Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
IMPROVEMENT(S) RELATED TO PARTICLE
MONITORS AND METHOD(S) THEREFOR
This application is a divisional of Canadian patent application Serial No.
2,543,567 filed internationally on October 20, 2004 and entered nationally on
April
21, 2006.
FIELD OF INVENTION
The present invention relates to the field of the detection, analysis and/or
determination of matter or particles suspended in fluid.
In one particular form, the present invention relates to smoke detectors,
which detect unwanted pyrolysis or combustion of material. In another form,
the
present invention relates to smoke detectors of the early detection type, and
which may be applied to ventilation, air-conditioning or duct monitoring of a
particular area. In yet another form, the present invention relates to
surveillance
monitoring, such as building, fire or security monitoring. In still another
form, the
present invention relates to environment monitoring, such as monitoring,
detection and/or analysis of a fluid, zone, area and/or ambient environment,
including commercial and industrial environments.
As will become apparent, the present invention has broad application and
thus the particular forms noted above are only given by way of example, and
the
scope of the present invention should not be limited to only these forms.
BACKGROUND ART
The present inventor has determined an understanding that the type of
smoke produced in various pyrolysis and combustion circumstances is different.
Fast flaming fires tend to produce a very large number of very small solid
particles
which may agglomerate into random shapes to form soot. In contrast, the early
stages of pyrolysis tend to produce a'much smaller number of relatively large
liquid particles (of high boiling point), typically existing as aerosols that
may
agglomerate to form larger, translucent spheres.
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The present inventor has also determined an understanding that the
detection of relatively large particles which slowly increase in quantity over
an
extended period of time would typically indicate a pyrolysis or smouldering
condition, whereas the detection of numerous small particles arising quickly
and
without earlier pyrolysis or smouldering could indicate arson involving the
use of
accelerants.
The present inventor has also determined an understanding that dust
particles are generated by the abrasion or non-thermal decomposition of
natural
materials or organisms in the environment and that such particles are in
general
very large compared with smoke particles.
The present inventor has also determined an understanding of the
following:
Conventional point type smoke detectors are primarily designed for ceiling
installation in a protected area. These detectors have relatively low
sensitivity
and have difficulty in detecting the presence of unwanted pyrolysis where
large
volumes of air pass through the area being monitored, thus diluting the
ability for
the detector to sense the presence of unwanted pyrolysis.
To overcome these disadvantages, highly-sensitive aspirated smoke
detectors where developed, and are often deployed on ducts for the purpose of
monitoring an area. These detectors provide a measure of sensitivity some
hundred times greater than convention point detectors. These aspirated systems
employ suction pressure via an air pump and also employ a dust filter to
reduce
unwanted dust pollution from soiling the detector or from being detected
indistinguishably from smoke and causing the triggering of a false alarm.
The smoke detector preferably employed in an aspirated system is a
nephelometer. This is a detector sensitive to many sizes of particles, such as
the
many smoke particles produced in fires or during the early stages of
overheating,
pyrolysis or smouldering.
Optical type smoke (or airborne particle) detectors of the prior art typically
use a single light source to illuminate a detection zone that may contain such
particles. The use of two light sources has been proposed for some detectors.
A
proportion of this light may be scattered off the particles toward a one or
more
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receiver cells (or sensors). The output signal(s) from the receiver cell(s) is
used
to trigger an alarm signal.
Other detectors use a laser beam, providing a polarised monochromatic
light source, typically in the near infrared wavelength. These detectors,
however,
are not considered to be true nephelometers as they are prone to being overly
sensitive to a particular particle size range at the expense of other size
ranges.
The disadvantage suffered by the above detectors is their relative
insensitivity to very small particles characteristic of early pyrolysis and
incipient
fires, as well as certain fast flaming fires.
lonisation smoke detectors, on the other hand, utilise a radioactive element
such as Americium, to ionise the air within the detection chamber. These
detectors are relatively sensitive to very small particles produced in flaming
fires,
but relatively insensitive to larger particles produced in pyrolysis or
smouldering.
They have also been found relatively prone to draughts, which serve to
displace
the ionised air within the detection chamber and thus trigger a false alarm.
This
places a practical limit on their useful sensitivity.
Other smoke detectors have used a Xenon lamp as a single light source.
The Xenon lamp produces a continuous spectrum of light similar to sunlight,
embracing ultraviolet, visible and infrared wavelengths. Use of this light
source
can detect all sizes of particles and the detectors produce a signal that is
proportional to the mass density of the smoke, which is characteristic of a
true
nephelometer. However, the type of fire cannot be characterised because the
particular particle size cannot be discerned. The Xenon light also has only a
relatively short life-span of some 4 years and its light intensity is known to
vary,
which affects the sensitivity.
