&ER&
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-144 Sept. 1981
Project Summary
Small Size-Range
Extension of an Optical
Particle Counter
James T. Brown, Jr.
The object of the study described in
this report was to improve the small
particle sensitivity and resolution of a
white light optical particle counter.
The particular counter chosen for
study was the Model 208 manufac-
tured by Climet Instruments. The
investigation involved three distinct
approaches: (1) a comparison of the
effect of several different photomulti-
plier detectors and operating param-
eters, (2) the effect of higher tempera-
ture operation of the white light
source, and (3) modification of the
detector and scattering chamber
geometries. Of the three possibilities
for performance improvement the last
was the most useful.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
In many applications the characteris-
tics of single particle optical counters
are desirable because they are able to
provide both size and number
information on a near real-time basis.
Typically, the operating (size) range for
these instruments extends from tens of
microns down to several tenths of a
micron. (Particle size refers to geometri-
cal diameter throughout this report.)
Because the physiological impact of
particles as small as 0.1 micron may be
significant, it would be advantageous to
apply single particle optical detectors to
problems and research in this size
range.
The instrument chosen for this study
is the Climet 208 (Climet Instruments
Co., Redlands, CA); its nominal size limit
is 0.3 to 10 microns. In practice it is diffi-
cult to resolve distributions for which
the mean size is less than 0.5 micron.
Familiarity with the Climet 208 has
indicated several potential modifica-
tions that show promise for improving
its resolution below 0.5 micron and for
increasing its sensitivity for particles as
small as 0.1 micron. The increased
sensitivity was expected to result
primarily from decreases m the noise
intrinsic to the design of the system. The
improvements in resolution were pre-
dicted to result from size-dependent
gain characteristics including the
photomultiplier tube (PMT) spectral
response and the spectral distribution of
incident light.
In order to obtain a relatively unam-
biguous evaluation of the sub-0.5
micron response of this instrument it
was necessary to have access to an
aerosol with a well defined peak in its
size distribution. Of equivalent impor-
tance is the absence of a large back-
ground of smaller particulates. A
parallel plate electrostatic classifier of
recent design was available and could
provide the required monodisperse
aerosol by separating the off-sized
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residuals from an aerosol generated
from Dow latex spheres (Dow Diagnos-
tics, Indianapolis, IN). The refractive
index of such particles (at a wavelength
of 0.59 micron) is 1.59, which is similar
to values commonly assumed for
ambient aerosol.
Description
Figure 1 presents a schematic view of
the Climet 208 sensing head. Light from
a 3000° K quartz-halogen lamp is
scattered from a collimated beam. The
scattered light is collected by an ellipti-
cal mirror and focused on a PMT
detector. The scattering volume, which
is the intersection of the particle beam
with the collimated light beam, isplaced
at one focus of the elliptical mirror and
the PMT is placed near the other focus.
In the application under consideration
the electronics which are supplied by
the manufacturer are by-passed and the
output of the PMT is fed directly to an
amplifier and a multi-channel analyzer
(MCA).
Three distinct approaches were used
in attempting to improve the small
particle sensitivity and resolution of the
Climet 208. These are described below.
1. Photomultiplier intercomparison:
Qualitative observations were
made with three different photo-
multiplier tubes: RCA 931 A, RCA
931 B, and RCA 4832. Best results
were obtained with the 93J B tube,
which is now normally supplied
with the Climet 208. The perform-
ance with the 4832 tube was
disappointing, as its very broad
spectral response characteristic
was initially thought to make it an
attractive candidate for considera-
tion. Since only one tube was tried
the 4832 results may not be
representative of this type.
2. Light source evaluation: The
standard white light source
provided with the Climet 208 is a
45 watt, 6.6 ampere, quartz-
halogen lamp. The effective fila-
ment temperature for the
standard operating conditions is
~3000° K. The rationale behind
light source modifications was
primarily that it would be desirable
to increase the operating temper-
ature of the filarnegt because of
the increased output in the short
wavelength portion of the spec-
Photo
Cathode
Photo -
multiplier
Tube
Elliptical
Mirror
Aerosol
Out
Quartz- <
Halogen
Lamp
Figure 1. Climet 208 sensing head.
trum. The results of operating the
quartz-halogen lamp at elevated
temperatures can be seen in
Figure 2. The five spectra were
obtained using 0.460 micron latex
spheres and by varying the lamp
voltage from 6.0 V. to 10.0 V. in
steps of 1.0 V. In Figure 2 some
spectra occur with different verti-
cal scales; however, this does not
influence the conclusion that the
resolution is substantially im-
proved with increasing lamp
voltage and filament temperature.
