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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to particle size measuring instruments in general
and in particular to devices for checking the calibration, sensitive
volume, and particle counting efficiency of optical, single particle size
spectrometers.
During the last decade or so, instruments have become commercially
available for measuring the size of individual, microscopic, airborne
particles such as cloud droplets or other particulate aerosols. These
instruments, called particle size spectrometers, are generally based on
the principle of visible light scattering by particles aspirated through
an illuminating beam. For particles larger than about 20 .mu.m diameter
some spectrometers are based on light interception or shadow imaging
techniques.
In the light-scattering models, light scattered out of the illuminating
beam by passing aerosol particles is collected by a mirror or lens and
directed to a sensitive photodetector. The photodetector current pulses
are amplified and, by means of electronic pulse height analysis
techniques, they are sorted and counted in histogram form. The number of
counts indicates the particle concentration and the histogram provides a
size frequency distribution for the detected particles. A given
spectrometer will respond to particles within some preselected interval of
sizes, such as 1 to 30 .mu.m diameter, for example. The histogram readout
further divides this interval typically into five to fifteen sub intervals
or size categories.
A problem common to all such spectrometers is the need for calibration so
that the instrument will properly indicate the size of each validly
sampled particle. A good primary calibration requires so much care and
special equipment that practically all instrument users rely on the
manufacturers to provide such services. For spectrometers sensitive to
particles larger than about 10 .mu.m diameter, a rough calibration can be
performed in the field with the use of commercially available, microscopic
glass beads. However, the available beads are sufficiently varied in size
within a sample that the finer size resolution capabilities of some
instruments cannot be properly tested.
Related, important problems include the need for measuring the optical
depth of field (the length of that portion of the illuminating beam which
is sensitive to transiting particles) and the efficiency of the instrument
in registering every validly sampled particle.
The prior art for accomplishing these objectives is described below, along
with the disadvantages inherent in the prior art.
a. Primary Calibrations
A primary calibration basically consists of adjusting amplifier gains and
or noting the size categories into which appear particle counts from a
sample of known, uniform (monodisperse) sized particles of the type to be
measured in application. By selectively changing the monodisperse size of
the test particles, the total instrument response (a determination of the
particle diameters corresponding to each of the available size categories
for the instrument) can be obtained.
The principal difficulties involved here are in the production of test
particles of a known and stable, monodisperse size. For most common
applications water droplets would be the preferred test particles but
monodisperse droplets are difficult to produce in the diameter range of
0.1 to 10 .mu.m covered by a majority of the optical, single particle
spectrometers in use. Even if a reasonable monodispersity can be initially
achieved by some type of droplet generator, it is difficult to prevent
droplets of this small size from evaporating partially during the transit
from the generator to the sensitive volume of the particle sizing
instrument.
To avoid these difficulties, various types of microscopic latex spheres,
oil droplets, or other non volatile particles are generally used. However,
the special equipment (nebulizers, air pumps, drying chambers, etc.)
required is sufficiently elaborate and the procedures sufficiently tedious
that most instrument owners rely upon the manufacturers to provide this
primary calibration service. Such equipment is definitely impractical for
routine use in the field.
The only existing method that is simple enough for use in the field
involves the use of microscopic glass spheres or "beads." These are
commercially available but are not very practical in sizes smaller than
the nominal 13 .mu.m diameter size class produced by the principal
manufacturer. The available sizes are not very monodisperse either, with
the result that the range of bead diameters is wider than the resolution
capabilities of some instruments. Other difficulties with the glass bead
method are:
First, the test results are generally not as reproducible as is desired for
an accurate calibration check.
Second, for instruments not equipped with an internal blower or pump for
pulling sample air through the sensor unit, extra blowers, pumps,
connecting tubes and fittings are necessary for preparing the probe for
the calibration checks.
Third, since the microscopic beads have the consistency of a fine powder
which almost floats in the air, the procedure must usually take place
inside a room or other shelter that is free from drafts or air currents
that interfere with the dispensing and use of the beads, and thus,
Fourth, the procedure usually requires dismounting the probe from its
normal outdoor sampling location (ship, tower, aircraft, roof, etc.),
carrying it inside along with its power and data cables or some auxiliary
set of cables, and generally proceeding through an annoying, if not
difficult, exercise.