The present inventor has also realised that in order to provide a wide
output range in sensitivity, prior art detectors provide an analog to digital
converter (ADC) used to apply the smoke level data to a microprocessor. With
careful design, substantially all of the capacity of the ADC is used to
represent the
maximum smoke level, such as (typically) 20%/m. ADC's operating at 8-bit
resolution are useful, whereas a 10-bit or larger ADC's are more expensive and
require larger microprocessors. A 10-bit ADC has been found to allow this
20%/m level to be divided into 1024 steps, each step representing an increment
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of 20/1024 = 0.02%/m. So the steps are 0, 0.02, 0.04, 0.06, etc, with no
opportunity for finer increments. At low smoke levels this is considered a
very
coarse resolution, making it difficult to set alarm thresholds finely. However
at
high smoke levels, a resolution of 0.02%/m is unnecessary - the ability to set
an
alarm threshold at 10.00%/m or 10.02%/m for example, has little if any
benefit.
So the resolution of the prior art detectors is considered too coarse at low
smoke
levels and too fine at high smoke levels.
Any discussion of documents, devices, acts or knowledge in this
specification is included to explain the context of the invention. It should
not be
taken as an admission that any of the material forms a part of the prior art
base or
the common general knowledge in the relevant art in Australia or elsewhere on
or
before the priority date of the disclosure and claims herein.
An object of the present invention is to provide a particle detection
apparatus and method(s) which enable an improved detection, discrimination
and/or analysis of particles, pyrolysis, smouldering and/or flaming events and
dust, thus providing a corresponding improvement in fluid-borne particle
detection.
A further object of the present invention is to provide a particle detection
apparatus and method(s) suitable for use with ducts or as a stand-alone
detector
and /or monitor.
A still further object of the present invention is to alleviate at least one
disadvantage associated with the prior art.
SUMMARY OF INVENTION
In accordance with aspects of the present invention, the monitoring,
surveillance, determination, detection and/or analysis of particles,
environment,
fluid, smoke, zone or area may comprise determination of the presence and/or
characteristic(s) of the particles as is required given the particular
application of
the present invention.
In this regard, an aspect of invention provides, a method of and device for
determining, in a fluid sample, the presence of particle(s) having
substantially a
predetermined size or range of size(s), the method comprising the steps of
illuminating the sample with a first wavelength of light, obtaining a first
response
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signal indicative of the first illumination, illuminating the sample with a
second
wavelength of light, obtaining a second response signal indicative of the
second
illumination, and determining the presence of the particles having the size or
range of size(s) by comparing the first and second signals.
5 Preferably, the illuminations are horizontally and/or vertically polarised.
In another aspect of invention, there is provided a gain control apparatus
adapted for providing gain control in a particle monitor, said apparatus
comprising
a first gain stage having a first amplifier, a second gain stage having a
second
amplifier, and a voltage or current-controlled feedback from the output of the
second stage to the input of the first stage so that the frequency response of
the
amplifier is unaffected by said feedback.
In still another aspect of invention, there is provided a method of
determining a service interval for a particle monitor, the method comprising
the
steps of determining the presence of dust particle(s), providing a measure of
the
presence of the particle(s), and providing a service indication when the
measure
has reached a predetermined threshold.
In yet another aspect of the invention, there is provided a particle monitor
chamber, comprising a first lens operable in association with a source of
illumination, a second lens adapted to focus impinging light toward a receiver
cell,
and a primary iris configured to substantially prevent light emanating
directly from
the first lens to impinge on the second lens.
In a further aspect of the invention, there is provided a method of and
device for determining the velocity of fluid flowing through a given area, the
method comprising the steps of providing a first sensor in the path of the
fluid flow
at a point of relatively low fluid velocity, providing a second sensor in the
path of
the fluid flow at a point of relatively higher fluid velocity, the second
sensor having
substantially similar temperature characteristics as the first sensor, and
determining the fluid velocity based on a measure of the cooling effect of the
fluid
passing the first and second sensors.
Additionally, there is provided in accordance with another aspect of
invention, a method of and device for mounting a housing on a duct, the method
comprising the steps of providing at least one tab element in association with
the
housing, locating the housing proximate the mounting area of the duct, shaping
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the tab element to substantially fit a profile of the duct proximate the
mounting
area, and attaching the housing using the tab element.
The device for mounting a housing on a duct according to the above
aspect of the invention comprises at least one tab element associated with the
housing; and the tab element being adapted to be shaped to substantially fit a
profile of the duct proximate a mounting area.
The present invention also provides a monitor for monitoring the presence,
concentration and characteristics of particulates in fluid medium.
The present invention also provides, as an output triggering the threshold
or alarm of the detector, a logarithmic signal. This means a signal, the
amplitude
of which may be compressed according to a logarithmic function or scale. The
logarithmic signal may represent various attributes of particles detected,
such as
the presence, number, frequency, concentration and / or duration.
