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Description  |
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The invention relates to the in situ electro-optical measurement of
particles suspended in a fluid medium. More particularly the invention
relates to a method and apparatus in which electromagnetic radiation such
as light is directed into a fluid medium containing suspended particles
and the intensity of backscattered light is utilized to indicate the size
and distribution of the suspended particles.
In in situ particle measurements, a beam of electromagnetic radiation such
as light is directed into the particle carrying fluid medium and the size
of the particle is inferred by interpreting the backscattered light.
Accuracy in interpreting the size of the particles is dependent in part
upon the intensity of the illumination incident on the particles being
constant. In devices in which flow of the particles contained in the fluid
medium is controlled, uniform illumination of particles can be assured as
by passing a fluid medium laden with particles through a tube for example,
into a preferred scattering volume or region of substantially uniform
illumination. In the in situ device, however, no control is exerted over
the flow of particles, and particles randomly occupy the volume defined by
illuminated portions of the fluid medium of interest. If the illumination
is not constant across the portion of the fluid medium, containing
particles under surveillance, different amounts of light will be scattered
from identically sized particles and specious data may result. For
example, where the source of illumination is a beam of light such as that
produced by a laser, the intensity of the illumination decreases at
locations radially spaced from the axis of the beam. For this reason, a 10
micron particle, for example, will scatter a different amount of light if
positioned at radially spaced locations within the beam. A straight
analysis of the amount of light scattered would result in an
interpretation that identically sized particles were of different sizes if
disposed at locations where the incident illumination is of different
intensities.
Certain attempts have been made in the past to provide systems and methods
for taking in situ measurements of the size and distribution of particles
suspended in a fluid medium. One such system is described in a publication
entitled, "Open Cavity Laser `Active` Scattering Particle Spectrometry
From 0.05 to 5 Microns", coauthored by Robert G. Knollenberg and Robert
Luehr of Particle Measuring Systems, Inc., of Boulder, Colorado. This
system utilizes a continuous laser for tracking particles through a
pedetermined two-dimensional region and the size and distribution
measurements are made during residence of the particles of interest within
that section of the predetermined region having substantially constant
illumination. Utilization of a continuous illumination source suffers
certain disadvantages, however, and in certain applications it is
preferable to use an intermittently interrupted illumination source. For
example, the data rate with a continuous illumination source is random in
that particles randomly enter and exit the sampled region which makes
processing the data more difficult than with a light source which is
interrupted at a known rate and produces data at a known rate. Moreover,
the power consumption is less with an intermittently interrupted source
for a preselected illumination intensity which enables packaging the
interrupted source system in a smaller volume for a given intensity
requirement.
It is therefore an object of this invention to provide an improved in situ
particle size and distribution measurement system. It is another object of
the invention to provide an improved method for making in situ particle
size and distribution measurements. It is a further object of this
invention to provide such a system and method utilizing an intermittently
interrupted illumination source which illuminates a portion of the fluid
medium in which the particles of interest are suspended. Another object of
the invention is to provide an optically defined sampling volume in which
the illumination is constant. A further object of the invention is to
provide means to assure that the particles under surveillance are within a
predetermined region or sampling volume having substantially uniform
intensity at the time measurements regarding the size and distribution of
the particles are made.
Other objects and advantages will become apparent upon reading the detailed
specification together with the drawings wherein:
FIG. 1 is a schematic representation of a particle measurement system
embodying various features of the invention;
FIG. 2 is a schematic representation of a cross section of the sampling
volumes, the smaller sampling volume being the location of the particles
under surveillance;
FIG. 3 is a cross sectional view of one embodiment of a sensing head
carrying various of the elements of the system and adapted for being
positioned proximate a fluid medium having particles of interest suspended
therein; and
FIG. 4 is a flow diagram illustrating various of the operational steps of
the system.
The present disclosure is directed to an electro-optical system and method
for in situ determinations of the sizes and distribution of particulate
matter suspended in a fluid medium. A beam of intermittently interrupted
electromagnetic radiation is directed through a portion of the medium and
radiation backscattered from first and further optically defined regions,
the first region being contained in the further region, is detected and
utilized to provide an indication of the size and distribution of
particles suspended in the fluid medium. To assist in assuring residence
of particles under surveillance within the first volume which is
transilluminated by substantially uniform intensity, the radiation
backscattered from the first and further regions or volumes respectively
are compared prior to making measurements concerning the size and
distribution of particles.
