NIOSH
EPA
National Institute for
Occupational Safety and
Health
Robert A. Taft
Laboratories
Cincinnati OH 45226
United States
Environmental Protection
Agency
Office of Environmental EPA-600/7-81-145
Processes and Effects Research August 1981
Washington DC 20460
Research and Development
Collection
Efficiency of Field
Sampling Cassettes
Interagency
Energy/Environment
R&D Program
Report
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LA-8640-MS
Collection Efficiency of Field Sampling Cassettes
OS
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a ^nvlroTi'apntal Protection Agency
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Chicago, 1L bOuU
LOS ALAMOS SCIENTIFIC LABORATORY
Post Office Box 1663 Los Alamos. New Mexico 87545
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An Affirmative Action/Equal Opportunity Employer
This report was not edited by the Technical
Information staff.
This work was supported by the National
Institute for Occupational Safety and
Health.
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Govern-
ment. Neither the United States Government nor any agency thereof, nor any of their employees,
makes any warranty, express or implied, or assumes any legal liability or responsibility for the accur-
acy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or rep-
resents that its use would not infringe privately owned rights. Reference herein to any specific com-
mercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or favoring by the United States
Government or any agency thereof. The views and opinions of authors expressed herein do not nec-
essarily state or reflect those of the United States Government or any agency thereof.
UNITED STATES
DEPARTMENT OF ENERGY
CONTRACT W-7405-ENG. 36
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LA-8640-MS
UC-41
Issued: December 1980
Collection Efficiency of Field Sampling Cassettes
C. I. Fairchild
M. I. Tillery
J. P. Smith*
F. O. Valdez
^National Institute for Occupational Safety and Health, Cincinnati, OH 45226.
TLS, Unvircnrr.sr/laT Protection Igency
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COLLECTION EFFICIENCY OF FIELD SAMPLING CASSETTES
by
C. I. Fairchild, M. I. Tillery, J. P. Smith, and F. O. Valdez
ABSTRACT
Industrial hygiene particulate samples are often collected under
anisokinetic sampling conditions and in crosswinds. Experiments were con-
ducted to quantitate errors associated with sampling under these nonideal
conditions.
Three types of field sampling cassettes were tested to determine particle
sampling efficiencies for 0, 2, and 5 m/s winds at incidence angles of 0, 90 and
180°. Sampling at one-half or full standard flow rate, 37-mm-diam filters in
both open-face and closed-face cassette configurations and a 13-mm
polyethylene filter cassette were challenged by monodisperse particles of 4-,
8-, and 21-/un nominal aerodynamic diameter (Dae). Challenge aerosols of
Eosin-Y fluorescent dye were generated by a Berglund-Liu vibrating orifice
generator, and monodisperse pollen aerosols were generated by nebulization
from a water suspension.
Tests were conducted in a chamber in which the cassettes or filter holders
were moved to generate a relative wind. Eight filter cassettes mounted sym-
metrically on a horizontal arm which rotated about a vertical shaft at 250
rpm were positioned so that four cassettes moved at 2 m/s and four at 5 m/s.
In addition to two each of the test cassettes, two in-line filter cassettes
equipped with isokinetic probes were mounted on the arm. Mass, or number
of particles, collected by test cassettes and isokinetic samplers was com-
pared to that collected by stationary filters placed near the rotating arm.
Results from tests in the rotating arm chamber were checked against similar
tests made in a turbulent flow wind tunnel.
Sampling efficiencies were compared to theoretical values calculated from
Belyaev and Levin's theory, and Davies1 modification for calm air sampling.
-------
Satisfactory agreement was obtained for 5-jum Dae particles; however, con-
siderable discrepancy was noted with larger particles. The results indicate
that corrections must be made to determine concentration when particles
collected under nonideal sampling conditions are larger than ~5-/mi Dae.
Worst case conditions were:
The 37-mm open-face cassette oversampled by factors of 2 or more when
facing into a wind.
All cassettes undersampled when the relative wind was at large angles
to the cassette's inlets.
INTRODUCTION
Collection of aerosols by filtration for analysis of concentration, composition, and size distribu-
tion is used widely in health hazard analysis and air pollution studies. It is usually assumed that
the high efficiency filters collect representative samples of the environment. This is particularly
true if the sampling is from a still atmosphere. However, unless a sampling system is carefully
designed, it is possible that the sample obtained is not representative.
Probably the most important component of a sampling train is the inlet. The inlet may be some
type of probe, isokinetic or not, with transfer lines leading to an in-line filter; or the inlet may be
an integral part of the filter holder. Numerous types of filter holders are available commercially
and large quantities of molded plastic, disposable filter cassettes are used in field sampling
because of their convenience and low cost. The cassettes have a variety of inlets and are usually
designed for multipurpose sampling.
This study concentrated on three filter cassettes that are used extensively in industrial hygiene
field sampling. These cassettes were an in-line (IL) type, 37-mm-diam filter holder, an open-face
(OF) type, 37-mm-diam filter holder, and an IL type, 13-mm-diam filter holder. These cassettes
are shown in Fig. 1 along with two probes designed specifically to fit a 37-mm in-line cassette and
sample isokinetically at 2 and 5 m/s airstream velocity. Little is known of the collection efficiency
of these holders (cassettes) in sampling situations such as sampling into a wind, crossflow, or fac-
ing 180° to the wind. Also, the flow rate recommended for use with these cassettes is usually set
according to the amount of sample required for analysis with little consideration of sampling
problems.
THEORY
Three sources of error in sampling particles from airstreams are: (1) particles that deposit on
the walls of the sampler inlet may be a significant fraction of the particles entering, (2) particles
that rebound from the frontal lip of the sampler inlet may enter the sampler, producing an over-
sampling of particles; and (3) a disproportionate number of large particles enter or pass the sam-
pler inlet because the particles cannot follow the fluid streamlines.
The source of error due to wall deposition is generally not amenable to theoretical analysis.
Some work, notably Sehmel's,1 has been reported wherein deposition in long ducts has been
treated theoretically; however, no good predictive equations are available for complex inlet
geometries. The amount of particulate material depositing on internal surfaces is dependent upon
-------
FILTER
POSITION
8= 2.3
mm
=2.7 mm
7.4.3 mm
PROBE
X/2=5°
2 m/s PROBE
37-mm IN-LINE
SAMPLER
FILTER
POSITION
p!
V////A
1
T oU
?=3l.8mm
11
mm
8= 3.2 mm
37-mm OPEN"FACE
SAMPLER
FILTER
POSITION
0=7.4 mm
D/i = 1.9
8= 1.7 mm
f=4.0 mm
13- mm SAMPLER
AIR FLOW
«
D- INLET OUTSIDE DIAMETER
I - INLET INSIDE DIAMETER
8- INLET WALL THICKNESS
X-IN LET CONE ANGLE
Fig. 1.