The present invention also provides for method of enabling fluid to flow
through a detection zone of a particle detector, the method comprising:
providing
an inlet to the detector through which the fluid is adapted to flow; diffusing
the
fluid flow prior to entering the particle detection zone; and passing the
diffused
fluid flow into the particle detection zone.
Moreover, the present invention provides for a chamber configuration
adapted for a particle detector having an inlet through which fluid is adapted
to
flow at a first velocity, the chamber comprising: a first diffuser adapted to
diffuse
the fluid flow of a first velocity and provide a fluid flow at a second
velocity; and
the first diffuser providing to a particle detection zone, the fluid flow at
the second
velocity.
In essence, in one aspect of invention, different wavelengths, various
ranges of wavelengths and/or polarisation are used to detect predetermined
particles in fluid.
In essence, in another aspect of invention, subtraction or providing a ratio
of two signals enables a more measurable output indicating the detection of
particles and the particle sizes.
In essence, in another aspect of invention, this output indicating the
detection of particles is amplified in accordance with the two signals.
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Other aspects and preferred aspects are disclosed in the specification
and/or defined in the appended claims, forming a part of the description of
the
invention.
The present invention has been found to result in a number of advantages,
such as reduced size, cost and energy consumption while achieving the highest
industry standards for sensitivity, reliabiiity, maintenance period and false
alarm
minimisation, and/or monitoring of an environment for the presence of smoke
and/or dust particles such that very high sensitivity to smoke may be provided
without suffering false alarms due to dust.
Throughout this specification, reference is made to a number of different
light sources having certain wavelengths. Reference to the light sources and
wavelengths is made only as they are current commercially available light
sources. It is to be understood that the principles underlying the present
invention have equal applicability to light sources of different
wavelength(s).
A monitor may include reference to a detector or similar apparatus.
Further scope of applicability of the present invention will become apparent
from the detailed description given hereinafter. However, it should be
understood
that the detailed description and specific examples, while indicating
preferred
embodiments of the invention, are given by way of illustration only, since
various
changes and modifications within the spirit and scope of the invention will
become
apparent to those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
Further disclosure, objects, advantages and aspects of the present
application may be better understood by those skilled in the relevant art by
reference to the following description of preferred embodiments taken in
conjunction with the accompanying drawings, which are given by way of
illustration only, and thus are not limitative of the present invention, and
in which:
Figure 1 illustrates results of blue 430nm and red 660nm wavelengths to
particles over a range of particle sizes,
Figure 2 illustrates results of blue 430nm and green 530nm wavelengths to
particles over a range of particle sizes,
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Figure 3 illustrates results of blue 470nm and infra-red 940nm wavelengths
to particles over a range of particle sizes,
Figure 4 illustrates the result following the relative subtraction of red from
blue signals,
Figure 5 illustrates the result following the relative subtraction of green
from blue signals,
Figure 6 illustrates the result following the relative subtraction of infrared
from blue signals,
Figure 7 illustrates the development of particle size over time for various
types of fuels,
Figure 8 illustrates a comparative response of infrared and blue channels
to smoke from various fuels and/or stages of fire growth,
Figure 9 illustrates relative ratios of channel B and channel A outputs in
response to airborne particles from given fuels during a trail,
Figure 10 illustrates a schematic block diagram of a smoke monitor
according to one embodiment of the present invention,
Figure 11 illustrates a circuit diagram of one form of a gain controlled
amplifier according to one embodiment of the present invention,
Figures 12, 13 illustrate a preferred chamber geometry including indicative
light pathways,
Figure 14 illustrates the use of a biconvex lens according to an aspect of
invention,
Figure 15 illustrates the relative operation of an aspheric lens according to
an aspect of invention,
Figure 16 illustrates the use of an aspheric lens according to an aspect of
invention,
Figure 17 illustrates the relative operation of a biconvex lens according to
an aspect of invention, and
Figure 18 illustrates an example of the mounting of a detector unit onto a
duct arrangement.
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DETAILED DESCRIPTION
In the embodiment described, at least two channels are referred to, one
being channel A, which uses wavelengths such as red or infrared wavelengths,
the other being channel B, which uses wavelengths such as blue wavelengths.
Additional channels could be employed such as channel C, which uses
wavelengths such as green wavelengths. Other wavelengths may also be
employed in accordance with the present invention, as will become apparent in
the following description. Generally, it is preferred if a reading established
from a
longer wavelength is compared with a reading establish from a shorter
wavelength. Most preferably, the longer wavelength is subtracted from the
shorter wavelength. A ratio may also be used to compare wavelength readings.
WAVELENGTHS OF LIGHT
In one aspect of invention, it has been determined by the present inventor
that the wavelengths of light employed have an important bearing on the
sensitivity of the present device to particle sizes. The scattering of light
from
particles over various size ranges has been described in 'Absorption and
Scattering of Light by Small Particles' by Bohren CF and Huffman DR, ISBN 0-
471-05772-X.