Referring now to the Figures, the system illustrated schematically at 10 in
FIG. 1 finds particular application in measuring in situ the size and
distribution of particles suspended in a fluid medium, such as process
gases. The system 10 is adapted for being positioned proximate a fluid
medium containing particles of interest and in this connection may be
mounted in a portable housing.
The system 10 includes a source of substantially monochromatic
electromagnetic radiation indicated generally at 12 which serves to
generate a beam of light for illuminating a portion of the fluid medium
containing the suspended particles. The source 12 is intermittently
interrupted for illuminating particles passing through the beam at
predetermined intervals. Utilization of an intermittently interrupted or
pulsating source 12 serves to reduce the power consumption and operational
cost of the system. One particularly suitable radiation source is a
pulsating laser of the junction diode type adapted for emitting infrared
radiation such as a GaAs PN-junction laser which emits 200 nanosecond
pulses. Preferably, the rate of pulsing is chosen in conjunction with the
velocity of particle and the cross sectional dimension of the region in
which the particles under surveillance reside when the measurements are
made to the end that there is a single exposure of a given particle during
the period of its transience across the scattering volumes. Accordingly,
no particle is exposed to the intermittently interrupted light beam more
than once during its passage through this region or sampling volume in one
embodiment of the invention. Consequently, only one measurement is taken
for each particle under survelliance.
It will be recognized, however, that the pulsing rate of the source 12 may
be chosen such that a single particle is illuminated a plurality of times
during its residence within the sampling volume and the size and
distribution of the particles determined by a different interpretation of
the resulting data. For example, if a single particle is illuminated a
plurality of times such as by multiple firing of the laser during the
particles transience across the sampling volume, a gaussian shaped
intensity curve would result. In this connection the system is inherently
isokinetic in that there is no dependence on the flow velocity of the
fluid medium carrying the particles of interest.
Light generated by the source 12 is directed through a portion of the fluid
medium containing the particulate material for transillumination of the
sampling volume. In this connection, light emerging from the source 12 in
FIG. 1 passes through beam forming optics including substantially parallel
disposed collimating lenses generally indicated at 14 which serve to form
a substantially collimated beam 16 which exits the system 10 through a
suitable aperture (not shown in FIG. 1) and is directed through a portion
of the medium through which the particulate matter under surveillance
passes. As necessary or desired, the collimating lenses may be adjusted to
vary the width of the beam 16.
The beam 16 produced by the source 12, such as a laser, is preferably
circular in cross section and the intensity variations of the beam are
kept within acceptable limits. As is common knowledge, however, the
intensity across the cross section of the beam varies radially with
respect to the axis of the beam, i.e., the intensity near the outer margin
of the beam is less than the intensity near the axis of the beam. In order
to assist in assuring substantially identical illumination of the
particles under surveillance, radiation is accepted for measurement
purposes only if the particles reside in a predetermined sampling volume
18 which is optically defined and substantially uniformly illuminated.
More specifically, only particles residing in the optically defined three
dimensional region or sampling volume 18 backscatter light which is
accepted for determining the size and distribution of the particles within
the fluid medium. To this end, the sampling volume 18 is defined by first
optical means which serve to collect light backscattered along
predetermined paths from the sampling volume 18 and direct such light
toward a photodetector to be described in greater detail hereinafter. As
shown in the schematic diagram of FIG. 1, the illustrated optical means
include lenses 22 and 24 which serve both to collimate the beam 16 and to
focus light passing through their respective annular margins onto an image
plane generally indicated at 32 for reasons which will be pointed out in
more detail hereinafter. A portion of the light backscattered by particles
in the fluid medium passes through the annular margins of lenses 22 and 24
and through the annular aperture 25 defined by the aperture means 28. In
the illustrated embodiment the aperture 25 serves to collect
electromagnetic radiation backscattered substantially along angles A and
A' and within the ranges of .DELTA.A and .DELTA.A' and is coaxially
aligned with the beam 16 for collecting backscattered radiation
symmetrically around the beam. In the embodiment illustrated in FIG. 1
angle A is equal to angle A', and .DELTA.A is equal to .DELTA.A'. It has
been found that by collecting the radiation backscattered by about
170.degree. as measured clockwise and counterclockwise, respectively, from
the axis of the beam exiting the sample volume, and within a range of
approximately 5.degree., accurate measurements can be made concerning the
size and distribution of particles suspended in the fluid medium.