Cross sections of sampling cassettes and isokinetic probes.
particle inertia and a number of factors specific to the aerosol and sampling system. These factors
include aerosol material, sampler material, relative humidity, wall roughness, and inlet channel
geometry (bends, expansions, etc.). In practice, either the deposition losses of a sampling system
are determined in preliminary experiments, or else deposited particles are cleaned from internal
surfaces after each sampling period and included in the analysis.
Rebound of particles from the sampler inlet lip generally contributes little error to the measure-
ment of aerosol mass concentration, but it may bias a size distribution measurement significantly
because only the large particles strike and rebound. Rebound particles tend to be carried into the
sampler under superisokinetic conditions, whereas, in subisokinetic sampling they are primarily
carried away from the inlet (Fig. 2). Some rebound error may occur even at isokinetic sampling
conditions with thick wall inlets because of turbulence at the inlet.
The largest sampling errors are produced by anisokinetic sampling or sampling at an angle to
the airstream. These errors, caused by particle inertia, result in a failure of particles to follow the
airflow. Primary parameters defining whether particles are captured by the inlet airflow are parti-
cle size, airstream velocity, sampler inlet air velocity, sampler inlet dimensions, and angle of the
inlet to the airstream. Particle size is defined better as particle inertia in the airstream according
to the Stokes number:
-------
-DIRECTION OF FLOW
SAMPLER UJ. "°-
INLET u _ -- ~~~ STREAMLINES
_^.-PARTICLE PATH
a SUB-ISOKINETIC SAMPLING (uUQ) REBOUNDING
PARTICLES TEND TO BE CARRIED INTO INLET.
Fig 2.
Conditions prevailing for sub- and super-isokinetic sampling.
STK = (v. uJ/U - g) , (1)
where vs is the particle settling velocity, u0 the airstream velocity, g the acceleration of gravity,
and I a characteristic linear dimension of the system, in this case the sampler inlet inside
diameter. The ratio of the sampler flow velocity and airstream velocity is the isokinetic
parameter, u/u0, which defines flow conditions at the sampler inlet.
Belyaev and Levin2 found in analyzing sampling errors that samplers must also be divided into
"thick-wall" and "thin-wall" types. If the outer diameter is D, <5 the wall thickness, and X the in-
cluded angle of taper of the wall from the inlet, then the sampler is thin-walled at any value of
STK and X, if d/l <0.05 (D/l is <1.1). According to Belyaev and Levin's experiments, a fractional
sampling efficiency of 1 cannot be achieved with thick-wall samplers in moving airstreams.
Cassettes tested in this study were not of a simple tubular design (Fig. 1), and according to the
criteria of Belyaev and Levin all of the samplers have thick-walled inlets, that is, for all three
cassettes D/l > 1.1 and in addition d/l > 0.05.
The sampling efficiency of the cassettes tested is defined as the fractional ratio of the measured
to the true aerosol concentration,
E = c/c0 , (2)
where E is fractional efficiency, c the aerosol concentration determined from the mass collected
on the cassette filter, and c0 the true or reference aerosol concentration determined independently
by a method known to yield true concentration. The efficiency is defined by Belyaev and Levin2
as an aspiration coefficient
A = c/c0 . (3)
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The aspiration coefficient can be further represented by
A = A, Aa Ay , (4)
where A] is the component of the aspiration coefficient characterizing concentration changes due
to aerodynamic and inertial forces (represented by STK and u/u0), Ad, the component charac-
terizing the concentration decrease in the sampler due to wall deposition, and Ay the component
characterizing particle rebound. Ay and Ad can only be determined by experiment, consequently
in Belyev and Levins' theoretical analysis, only At is considered:
A, = c/c0 . (5)
In a subsequent study of sampling inlets Belyev and Levin3 proposed a semiempirical
relationship for the inertial efficiency coefficient. This coefficient, A,, based upon their experi-
ments as well as those of Badzioch,4 and Voloschuk and Levin,6 related the efficiency of thin-wall
samplers to STK and u/u0:
A, = 1 + [(u0/u) - 1] 0 (STK, u/u0) , (6)
where u0/u is the inverse of the isokinetic parameter, and /3 was found experimentally to be a func-
tion of both STK and u/u0,
|8(STK, u/u0) = 1 - {!/[! + (2 + 0.62 u/u0)STK]} . (?)
Substituting the expression for j3, the final equation for A! is
A, = 1 + (u0/u -1)A- {!/[! + (2 + 0.62 u/u0)STK]}7 . (8)
Strictly speaking, Eq. (8) applies only to thin-wall inlets in significant winds and at zero in-
cidence angle (a = 0°).
The calculated theoretical coefficient for calm wind, A0, must be defined differently because at
u0 = 0 Eq. (8) goes to A! = 0 for all sampling conditions. Davies6 recommends another relation for
u0>o using Stokes number, Ku, based on flow within the sampling nozzle rather than STK based
on the airstream. Then as u0 approaches zero, A4 approaches A0:
-------
= 1 - {0.62 Ku/tO.05 Ku + (1 + 0.62 Ku]} (9)
A0 is independent of u0 and depends only on Ku in no wind conditions.
The other sampling condition which causes significant error, sampling at an angle a to the wind
(a = angle in degrees between wind direction and sampler inlet axis), had been studied little until
recently. Durham and Lundgren7 introduced empirical equations based upon their experiments
as well as those of Belyaev and Levin. These equations predict that the sampling efficiency
decreases rapidly as a goes to 90°, becoming essentially 0 at 90°. Raynor8 found similar ex-
perimental results for a up to 90° but also found the efficiency to increase slightly as a continued
to increase from 90 to 120°. He found for an open-face, thick-wall sampler (3.2-cm o.d. by 1.9-cm
i.d.) the minimum efficiency against 6-/um particles was near zero at a = 90° but increased to ap-
proximately 50% at a = 120°. However, Raynor did not correct for wall deposition in the sampler.
Rajendran,9 using models based upon the Navier Stokes equations of fluid motion and equations
of particle dynamics obtained numerical solutions which showed, in agreement with Durham,
that the collection efficiency of samples decreases to very low values at angles of incidence (a) ap-
proaching 90°.
EXPERIMENTAL
The experimental plan for this investigation called for examining the performance of the three
cassettes against three aerosol particle sizes, three airstream velocities, and three airstream in-
cidence angles using two sampling flow rates. One flow rate, designated the standard flow rate,
was an average of the rates recommended by the National Institute for Occupational Safety and
Health (NIOSH) in present sampling practice. The other flow rate was one-half of this standard
and was used to simulate an extreme condition of poorly calibrated flowmeter, or an overloaded,
high pressure drop filter. Sampling flow rates ranging from 1 to 5 L/min for 37-mm filter holders,
and 0.1 to 1 L/min for 13-mm filter holders are used in present field sampling practice. For this in-
vestigation representative average sampling flow rates of 1.8 L/min and 0.5 L/min for the 37- and
13-mm cassettes respectively were chosen as standard flow rates. Three replicates of each condi-
tion were planned (Table I), although for calm wind sampling conditions either two or four sam-
ples were obtained because of test equipment limitations. Thus a total of 396 samples for three
test cassettes, and 300 reference samples collected by four reference samplers were required. In
addition to these filter samples, it was planned to analyze about 25% of the cassettes for particle
deposition on the inlet wall. An additional 32 samples were obtained in a separate wind tunnel.
