Standard Evolution
In the era of the metal mesh filter, testing of filter performance was based upon a gravimetric differentiation measurement of a synthetic test dust with a broad particle size distribution. Even though such filter may have been 50% efficient on synthetic test dust, soiling in control areas, continued, as demonstrated in the following example:

Suppose a filter is efficient at removing 50% of the synthetic test dust by weight. Assume further that the filter is 100% effective removing particles 5 micron in size, but 0% effective removing 1 micron size particles. Let's examine the overall effectiveness of this filter.

· A one to five micron size particle has a relative weight of 125.
· 125 one micron size particles have a relative weight of 125.

If 126 particles (one 5 micron & one hundred twenty five 1 micron particles) are fed to this filter, the 5 micron size particle is captured, then the filter may be deemed efficient at removing 50% contaminant by weight. However, the efficiency of this filter by particle count is 1/126 of 0.088% or less than 1%.

Such a hypothetical filter would be designated by this method a 50% efficiency filter by weight. All of the smaller airborne particles (1 micron), would pass through the filter. It is this fine airborne particulate which accounts for the soiling of surroundings.

The ability to determine the efficiency of removing these fine airborne contaminants became even more crucial as technology advanced. As HVAC systems became more efficient, coil fins on these systems moved closer together. Fine airborne matter caused increased coil fouling, decreased thermal transfer, and increased energy usage. The Dill Dust Spot Test and the NBS (National Bureau of Standards) addressed the methods for testing these fine airborne particulates. Both test rated filter efficiencies by different methods:
1. An efficiency by removing only airborne atmospheric particles.
2. An efficiency by removing synthetic test dust consisting of Cottrell Precipitate and lint.

ATMOSPHERIC DUST SPOT EFFICIENCY
A measure of the ability of the filter to remove atmospheric dust from the test air. The method of determining this quantity is based upon light transmission through previously evaluated target paper. This is accomplished by adjusting the ratio of quantities of air sampled through targets upstream and downstream of the test filter so that equal changes in light transmission occur. The ratio is converted to an efficiency which is expressed as a percent. A high dust spot efficiency results in a high resistance to staining.

ARRESTANCE
A gravimetric measure of the ability of a tested filter to remove injected ASHRAE synthetic dust from the test air. Also expressed as a percent.

DUST HOLDING CAPACITY
Determined by the product of the quantity of synthetic test dust fed to the test filter, expressed in grams, and its average arrestance.

DETAILED ASHRAE TEST PROCEDURES

ATMOSPHERIC DUST SPOT EFFICIENCY TEST
1. Weigh the test filter.
2. Install the test filter in the test duct, and obtain clean air filter resistance or pressure drop.
3. Dust Spot efficiency is a soiling index that utilizes the opacity of a target paper as it means for measurement.

a. Zero the opacity meter using a standard light blockage.
b. Install a clean target paper in the target holder and measure its light transmission. This normally results in an 80-85% light transmission or a 15-20% light blockage.
c. Match two target samples with similar light transmission readings (i.e., 81% or 82%).
d. Install target papers in the target paper holders. One target paper is installed upstream of the test filter and one target paper downstream of the test filter. There are certain criteria which must be satisfied during this test:

· The sampling of atmospheric air must be long enough that the opacity of the target has significant change. Since the downstream target will be seeing the cleanest air, the minimum sampling time is longest on clear days and with higher efficiency filters. The minimum opacity change by Standard 52.1-1992 is 10%.
· The sampling time cannot be so extensive that the opacity change exceeds 40%.
· Anticipated filter efficiency must be considered to properly schedule opacity readings of the target papers. Final opacity readings of the target papers must be within 20% of each other.

ARRESTANCE
This Test is also performed as part of the dust loading procedure in which approximately four equal dust loading increments are used.

ASHRAE SYNTHETIC DUST
ASHRAE synthetic Dust consists of the following:
72% Standardized fine test dust by weight (Arizona Road Dust) 23% Molocco Black by weight 5% No. 7 Cotton Linters by weight, ground in a Wiley mill with a 4mm mesh screen.

ARRESTANCE PROCEDURE
1. Cap off or cover the dust spot samplers.
2. Weigh the high efficiency final filter (95% dust spot efficiency or above) and install it downstream of the test filter.
3. Estimate the total amount of dust feed required to bring the test filter to the final resistance as determined by the manufacturers literature. Introduce 1/4 of this requirement to the dust feeder.
Notes

The dust spot test is alternately tested with the arrestance test. The dust spot tests readings will total five. The arrestance tests readings will total four.

If the results of the dust spot efficiency test is less than 20% then the initial efficiency shall be reported as "Less than 20%" and the average efficiency need not be reported.