United States
 Environmental Protection
 Agency
 Environmental Sciences Research
 Laboratory
 Research Triangle Park NC 27711
 Research and Development
 EPA-600/S2-81-096 Aug. 1981
 Project  Summary
 Modification  of  Optical
 Instrument for  In-Stack
 Monitoring  of  Respirable
 Particle  Size
 A. L. Wertheimer
  A light scattering instrument for in-
 situ measurements of particulates in
 the 0.2 to 20 micrometer diameter
 size range is described, and field test
 results are presented. The instrument
 is a modified version of a prototype
 built during a prior EPA contract.
 Number 68-02-2447. The upper limit
 of the size response has been extended
 from 10 to 20 micrometers, and several
 component and packaging changes
 have been incorporated to make the
 unit more suited to stack particulate
 survey applications. Low forward
 angle and 90° polarization dependent
 scattering is employed to make the
 measurements.
  The completed instrument was tested
 at a coal-fired electric power generat-
 ing facility. During the test a cascade
 impactor was used as a referee device
 and both instruments were run side by
 side in the outlet duct of the electro-
 static precipitator.

  The results show an excellent cor-
relation between the two instruments
with regard to the identification of a
"\fjrn diameter peak in the particle size
distribution. A second peak around 20
/urn was defined by the optical instru-
ment, but could not conclusively be
confirmed through the impactor data.
The optical instrument  handled well
during the field test and was delivered
to EPA for additional testing.
  This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).

Introduction
  A prototype real-time in-situ monitor
was developed and constructed on EPA
Contract 68-02-2447 to measure particle
size distribution of respirable particles
in the 0.2 to 10 /t/m range. The purpose
of this project was to add a channel to
cover the  15 yum size range so as to
include the upper cut-off of the inhalable
particulate emissions from stationary
sources.
  The addition of the large particle
channel required a series of changes in
the optical and electronic assemblies of
the original instrument. In the process
of incorporating these changes, the
latest available components were se-
lected and packaging improvements
were made, resulting in an instrument
optimally suited for  survey  work and
stack particulate analyses.  The new
instrument measures the size distribu-
tion in the 0.2 to 20 yum range in five size
fractions, using a low power helium
neon laser light source.
  The modified prototype instrument
was tested at a coal-fired electric gen-

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erating plant. Referee measurements
were made with a cascade impactor.
Both instruments reported a strong
peak in particle size around one /um in
diameter.

Procedure

Principles of Operation
  The instrument was designed by
using simple diffraction theory for the
low  angle forward scattered light, and
rigorous Mie theory for the light scattered
at 90° to the probe beam. By adding high
angle scattering capabilities, the use of
light scattering for particle analysis can
be extended to the sub-micrometer size
range.
  The stack particulate monitor mea-
sures the light scattered by particles
passing through a 2.5 cm by 36 cm slot
at the end of a  152 cm (5 foot) long
probe. The light source is a 2 milliwatt
helium neon laser, which emits a co-
herent beam at 0.6328 //meters. The
scattered light signals are proportional
to the volumes of particulate  material
present in each of five size fractions. Six
scattered  light  readings are taken at
precisely determined angles. The light
signals are acquired through fiber optic
cables and transmitted to detectors
located in the transceiver. A digital
microprocessor calculates a five chan-
nel, volume-by-size histogram, covering
the size range from 0.2 //m to 20 /um.

Modification of the Prototype
  Modification of the original instru-
ment to add a 15/um channel involved a
number of significant changes. When
appropriate, these changes were made
so as to accommodate improvements
suggested from field trial experience
with the first unit. The pertinent aspects
of the new design are discussed in the
following paragraphs.
  The xenon arc source was replaced by
a low power (2 milliwatt) helium-neon
laser, which provides better collimation
of the source, and eliminates a trouble-
some electrical transient starting prob-
lem. A slightly larger collection lens
system was designed to accommodate a
wider range of forward scattering angles.
However, the 90° collection system
used in the earlier unit remains the
same.
  A beam alignment sensor was added
to the tip  of the probe to monitor any
thermally induced shifts. Through ports
accessible from the  rear of the probe,
the beam can be aligned in or out of the
stack by  maximizing the reading on a
meter adjacent to the adjustment ports.
  A Z-80 microprocessor  system re-
placed the original 8008 based elec-
tronics. The new system allowed for
rapid and efficient implementation of
the hardware and software changes
required in modifying the unit. The new
electronics is much more compact than
the earlier version, and is combined
with a small digital printer in a 20 pound
transportable electronics console. A
second, smaller box, contains the elec-
tronics power  supply,  packaged sepa-
rately to avoid heat  build-up  on  the
control console box.
  A summary of operational character-
istics of the prototype is shown in Table
1. The measurement time can be set by
the user and ranges from 5 seconds to
12 minutes. Immediately following the
data collection, the size distribution is
printed out at the console.