It will be recognized that the angle between the incident illumination,
i.e., the laser beam 16, and the collected radiation which is scattered
from the particles may vary from approximately 0.degree. to approximately
180.degree.. For example, the collecting optics could be positioned on the
opposite side of the sampling volume 18 from the source 12 as necessary or
desired. The particular arrangement shown in FIG. 1, however, utilize
lenses 22 and 24 both in forming the beam 16 and in collecting the
scattered radiation.
To assist in assuring that particles under surveillance are illuminated by
a substantially identical amount of incident illumination, i.e., to assure
location of particles within the first region or sampling volume 18 at the
time measurements concerning the size and distribution of the particles
are made, light backscattered from particles within the sampling volume 18
and collected by the optical means is separated into predetermined amounts
of electromagnetic radiation by a beamsplitter 30. More specifically, the
illustrated beamsplitter 30 is substantially coaxially aligned with the
beam 16 and is interposed in the path of the backscattered radiation
focused on the image plane 32 of the sampling volume 18 by lens pair 20.
The illustrated beamsplitter 30 is disposed substantially parallel with
respect to lenses 22 and 24 and fabricated from a partially reflective and
partially transmittal material which serves to pass and to reflect
predetermined portions of the incident backscattered radiation. The angle
of incidence of the backscattered radiation on the beamsplitter 30 is
substantially identical to the angle of emergence of the portion of
radiation transversing the beamsplitter toward the image plane 32.
Moreover, the angle of incidence of the reflected portion of the radiation
is substantially identical to the angle of reflection of this radiation.
It will be recognized that the image planes of the sampling volume 18 as
defined by the portions of the backscattered radiation transversing and
reflected from the beamsplitter 30 lie along the imaginary planes 32 and
34, respectively, which are aligned substantially parallel with respect to
beamsplitter 30 and disposed equal distances from the beamsplitter.
A portion of the light passing through beamsplitter 30 is incident on
photodetector means 36 disposed behind the beamsplitter 30 with respect to
the sampling volume 18. The photodetector means 36 is of known design, one
suitable photodetector means including a silicone photodiode-preamplifier
combination, for example, the MDA321 photodector manufactured by Meret,
Inc. This photodetector means 36 includes a first photosensitive medium 38
which serves to produce electronic signals systemmatically related to the
intensity of the electromagnetic radiation backscattered from the sampling
volume 18 and falling on the photosensitive medium. The electronic signals
generated by the photosensitive medium are fed to a conventional amplifier
40 which serves to amplify the electronic signals and produce such
amplified signals at its output 41.
The portion of the light accepted by the photosensitive means is determined
by aperture means 42 interposed in the path of the backscattered radiation
directed from the beamsplitter 30 to the photodetector means 36. This
aperture means 42 serves to determine the amount of light accepted from
the sampling volume 18 and is disposed within the image plane of such
sampling volume as determined by lenses 22 and 24. More specifically, the
sampling volume 18 comprises the projected image of the aperture 44 with
the widest portion 45 (See FIG. 2) of the sampling volume 18 being
identical to the width of the aperture 44 if the magnification of the
lenses 22 and 24 is equal to one.
Similarly, a portion of the backscattered radiation reflected by
beamsplitter 30 is incident upon further photodetector means 46 of
conventional design which is disposed in the illustrated embodiment on the
opposite side of beamsplitter 30 from photodetector 36. The photodetector
46 includes a photosensitive medium 48 which serves to produce electronic
signals systemmatically related to the intensity of the incident
electromagnetic radiation. The electronic signals generated by the
photodetector means 46 are fed to a conventional amplifier 50 which serves
to amplify the electronic signals and produce such amplified signals at
its output 53.
The portion of the electromagnetic radiation incident on the photodetector
means 46 is determined by the aperture means 52 interposed between the
photodetector 46 and the beamsplitter 30 along the path of light reflected
from the beamsplitter 30 toward the photodetector 46. Aperture 56 defined
by aperture means 52 serves to determine the shape and size of the
sampling volume 54 within the fluid medium from which backscattered
radiation is accepted by the photodetector 46 and is disposed within the
image plane of such sampling volume 54.