The number of samples and analyses required in the allotted time introduced a problem con-
cerning equipment. As described in a review by Fuchs,10 experiments to determine sampling ef-
ficiency against controlled winds have usually been performed in wind tunnels. In the present in-
vestigation, where a minimum of three cassettes plus a reference sampler were to be tested in
identical conditions, a tunnel diameter of 20 cm was required to prevent interference effects bet-
ween the four samplers. At a wind velocity of 5 m/s a volume of 10 000 L/min (340 cfm) of aerosol
was required. Then to limit each test run to 2 h assuming 25 fig of mass is required on each filter
for reasonable analytical accuracy (fluorometric analysis precision of ±2 ng) an aerosol concen-
tration of approximately 0.4 ngfL for the 500 cc/min sampling rate of the 13-mm filter cassette
was required. To produce this concentration an aerosol generation rate of 4 mg/min was required.
This is an excessive generation rate for monodisperse particles >l-/um diam. Another approach
-------
TABLE I
EXPERIMENTAL PLAN
Rotating Arm Chamber
Angle of
Incidence
0
90
180
0
180
Sampling
Flow Rate
Stda
1/2 Std
Std
1/2 Std
Std
1/2 Std
Std
Std
Nominal Particle Diameter, D._
ae
4 urn
0 _2_
4b 3
4 3
3
3
Wind
Airstream
5 0
3 4
3 4
3
3
Tunnel
4
8 pro
Velocity
2
3
3
3
3
3
>
, UQ (mis)
5 0
3 4
3 4
3
3
3
3
4
4
21 vim
2
3
3
3
3
5
3
3
3
3
4
Total number of test samples in RAC: 372
Total number of reference samples in RAC: 60 isokinetic + 240 stationary: 300
Total number of wind tunnel samples: 16 test sampler + 16 isokinetic: 32
Total number of cassettes analyzed for wall losses: ~200
aStd flow was 1.8 L/min for 37-mm cassettes and 0.5 L/min for 13-mm cassettes.
"Numbers in table indicate number of replicates for each condition.
considered was to employ a closed cycle (recirculating) wind tunnel to permit increasing the con-
centration to an equilibrium value. Disadvantages of this technique are (1) particle loss on wall
and air mover, particularly for large particles, (2) length of time to build to equilibrium concen-
tration, and (3) possible heat and moisture removal problems.
An alternative to moving the aerosol past the samplers is to move the samplers through the
aerosol to achieve a desired relative wind velocity. If the motion of the samplers is confined to a
relatively small volume then the requirements for aerosol generation rate are correspondingly
decreased. This type sampling chamber was constructed for this investigation using a cylindrical
chamber (barrel) with the samplers mounted on a vertical shaft and crossarm so as to move in a
horizontal circle around the shaft. In addition to decreasing the demands on the aerosol genera-
tion system, the moving sampler system had other advantages. The system was more compact
than a wind tunnel, several samplers could be tested simultaneously, and the turbulence
generated within a relatively large diameter chamber simulates the scale and intensity of tur-
bulence found in normal industrial sampling environments better than does a small diameter
wind tunnel.
The primary disadvantage to moving the samplers rather than the air arises from the necessity
of proving the two methods equivalent. It was found to be impractical to measure the relative
wind velocity directly in front of each sampler with sensors requiring leads, because of twisting or
sliding contact electrical noise from the rotational motion, or in the case of remote measurement
(laser doppler velocimeter), because of the expense. Consequently, results obtained in the moving
-------
sampler apparatus and results obtained from limited tests in a small conventional wind tunnel
were compared to establish the validity of the moving sampler concept.
The primary apparatus used to test the sampling efficiency of the filter holders was a plastic
drum with an internal rotating arm. The drum was 0.6-m diam by 0.8-m high with a 3/4-in. thick,
removable Plexiglas top, which could be clamped to the drum flange gasket to form an airtight
seal. This Plexiglas top served to support an inverted "T" shaped rotatable arm (Fig. 3), which
supported up to eight filter cassettes. The inverted "T" arm was rotated about its vertical shaft by
a motor-pulley-belt drive above the Plexiglas top. The shaft was supported by ball bearing
bushings and a rotating seal, which prevented air leakage to or from the drum. The inverted "T"
arm, or rotating arm, was made of hard drawn copper tubing which had four tubular, threaded
filter holder mounts for two filter cassettes silver soldered symmetrically to the arm at 7.5 and 20
cm from the center of the horizontal cross tube (Fig. 4). Thus, a total of eight cassettes could be
mounted on the rotating arm facing any horizontal direction, four at 7.5 cm and four at 20 cm
from the center of rotation. A calibrated critical flow orifice was positioned behind each filter
cassette. At the top of the vertical shaft another rotating seal connector (not visible in Fig. 3)
joined the shaft to a flexible vacuum hose leading to a pump. The pump maintained a vacuum of
<150 torr absolute on the tubular rotating arm manifold so that all eight orifices operated in the
critical flow pressure range. Above and below the rotating arm two similar fixed arms held two
filter holders each to serve as stationary reference samplers. The stationary arms were positioned
as close to the rotational plane of the samplers as possible. The stationary samplers, which sam-
pled at 5 L/min, were also controlled by critical flow orifices. Two additional tubes, an aerosol in-
let and an exhaust tube, extended through the drum sidewall to near the drum center. The upper
tube near the chamber top had a baffle on the intake and two parallel filters on the exhaust end
external to the chamber. The lower tube, near the chamber bottom, served to introduce aerosols
into the chamber. A circular baffle plate immediately above the aerosol inlet prevented aerosol
SAMPLER
AIR OUT
EXHAUST
FILTERS
STATIONARY
SAMPLERS
CIRCULATING
FAN
\
\
ROTATING
ARM
\ /AEROSOL
~T IN
ROTATING ARM CHAMBER
Fig. 3.
Rotating arm chamber (RAC) test apparatus.
Fig. 4.
Rotating sampler arm.
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from jetting up into the chamber. At the bottom of the chamber a 100-cfm fan provided a cir-
culating, mixing airflow to produce a uniform concentration of aerosol within the chamber. Ad-
ditional tubular probes were inserted through the side wall of the chamber to withdraw samples
for particle sizing.
A small wind tunnel (WT) was also used to duplicate some tests made in the rotating arm
chamber (RAC). This tunnel was 8-cm diam by 2-m long and had a high-efficiency particulate air
(HEPA) filter on the inlet and exit (Fig. 5). Aerosol was introduced through a side arm near the
tunnel inlet into the center of the tunnel and a Stairmand disk immediately downstream of the
aerosol inlet promoted mixing with the inlet dilution air. The length to diameter ratio of the tun-
nel insured that well developed turbulent flow existed at the test section. Only two test samplers
were accommodated in the WT, one in-line type 37-mm holder with an isokinetic inlet and one
without an isokinetic inlet. These were tested at only one velocity (5 m/s), two particle sizes (4-
and 8-Atm Dae), and two sampler orientations (0 and 180°). The test cassette was placed alter-
nately in the upstream and downstream positions of the mounting fixture (Fig. 5).