It has been determined that Mie equations are appropriate for considering
particies of a size range appropriate to common smoke and dust. Fast flaming
fires tend to produce a very large number of very small carbonaceous particles
which may agglomerate into random shapes to form soot. In contrast, the early
stages of pyrolysis tend to produce a much smaller number of relatively larger
liquid particles (of high boiling point), typically existing as aerosols that
may
agglomerate to form even larger, translucent spheres or droplets. Dust
particles
generally result from mechanical abrasion and have random shapes that can be
approximated as yet larger spheres for modelling purposes. A source of smoke
or dust is unlikely to be mono-disperse (contain one particle size), but is
more-
likely poly-disperse, with a size range that may follow a Gaussian
distribution. It
has been found by the inventor that a typical standard deviation for the size
distribution is in the vicinity of 1.8 to 2.
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It has also been found that airborne particle distributions in cities are
bimodal, peaking at around 0.1 micron and 10 micron. Typically, smoke
particles
lie in the range 0.01 to 1 micron, whereas airborne dust particles lie in the
range 1
to 100 micron. However there is some overlap at the 1 micron boundary because
5 the smallest dust particles in nature are smaller than the largest possible
smoke
particles.
The present inventor has also determined that certain particle sizes are
more easily discerned by particular (different) wavelengths of light. Given
this, we
use two wavelengths of incident light. The light can range anywhere from blue
to
10 red (and infrared). An example is light ranging from 400 nm (blue) to 1050
nm
(red). For example, 430nm (blue) and 660nm (red) could be used.
By application of Mie theory to particle sizes ranging from 0.01 to 10
micron mass mean diameter and using a standard deviation of 1.8, Figure 1
shows the results for two wavelengths of incident light, 430nm (blue) and
660nm
(red), each being unpolarised, vertically polarised or horizontally polarised
and
projected at the same angle relative to the optical axis.
In Figure 1, the blue family of results (B = Blue unpolarised, BV = Blue
Vertically polarised, BH = Blue Horizontally polarised) are quite suitable for
the
detection of smoke and dust, whereas the red family of results (R, RV and RH)
are equally suitable for the detection of dust but are comparatively poor at
detecting a wide range of smoke particles due to the lack of response to small
particles. All of the graphs of Figure 1 come together above about 0.8 micron
whereas there is a significant difference between the graphs for particle
sizes
smaller than this. The best separation is achieved for blue-vertical (BV) vs
red-
horizontal (RH). The graphs cannot be effectively separated at larger
diameters.
The periodicity (fringing or resonance) in the curves is caused by phase
cancellation and reinforcement due to interaction between the given wavelength
and the given particle size.
If instead the combination of wavelengths 430nm (blue) and 530nm
(green) is examined, the results shown in Figure 2 are obtained. Here the
various
graphs are much more similar to each-other and it is difficult to separate the
graphs above about 0.5 micron.
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The wavelengths chosen to be exemplified have been limited to those of
commercially available projectors. Based on the information obtained in Figure
2
(530nm), the results for orange (620nm) would be similar to Figure 1 (660nm).
The results for blue (470nm) vs infrared (940nm) are now presented in
Figure 3. In Figure 3, the wavelength separation is substantially one octave.
It
can be seen that there is a more clear separation of the graphs in the region
below 1 micron, the nominal boundary between smoke and dust.
There would be some merit in operating the monitor at even more-widely
separated wavelengths, but currently available technology is a limiting
factor. The
receiver cell used to detect the scattered light is a PIN photodiode that has
enhanced blue response. With a peak response at 850nm, it's response falls to
about 30% at 400nm and at 1050nm, so for practical purposes the projector
wavelengths are currently limited to this range. Of course, if another
receiver cell
was able to be used, the wavelengths of light impinging the particles could be
altered to have a larger separation.
From the above results, it can be seen that in one embodiment of the
present invention, the wavelengths for two projectors for irradiating
particles to be
detected should preferably lie in the range 400 to 500nm for blue/violet and
650
to 1050nm for red/infrared.
In another aspect of the invention, it has been found that if the results of
the received signals are compared with each other, such as by comparison of
ratio or by being subtracted from each other, that is one signal is subtracted
from
the other signal, a more reliable 'trigger' or detection signal can be
produced
indicative of the presence of particles having a size of interest to the
application to
which the monitor of the present invention is applied. Thus, for example, if
the
monitor of the present invention is configured as a 'smoke' monitor, then
relatively
small particles would be of greater interest than larger (dust) particles.
Thus, the
inventor has realised that for a smoke monitor, for example, blue light has
been
found to be responsive to relatively small as well as large particle sizes,
and that
infrared light has been found to be responsive to relatively large particles
only.