As illustrated in FIG. 1, the image planes of sampling volumes 18 and 54
are spaced equal distances of the beamsplitter 30. Preferably, the
apertures 44 and 56 are circular in outline and define the conical shaped
sampling volumes, as shown in FIG. 2 which have diamond shaped cross
sections. As necessary or desired, however, different shaped apertures and
sampling volumes may be used.
The outputs of the photodectors 36 and 46 are fed through leads 60 and 62
to indicator means 58 adapted to convert the signals to a usable form for
indicating the size and distribution of particles suspended in the fluid
medium. The illustrated indicator means may comprise any one of several
possible devices, for example, the output signals may be fed to a
conventional oscilloscope, a tape recorder, a digital counter, a pulse
height analyzer or a mini-computer with or without a readout. In the
illustrated embodiment the indicator means 58 comprises a microprocessor
based pulse height analyzer 59 of conventional design. The pulse height
analyzer is programmed to produce an output at 61 which may be adapted to
drive a printer, for example, for producing a usable form of information
as will be pointed out in greater detail hereinafter.
The signals representative of the intensity of the radiation backscattered
from the first and further regions 18 and 54 shown in FIG. 2 are compared
to determine the location of the particles prior to making measurements
indicative of the size and distribution of the particles suspended in the
fluid medium. Assuming that the transfer functions including gain for the
apparatuses associated with each channel are identical only when the
magnitude of the signals generated by the first and further photodetector
means 36 and 46, respectively, are equal will the measurement be made.
(Compensations are made in the physical apparatus to accommodate
variations in the transfer functions for the apparatus associated with
each sampling volume, as will be pointed out in greater detail
hereinafter.) Assuming identical transfer functions, it will be recognized
that where the magnitude of the signals representative of the intensity of
the radiation backscattered from the first and further regions 18 and 54,
respectively, are equal, no particles lie within the region 62 which is
within the further region 54 and outside the first region 18. Particles
residing within the first region 18 are illuminated with electromagnetic
radiation of substantially identical intensity and therefore there is a
systemmatic correlation between the size of the particles and the
backscattered radiation. Moreover, the volume of the fluid medium
contained within the region 18 is predetermined inasmuch as it is the
projected volume defined by the aperture 44 interposed between the
beamsplitter 30 and the photodetector 36. In this connection the
volumetric concentration of the particles is determined by comparing the
volume of the fluid sampled with the number for particles detected. More
specifically, the concentration of particles suspended in the fluid medium
is determined by dividing the number of particles by the volume of the
fluid sampled, which is equal to the volume of the region 18 multiplied by
the number of times the laser 12 fires. Similarly, the mass concentration
of the particles suspended in the fluid medium is determined by dividing
the mass of the particles by the volume of the fluid sampled, i.e., the
volume of region 18 multiplied by the number of times the laser 12 has
fired. The mass of the particles can readily be determined by multiplying
the density of the particles by the particle size, which is proportional
to the intensity of the backscattered radiation. Such calculations are
made by the indicator means 58, suitable indicator means as pointed out
before comprising in one embodiment of the invention a
microprocessor-based pulse height analyzer 59. This information may be
printed on hard copy tape or the like generated by the printer 64 which is
drivingly connected to the indicator means 58.
A flow diagram illustrating the operational steps of the
microprocessor-based pulse height analyzer during assimulation and
processing of the information signals produced at the outputs of
amplifiers 40 and 50 is shown in FIG. 4. More specifically, the first
block 150 serves as an interrupt to begin the operation sequence upon
receipt of an input or command signal from leads 60 and 62 as when the
laser 12 fires. The operational mode of the pulse height analyzer 59 is
entered upon receipt of such a command signal and further command signals
are blocked until the operational mode is completed.
Upon entering the operational mode, the analogue output or voltage pulse
with a height V'.sub.A of the amplifier 40 which is representative of the
intensity of the radiation backscattered from region 18 is converted into
a digital signal by a conventional analogue to digital converter (not
shown) housed in the pulse height analyzer 59 in the illustrated
embodiment. The digital information is then fed into a microprocessor as
indicated at the output of block 152. The microprocessor then performs
background subtraction as indicated at 154 and discussed in greater detail
hereinafter to produce a net pulse height V.sub.A. The height or magnitude
(V.sub.A) of the digitized signal representative of the intensity of the
radiation backscattered from the sampling volume or region 18 is then
stored in the microprocessor memory as indicated at 156.