Monodisperse particles <20-/um Dae were generated from isopropyl alcohol solutions of Eosin-Y
fluorescent dye. A Berglund-Liu vibrating orifice generator with a pneumatic system to feed dye
at a constant rate produced spherical particles with geometric standard deviations (20 fj.ru used in these tests, short ragweed and giant ragweed pollen, were nebulized
from a water suspension in a manner similar to that used by Lautner and Fisher.11 Due to the
large aerodynamic size of these particles a large fraction redeposited within the nebulizer or the
AEROSOL THERMAL ANEMOMETER TEST PUMP
IN VELOCITY PROBE SAMPLER
HEPA STAIRMAND
FILTER DISK
WIND TUNNEL
Fig. 5.
Wind tunnel (WT) test apparatus.
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short transfer line to the test chamber. This produced a very low concentration (<1 particle/cc) of
pollen particles within the test chamber. Rather than sample long enough to obtain sufficient
mass for weighing, the test filters containing pollen particles were counted using a Zeiss
microscope at 60X with low angle side lighting. Initially, the entire filter area was searched;
however, it was found that the counts from four traverses across the diameter in different direc-
tions, multiplied by an area factor, provided total counts within 15% of counts obtained by scan-
ning the entire filter.
Sizing pollen particles was performed with the same Zeiss microscope using a Porton graticule
in one eyepiece at a magnification of 160X. Short ragweed pollen was found to be 20.3-jttm diam
with ffg <1.05 and giant ragweed pollen was 24.2 /urn with ag = 1.04. Aerodynamic diameters were
obtained independently from a falling particle-flash photography apparatus. A stroboscopic light
provided periodic illumination of pollen falling within a glass column of clean, still air. Time-
exposure photographs of the particles were not used unless a minimum of three images were
equidistant vertically from one another. Aerodynamic diameters calculated from the images were
21.2 ± 2.7 yum and 25.3 ± 1.9 nm for short ragweed and giant ragweed spores respectively. The
particle densities (assuming perfect spheres) of 1.08 and 1.09 calculated from the measured
physical and aerodynamic diameters were corroborated qualitatively by the observation that the
particles settled to the bottom of a water suspension over a period of 24 h. Considering the preci-
sion of the aerodynamic diameter measurements, nominal diameters of 21 and 25 jum were used
for all calculations.
RESULTS
a * 0°
Results from cassette inlet tests for a = 0° are compiled in Tables II and III. Table II, contain-
ing results for calm wind (u0 = 0), is discussed separately because Davies' method of calculation is
used to calculate A0 in the interval 0.06 < u0/u < 1 and 0.05 < Ku < 6.3 (recommended by
Davies). Some Ku and u/u0 values are below the recommended Ku range, but because Eq. (5) ap-
proaches 1 as Ku decreases, the values in Table II should be valid. Table III contains results from
all tests at u0 > 0 and zero angle of incidence for both one-half and full standard sampling flow
rates. Individual sample values have been averaged into groups of three or more samples (oc-
casionally only two samples were available/group) which have closely matched parameters (u/u0,
STK). Most of the columns tabulated are dimensionless; only the airstream velocity, u0, and
aerodynamic diameter, Dae, have dimensions. The experimental values of concentration
measured by each filter sampler divided by true concentration (measured by stationary sampler),
c/c0, are compared to A, (or A0) in the final column of the tables. Thus the final column indicates
the ratio of the experimental results to the theoretical value. A, is the theoretical sampling ef-
ficiency only for inertial effects and accounts neither for particles deposited on the cassette walls
nor for particles rebounding from the wall (lip) of the cassette. For this reason wall losses were
determined independently, and c/c0 in all tables has been adjusted for cassette wall loss, but not
for rebound loss (or gain).
The tabulated values of c/c0 and c/c0/Ai from Tables II and III are plotted in Figs. 6 and 7. Data
from Table II for u0 = 0 are shown as c/c0 vs Dae in Fig. 6 because the large range of Ku (10'6 to 3.7)
makes an abscissa scaled to Dae preferable. The results from Table III for u0 > 0, including full
and one-half standard sampling flow, are shown as c/c0 vs STK (Fig. 7). Several of the data from
the tables have been combined in Figs. 6 and 7 to avoid the confusion of multiple points and
overlapping error bars at the same Dae or STK. Data from the isokinetic (IK) samplers are not
plotted in Fig. 7 but are included in later graphs.
10
-------
TABLE II
SAMPLING RESULTS FOR U0 = 0
Sampling ,,
Conditions u ae ..a . b . c , /A
and Cassettes (m/s) ym Nu wc-o Mo L/V"o
STANDARD FLOW RATE
lLd 2.4 2.3 0.01 (8) 1.05 * 0.05 (5) 0.99 1.06
0.94 1.18
0.60 2.7
OF 0.04 2.3 2.2 x 10~5 (12) 1.10 * 0.09 (8) 1.00 1.10
1.00 1.19
1.00 1.3
13-ron IL 0.7 2.3 3.0 x 10--* (8) 1.01*0.19(19) 1.00 1.01
0.98 0.69
0.83 1.01
1/2 STANDARD FLOW RATE
IL 1.2
OF 0.02
13-mm IL 0.3 7.4 0.03 (12) 0.51 * 0.10 (20) 0.98 0.52
2.3
7.8
25
2.3
7.8
25
2.3
7.8
25.0
0.01
0.11
1.13
2.2 x lO-5
2.3 x 10-4
2.0 x lO-3
3.0 x lO-3
0.03
0.33
(8) 1.05 * 0.05 (5)
(6) 1.11 * 0.10 (9)
(4) 1.6 * 0.81 (51)
(12) 1.10 ± 0.09 (8)
(6) 1.19 * 0.04 (4)
(4) 1.3 * 0.21 (5)
(8) 1.01 * 0.19 (19)
(6) 0.68 * 0.19 (28)
(4) 0.84 * 0.49 (58)
6
7
6
7
.2
.8
.2
.9
0.07
0.11
1.5 x 10-4
2.4 x 10~4
(4)
(8)
(4)
(8)
0
0
1
1
.93 *
.78 ±
.43 *
.20 *
0
0
0
0
.08
.27
.12
.28
(9)
(35)
(9)
(24)
0.
0.
1.
1.
96
94
00
00
0.97
0.83
1.43
1.20
aFor u0 = 0,KU is based on u for each cassette.
bc/Cq is in the format: (No. of samples) mean * std dev (COV).
CA0 is calculated from Eq, (5),
dCassettes are: IK, 37-mm ISOKINETIC; IL, 37-mm IN-LINE; OF, 37-mm OPEN-FACE; 13-mm IL,
13-flim IN-LINE.
