Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE FOR M~ASURING
THE CONCENTRATION OF AIRBORNE FIBERS
Cross-reîerence To Related App~ication
This application is a continuation-in-part application of U.S. Patent
Application Serial No. 08/743,554, entitled "Device For Measuring The Dimension Of A
Airborne Fiber", filed on November 4, 1996 and U.S. Patent Application Serial No.
08/743,555, filed on November 4, 1996, which applications are assigned to the same slccignce
hereof, and are hereby incorporated by reference.
Field Of The Invention
This invention relates to methods and devices for estim:~ting the concentration
of airborne fibers, and particularly to devices which can decipher between respirable fibers
and non-fibrous respirable fibers.
Back~round Of The Invention
At present, two primary methods for monitoring airborne fiber concentration
exist. In the first method, airborne fibers are collected on a filter. This filter is analyzed by
microscopy or chernic~l methods to deterrnine the type of fibers present and to estimate
airborne fiber concentration. This method suffers from the drawbacks of delayed availability
of information, tediousness, inconvenience, high cost per sample, and lack of precision.
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Also, irl~ntific~tion of fibers typically is performed by visual inspection, adding uncertainty
to measurements for particular species of airborne fibers.
In the second method, real-time airborne fibers concentration is determined .,
using optical techniques, in which light, ~ n~ ed by fibers passing by a light source, is
analyzed. However, most of these devices do not discriminate between different species of
airborne fibers and, in particular, may not provide an accurate measurement of potentially
respirable fibers, particularly small glass fibers.
Because of the significant health problems posed by airborne asbestos fibers,
current real-time airborne fiber monitors typically are aimed at selectively deterrnining
asbestos fiber concentration in an air sample having asbestos and other fibers. Because
asbestos fibers exhibit paramagnetic properties, some existing devices preferentially align
and oscillatc asbestos fibers using, for example, a time-varying electric field quadrupole, a
hybrid electric/magnetic field, or both. The induced oscillations tend to create a
characteristic scattering of an impinging light, thus identifying the oscillating fiber as
asbestos. Electrostatic techniques also may be used. Examples of such devices and methods
for measuring airborne particulate concentration are found in U.S. Patent No.3,692,412 to
Chubb (1972), entitled "Apparatus for Analyzing Suspended Particles"; in U.S. Patent No.
4,940,327, to Lillienfeld (1990), entitled "Method and Apparatus for Real-Time Asbestos
Aqonitoring"; and in U.S . Patent No. 5,319,575, also to Lillienfeld (1994), entitled "System
and Alethodfor Det~rrr~irlir~ and OutputtingAirborne Particle Concentration. " ~lso see
MIE ~iber Monitor Model FM-7400 User's Manual by MIE~, lnc., Billerica, MA.
However, because potentially harmful respirable fibers including, for example, ,
glass fibers, often do not exhibit par~m~n~ti.~m, such methods may not be a~ pliate.
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What is needed, then, is an airborne fiber concentration m~enrin~ device that can acc~rately
rl~t~rmine the conc~ntration of respirable fibers suspended in an air sample, in real time,
without the need for electrostatic, magnetic or hybrid electromagnetic components.
Additionally, the Lillienfeld's device is more complicated, detects only a small
percentage of fibers in a given sarnple, and if the concentration of fibers in the sample is low
or not representative of the fiber concentration in the air flow, measurement errors can result.
There therefore remains a need for a fiber concentration measuring device which takes a
more significant s~mr)ling of the fiber population and which is accurate at low concentration
re~-lingc
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Summarv Of The Invention
This invention provides devices and methods for measuring the concentration
of airborne fibers in a fiber-contSIining air sample. The preferred device includes flow means
for providing laminar flow to at least a portion of the fibers in the air sample. These
laminarly flowing fibers are then illllmin~tecl with a light source to produce sc~U~red light. A
portion of the scattered light is then sensed to produce an output from which a fiber
concentration estimate can be measured. Additionally, separation devices can be used to
preselect fibers having a particular size~ so as to measure only respirable fibers, for example.
This invention provides an inexpensive way of measuring respirable fibers in a work
environment, such as a glass insulation or mat-making facility.