Calibration
  The calibration process involved
several steps and used a variety of
materials. To properly fill the sample slot
region under operating conditions sim-
ulating a flowing gas stream, an aerosol
test chamber was constructed in the
laboratory.
  The major  steps of the calibration
process are outlined here.
  (1) During assembly, the light collect-
      ing apertures were checked for
      alignment and adjusted to insure
     that the correct angles were being
      measured.
 • (2)  Di-octyl phthalate (OOP), a trans-
      parent  liquid with an index of
      1.49, was dispersed as a droplet
     suspension in the aerosol test
     chamber by a Phoenix Precision
     Aerosol Generator. This created a
     well-controlled size and loading of
     particles in the 0.2 to 3 /um size
     range. From the measured signal
     levels and knowledge of the load-
     ings, detector gain adjustments
     were made to accommodate a
     uniform distribution of particles at
     40 parts per billion.
  (3) The collection geometry and fiber
     transmission product at each
     angle was determined by measur-
     ing fresh, filtered cigarette smoke.
     Because the majority of the par-
     ticulate volume is well below one
     /um  in  diameter,  the  forward
     scattering pattern does not change
     with particle size. A correction
     constant is thus defined for each
     scattering angle, based on
     difference between scattered light
     strengths observed and those
     predicted by theory.
Results

Laboratory Tests
  As a check for consistency, the instru-
ment was then used to measure the
aerosol  distributions employed to cali-
brate it. Figure 1  shows the filtered
cigarette smoke distribution, indicating
a large percentage of the material in the
0.3  yum size channel, while Figure 2
shows the measured and manufacturer's
specifications for the OOP aerosol
suspension. In both cases, agreement
between expectation and observation is
quite good.
  To further check the performance and
calibration, two other materials were
run, burning red phosphorous, and solid
glass spheres. The red phosphorous is
used for tactical smoke screens, but no
referee  data was available. The instru-
ment readings indicated roughly equal
amounts of material in the 0.3 and 1.0
/urn  size channels.  This is  consistent^
with its  intended tactical use since par-V^
tides in this size range are the most
efficient scatters per unit volume and
thus provide good obstruction.
  The solid glass spheres, from Potters
Industries, Inc.,  were  used to check
performance of the larger size channels.
The  spheres are  specified as "3 to 10
micron" size, but no additional data was
provided or available. No material  is
reported  in the 0.3 /um channel, as
expected, and most of the material is in
the 3.5  or 7.5 fjim region. The material
reported in the 15 //m channel may be
caused by clumping of the beads due to
electrostatic charges introduced in the
suspension process. Microscopic exam-
ination  of a bead  sample collected
during the test confirmed this, showing
occasional clumping.

Field Test Performance
  During July, 1980, the prototype
instrument was tested at an east coast
coal-fired electric power generating
station.  L&N personnel used the proto-
type instrument to measure particle size
distribution in a duct leading to the
smoke stack. Personnel from Northrop
Services, Inc., Environmental Science
(NSI-ES), participated in the tests,
taking data with a  cascade  impactor, /*
and provided the necessary data analysis V

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Table 1.    Operational Characteristics of Stack Paniculate Monitor
 Size Range (Particle Diameter)

 Size Discrimination
 Mode of Operation


 Loading Range



 Measurement Time



 Duct Velocity

 Duct Temperature

 Instrument Temperature

 Power Requirements


 Probe Dimensions


 Sample Slot Dimensions

 Transceiver-Probe Assembly


 Control Console


 Electronics Power Supply


 Blower


 Probe Material
    0.2 to 20.0 urn

    Five volume fractions with centers at 0.3, 1,0,
    3.5. 7.5, 15 urn

    Low angle forward scattering and 90°
    polarization dependent scattering

    0.01 to 1.0 grams of material/meter3 (.023
    to 2.3 grains/ft3) or 4 to 400 pans/billion by
    volume (with s.g. of 2.5)