By obtaining a signal which is based on a 'blue' response signal less an
'infrared'
response signal, the monitor can be configured to have a relatively higher
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responsiveness to small particles and a lower or null responsiveness to
relatively
large particles.
For example, Figure 4 shows the result of subtracting the red-horizontal
(RH), the red unpolarised (R) or the red vertical (RV) from the blue (B) data.
A
monitor configured in these ways would respond with more sensitivity to
particles
smaller than one micron, with best sensitivity from the B-RH combination. To
avoid clutter the BH and BV results are not shown but they are consistent.
For comparison with Figure 4, the subtraction of GH, G and GV from B
produces the results of Figure 5. Again, relatively smaller particle sizes are
more
clearly discernable than larger (dust like) particle sizes although the
fringing
effects are significant.
Figure 6 presents the results after subtraction of IRH from B. Other results
are omitted for clarity. In addition, some published data on particle mean-
sizes
obtained for incense, cotton lamp wick, toast and Portland cement (a dust
surrogate) are also shown. It can be seen that a monitor configured to perform
this subtraction would have appropriate sensitivity to common smoke types
while
being able to reject (relatively) dust to a significant extent.
Following on from this subtraction aspect, a further aspect of invention has
been developed in that an appropriately constructed gain amplifier can be used
to
provide appropriate output signals for use by alarm or other warning devices
or
systems. This aspect will be more fully disclosed below.
If a third or further wavelength(s) is used in addition to the two wavelengths
disclosed above, it will be possible to identify not only small and large
particles,
but other (intermediate) sized particles, dependent on the wavelength(s) used.
TWO CHANNEL DESIGN
Another feature that is provided by the use of a two channel design in
accordance
with an aspect of invention, is that by subtracting the A (reference) channel
from
the B (sample) channel (or vise versa) as described herein, we can achieve a
null
balance. This balance has been found to not significantly vary if the
background
of the chamber changes with time. The inventor has realised that as the
chamber
ages or soils over a long time span (a time that is greatly extended by the
use of
a dust filter), the background light level may change. The benefit of
subtracting
the channels is that because the response of both channels (especially to dust
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build-up) is substantially the same, then the effect is self-cancelling, which
minimises any change in the resultant output from the adder circuit over time.
Note that the signal obtained from dust is not dependent on it being airborne -
it
can be settled on a surface. The same is true for anything larger than dust -
like
dust agglomerates or even the walls.
This nulling of drift due to soiling is considered a valuable feature in terms
of
maintaining calibration.
SIGNAL LEVEL ANALYSIS
Further disclosure of the present invention is made with reference to a
smoke monitor application. It is to be noted, however, that the present
invention
should not be limited to only this application.
Conventional ceiling-mounted "optical" smoke detectors typically provide a
sensitivity equivalent to about 10%/m (3%/ft) obscuration, to generate an
alarm.
The established benchmark for very-high-sensitivity smoke detection requires a
sensitivity at least two orders of magnitude higher, equivalent to 0.1 %/m
obscuration at full-scale with alarm set-points below this level. Eccleston,
King &
Packham (Eccieston AJ, King NK and Packham DR, 1974: The Scattering
Coefficient and Mass Concentration of Smoke from some Australian Forest Fires,
APCA Journal, v24 no11) have shown that for eucalypt forest fire smoke, the
0.1 %/m level corresponds to a visual range of 4km and a smoke density of
0.24mg/m3. Such high sensitivity enables the detection of early stage
pyrolysis
and thereby provides for the earliest warning of a potential fire in
buildings,
commensurate with a low rate of false alarms.
Most very-high-sensitivity smoke detectors today utilise an optical chamber
with an infrared solid-state laser diode. The long wavelength of infrared
light is
useful for detecting the relatively large airborne particles characteristic of
dust, as
well as smoke aerosols from certain types of fires, but comparatively poor at
detecting the very small particles evolved in other fires. Conventional solid-
state
lasers operating at preferably shorter, visible wavelengths would be expensive
or
could not operate reliably at elevated ambient temperature (60 C). To overcome
these difficulties it was decided to use, in a preferred embodiment of the
present
invention as applied to smoke monitors, a light-emitting diode (LED) projector
operating at the blue end of the visible spectrum (470nm).
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The monitor configuration incorporates this blue projector set at 600 to the
receiver cell axis within an optical chamber, as will be further explained
below. It
also includes a reference projector at 940nm (infrared) set at the same angle,
but
horizontally opposed to the blue projector. With an effective projector
irradiant
cone angle of 10 , the arrangement offers a relatively optimum configuration
for
maximising system sensitivity while minimising background light that may
otherwise swamp the receiver cell.