In a more detailed embodiment of this invention, a device is provided for
analyzing air having respirable fibers, and non-respirable fibers or non-fibrous particulate
matter, or both. This device includes separation means for selectively removing respirable
fibers from non-respirable fibers to produce a filtered air sample co~ ,it~g aligned
respirable fibers. These aligned fibers are thcn illlln~in~ d to produce scattered light, which
is collected by a light sensor to produce an electrical output. The device further includes
processing means for providing a concentration estimate for the respirable fibers from the
output of the light sensor.
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Brief Descril)tion Of The D~
The accompanying drawings, referenced to herein and constituting a part
~ hereof, illustrate preferred embodiments of the device of the present invention and, together
with the description, serve to explain the principles of the invention.
Figure 1 is an illustration of an airborne fiber concentration measuring device
in accordance with the present invention.
Figure 2 is an illustration of one presently preferred embodiment of a sensor in
accordance with the present invention.
Figure 3 is an illustration of another presently preferred embodiment of a
sensor in accordance with the present invention.
Detailed D~se. ;~,tion Of The Invention
Figure 1 illustrates one embodiment of the airborne fiber concentration
m~ rinE device 100 according to the principles of the invention herein. Device 100 can
include a sensor 1 for ~letectinE fibers and separation means, for exarnple, virtual impactor 2,
for separating respirable from non-respirable fibers or non-fibrous particulate matter. As used
herein, "respirable fibers" means fibers which are less than about 3 IlM in diarneter, and
preferably those with an aspect ratio of at least about 5:1 (length:diarneter~. Additionally,
the term "light" refers to both visible and invisible electromatic waves, including x-ray and
infrared.
A skilled artisan would recognize that virtual impactor 2 can use well-known
techniques to se~ the respirable particles from non-respirable particles, and therefore, the
skilled artisan could employ other separating means for isolating respirable fibers from non-
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respirable fibers. One exemplary virtual impactor 2 that has been found suitable is shown in
FIG. 1. This device takes in fiber-cont~inin~ ambient air and draws off smaller respirable
fibers 20 laterally at a venturi's mouth. Larger fibers 41, greater than about 3~Lm, are drawn
into the center tube of the virtual impactor 2.
In generah the air entering the device can have respirable fibers~ non-
respirable fibers, and other particulate mattcr mixed therein. Sensor 1 preferalaly senses
aligned respirable fibers in the air but is substantially insensitive the other non-fibrous
particulate matter. In operation, respirable fibers 20 that may be present in the air are drawn
from virtual impactor 2 through hose 3 which connects virtual impactor 2 to sensor 1. Air is
drawn through the system by a small vacuum pump 22 to outlet 4 of lower flow tube 6. The
air flow rate, and lengths and diameter of the upper and lower flow tubes 5,6, are preferred to
be such as to produce a laminar flow of air through tubes 5,6. This laminar airflow tends to
cause the fibers 20 in the air within tubes 5,6 to become substantially aligned with the airflow
and, hence, with the longitudinal axis 30 of flow tubes 5,6. Flow tubes 5,6 preferably are
separated by a small gap 7 within sensor 1. Alternatively, a single tube having a pair of slots
through its side wall perpendicular to its axis could work as well. This gap 7 is pre~erably
positioned syrnrnetrically about axis 8 of sensor 1. Flow tubes 5,6 and gap 7 constitute the
"flow channel" for this embodiment of the invention.
Within sensor 1 is a light source 9 which can be a coherent light source such
as, for exarnple, a diode laser. Light source 9 can produce a bearn 12, preferably with a
preselected cross-section along the beam path. It is preferrcd that light source 9 produce a
collim~ted beam of light, ideally with an elliptical cross-section directed at light sensor 14.
Light sensor 14 is preferred to be a phot-~detector. ~3earn 12 can be aimed along axis 8 of
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sensor 1 with the ma~or axis of the ellipse of light preferably being ~ub~ Lially parallcl to
gap 8 between flow tubes 5,6. The width of beam 12 need not be as wide as the diarneter of
flow tubes 5,6.