Similarly, the analogue output or voltage pulse with a height V'.sub.B of
the amplifier 50 representative of the intensity of the radiation
backscattered from the further region 54 is converted into a digital
signal by a conventional analogue to digital converter (not shown) housed
in the pulse height analyzer 59 as indicated at block 158 of the flow
diagram shown in FIG. 5. This digital signal is then fed into the
microprocessor (not shown) of the microprocessor-based pulse height
analyzer as indicated at the output of block 158. The microprocessor then
performs background subtraction as indicated at 160 and discussed in
greater detail hereinafter and the net pulse height signal V.sub.B is
produced. The magnitude or height of the digitized signal (V.sub.B)
representative of the intensity of the radiation backscattered from the
sampling volume or region 54 is then stored in the memory as indicated at
162.
The two pulses are then compared by the microprocessor in order to
determine the location of the particles which backscatter radiation from
the scattering volumes to the end that measurements are made only if the
particle or particles under surveillance are with region 18 which is
substantially uniformly illuminated. In this connection, measurements are
made only if the radiation backscattered from the first and further
sampling volumes are substantially identical. The apertures which
determine the sampling volume geometry and the amplifiers are adjusted to
the end that when the net signal V.sub.A is greater than the net signal
V.sub.B, the yield from the respective sampling volume is substantially
identical. More specifically, if V.sub.A is less than V.sub.B, i.e., if
the magnitude of the intensity of the radiation backscattered from the
inner sampling volume 18 is not greater than the magnitude of the
intensity of the radiation backscattered from the outer sampling volume
54, then the particles under surveillance are not confined within the
inner sampling volume 18 having a substantially uniform illumination at
the time the laser 12 is fired and no measurement of the particle size is
made and a signal will be produced at branch 165 connected to block 164
which qualifies channel "0" only, channel "0" being the channel which
increments upon each firing of the laser 12 regardless of the presence of
a particle within the sampling volume 18. To this end, channel "0" is
incremented by the operational steps generally indicated at 167 and
described in greater detail hereinafter. In this event, only channel "O"
will qualify and increment.
If a particle under surveillance resides within the inner sampling volume
18, the particle is classified according to size into one of a number of
predetermined channels from 1 to N+1 with each of the channels being
representative of particle sizes within a predetermined range. To this
end, the string of decision blocks generally indicated at 168 serve to
classify the particles residing within the substantially uniformly
illuminated region 18 by sequentially determining if the voltage V.sub.B
falls within a given range. If one of the inequalities indicated in the
decisional string 168 is satisfied, the corresponding information is
stored in an appropriate channel counter for display to indicate that a
particle having a size within a predetermined range is detected. For
example, if the particle size as represented by the digitized voltage
signal V.sub.B is less than a predetermined value V.sub.1, as illustrated
by decisional block 170, then channels "0" and "1" are qualified as
indicated at 172 by a command produced at branch 174 connected to block
172 which orders the microprocessor to increment the qualified channels as
indicated at 176. More specifically, upon determining that V.sub.B is less
than V.sub.1, channel "0" is qualified (Block 172) and incremented (Block
176) to increase a conventional counter providing a cumulative count of
the number of times the laser 12 has fired. Moreover, channel 1 is
qualified and incremented by one to indicate that a particle is detected
having a size smaller than the size represented by the voltage V.sub.1.
Similarly, if the size of the particle within the region 54 is greater than
or equal to the particle size represented by V.sub.1, a "no" command is
produced at branch 178 connected to block 170 and a "yes" command is
produced at branch 180 connected to block 170, to the end that the
particle size will be classified within predetermined ranges greater than
particle size represented by V.sub.1.
If branch 180 connected to block 170 indicates the particle to be
classified according to size is equal to or larger than the size
representated by V.sub.1, and not less than the particle size represented
by V.sub.2 then channels 0 and 2 will be qualified and incremented. If the
particle to be sized is larger than or equal to the size represented by
V.sub.2 then neither channels 0, 1 nor 2 will be qualified and the signal
information (V.sub.B) will pass through to the further decisional blocks
182 and 184, for example, for classifying the particle according to size
upon satisfaction of the illustrated inequalities. If the particle to be
sized is equal to or larger than the upper limit of the size range
required for qualifying channel N, channel N+1 and channel 0 are qualified
and incremented. Channel N+1 provides an overflow which is incremented
upon the detection of a particle larger than the size represented by
V.sub.N.
