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
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.
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
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.
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.
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
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.
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.