For calm air sampling (u0 = 0), Table II and Fig. 6 show that both the 37-mm IL and OF
cassettes oversample increasingly for increasing Dae. This is contrary to Davies' sampling theory
for calm conditions. The error bars at Dae = 25 nm suggest that experimental precision may ac-
count for the contradiction; however, other conditions such as nonzero airstream velocities pre-
sent at the test or reference samplers could affect the results. In contrast to the 37-mm cassettes,
the 13-mm IL cassettes exhibited good agreement with expected collection efficiency, A0, even
though the precision of the data is poor.
Sampling into a wind (Table III and Fig. 7) produced data consistent with Belyaev and Levin's
theory [Eq. (4)] in that all samplers exhibited increasing oversampling with increasing STK for
u/u0 < 1. The oversampling is pronounced even at STK < 0.1 for the OF cassette because of the
low u/u0 ratio observed for this configuration.
To compare the test results obtained at a = 0° against the sampling theories outlined
previously the collection efficiency, c/c0, was normalized to the Af (or A0) value by dividing by AI
(or Ao). This normalized value, c/Co/A,, is listed in the final column of Tables II and in. If the ex-
perimental value (c/c0) agrees with theory (At or A0) then this final column shows a value near
one, as it does for most isokinetic or small particle sampling. The normalized values, C/CO/A! from
Tables II and III are plotted vs the corresponding A0 or Aj on the abscissa (labeled AI) in Fig. 8.
11
-------
TABLE III
SAMPLING RESULTS FOR
,0"
Sampling
Conditions
and Cassettes
STANDARD FLOU
U, . 5 m/s, « . 0°
(i»/si "/uo
13-nm
IL (HT)
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
5.1
0.96
0.95
0.94
0.43
0.45
0.49
7 x 10-3
7 x 10-3
8 x 10-3
0.20
0.13
0.15
0.50
0.46
ae
L-l
4.6
7.2
21
4.6
7.2
21
5.0
7.5
21
4.5
7.4
21
3.5
7.4
0.2
0.3
2.3
0.06
0.22
1.93
0.02
0.025
0.27
0.07
0.23
1.93
0.05
0.22
(6) 0.97 * 0.23 (24)
(10) 0.97 * 0.17 (17)
(3) 3.9 * 1.8 (46)
5) 1.10 * 0.08 (7)
6) 1.25 * 0.07 (6)
3) 5.4 * 0.40 (8)
2) 1.42
9) 1.40* 0.25 (18)
2) 12.0
(7) 1.29 * 0.58 (45)
(5) 1.42 * 0.86 (61)
(3) 7.6 * 1.7 (23)
(5) 1.07 * 0.30 (28)
(7) 1.08* 0.07 (7)
1.01
1.03
1.07
1.13
1.41
1.86
4.6
8.0
45.0
1.65
3.19
5.84
1.08
1.35
c/c/A,
0.96
0.94
3.7
0.97
0.89
2.92
0.31
0.18
0.27
0.78
0.45
1.3
1.0
0.80
OF
13-m
1/2 STANDARD FLOU
IL
OF
13-M
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
1.9
5.2
5.2
1.9
5.2
1.9
5.2
1.9
5.2
1.9
5.2
1.04
1.02
1.02
1.25
1.28
1.36
0.02
0.02
0.02
0.27
0.30
0.28
0.63
0.22
0.24
0.01
3.7 x 10-3
0.01
3.5 x 10-3
0.15
0.50
0.12
0.06
4.6
7.4
21
5.9
7.6
21
4.6
7.4
21
5.9
7.9
22
7.1
7.2
21.0
7.0
7.1
21.0
21
7.1
7.0
21.0
21.0
0.03
0.08
0.60
0.06
0.09
0.71
5 x 10-3
0.01
0.09
0.06
0.10
0.71
.08
.20
1.9
0.01
0.03
0.09
0.24
0.08
0.20
0.71
1.9
(5) 0.80 * 0.06 (8)
(5) 0.93 * 0.16 (18)
(3) 2.23 * 0.40 (18)
1.03* 0.05 (5)
1.01 * 0.25 (25)
5.7 t 1.21 (21)
(5) 0.94 * 0.07 (8)
(5) 1.15 * 0.28 (25)
(3) 8.1 * 0.35 (4)
(2) 1.68
(7) 0.93 * 0.39 (41)
(3) 4.3 * 2.5 (59)
(3) 0.82 * 0.31 (37)
(5) 1.6 * 0.5 (31)
(3) 3.2 * 2.1 (66)
(2) 0.88
(3) 0.83 * 0.10 (12)
(5) 9.9 * 6.0 (60)
(2) 10.4
(3) 1.11 * 0.08 (7)
1.41 * 0.23 (16)
4.8
3.0 * 0.9 (30)
1.00
1.00
0.99
0.98
0.96
0.82
1.22
2.04
8.6
1.28
1.40
2.4
1.09
2.12
3.6
2.9
14.2
17.1
93.6
1.9
6.3
5.5
14.5
0.80
0.93
2.3
1.07
1.05
7.0
0.77
0.56
0.94
1.31
0.66
1.8
0.75
0.75
0.89
0.30
0.06
0.58
0.11
0.58
0.23
0.09
0.21
"c/c,, 1s In the format: (No. of samples) mean * std dev (COV).
bCassettes are: IK, 37-nm ISOKINETIC; IL, 37-ma IN-LINE; OF. 37-aM OPEN-FACE; 13-4m, 13-m IN-LINE; (UT), tested 1n wind tunnel.
CA( 1s calculated from Eq. (5) It Uo - 0.
Again, if c/c0 equaled At, the plotted points would lie along a horizontal line at c/Co/Aj = 1 for any
AI. In fact, the c/c0/A( values decrease monotonically as A4 increases. Also discernible in Fig. 8 are
two distinct groups of data. Examination of the data reveals that the upper group contains only
data for particles with Dae > 20 ^m, whereas the lower group contains data only for Dae < 12 jim.
Most of the points which do not lie distinctly in one or the other group are from tests at one-half
standard sampling rate. Therefore, only points resulting from full standard sampling flow rate
tests are shown in Fig. 9. With the exception of 1 or 2 points the groups are distinct and have
nearly equal slopes. Regression analysis of the two groups of data provides analytical expressions
in the form of power functions. The specific functions found to describe sampler efficiency at a =
o° are:
c/c0 Af1 = 1.1 Af°-72 for 2-MHi < Dae < 12 /mi; r = 0.82 ,
or c/c0 = 1.1 V-28 ,
(10)
12
-------
10
08
06
o
O
°04
02
01
COLLECTION EFFICIENCY IN CALM WIND
1 1 1 1 1 1
1 37 mm OPEN-FACE
I 8 L/min
' 37 mm IN-LINE
1.8 L/min
i 13 mm IN-LINE
0 5 L/min
I I I I I
!
0 2 4 6 8 10 20 26
AERODYNAMIC Dl AMETER ,Dae (p.m )
Fig. 6.
Collection efficiency (c/c0) as a function of
aerodynamic diameter in no wind conditions.
10 p
SAMPLING EFFICIENCY AT 0° ANGLE OF INCIDENCE
°01
STK
Fig. 7.