    Signal integration time selectable from 5
    seconds to 12 minutes (including a 6-minute
    position)

    1.5 to 18 meters/second (5-60 feet/second)

    260° C maximum (500° F)

    2°  C to 43° C (35 to  110° F)

    One 20A, 115 volt, 60 Hz outlet

Physical Specifications

    152 cm long (60 inches) by 9 cm diameter
    (3'A inches)

    2.5 x 36 cm (1 x 14 inches)

    203 x 25x25 cm, 31.8 kg (80 x  10 x 10 inches.
    70 pounds)

    38x41 x 25 cm. 9.1 kg (15 x 16 x 10 inches,
    20 pounds)

    23 x 41 x 25 cm, 6.4 kg (9 x 16 x 10 inches,
    14 pounds)

    74x48x 43 cm, 22.7kg (29 x 19 x 17 inches,
    50 pounds)

    Type 316 Stainless Steel (except for optical
    components)
for that method. Six separate data sets
were collected over two days. One set of
data from each day is presented here.
  All testing was performed at the
outlet of the electrostatic precipitators
and  prior to the final exhaust fan. The
testing section was a vertical flow duct,
approximately 32  1 /2 ft. wide  by 7 ft.
deep. Sampling ports are located hori-
zontally across the wide side of the duct.
Each port is a 6-inch diameter  flanged
pipe, approximately 14  in. long. Two
adjacent ports were selected  as test
points. A summary of the stack condi-
tions appears in Table 2.
  All aerodynamic particulate sizing
was  performed using a  University of
Washington Mark III Cascade Impactor
and necessary support equipment. Prior
to actual source testing, all in-stack
atmospheric measurements necessary
for isokinetic and other calculations
             were recorded. Velocity head and stack
             differential pressure measurements
             were peformed using a type "S" pitot.
             In-stack temperatures were measured
             using a thermocouple system attached
             to the end  of the pitot tube. Velocity
             profile measurements were made up to
             4.5 ft. into the duct at both test ports,
             with  the impactor sampling conducted
             at the point of both average velocity and
             close proximity to the optical instrument.
             The point used for sampling was ap-
             proximately the mid-point of the duct or
             4 ft. from the lip of the port flange.
               The impactors were preheated to
             stack temperature before sampling to
             avoid moisture condensation within the
             impactor body. The duration of each test
             was varied according to the stack opacity,
             knowledge that this coal unit was within
             particulate emissions  standards, and
             the visual inspection of the previous
    80-

    70

    60-
    30-

    20-

    10-
             .3  1.0 3.5  7.5   15

                    It
Figure 1.
Calibration   run
cigarette smoke.
                              using
impactor test. Sample runs varied from
20 to 40 minutes in length.
  Several hours were required to make
the preliminary measurements before
the impactors were inserted.
  The prototype optical particle size
monitor was prepared within approxi-
mately one hour. All electrical cables
were connected and the instrument
was turned on to warm-up the electron-
ics. The optical  alignment of the unit
was adjusted using the external meter.
The stack velocity,  measured for the
impactor runs, was used  to set the
purge flow rate on the blower. To facili-
tate insertion and removal from the
stack during the tests, a suspension rail
designed  and built previously for this
unit by NSI-ES was erected. A typical
sample run lasted 6 minutes, and sev-
eral runs were made during the impactor
sample collection period.
  On the  first sampling  day the boiler
unit was operating at maximum output.
On the second day, the boiler was
operating at reduced output, and the
particulate emissions were distinctly
lower, dropping from around 0.02g/Nm3
the first day, to 0.007g/Nm3 the second,
as measured by the impactor.
  Results  for both optical and inertia!
instruments are shown in Figures 3 and
4, plotted as  histograms of volume
fraction per unit log  interval of panicle-
size. The optical data in each figure are
indicated by the cross hatched histogram,
while the  impactor data  are shown as
the heavier outlined histogram. Varia-
tions in the individual channel widths

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   so-

   so-

   70-

   60-

|  50-\



   30-

   20-

   10-
                                        Table 2.    Stack Conditions During Field Test
                     Measured