For the beam attenuation or shadow imaging type of particle size
spectrometers a calibration technique involving the use of opaque wires
has recently been disclosed in U.S. Pat. No. 4,135,821 entitled
"CALIBRATION OF OPTICAL PARTICLE SIZE ANALYZER" issued in the names of
applicants William H. Pechin, Louis H. Thacker, and Lloyd J. Turner. It
must be pointed out here that the translucent fibers of the present
invention are suitable for use with these types of analyzers as well as
with the light-scattering types, but the opaque wires are not. That is,
translucent fibers of an adequate diameter will cast a measuring shadow as
will opaque wires, but the latter will not scatter light in a way that is
uniquely related to the wire diameter. Light-scattering spectrometers are
designed to measure the intensity of light deflected out of the beam,
usually in a near forward direction, by passing particles. For the
microscopic particles of interest, this deflection is due to both
scattering and refraction by the translucent particles. The resultant
deflected intensity is a unique function of the particle diameter, index
of refraction, wavelength of the light in the illuminating beam, and the
angle of deflection. Opaque objects can only reflect and/or absorb the
illuminating beam, and the reflection mechanism does not deflect light in
a way that is unambiguously related to the diameter of the object.
For these reasons it is to be further emphasized that the invention of an
opaque wire technique for performing primary calibrations on beam
attenuating type analyzers does not imply the use of translucent fibers as
an obvious technique for calibrating light-scattering spectrometers. The
latter are based on the teachings of light-scattering theory as applied to
spherical particles. The theory is very complex, mathematically, even when
applied to this simplest of symmetries--the sphere. Non-spherical
particles not only enormously complicate the mathematics of the theory but
are also unsuitable for use with existing particle size spectrometers.
This is because the intensity of light scattered into the detector by
non-spherical particles depends on the shape of the particle and on the
angle the particle symmetry axes make with the illuminating beam as the
particle passes through. Since this angle is generally random there is no
longer a unique relationship between the detected light signal and the
size of the particle as there is for spherical particles. Fortunately,
many of the naturally occurring particles of interest are spherical in
shape. Thus, those skilled in the art of light-scattering particle size
spectrometry have logically chosen spherical test particles for use in the
calibration and testing of these instruments. Calibration tests have
therefore been thought of only in terms of primary calibrations since the
test particles were of the same size and shape as the particles to be
measured in applications. Until the disclosure of this present invention,
it was apparently not realized that aligned, translucent, cylindrical
fibers could be used in place of microscopic spheres, not as primary
calibration particles but as secondary, or transfer standards for checking
or adjusting the calibration once a primary calibration with translucent,
microscopic spheres had been performed.
b. Determination of the Optical Depth of Field
In addition to accurate particle size determination, practical use of the
data obtained with these instruments requires a knowledge of the particle
number density--the number of particles per unit volume of the sampled
air. This information is easily available if the air samples are aspirated
through the probe at a known flow rate and if the sensitive volume of the
illuminating beam is known. The sensitive volume is simply the product of
the effective beam width and the depth of field (d.o.f.) determined by the
imaging optics and the detection system in the instrument. The d.o.f. is
preset by the manufacturer but since it may be defined and controlled by
optical and electronic components it is subject to change if these
components change or drift.
The prior art for measuring or checking the position and length of the
d.o.f. involves the insertion of a suitable, usually static, test object
in the particle illuminating beam while an electrical or optical response
somewhere in the instrument is monitored. The disadvantages of the prior
art are:
First, the particle analyzer must usually be opened up to measure voltages
on electronic circuits that are normally inaccessible, and therefore,
Second, the probe must usually be dismounted from its normal, outdoor
sampling location as in the case of prime calibration checks.
Third, the measurements usually require an oscilloscope or other laboratory
instruments which therefore make field checks less practical.
c. Efficiency Checks of Particle Counting Circuitry
It is not unusual that during extended field deployments of optical
particle counters the indicated particle counts may sometimes decrease
significantly without any apparent reason. Sometimes this decrease is a
natural result of unusual atmospheric conditions, for example, but it can
also be due to instrument malfunction.
There is no known prior art that is capable of providing an exactly
countable sequence of individual test particles with diameters less than
about 100 .mu.m for testing the counting efficiency of optical scattering
spectrometers.
BRIEF SUMMARY OF THE INVENTION
The general purpose of this invention is to provide a single, quick and
reliable calibration checking device for use with optical, single
particle, size spectrometers.
Accordingly, it is an object of this invention to provide a repeatable
sequence of scattered light pulses or shadow images for use in adjusting
the spectrometer calibration or for detecting any change or drift in
calibration of said spectrometers.
Another object is to provide a simple, easily used calibration standard for
field adjustment of the spectrometer calibration.
A further object of the invention is to provide a simple, easily used
method for locating and measuring the limits of the optical depth of field
in the sensitive volume of the particle illuminating beam in suitable
spectrometers.
Still another object is to provide a reliable means for testing the
efficiency of these spectrometers in detecting and counting single
particle events.
These and other objects and advantages of the invention will appear more
fully from consideration of the detailed description, which follows, in
conjunction with the accompanying drawing wherein two embodiments of the
invention are illustrated. It is to be expressly understood, however, that
the drawing is for the purposes of illustration and description and is not
to be construed as defining the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a view in perspective of an embodiment of the invention suitable
for use with the ASSP and FSSP models of particle size spectrometers
manufactured by Particle Measuring Systems (PMS), Inc., of Boulder, Colo.