For the specified smoke density (0.1 %/m), comprising particles of (say)
0.3pm mass median diameter (with a practical, geometric standard deviation of
1.8), Weinert (Weinert D, 2002: Assessment of Light Scattering from Smoke
Particles for a Prototype Duct-mounted Smoke Detector, unpublished) has
determined that in the monitor configuration used, the signal strength
received
due to irradiance of this smoke by an unpolarised blue light source, is on the
order of 4.5E-8 per unit irradiance. The Weinert data at 470nm and 940nm has
been graphed and presented in Figure 3. Crucially, this means that the
"background" light intensity received by the cell, due to unwanted remnant
reflections off the chamber walls, must be at least eight orders of magnitude
lower
than the projector beam intensity, so that the wanted light signal (scattered
off
smoke) is not swamped.
The blue projector, in one form, is specified to have a luminous intensity of
40 candela (cd) at a drive current of 500mA. By definition, at 1 cd the power
level
is 1.464mW per steradian (sr) so the rated power is 1.464*40 = 58.6mW/sr. The
5 half-angle converts to 2n(1-cos(5)) = 0.024sr so the output power is
58.6*0.024
= 1.4mW. Incidentally, at this drive current, the projector voltage drop is
4.OV so
using a 0.1% duty cycle, the input power to the projector is 0.5*4.0*0.001 =
2.0mW which is less than 1% of its maximum power dissipation rating.
Accordingly, at a pulsed projector power output of 1.4mW, the scattered
light signal directed toward the cell is 1.4*4.5E-8 = 6.3E-5pW for the
configuration
used. This level of illumination is directed and focused to fall upon the
receiver
cell, which is a PIN photodiode within the receiver module. The sensitivity of
the
cell is specified as 0.2A/W at 400nm, converting to 0.31 pA/pW at 470nm. With
a
CA 02598926 2007-09-12
specified lens transmission of 92% (uncoated), the signal developed by the
illuminated cell is therefore 0.31 *6.3E-5*0.92 = 1.8E-5pA.
The receiver module, in one form, includes a three-stage, AC-coupled
pulse pre-amplifier comprising a current-to-voltage converter followed by two
5 voltage amplifiers. The converter is an operational amplifier with the PIN
photodiode connected differentially between the inverting and non-inverting
inputs, with negligible series resistance. The feedback resistor may be 3.9M.
(shunted by 3.9pF) so at mid-band frequencies, for an input signal of 1 NA,
the
output from this stage would be 3.9E6*1 E-6 = 3.9V/pA. In response to the
10 specified cell illumination, the output becomes 3.9*1.8E-5 = 7.OE-5V or
70pV.
The following two stages, in one form, are operational amplifiers each
having a mid-band gain of 10, so the receiver module output should be 7.OmV at
the specified illumination. The calibration full-scale output level for signal
processing may be 3V, so the main amplifier voltage-gain would be 3/7.0E-3 =
15 429. Employing two similar stages, this amplifier would require a gain of
21 per
stage. In practice a gain of 17 per stage has been found adequate to produce a
sensitivity consistent with a nominal 0.1 lo/m at full scale, as required.
Clearly, the sensitivity of all smoke detectors depends on the particle size
and a meaningful standard would require this size (or a range of sizes) to be
specified. Nevertheless, the well-established international benchmark of
performance is the VESDA Mk3 monitor most recently produced by Vision
Systems Australia, using a Xenon light source. In fact this source is
comparable
with the blue projector, because the spectral characteristic of the Xenon
lamp,
combined with the spectral response of a PIN photodiode and the light
scattering
off small aerosol particles or molecules (which favours short wavelengths as
1/2,4), determine that the characteristic wavelength for calibration of Xenon-
based
monitors is 470nm - the same as the blue projector. For this reason, reliable
gases such as Nitrogen and FM200 can continue to be used for calibration
(which
is not possible for infrared laser based detectors).
As stated earlier, the monitor employs two projectors operating at different
wavelengths. With reference to Figure 3, for relatively large particles (>1 p)
it is a
design objective that the same signal level is generated at the cell by the
infrared
CA 02598926 2007-09-12
16
signal, as is the case for the blue signal. At the infrared wavelength of
940nm,
the receiver cell has a sensitivity of 0.55pA/pW (compared with 0.31 uA/uW at
470nm). Since at 940nm the lens transmission remains 92%, then because all
the relevant equations are linear and the geometries are relatively identical,
the
infrared projector output power can be reduced by a factor of 0.31/0.55 =
0.56. At
a current of 500mA, the infrared projector has a power level of 343mW/sr
(compared with 58.6mW/sr for the blue projector), so the required drive
current for
the infrared projector becomes 500*0.56*58.6/343 = 48mA. This drive current
would need to be increased if a polarising filter was used, in order to
overcome
the loss in this filter.