                     Manufacturer's
                     Spec.
Figure 2.
            .3   1.0 3.5 7.5   15
Calibration using dibutyl
phthalate  aerosol  from
Phoenix generator.
are due to the different principles
involved in measuring the  particle
distribution.
  The impactor data, provided by NSI-
ES, were derived by plate weighings and
computer assisted data reduction. A
material density of 2.5g/cm3  was as-
sumed, and the channel edges were
based on the aerodynamic separation
properties of the individual stages of the
impactor.
  During each impactor run, continuous
optical data measurements were made.
The histograms shown are compiled
from the time weighted average of the
sequential optical  data, which  involved
from 5 to 8 optical runs, depending on
the length of  the impactor run. The
boundaries of the  optical histogram are
determined by the instrumental response,
as calculated from scattering theory.

Discussion
  The optical and inertia! measurements
agreed in some significant respects. In
all runs both instruments reported a
significant size fraction to be around
one fjm in diameter with, in most cases,
substantial reductions in the amount of
material above the  one urn size. The
optical instrument consistently indi-
cated a good  deal of material in its
largest size channel, which made the
distribution appear bimodal. This could
not be definitely confirmed by the
impactor data  available, although im-
pactor runs from some tests show a
                                          Stack gas velocity:
                                          Stack gas temperature:
                                          Gas pressure:
                                          Direction of flow:
                                                                 45 to 50 feet/second
                                                                 230 to 300° F
                                                                 -2 inches of Hg
                                                                 Vertical downward
leveling off of the distribution, and run 4
does indicate a secondary peak in its
largest particle channel.
  The general agreement between the
two methods is good. The size response
question could well be resolved through
further testing at other  sites. There
were some relatively minor technical
problems, but none that should prevent
the optical instrument from being used
in other field tests. At the conclusion of
this test, the prototype stack participate
monitor and its associated  equipment
were turned over to the EPA.

Conclusions and
Recommendations
  The primary goal of this work was to
modify and test a prototype optical stack
particulate monitor by the addition of a
channel responding  to particles  in the
15 fjm size range. This was successfully
accomplished. Tests in the laboratory
showed results that agreed with ex-
pected size distributions of several
                              40-
sample materials which were in the 0.2
to 20 yum size range of the instrument.
The field tests, conducted at a coal fired
electric utility plant, provided size distri-
bution data which were in  excellent
agreement with  results reported by a
referee inertia! impactor. An additional
advantage with the optical instrument is
that size distribution data are computed
and  displayed immediately  upon the
conclusion of the  signal collection
sequence.
  A secondary  goal of this project was to
improve the reliability and portability of
the instrument to make it more suitable
for stack  survey  work. The modified
prototype  unit is lighter and smaller
than the original, and the operational
improvements, such as ease of align-
ment and  reliability of operation, were
demonstrated during the field trial.
  There may be some value in develop-
ing an on-site technique to provide the
operator with a quick means of checking
the calibration of the  unit. Although
                                                Run No. 1
                                                Impactor
                                                Optical Sizer
                                       .2
                            Figure 3.    Particle size vs. volumetric concentration distribution during Day 1 of
                                        the field test.

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  c
  01
  se
     50-
     40-
     30-
 < 20-

 <
     10-
                                               Run No. 4
                                               Impactor
                                               Optical Sizer
                          .5        J      2

                                     Diameter, D^irri)
 i
5
20
 Figure 4.     Particle size vs. volumetric concentration distribution during Day 2 of
              the field test.

 there are no moving parts in the proto-
 type which would affect the calibration,
 some form of indicating calibration
 status is desirable.
  Another area for future consideration
 is modification of the electronics to
 optimize the gain for  loadings at or
 below the originally specified range of
 0.01 to 0.1 grams/meter3. This could be
 done by changing the feedback resistors
 at the detector board and trimming the
 electrical offsets to lower values.
  In its present form, however, this type
of instrument should prove to be very
 useful for field survey work for analysis
of size distributions from stationary
sources. Recommendations for future
work involve  additional field trials at
 sites with different types of fuel, clean-
 up devices,  and loading conditions. To
gain confidence in this type of instru-
mentation, measurements with referee
sizing  instruments should be taken in
parallel.
  > U.S. GOVERNMENT PRINTING OFFICE 1W1-757-012/7282

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