Measured collection efficiency (c/c0) as a func-
tion of STK in three wind conditions and two
sampling flow rates at a = 0°.
C/C0
10-
37mm IL
13mm IL
37mm IK
.fl 4
»0
10"
10'
c/c
C/ C0
10*
10'
10'
a 37mm Of
IL
37mm
10*
10'
10'
Fig. 8.
Measured collection efficiency (c/c0) nor-
malized to Ah plotted as a function of A\ for all
sampling conditions at a = 0°.
Fig. 9.
Measured collection efficiency (c/c0) nor-
malized to AI, plotted as a function of AI for
standard flow rate conditions at a = 0°.
Regression lines are shown for particles with
Dae > 20 nm (upper) and D&e < 12 nm (lower).
13
-------
and
c/c0 Af1 = 4.3 Ar°'74 for 21 pm < Dae < 25 Mm; r = 0.96 ,
(11)
or c/c0 = 4.3 Ai°-26 .
The two power functions are nearly parallel over the range of calculated Af and the primary dif-
ference in the two groups of data is their displacement from one another. Since C/CO/AI is nearly 1
for <12-jum Dae spheres when A] is 1 (Fig. 9), it appears that these spheres give more valid results
than do the larger spheres. At the present time no reason is known for the anomalous behavior of
Dae > 20-/um spheres. Nor is the reason for the decrease of c/c0/Ai with increasing At known unless
it is due to particle bounce from the thick wall of the inlet. This seems unlikely, however, because
the difference between c/c0 and Af is large to be caused by bounce, and because the isokinetic
samplers, which had very thin walls, also exhibit c/c0 much larger than Af when 21- to 25-Mm Dae
challenge aerosols are used (Table III). One known error in the measurement of the reference con-
centration (C0) of large particles in calm air occurs because a finite volume of air beneath the
cassette becomes devoid of particles after a few seconds of sampling. The settling velocity of large
particles prevents replenishment of this volume with particles. Calculations indicate however,
that this effect decreases C0 by only 9% for 20-Aim Dae particles; considerably less than the dif-
ference in Eqs. (10) and (11). The most probable explanation for the different results for large or
small particles lies in the fact that different techniques were used for small and large particle
tests. These include different particle material, generation method, and size measurement
method (see experimental section), and in combination could have produced significant dif-
ferences in results.
Using Eqs. (10) and (11) to calculate the collection efficiency of the cassettes for 5-, 10-, 15-,
and 20-jitm Dae particles, a set of empirical curves (Fig. 10) was derived to predict the performance
of each cassette at a = 0° against winds of 2 and 5 m/s. Because no data were obtained between
12-ium >Dae < 21 /um, and the equations differ above and below this range, 15-/um Dae was used as
the crossover between Eqs. (10) and (11); that is, c/c0 values from each of Eqs. (10) and (11) were
averaged to force the equations together at 15-/im Dae. Figure 10 illustrates the oversampling
phenomena of samplers facing into the wind and sampling at u/u0 < 1. According to Fig. 10, c/c0
for the 37-mm IL cassette increases above 11-jLtm Dae, but the theory for thin-wall samplers
predicts it should turn slightly downward because at 2 m/s u/u0 is > 1. This experimental result
conflicts with thin-wall theory, but cannot be considered erroneous because no accepted theory
for thick-wall samplers exists.
a * 0°
Results from sampling tests at 90 and 180° angles of incidence (a) are listed in Table IV and il-
lustrated as a function of STK in Figs. 11 and 12. Below STK = 0.1 all cassettes appear to have a
fractional collection efficiency close to 1; however, as STK increases all cassettes exhibit a
decrease in efficiency. The decrease is least for the 37-mm OF cassette both at a = 90° and a =
180°. This phenomenon may occur because the large inlet diameter of the OF cassette produces
eddies (vortices) which spill from the inlet lip to bring particles under the influence of the sampl-
ing airflow.
14
-------
PREDICTED COLLECTION EFFICIENCY FOR a=0°
13-mm IN-LINE
0 5 L/mm
10 15
AERODYNAMIC DIAMETER, DQe (fj.m)
Fig. 10.
Collection efficiency (c/cj predicted by Eqs.
(10) and (11) for a = 0°.
The same analytical approach used for a = 0° (relating c/c0 to A4) was tried for sampling results
from a = 90 and 180° tests even though no theoretical justification exists for a correlation between
AI and c/c0 at a ^ 0°. Surprisingly, c/c0 correlated with Aj better for a = 90 and 180° sampling
than it did for a = 0° sampling. However, no spheres larger than 11.2-jum Dae (Table IV) were
used in the 90 and 180° sampling.
Results from sampling at an angle to the wind are correlated as c/c0/Ai vs AI from Table IV for
90° samples in Fig. 13 and for 180° samples in Fig. 14. Although the results do not fall on a com-
mon line as they do for a = 0° samples (Fig. 9, Dae <12-/j,m data), the data for each particular
sampler cassette exhibit good correlation both at a = 90° and a = 180°. The data for the 37-mm
in-line and 13-mm in-line cassettes nearly coincide, probably because the inlet inside diameters
of these two cassettes are almost identical, whereas the 37-mm open-face cassette inlet is much
larger. The 37-mm OF cassette is least sensitive to incidence angle. For the OF cassette the curves
for 90 and 180° (Figs. 13 and 14) are only slightly steeper than that for 0° (Fig. 9, <12-/um Dae).
Regression analysis of the results for a = 90 and 180° orientation tests produced the functions
listed in Table V. All of the results follow a power function of A] with a minimum correlation coef-
ficient, r = 0.84. Equations from Table V were used to calculate the curves shown in Fig. 15 for U0
= 200 cm/s. These empirical, predictive curves indicate that collection efficiency is sensitive to
angle of incidence of the wind, more so for the 13-mm IL cassette than for the others. The
minimum c/c0 for the 13-mm IL cassette occurs at 90° incidence angle for all particle sizes,
15
-------
SAMPLING EFFICIENCY AT 90° ANGLE OF INCIDENCE
C.
C0
C_
co
10
01
37-mm IN-LINE
0 6
-------
TABLE IV
SAMPLING RESULTS FOR « . 90" and 180*
Sampling
Conditions
and Cassettes
STANDARD FLOW
o . 90°
!Lt>
OF
13-m
It.