Since increasing the filament
temperature will significantly
reduce the expected lifetime of the
bulb, any decision to routinely
operate the system under such
conditions will necessarily have to
consider the inconvenience of fre-
quent bulb replacement; at the
maximum voltage, the bulb life-
time is expected to be of the order
of several to tens of hours. Bulb-
life as a function of operating
voltage was not studied quantita-
tively.
3. Geometry modifications: The
object of this approach was to limit
as much .as possible the light
reaching the PMT to only that
which originated in the scattering
volume, i.e., the intersection of the
part|cle beam and the light beam
where the slit is imaged (see
Figure 1). This aim required
knowledge of the focal properties
of the elliptical mirror.
First, a piece of photographic paper
was fastened to the surface of the PMT
and a 0.005-inch wire was threaded
through the inlet and outlet ports inter-
secting the incident light beam. The
wire then served as a source of
scattered light originating in the
scattering volume. Exposing and
developing the film yields the desired
image information. Calculations based
on the geometry and the mirror configu-
ration agreed very well with the image
determination just described. In each
case, the image of the scattering volume
was a 5-mm-diameter circle at the focal
point of the mirror. Since the dimen-
sions of trie photocathode are large (8
mm x 24 mm), it was reasonable to
consider masking the surface of the
PMT to admit only the signals falling on
the image of the scattering volume. In
an effort to estimate the extent of the
image of the noise (the illuminated
portion of the photocathode due to light
scattered from regions outside the
scattering volume) the wire was
removed and the PMT covered again
with photographic paper. With no
signal, that is, no particulates passing
through, a long exposure was made. At
the focal point of the mirror the image
produced by the intrinsic noise was a
circle approximately 20 mm in diameter.
Also, in order to mask the PMT
optimally, the sensing head had to be
adjusted to place the surface of the PMT
at the focal point of the mirror, further
from the mirror housing than the
normal operating configuration. It was
not possible to adjust the Climet 208 to
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1
10V
9V
8V
7V
6V
Apparent Size
Figure 2. Particle size spectrum as
a function of lamp voltage
for 0.46 micron latex
spheres. The vertical
scale is arbitrary for each
spectrum.
this position because, with the required
extension of the sensing head, the
o-rings would no longer provide a reli-
able seal. A "collar" attachment was
fabricated to fit onto.the outside of the
sensing head to allow the additional
extension needed to focus the mirror on
the PMT surface. The masking of the
PMT was accomplished by taping a
piece of black-painted aluminum foil,
with the appropriate circular aperture,
onto the glass envelope of the PMT. As
can be seen in Figure 3, the
improvement in resolution due to the
masking is appreciable forO.31 2 micron
latex aerosols.
Conclusions
It is possible to extend the small-size
sensitivity of the Climet 208 to allow
resolution of a 0.312 micron aerosol
peak. The most significant modifica-
tions in the instrument leading to this
improvement are the refocusing of the
elliptical mirror on the PMT, the mask-
ing of the surface of the PMT, and
increasing the operating temperature of
the quartz-halogen lamp.
The anticipated improvement in the
response through the use of a broad
ectral response PMT (RCA 4832) did
>t occur. Lack of resources, however.
limited the thoroughness of this part of
the investigation.
Recommendations
The improved performance of the
Climet 208 resulting from repositioning
and masking the PMT should be of
interest to users of this instrument.
Both are relatively straightforward and
inexpensive modifications which can be
performed by the user. Higher tempera-
ture operation of the light source also
improves sensitivity and resolution.
This mode of operation cannot be
recommended in general, however,
because of the shortened lifetime of the
lamp and the increased danger of shat-
tering under the more extreme opera-
ting conditions.
The photomultiplier comparison was
not sufficiently quantitative nor com-
prehensive to be definitive. Additional
.work is needed to understand and con-
firm the result found for the 4832 tube,
and to extend the investigation to
additional tube candidates.
With Mask
Factory Configuration
Apparent Size
Figure 3. Particle size spectrum for
0.312 micron latex show-
ing the improved resolu-
tion due to masking the
PMT.
James T. Brown, Jr., is with the Department of Physics, Colorado School of
Mines, Golden, CO 80401.
Charles W. Lewis is the EPA Project Officer (see below).
The complete report, entitled "Small Size-Range Extension of an Optical Particle
Counter," (Order No. PB 82-103 623; Cost: $5.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
US GOVERNMENT PRINTING OFFICE. 1981 —559-017/7366
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