle a filter captures acts to absorb other particles. Yes, a filter is the weakest at the very beginning of its service life, and then as it traps particles the efficiency grows until the filters Dust Holding Capacity is reached to create a pressure drop. At this point the filter is considered too dirty and must be changed. The Green Screen's performance numbers increase dramatically half way through its long service life because of its' large Dust Holding Capacity. Here are the mid-point numbers of the Green Screen:
- 10 microns and above (arrestance) – Above 99%
- 3 to 10 microns – 95.65% average
- 1 to 3 microns – 75.3% average
- .3 to 1 micron – 29.1% average
- STRONG NUMBERS!!!!!
ASHRAE – American Society of Heating, Refrigerating and Air-Conditioning
- ASHRAE Standard 52.1(Tests)
- Arrestance - The percentage of the weight of ASHRAE dust caught by the filter compared to the total weight of dust fed into the filter throughout the test.
- Efficiency Dust Spot - The ability of the filter to remove stain causing particles from atmospheric air.
- Dust Holding Capacity - The total weight in grams of ASHRAE dust caught by the filter before reaching a predetermined pressure drop at a given rate of flow. Measures service life of a filter.
In 1999, ASHRAE came up with an additional test Standard 52.2 called MERV (Minimum Efficiency Reporting Value) ratings. This new test is not intended to be a replacement for Standard 52.1; instead, both will continue to be relied upon as the industry accepted measures of filter performance. The MERV rating is reporting the filters Minimum Efficiency, which is obviously at the very beginning of a filters service life. The test does not take into consideration the service life of the filter (Dust Holding Capacity).
ASHRAE Standard 52.2
This test is conducted by using 12 separate particle size tests between .3 up to only 10 microns. To capture the smallest of particles from the very beginning of a filters service life requires larger thicker filters and in many instances combinations of filters. The A/C units needed to pull the air through these filters are not found in most homes.