OF
13-mm
a ' 180°
1L
OF
13-ran
1L
OF
13-nm
1/2 STANDARD FLOW
a . 180*
1L
OF
13-mm
1L (WT)
1L
OF
13-nm
"o
5.1
5.1
3.9
3.9
3.9
3.9
3.9
1.9
1.9
0.5
0.5
0.5
1.8
5.1
5.2
5.1
1.9
1.9
1.9
5.1
5.1
5.1
5.1
5.0
1.9
1.9
1.9
1.9
1.9
u/ufl
0.62
0.59
0.01
0.01
5.0 x 10-5
0.18
0.13
4.7
4.7
2.5
0.08
2.5
0.73
0.25
8.0 x 10-3
0.12
1.14
0.21
0.29
0.21
3.5 x 10-3
3.7 X 10-3
0.25
0.24
0.59
0.58
0.01
0.68
0.68
°»e
3.7
7.7
5.0
7.5
11.2
4.7
7.2
4.7
8.0
11.2
7.5
7.8
11.2
7.8
7.8
7.7
7.8
7.8
7.8
7.3
6.2
7.2
7.3
7.6
6.2
7.2
7.3
6.2
7.2
STK
0.041
0.19
0.01
0.023
0.05
0.065
0.16
0.009
0.021
0.05
0.003
0.025
0.18
0.25
0.030
0.21
0.09
0.012
0.09
0.25
0.02
0.027
0.24
0.22
0.05
0.09
0.01
0.06
0.06
"«,*
(1) 0.89
(4) 0.31 * 0.03 (9)
(3) 1.58 * 0.13 (8)
(4) 1.13 * 0.21 (18)
(2) 1.08
(4) 0.09 * 0.06 (66)
(2) 0.08
(3) 0.50 * 0.02 (5)
(4) 0.80 * 0.16 (20)
(2) 1.25
(2) 1.37
(2) 0.95
(2) 0.15
(3) 0.31 * 0.06 (21)
(3) 0.72 * 0.12 (17)
(1) 0.08
(3) 0.73 * 0.10 (14)
(3) 0.62 * 0.04 (6)
(3) 0.26 * 0.03 (11)
(3) 0.28* 0.02 (6)
(6) 0.79 * 0.18 (22)
(3) 0.12* 0.03 (25)
(3) 0.20 * 0.15 (78)
(4) 0.32 * 0.09 (28)
(6) 0.66 * 0.09 (13)
(3) 0.49 * 0.22 (45)
(3) 0.62 * 0.21 (35)
(6) 0.61 * 0.07 (12)
(3) 0.54 * 0.14 (26)
A1
1.05
1.20
2.8
5.1
17.8
1.49
2.14
0.97
0.93
0.91
1.08
0.95
1.11
2.05
8.6
3.3
0.98
2.0
1.40
2.2
11.4
13.5
1.93
2.04
1.08
1.14
3.1
1.06
1.02
cVAi
0.85
0.26
0.56
0.22
0.06
0.06
0.04
0.52
0.86
1.38
1.27
1.00
0.14
0.15
0.08
0.02
0.74
0.31
0.18
0.13
0.07
0.01
0.10
0.15
0.61
0.43
0.20
0.58
0.53
is in the format: (Ho. of samples) wan * std dev (COV).
"Cassettes are: IK, 37-nw ISOKINETIC; IL, 37- K7
Range
0.9 £ A1
1.1 < Ai
0.9 <_ A,
1.0 <_ A1
2.0 < Ai
1.0 A, <
1 1-2
< !8
1 2.2
i 2-2
i 12
3.3
No. of
Samples
5
4
4
6
5
5
Correlation
Coefficient
r
0.84
0.99
0.88
0.99
0.87
0.99
17
-------
PREDICTED COLLECTION EFFICIENCY FOR UQ = 200 cm/s
10
C
c;
0 I
10
I 0
= 10/j.m
\
37-mm OPEN-FACE
1 8 L/mm
37-mm IN-LINE
I 8 L/mm
13-mm IN-LINE
0 5 L/mm
90
180
ANGLE OF INCIDENCE, a (deg)
Fig. 15.
Collection efficiency (c/c0) predicted by Eqs.
(10), (11), and those in Table IV for standard
flow rate and u0 = 200 cm/s.
whereas, for the 37-mm OF and IL cassettes minimum c/c0 occurs at a = 180°. Raynor8 found that
an open-face type, thick-wall holder (D/l = 1.7) also exhibited minimum collection efficiency at a
= 90°. The predicted results for 20-jum Dae particles in Fig. 13 are extrapolations since no experi-
ments at a = 90 and 180° were conducted with particles larger than 11-jum Dae.
18
-------
RAC AND WT COMPARISON
As stated earlier, limited tests were made in a small wind tunnel to determine the similarity in
performance of the cassettes in the WT and RAC. Only two cassettes, a 37-mm IL type and an
identical cassette fitted with an isokinetic probe, were tested and compared in the WT. Com-
parison with an isokinetic probe was necessary because no stationary reference samples could be
obtained in the WT, as they were in the RAC. A velocity of 5 m/s and challenge aerosols of 4- and
7-jum Dae were used in the WT tests.
Results from WT runs are designated in Tables III and IV with the notation WT under the first
column. No stationary reference samples were obtainable in WT runs; consequently, the WT
results cannot be compared directly to results in the RAC. However, since identical IK
samplers were used in both chambers, the results can be compared from the listed c/c0 and
Ai (Table III) for 7.2- and 7.4-/um Dae for the IK and IL samplers. Using Cn, and CIK to denote con-
centrations obtained from the 37-mm in-line and isokinetic samplers, respectively, the develop-
ment is:
CIK/C0 = 0.97 in RAC (Table III, row 2, c/c0) (12)
and CIL = 1.25 in RAC (Table III, row 5, c/c0) (13)
then C0 = CiK/0.97 (14)
and CIL/C0 = CIL/(CIK/0.97) = 1.25 in RAC , (15)
.'. CIL/CIK = 1.29 in RAC . (16)
Also CIL/CIK = 1.08 in the WT (Table III, 5 m/s, last row, c/c0) . (17)
But these concentration ratios are for slightly different At and must be normalized to the same A4
(equivalent conditions). Thus from (16) and Aj in Table III (rows 5 and 14):
CIL/CIK Af1 = 1.29/1.41 = 0.91 in the RAC , (18)
and CJL/CIK A'1 = 1.08/1.35 = 0.80 in the WT . (19)
Therefore CIL/CIK in the RAC, and (CIL/CIK) in WT are (0.91-0.80)/0.80 = 13.7% different for
equivalent AI. The minimum error in the experimental data may be estimated from the coef-
ficient of variation (CoV) from Table IE:
19
-------
Error = [(CoV of IK)2 + (CoV of IL)2] 1/2 = [(0.17)2 + (0.07)2]172 = 0.18 or 18%
This indicates that the difference between sampling efficiency determined in the two chambers
(13.7%) is less than the experimental error in measurement. Thus the RAC and WT give
equivalent sampling efficiencies for the test cassettes.
Throughout the cassette testing the wall losses (deposition) of a fraction of the cassettes were
determined. Losses due to deposition were generally high and variable, but proportional to the
challenge aerosol particle size. The losses at U0 = 5 m/s were slightly greater than at U0 < 5 m/s
but this difference was not quantitated. The 37-mm IL cassette with isokinetic probe exhibited
the largest losses at a = 0° (Table VI), probably due to the small diameter and long entrance
length of the probe. At 90 and 180° insufficient numbers of samples were measured to serve to
define losses at sizes other than 8-^tm Dae, but at a = 180°, these particles produced the greatest
wall losses measured. In all angles of incidence, average losses (for a particular cassette) were
used to estimate losses in collection efficiency tests for which no loss data were obtained. As noted
previously, data in Tables II through IV (and Figs. 6-13) have been corrected for actual or es-
timated wall losses. The average loss for each cassette is shown in Table VI, with number of sam-
ples in parentheses before the mean value, and CoV in parentheses after the standard deviation.