At the required projector drive settings, to a first approximation the small
background signal caused by the aggregate reflections off the chamber walls,
as
seen by the receiver cell, should be at the same (very low) level for either
projector. This requires that the reflection (or absorption) of the chamber
walls is
largely independent of the difference in wavelengths used. Therefore, in the
absence of any smoke in the chamber, the differential voltage between the two
channel outputs should be approximately zero (or can be so adjusted).
By introducing smoke into the chamber, the voltage on each channel
should rise, but the differential voltage between the channels may often be
non-
zero. This differential voltage provides an indication of the nature of the
airborne
particies. Figure 6 indicates the resultant sensitivity when the infrared
channel is
subtracted from the blue channel. This outcome could be used to highlight the
presence of particles smaller than 1 N mass mean diameter. Included in Figure
6
are lines identifying published data for the mass mean diameter of particles
produced from some available materials - Portland cement "dust", toast, cotton
lamp wick and incense. The differential voltage should be zero or slightly
negative in the first example (large particles), but significantly positive in
the other
three examples (small particles). This demonstrates the opportunity for
discrimination against dust while maintaining good smoke detection.
The particle size in smoke aerosols can vary substantially according to the
fuel used, the temperature and period of time, as well as the air flow
conditions
which determine the oxygen supply, cooling and smoke dilution. In Figure 7,
data
from Cleary, Weinert and Mulholland (Cleary TG, Weinert, DW and Mulholland
CA 02598926 2007-09-12
17
GW, 2001: Moment Method of obtaining Particle Size Measures of Test Smokes,
NIST) has been averaged to produce graphs of the aerosol sizes generated by
four fuels namely cooking oil (glass dish on hotplate), toast (toaster),
polyurethane foam (smouldering) and beech wood blocks (hotplate). It can be
seen that in each case the average particle is initially small, increasing in
size and
then falling as the fuel becomes fully consumed. As a generalisation it can be
said that the detection of small particles is important for the earliest
possible
warning of an incipient fire. Other data shows the aerosol mass concentration
peaking in the latter half of each period graphed, and falling at the end.
Figure 8 provides a more comprehensive comparison of the relative
response of the two channels, expected for a number of materials arranged in
order of particle size as published. Here the response has been normalised to
that of Portland cement (dust surrogate) by reducing the infrared projector
signal
by a factor of 0.64. Data for Douglas fir and rigid polyurethane (Bankston et
al;
Bankston CP, Zinn BT, Browner RF and Powell EA, 1981: Aspects of the
Mechanisms of Smoke Generation by Burning Materials, Combustion and Flame
no 41 pp273-292) demonstrate the progression of three different stages of
radiant
heat release rate, which should produce a commensurate differential voltage
signature.
To a first approximation and for the reasons stated earlier, Figure 8 could
be regarded as a comparison of expected performance between standard Xenon
and current laser based (infrared) detectors.
Moreover, in the case of the two-channel monitor, Figure 8 demonstrates
the opportunity for increased sensitivity compared with these infrared
detectors
(by up to a factor of four or five), to incipient fire events involving
pyrolysis and
smouldering, while greatly reducing the sensitivity to false alarms from dust.
This
could imply that a dust filter is not required. On the contrary, dust
filtration is
desirable to minimise soiling and thereby to maximise the maintenance period
and overall service life of the monitor. Given that a perfect filter for dust
would
also capture smoke, then the dust discrimination capability can be used to
avoid
unwanted alarms caused by the small quantity of dust that inevitably passes
through a practical filter.
CA 02598926 2007-09-12
18
Furthermore, because channel A is predominantly responsive to dust, the
output from channel A can be integrated over time (measured in months or
years)
to record the actual exposure of the chamber and the filter element to dust as
distinct from smoke, thereby enabling the service interval to be determined
and
annunciated in accordance with the (often unpredictable) ambient environment.
For example, a service interval may be determined for the dust filter based on
accumulating or counting the number of times a dust reading is detected. Once
the count reaches or exceeds a predetermined threshold, a service indicator
may
be illuminated or other wise communicated. Preferably, the service indicator
circuit should integrate the actual dust level and the period of its duration.
LOGARITHMIC OUTPUT
As noted above, in order to provide a wide output range in sensitivity, prior
art detectors provide an analog to digital converter (ADC) used to apply the
smoke level data to a microprocessor. With careful design, substantially all
of the
capacity of the ADC is used to represent the maximum smoke level, such as
(typically) 20%/m. ADC's operating at 8-bit resolution are useful, whereas a
10-
bit or larger ADC's are more expensive and require larger microprocessors. A
10-
bit ADC has been found to allow this 20%/m level to be divided into 210 = 1024
steps, each step representing an increment of 20/1024 = 0.02%/m. So the steps
are 0, 0.02, 0.04, 0.06, etc, with no opportunity for finer increments. At low
smoke
levels this is considered a very coarse resolution, making it difficult to set
alarm
thresholds finely. However at high smoke levels, a resolution of 0.02%/m
is unnecessary - the ability to set an alarm threshold at 10.00%/m or 10.02%/m
for example, has little if any benefit. So the resolution of the prior art
detectors is
considered too coarse at low smoke levels and too fine at high smoke levels.