FIG. 2 is a view in perspective of an embodiment of the invention suitable
for use with the model 241 optical particle counters manufactured by Royco
Instruments Co. of Menlo Park, Calif.
FIG. 3 is a diagrammatic illustration of the optical system of a PMS model
ASSP probe with an embodiment of the invention shown in situ in the
sampling volume of the illuminating beam.
FIG. 4 is a diagrammatic illustration of the optical system of a Royco
model 241 particle counter with an embodiment of the invention shown in
situ in the sampling volume of the illuminating beam.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 and 2 show two embodiments of the invention which, basically, has
two parts. First there is a set of one or more uniform, translucent fibers
10 which serve as substitute test particles. Second, there is a frame 12,
14 for holding the fibers in place across an unobscured opening. Frame 12,
14 need not entirely enclose the unobstructed opening but may instead be
bounded on three sides (U-shaped), or two sides (V-shaped), or gapped
(C-shaped), etc.
The fibers 10 may be nylon, glass, or any other natural or synthetic
material, and may be clear, colorless, or colored. A primary requirement
is that the optical scattering properties of the fiber must be uniform
along the length that is to be exposed to the illuminating beam. The
fibers should also be sufficiently thin or otherwise chosen so that the
optical signal resulting from their introduction into the light beam is
within the intensity range of the detection system for the instrument in
use. This intensity limitation may be overcome, however, by attenuating
excessively intense optical signals with neutral density filters placed
appropriately in the light beam.
Suitable fibers are conveniently obtained from small diameter nylon rope or
dental floss, for example. A single fiber from one of these sources may be
as narrow as 20 .mu.m in diameter. One or more of these fibers, with the
same or different diameters as desired, may then be positioned across the
opening in the holder 12, 14 and secured in place with a dot 16 of model
cement or other suitable means of securing.
The dimensions and shape of the holder are determined mainly by the
dimensions and shape of the sample inlet orifice of the particle sizing
probe in use. Typical outside dimensions are about one by four inches for
the embodiment of FIG. 1, and about one eighth by six inches for the
embodiment of FIG. 2.
In FIG. 3 the embodiment of FIG. 1 is shown in situ in the sampling chamber
18 of the PMS model ASSP spectrometer probe 22 shown partially cutaway.
The embodiment is shown positioned such that one of the fibers is within
the sensitive length of the illuminating laser beam 20. This fiber
scatters light in all directions and the detector assembly collects the
portion of scattered light that is contained within the solid cone 24
subtended at the intersection of the fiber and the laser beam. The probe
manufacturer uses a beam stop 26 to prevent the direct laser beam from
entering the detector.
Proper usage of the invention simply requires that the holder 12 be moved
within the sampling chamber 18 so that the fibers 10 cross the beam 20
within the sensitive length of said beam. This is easily accomplished by
gripping one end of the holder with the thumb and forefinger, or with a
suitably long clamp, tong, or plier, and pushing or pulling the holder
parallel to the length of the sampling chamber so that the fiber(s)
pass(es) one or more times, as desired, through the beam.
The holder 12 may be of any convenient length, but one that is at least two
or three inches long has the advantage of allowing a long enough opening
that the fibers may be attached to the holder an inch or more from either
end of said opening. This configuration makes it easier for the user to
move the fibers through the beam and stop the motion of the holder before
the edge of said holder reaches the beam. This avoids possible
complications from spurious reflections off the end of the holder.
As has been previously indicated, some fibers may require the use of an
optical neutral density filter to reduce the intensity of light scattered
by the fibers into the detector. Such a filter 28 is shown located in one
possible position just ahead of the detector assembly in FIG. 3. This
location is convenient since said filter may easily be placed on the front
end of the detector assembly by unscrewing cap 30 on the end of probe
housing 22 and sliding the detector assembly out of the probe.
The minimum speed with which the fibers must be passed through the laser
beam can be determined by trial, but generally said speed is comfortably
within the capability of manual movements. About ten centimeters per
second is typical.
Finally, the ASSP models generally contain an electronic "velocity reject"
feature which must be disabled before the test fibers 10 will be reliably
counted. This "velocity reject" feature is designed by the manufacturer to
discriminate against particles which just graze the edge of the laser beam
during their passage through the sampling chamber 18. In effect, signal
voltage pulses from the detector that are narrower than a running average
are rejected by the counting circuitry, since said narrower pulses are
normally those arising from beam grazing particles. Unfortunately, this
"velocity reject" feature will also discriminate against signals from
those test fibers of the invention which pass through the beam at a rate
faster than the average. Since the speed of travel of the fibers is
difficult to control, as the invention is normally manipulated manually,
it is better to disable the "velocity reject" circuitry so that all fibers
passing through the sensitive length of the beam will be counted. Such
temporary disablement capability is easily incorporated into the probe
electronics by the simple addition of a switch in an appropriate part of
the circuitry as reference to the probe owner's manual will show.