The large CoVs indicate the variability of the wall losses.
Cassette
IK
37-mm IL
37-mm OF
13-mm IL
25-tnm IL b
37-nrn IL
37-mm OF
13-nrn IL
37-mm IL
37-mm OF
13-nrn IL
a
0°
0°
0°
0-
0°
go-
go"
go-
iso"
180"
180°
TABLE VI
WALL DEPOSITION LOSSES (%) IN TEST CASSETTES
Nominal Particle Diameter, D (ym)
ae
4
(2)153
(3)8*6(76)
(2)5
(2)4
(1)46
6
(10)19*42(60)
(4)9*10(116)
(6)8*4(53)
(2)15
(2)4
(2)10
(1)21
8
(17)22*11(49)
(13)17*19(110)
(10)11*10(90)
(3)13*9(71)
(3)6*3(47)
(4)5*2(40)
(6)12*6(49)
(2)6
(6)47*13(28)
(3)59*20(33)
(3)45*17(37)
21
(9)47*26(56)
(10)19*20(102)
(9)34*17(50)
(7)36*22(59)
(12)14*8(55)
Wall losses are shown in the format: (Number of samples) mean wall loss +
standard deviation (CoV). All in %, except number of samples.
t>25-mm IL cassettes were used as stationary reference samplers, collecting at
5.0 L/min flow rate.
20
-------
SUMMARY OF RESULTS
Based on the empirical functions found to correlate test results with Eqs. (8) and (9), it is possi-
ble to predict the performance of the cassettes under various conditions. The predicted perfor-
mance was shown in detail in Figs. 10 and 15. A summary of predicted performance for various
sampling conditions is listed in Table VII. The conditions of wind, angle of incidence, and Dae are
listed for which each cassette can be expected to provide a sampled concentration within 20% of
the true or reference concentration. Thus, for calm conditions all three cassette samplers can be
expected to provide a representative sample for particles with Dae to at least 15 /j.m. On the other
hand, none of the samplers provide a valid sample if the wind is 200 cm/s or greater from behind
the cassette inlet, with the exception that the OF cassette will provide a valid sample of particles
not exceeding 5-jum Dae. The absence of entries for u0 = 500 cm/s and a = 90 or 180° indicates that
none of the cassettes sample efficiently under these conditions for any particle size tested. The
results from Table VII as well as Figs. 10 and 15 are valid only for cassette samples for which the
wall losses have been determined and included in the determination of concentration.
CONCLUSION
Aerosol concentrations determined by test samplers facing into the wind were generally within
20% of those determined by reference sampler for aerosols with Dae <5 /urn after correction for
wall losses. In calm wind conditions the cassettes were within 20% for Dae ^ 15 /um. However, as
particle size was increased, sampling efficiencies of the test cassettes differed increasingly from
theoretically calculated efficiences at both calm and windy conditions. This departure from
Belyaev and Levin's theory for thin-wall samplers was consistent even though the reason for the
disparity is unknown. In spite of the disparity, Belyaev and Levin's theory and Davies' modifica-
tion for calm sampling can be used to develop predictive equations for the test cassettes sampling
at angles of incidence of 0, 90, and 180°. Predictive equations can be based upon the "thin-wall"
sampler theory because it contains two of the same parameters that determine thick-wall sampler
performance, namely u/u0 and STK.
Using criteria described in the summary some specific conclusions concerning the cassettes
may be stated.
In calm winds, sampling error is <20% for all cassettes for Dae <15 /urn.
The 37-mm OPEN-FACE cassette exhibits large error when sampling into a wind (a =
0°).
The 37-mm IN-LINE cassette exhibits <20% error for Dae <9 /urn for all test conditions
except a = 180°.
The 13-mm IN-LINE cassette exhibits <20% error for Dae <25 /um in calm winds, but
when sampling against winds, particularly at a = 90 and 180°, its error exceeds 30%.
TABLE VII
SUMMARY OF CONDITIONS FOR WHICH THE CASSETTES
MEASURE WITHIN +20 PERCENT OF TRUE CONCENTRATION
Conditions Size range, aerodynamic dlam, Dap (iim)
Wind a
cm/s degrees
Calm
200 0
500 0
200 90
200 180
37-mm
OPEN-FACE
<15
TTone
none
<20
I 5
37-min
IN-LINE
<15
-------
The 37-mm IL cassette apparently provides the best sampling efficiency for most conditions in
spite of its complex inlet geometry. Finally, it must be cautioned that wall losses are significant in
all the cassettes for Dae > 6 jiim and must be accounted for in use.
ACKNOWLEDGMENT
Mrs. Bonnie Isom, Group H-5, Los Alamos Scientific Laboratory, performed the SEM
photomicrography required for this study.
REFERENCES
1. G. A. Sehmel, "Complexities of Particle Deposition and Reentrainment in Turbulent Pipe
Flow," J. Aerosol Sci. 2, 63-72 (1971).
2. S. P. Belyaev and L. M. Levin, "Investigation of Aerosol Aspiration by Photographing Particle
Tracks Under Flash Illumination," J. Aerosol Sci. 3, 127-140 (1972).
3. S. P. Belyaev and L. M. Levin, "Techniques for Collection of Representative Aerosol Sam-
ples," J. Aerosol Sci. 5, 325-338 (1974).
4. S. Badzioch, "Collection of Gas-Borne Dust Particles By Means of an Aspirated Sampling
Nozzle," Brit. J. Appl. Phys. 10, 26-32 (1959).
5. V. M. Voloshchuk and L. M. Levin, "A Study of Aerosol Aspiration," Trans. List. Exp. Met.,
No. 1, 84-105 (1969).
6. C. N. Davies, personal communication, 1980.
7. M. D. Durham and D. A. Lundgren, "Evaluation of Aerosol Aspiration Efficiency as a Func-
tion of Stokes Number, Velocity Ratio and Nozzle Angle," J. Aerosol Sci., 11, 179-188 (1980).
8. G. S. Raynor, "Variation in Entrance Efficiency of a Filter Sampler with Air Speed, Flow Rate,
Angle and Particle Size," Am. Ind. Hyg. Assoc. J. 31, 294-304 (1970).
9. N. Rajendran, "Theoretical Investigation of Inlet Characteristics for Personal Aerosol Sam-
plers," IIT Research Institute report, 128 pp. (1979).
10. N. A. Fuchs, "Sampling of Aerosols," Atm. Env. 9, 697-707 (1975).
11. W. K. Lautner and G. T. Fisher, "Ragweed Pollen Generation and Sampling Methods for
Filtration Studies," Am. Ind. Hyg. J. 36, 303-310 (1975).
*U S. GOVERNMENT PRINTING OFFICE: 1980777-022/220
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