Upon detection of a particle with the range of one of the channels 1 - N+1
illustrated in FIG. 4 that channel is qualified and a counter (not shown)
operatively associated with the channel together with a counter
operatively associated with channel "0" are incremented as illustrated at
block 176.
In order to selectively display the number of particles detected with a
given size range, a suitable multipositional switch 187 is provided on the
pulse height analyzer 59 to select a predetermined channel for display. To
this end, the microprocessor reads the position of the switch 187 (see
block 186) and displays the selected channel indicated at block 188 on a
suitable digital display (not shown).
After a particular particle has been sized or a determination is made that
a particle is not within the region 54, the sizing and incrementing
processes are halted and the interrupt is again enabled for processing
further information taken at the output of amplifer 40 and 50 upon the
laser 12 firing again.
Where a conventional pulse height analyzer 59 is used to drive a printer,
the microprocessor may be programmed to sort the particle sizes into
different ranges. One typical printout is shown below.
______________________________________
EXAMPLE PRINTOUT
______________________________________
1. 1.0-2.0 (.mu.m) 012345
2. 2.0-3.0 (.mu.m) 008511
.
7. 8.0-10.0 (.mu.m) 000358
8. (Laser Pulses) 999999
______________________________________
The left-hand column following the numbers 1, 2 . . . 7, indicates the
diameter range of the particles and the corresponding right-hand column
provides an indication of the number of particles within the corresponding
range. The number of laser pulses is indicated as the last entry in
right-hand column in the sample print-out. As indicated hereinbefore, the
volumetric concentration of the particles can readily be determined by
counting the number of particles and dividing the particle count by the
volume of the fluid sampled which is equivalent to the volume of region 18
multiplied by the number of times the laser 12 has fired.
In a further feature of the invention, the microprocessor-based pulse
height analyzer 59 is used to drive a conventional line printer 57
producing a graph 63 which indicates the size of the particles within the
sampling volume 18 at the time the source or laser 12 is fired. Each of
the peaks 65, 67 and 69 represent the intensity of the backscattered
radiation which is systemmatically related to the size of the particles in
the sampling volume 18 which is transilluminated by substantially
identical electromagnetic radiation. Preferably, the abscissa of the graph
is marked in units representing the diameter of the particles located.
In order to assist in preventing background radiation from producing
specious data at the time the measurements are made, the microprocessor
based pulse-height analyzer 59 serves to subtract the background
radiation. This is accomplished in the preferred embodiment by subtracting
the average of the background pulses, occurring when the source 12 is
fired and there are no particles in the sampling volume. More
specifically, the size of the particle indicated by peak 65 on the graph
would be reduced by the average size of the pulses 71 and 73, which
represent the background radiation occurring before and after the
measurement of a particle in the sampling volume 68.
In one embodiment various of the components of the invention are mounted in
a suitable sensing head indicated generally at 66 in FIG. 3. The
illustrated sensing head 66 is positionable proximate the fluid medium
containing the particles to be investigated and includes an elongated
frame 68 fabricated from a suitable rigid material. The illustrated frame
68 is substantially cylindrical and defines a cavity 70 along its length
which opens at its opposite ends 72 and 74, respectively, of the frame. It
will be noted that all of the necessary conducting wires are not shown in
FIG. 3 for the sake of ease of understanding the drawings. Also, it should
be noted that the various means for mounting the various elements of the
sensing head are of small area relative to the total area so as not to
present substantial overall obstructions to the passage of light.
In the embodiment of the sensing head illustrated in FIG. 3, the light
source 12 is mounted in a suitable cylindrical housing 76 which defines a
recess 78 for carrying the portion of the light source 12 such as a diode
from which the light is emitted. This housing 68 is provided with a
suitable cover 80 and defines an opening 82 therethrough which is adapted
for receiving one end portion 84 of the fiber optics cable 86. Light
emitted by the source 12 is fed through the optical fiber cable 86 which
is positioned at its opposite end 88 substantially along the longitudinal
axis of the sensing head 66.
