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The particulate filtration efficiency of test fabrics is evaluated
using an aerosol generator. This generator and the sample chamber
are contained in a class 10 cleanroom hood. One size of polystyrene
latex spheres is used as the challenge aerosol as it better represents
current cleanroom concerns. The aerosol is drawn into a sampling
chamber at a known flow rate by a laser particle counter (LPC)
creating a known face velocity through the fabric. The LPC counts
and records the aerosol particles before and after the test fabric
has been inserted into the sampling chamber. From this data,
the fabric's barrier efficiency can be determined. The standard
sample size of 6 in. x 6 in. is used and replications are conducted.
A schematic of the test apparatus is shown in the figure below.
The challenge aerosol is generated by a Climet Model CI-295 Aerosol
Generator. The aerosol
is drawn into a sampling chamber at a flow rate of 0.1 ft/min by a Met One,
Inc., Model 2300, laser particle counter (LPC). At this flow rate, the face
velocity through the fabric is 1.8 cm/sec. The aerosol concentration will be
set at 15,000 0.5 µm particles per minute with the circular exposed part
of the sample being 5.9 cm in diameter. Polystyrene latex spheres (PSL spheres)
with a mean diameter of 0.477 µm are used as the challenge aerosol. The
sample chamber and aerosol generator are operated in a class 10 cleanroom hood. 
Test Procedure
1.) Fifteen mL of HPLC grade water is added to an ultrasonically
cleaned nebulizer in the aerosol generator and the number of particles
is counted for two minutes to insure that the water is not contaminated.
2.) Two drops of the PSL 0.48mm spheres are added to the nebulizer water
which is in the aerosol generator and particle counts are recorded every
minute for four minutes.
3.) The fabric sample is inserted into the sample chamber, and particle measurements
are made every minute for seven minutes. A control is set on the generator
so that approximately 60,000 particles/ minute are generated. Replicate measurements
are made for each fabric sample tested.
4.) The fabric sample is removed from the sample chamber and particle measurements
are made every minute for four minutes. (This serves to check and possibly
correct for any change in the system).
Calculations
Particle counts for each fabric sample are stored in the LPC
as a text file in units of particles/ minute. After samples have
been tested, the text file is imported into a spreadsheet where
the calculations and graphing takes place.
1.) The first measurement made
are discarded because the sample chamber has to be opened to insert and
remove fabrics. This
may allow alien aerosol particles to enter the system. The first count
of each run is used to "flush" the system of these,
possibly incorrect counts of aerosol particles.
2.) The average of the six, one minute measurements are calculated
with the sample in the holder. This count is considered the "upstream" count.
3.) The three one minute measurements are taken before and after
the fabric is placed in the holder (six measurements in all)
are averaged. This count
is considered the "downstream" count.
4.) The penetration through the fabric (P) is calculated from the counts
obtained in 2 and 3 from equation 1.
Equation 1:
% Penetration = ( Cd \ Cu)
x 100%
The filtration efficiency can be obtained from equation 2.
Equation 2:
Filtration efficiency = ( 1 - (Cd \ Cu ))
x 100%
where:
Cd = downstream particle count
(average particle count with sample in holder)
Cu = upstream particle count
(average particle count without sample in holder).
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