In FIG. 4 the embodiment of FIG. 2 is shown in situ in the sampling chamber
40 of the Royco model 241 particle counter partially shown in a schematic
view. In the actual instrument the assemblies shown are all enclosed
inside a light tight container (not shown). The embodiment is shown
positioned such that one of the fibers 10 is within the sensitive volume
of the illuminating beam 42. This fiber scatters light in all directions
and the detector lens assembly collects the portion of scattered light
that is contained within the solid cone 44 subtended at the intersection
of the fiber and the illuminating beam. The probe manufacturer uses a
light trap to prevent the direct illuminating beam from entering the
detector.
Proper usage of the beam simply requires that the holder 14 be moved
lengthwise in the sampling chamber 40 so that the fibers 10 cross said
illuminating beam one or more times, as desired. This is easily
accomplished by grasping the end of the holder 14 that protrudes from the
sampling chamber 40 and moving the holder with a push-pull motion.
Some fibers may require the use of an optical neutral density filter to
reduce the intensity of light scattered by the fibers into the detector.
Such a filter 46 is shown conveniently located in a gap just outside the
lamp housing and lens assembly. This location is easily accessible by
removing the cover to the light-tight container which houses these
assemblies.
The minimum speed with which the fibers must be passed through the
illuminating beam can be determined by trial, but said speed is
comfortably within the capability of manual movements. About ten
centimeters per second is typical.
It should be understood, of course, that the foregoing disclosure relates
to only two possible embodients of the invention and that numerous
modifications or alterations may be made therein, as suits the
application, without departing from the spirit and scope of the invention
as set forth in the appended claims. It should also be understood that the
invention relates in general to all single particle optical scattering
spectrometer type instruments and not to just the two models for which the
present embodiments were designed. In addition, the invention relates as
well to single particle size spectrometers based on the principle of beam
attenuation or shadow imaging techniques in the same way as the opaque
wire method of calibration.
The invention may be used in the following ways:
a. Instrument Calibration
The present invention will not normally be used for primary calibrations
since the shape and size of the test fibers are completely different from
the particles (cloud droplets, smoke, powders, or other aerosols, for
example, that are normally measured by these instruments. However, a given
test fiber will produce an optical signal of fixed, repeatable amplitude
each time the fiber is passed through the sensitive volume of the particle
illuminating beam. The fiber will thus simulate a particle event which
will be registered in the data readout system of the instrument as a
"count" corresponding to some, perhaps originally undetermined, particle
size. If the instrument is in proper calibration when the response to one
or more test fibers is first documented, then the test fiber(s) will serve
as an accurate transfer standard for future calibration checks and
adjustments.
In succeeding calibration checks the operator simply passes the fiber(s)
across the sensitive volume of the particle illuminating beam and observes
in the readout the "counts" indicated for the various particle size
categories of the instrument. If the counts are distributed among the size
categories in a way that is identical to the distribution that was
documented when the instrument was known to be properly calibrated by
other means, then it will have been demonstrated that the instrument is
still in calibration.
If an optical filter is needed to reduce the scattered light intensity,
said filter is chosen with an appropriate attenuation factor such that the
instrument registers each test signal as a count in one of the available
size categories. In such cases, the same optical filter must be used
whenever the present invention is used in order to ensure that the net
effect will be the same.
b. Measuring the Depth of Field
The present invention may be easily used to locate and measure the optical
depth of field (d.o.f.) in the particle illuminating beam of suitable
instruments. Basically, the operator simply passes the fibers across the
beam at various positions along the length of the beam in the sampling
volume of the instrument while observing the instrument readout for an
indication of "particle" counts. Within the d.o.f., each test fiber will
be counted as a valid particle event as the fiber passes across the beam.
When the fibers cross the beam at location outside the d.o.f., no particle
counts will be registered. Depending on the accuracy desired, the operator
may use a jig or positioning device to obtain more precision in locating
the points of passage of the test fibers across the beam.
c. Performance Checks of the Particle Detection and "Single Particle"
Counting Capabilities of the Instrument
The use of the invention for this application is straight forward. The
operator simply passes the fibers across the illuminating beam within the
d.o.f. and observes the instrument readout for an indication of one
"particle" count for each fiber crossing the beam.
From the foregoing description, one skilled in the art can easily ascertain
the essential characteristics of this invention and, without departing
from the spirit and scope thereof, can make various changes and
modifications of the invention to adapt it to various usages and
conditions.
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