A suita~le light source for this embodiment can be, for example, a model LPM
03(670-5) laser diode from Power Technology, Inc., Little Rock, ~rk~n~c Similarly, a
suitable photodetector is, for e~ample, Devar Model 509-l, Bridgeport, ~onnecticut. A
skillcd artisan could employ other suitable light sources and light sensors to provide and
detect light signals indicative of the presence of respirable fiber;
Figure 2 presents a cross-sectional view of a plc;r~ d sensor 1, which is
positioned generally perpendicular to the airflow. After passing through gap 7, bearn 12
enters an optical lens assembly l O. Lens assembly 10 can be a pair of con~ in~: lenses, for
example. This combination of lenses tends to have a short focal length, permitting a portion
23 of beam 12 to be directed to the back surface 24 of the second lens 25. Beam block I 1 can
be used to substantially block the collimated light 23 from being sensed by photodetector 14.
It is preferred that the beam block 11 be umbrageously situated relative to photodetector 14
so that beam block 1 1 can shield photodetector 14 from light not indicative of the presence of
a sensed fiber.
As fibers 20 pass though the beam 12 between the flow tubes 5,6, some of the
fibers 20 will scatter the light, as shown in FIG. 2. When a cylinder, such as a glass fiber, is
illllmin~tf~cl at a normal incidence by light, it typically scatters the light in a preselected
orientation in the flow channel, i.e. in a plane that is normal to the cylinder. Because fibers
20 have been aligned by the laminar airflow, these fibers 20 are generally oriented
perpendicularly to the direction of beam 12. Therefore, beam 12 can be scattered in a plane
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that is generally parallel to planes forrned by the ends of flow tubes 5,6, thus permitting
scattered light 26 to pass through gap 7 between flow tubes 5,6.
For the laminar flow of this invention it is generally recognized that two conditions
must be met. These are that the Reynolds number should be less than about 2000 and there
must be suff1cient distance for the flow to become laminar. In the case of the claimed device,
a flow of about 4 liters/min. and a fiber diameter of .44 in. (1.1 cm) produces a Reynolds
number of about 500, which is well into the laminar flow regime. The length of the flow tube
before the fibers reach the laser bearn is about 5-50 in. (12.7-127 cm), preferably about 10 in.
(25.4 cm) which is more than 22 times the fiber diameter. Since laminar flow should develop
within 10 diarneters from the entrance of the tube the flow in the device should have arnple
time to assurne a laminar condition.
A visual confirmation of the ~ nrnent of fibers during the transition between
turbulent flow and larninar flow can be made. It can be seen that: in the case of glass f1bers
in a turbulent flow, the diffracted laser bearn is dispersed into separated spots of light in
random directions; while in the case of glass fibers in a laminar flow, the diffracted laser
bearn is concentrated in approximately one direction (area), thus showing that the fibers are
aligned in a direction substantially parallel to the flow.
Light that is scattered in a forward direction 13 can be collected by lens
assembly 10 and focused on photodetector 14. Because this light typically is not colltm~t~.1
when it cnters the lens assembly 10, it can be focused to a point some distance beyond lens
assembly 10, thereby passing around beam block 11. Thus, while both the beam 12 and
scattered light 26 enter lens assembly 10, beam 12 typically is blocked from impinging on
-
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photodetector 1~ while scattered light 26 is, for the most part, focused onto the photodetector
14. Overall, only a small fraction of scattered light 26 is blocked by beam block 11.
It is preferred that photodetector 14 have a sensing region with a finite width
which is wide enough to receive the scattered light 26. Within this width, it w;ll respond to
light scattered by fibers 20 that are some distance to either side of, as well as in front and in
back of, axis 30 of flow tubes 5,6. Therefore, fibers 20 are not required to pass through beam
12 single-file or closely aligned with axis 30. When beam 12 is scattered by fiber 20, it is
focussed though lens assembly 10 to impinge upon photodetector 14, thus generating a brief
electrical pulse therefrom. In general, the amplitude of this pulse is preferred to be
proportional to the amount of light scattered by the fiber. The resultant pulse can be sent to
an ~pro~l iate electronic measurement circuit 31 where the pulse is recorded. Using other
~uantitative information, such as, the flow rate of the air through sensor 1, and clet~rrnining
the rate at which the pulses are received, the concentration of respirable fibers in the air can
be determined.