In order to position end 88 of the optical fiber cable 86 along the axis of
the sensing head 66 for directing light emitted from the end 88 of the
cable, this end 88 of the cable fiber is secured in the substantially
cylindrical support 90 defining an opening 92 at its central portion for
receiving the end 88 of the cable and positioning this end of the cable
along the axis of the sensing head. This support 90 is secured to the
inner surface 94 of the wall 68 of the sensing head by an arm (not shown)
proportioned to assist in preventing blocking backscattering radiation
from passing through cavity 70 toward end 74 of the sensing head 66.
Lenses 14 are positioned in the nose section 94 of the sensing head 66 and
adapted to assist in collimating light exiting the end 88 of the optical
fiber cable and directing the beam into the fluid medium containing the
particles under surveillance.
Light backscattered from particles contained in the fluid medium pass
through the annular margins of the lens pair 22 and 24 which serve to
direct the backscattered radiation toward the beamsplitter 30. More
particularly, a portion of the backscattered radiation passing through the
lenses 22 and 24 is directed through the aperture means 28 defining spaced
annular apertures 96 and 98 which circumscribe the axis of the sensing
head 66 in the illustrated embodiment. Light passing through these
apertures 96 and 98 is incident upon a beamsplitter 30 positioned
substantially perpendicular to the axis of the sensing head 66. The
illustrated beamsplitter 30 is carried by a suitable support 100
adjustably positioned along a shaft 102 mounted on the sensing head frame
68. In this connection, the shaft 102 extends through registering openings
104, 106 and 108 defined in the wall 68 of the sensing head frame 66 and
is secured at its opposite ends against supports 110 and 112 by nuts 114
and 116, respectively, received on the opposite ends of the shaft 102.
Further, nuts 118 and 120 serve to position the beamsplitter support 100
at a desired location and assist in moving the beamsplitter to preselected
positions with respect to the axis of the sensing head.
Light passing through the beamsplitter 30 is incident upon a photodetector
means 36 which is carried on a detector support 122 secured at its end 124
to the shaft 102. More specifically, the photodetector means 36 is carried
within the cylindrical wall 126 of the detector support 122 and is
connected through conductors (not shown) to suitable indicator means 58
shown diagrammatically in FIG. 1.
The diameter and length of the portion of the fluid medium from which
backscattered radiation is accepted by the photodetector means 36 is
determined by suitable aperture means 42 positioned substantially
perpendicular to the axis of the sensing head 66 and secured at end 128 of
the cylindrical opening 130 in which the photodetector means 36 is
positioned.
The backscattered electromagnetic radiation reflected by the beamsplitter
30 is directed toward further photodetector means 46 positioned
substantially along the axis of the sensing head 66 and mounted in a
suitable opening 132 provided in support 134. This photodetector means 46
is connected through suitable electrical connectors (not shown) to the
indicator means 58 which serves to compare the signals generated by the
first and further photodetector means 36 and 46, respectively, for
indicating the size of a particle contained in the region 18. More
specifically, to assist in assuring particles under the first region or
sampling volume 18 at the time the measurements are made, the intensity of
the light backscattered from the further region or sampling volume 54
disposed in the fluid medium and containing the first region 18 is
compared with the intensity of the light backscattered from the first
region. Measurements are made if and only if the intensity from the
further region 54 is substantially identical to the intensity from the
first region 18, (where the transfer functions of the measuring apparatus
associated with each channel is identical) thereby assuring the absence of
particles within the region 62 outside the first region. It will be
recognized that where the transfer function of the measuring apparatus or
the amount of light passed and reflected by the beamsplitter 30 is not
identical then corresponding adjustments may be made in the programming of
the microprocessor based analyzer to avoid the accumulation of specious
data.
The size and shape of the sampling volume 54 is defined by aperture means
52 which is positioned substantially perpendicular to the axis of the
sensing head 66 at a spaced location from the surface 47 of the
photosensitive means 46. The dimensions of the aperture 56 define the
length and width of the area in fluid medium from which light
backscattered from particles positioned in such sampling volume is
received by the photosensitive means 46 as pointed out in more detail
hereinabove.
From the foregoing detailed description it will be recognized that an
improved in situ electro-optical particle measuring system and method are
provided which have certain advantages over prior art devices. For
example, the disclosed system includes an intermittently interrupted
source of electromagnetic radiation for providing the incident
illumination of pa | | |