It is preferred that sensor 1 be substantially insensitive to non-fibrous
particulate matter. Presently preferred embodiment of the current invention accomplish this
selectivity by analy~ing, for example, the optical differences between the typically cylindrical
respirable fibers, and particulate matter having other shapes. That is, if a spherical or
irregularly-shaped dust particle is drawn into sensor 1, the particulate matter will also scatter
light from beam 12. However, such a particle tends to scatter light into a spherical volume.
Much of this scattered light will impinge on, and be absorbed by the walls of flow tubes 5,6.
In general, only a small fraction of the light scattered by these particles tends
to pass through the gap 7 between flow tubes 5,6. This small amount of scattered light tends
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to produce only a weak signal in photodetector 14. Circuit 31, receiving pulses from the
photodetector 14, can be designed to ignore low amplitude pulses resulting from particulate
matter. Therefore, device 100 can be made to respond only to respirable fibers while
ignoring other non-fibrous particulate matter that may be present. Unlike prior art devices,
the invention herein does not re~uire the use of electrostatic or electromagnetic components
to induce movement in the matter suspended in the air in order to determine whether or not
the matter is a respirable fiber.
Indeed, the ability of device 100 to discriminate between respirable fibers and
other particles could optionally use the following principles. First, non-respirable fibers are
climin~t~ from the airflow by separation means, i.e. virtual impactor 2, before the air enters
sensor 1. Second, the rem~ining fibers tend to be aligned with flow tube axis 30 by the
laminar flow of air through tubes 5,6. Third, beam 12 generally is oriented to be norrnal to
the axis of tubes 5,6. Fourth, light scattered by fibers 20 tends to be scattered in a plane
which passes between the ends of flow tubes 5,6, and a portion of the scattered light is
focused onto photodetector 14. Fifth, light scattered by other particles tends to be scattcred
more omni-directionally than is the case with cylinders. Most of this light is absorbed by the
walls of flow tubes 5,6 and only a small amount of light remains to be focused on
photodetector 14. Sixth, by discrimin~ting between the amplitude of signals received from
photodetector 14, device 100 can discriminate between fibers and other particles.
In Figures I and 2, lens assembly 10 and photodetector 14 are shown as being
substantially in-line with, or in opposit;on to, beam 12. In view of the te~chings of this
invention, a skilled artisan would recognize that lens assembly IQ and photodeteotor 14 may
be placed anywhere around axis 30 of flow tubes 5,6, as long as they are still in the plane of
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light scattered from fibers 20. Although the amount of light collected by lens assembly 10
can depend upon the location of lens assembly 10, sensor 1 can c~ . ;."iuille between
respirable fibers and other particles even with these ~it~m~tive configurations.
In Figure 3, for exarnple, the components of device 100 are substantially the
sarne as those in Figures 1 and 2, with the exception that lens assembly 10 and photodetector
14 have been rotated in orientation by 90 degrees. Also in Figure 3, beam block I 1 seen in
Figures 1 and 2, may be elimin~icd because bea~n path 12 no longer is in-line with, or in
opposition to, photodetector 14.
All publications mentioned in this specification are indicative of the level of
skill of the skilled in the art to which this invention pertains. All publications are herein
incorporated by reference to the sarne extent as if each individual publication was specifically
but individually indicated to be incorporated by reference.
While specific embodiments of practicing the invention have been described
in detail, it will be appreciated by those skilled in that art that various modifications and
alternatives to those details could be developed in light of the overall teachings of the
disclosure. Indeed, a skilled artisan would recognize that, although the invention has been
described in terrns of clf termining the concentration of airborne respirable fibers, the
apparatus and method illustrated in detail herein also can be used to detect, characterize, and
visualize other types of particles having specific optical properties. Accordingly, the
particular arrangements of the methods and ~y~lus disclosed are meant to be illustrative
only and not limiting to the scope of the invention, which is to be given the full breadth of the
following claims, and any and all embodiments thereof.
t~ . " i~