In accordance with this aspect of invention, however, these prior art
disadvantages as noted above are overcome by providing a logarithmic or decile
output range. In accordance with the present invention, it has been found that
the resolution is appropriate to the given smoke level, namely fine at low
smoke
levels and coarse at high smoke levels. As an illustration, with the present
invention, using a logarithmic output range, at low smoke levels, an alarm
threshold could be set at 0.010 or 0.011 %/m but with equal ease, at high
smoke
levels, an alarm threshold could be set at 10%/m or 11 %/m.
CA 02598926 2007-09-12
19
In other words, realising that smoke is a very variable substance, and there
is little benefit in measuring its density (concentration) to an accuracy
better than
2 significant figures, the adoption of a logarithmic output provides a
beneficial
sensitivity resolution over a relatively wide range of smoke levels and / or
threshold settings.
SMOKE TEST RESULTS
A series of trails were conducted using the present invention configured as
a smoke monitoring apparatus and constructed and set up in accordance with the
Signal Level Analysis disciosure above. The monitor was mounted onto a
200mm diameter ventilation duct, while a probe was inserted into the duct to
sample the air passing through the duct. An inlet fan maintained a relatively
continuous flow through the duct, while ensuring that airborne particles were
thoroughly mixed with the incoming fresh air. The outlet from the duct was
exhausted via a flue. A hotplate operating at approximately 350 C was
positioned at the fan and duct inlet so that small fuel samples could be
placed on
the hotplate.
The arrangement was such that considerable dilution occurred, because
the smoke was entrained and mixed with the predominant flow of fresh air that
was continuously drawn into the duct from within the laboratory. This
situation
was intended to simulate a real protected environment where high levels of
dilution would be expected during the early stages of incipient fire growth.
Several different fuel samples were separately heated on the hotplate to
generate
smoke aerosols. In addition, some dust samples were also evaluated without the
hotplate by agitating and releasing the dust at the fan and duct inlet.
The output of the two monitor channels A and B were measured to provide
the voltage excursion beyond the quiescent (clean air) condition after
airborne
particles were introduced to the monitor.
It was observed that the various fuel types produced smoke aerosols at
differing rates and concentrations. As various fuels were heated and consumed,
it was expected that the aerosol particle size would vary with time and so the
relative output from channels A and B should vary in sympathy. Figure 9
presents the channel B output expressed as a ratio of the channel A output, in
response to a number of particle sources (after making allowance for
CA 02598926 2007-09-12
measurement settling transients). These data are presented as ratios in order
to
account for the different airborne particle densities involved, given that our
current
interest is in particle size. The length and position of each horizontal bar
represents the range in ratios that occurred during the course of each trail.
In
5 many cases the ratio quickly rose to the highest value, then fell slowly. In
some
cases the ratio rose again after a period at lower values. Some such patterns
(signatures) were observed to be distinctly bimodal.
Figure 9 also represents the relative sensitivity of the monitor to these fuel
and dust sources, arranged in apparent order of average particle size.
10 Accordingly, Nylon tubing initially produces the smallest size particles
(peak ratio
5.3). After the trial was half completed, the ratio fell slowly, indeed the
fuel melts
on the hotplate and produces an aerosol for a comparatively long time. Styrene
foam has a similar effect. Fuels further down the chart tend to char and
produce
a solid carbonaceous residue.
15 The hot wire test consisted of a 2m length of PVC insulated wire that was
heated by passing a high current delivered by a 2V AC "scope" transformer, to
simulate an overheating cable that results in early stage pyrolysis.
The result for solder resin comes from the melting of a short length of
resin-cored solder and its place in the chart indicates that comparatively
large
20 particles (high melting-point droplets) are produced.
The result for steam is anomalous inasmuch as the output readings
obtained from a boiling kettle source were of very small magnitude and did not
generate an alarm condition, but the ratios involved placed the particle size
at the
low end of the chart. In contrast, all the other sources produced large output
readings and it is only the channel output ratio that is small, in the case of
various
dust sources (including talcum powder).
Clearly there is a strong differentiation made between smoke aerosols and
dust, based upon particle size, so it is possible, with the present
embodiment, to
discriminate between wanted smoke sources and unwanted dust sources in the
process of generating an alarm.
Where the ratio is close to unity, it may be understood that subtraction of
channel A (such as infrared) from channel B (such as blue) would result in a
greatly reduced reading, such that unwanted alarms from these sources can be
CA 02598926 2007-09-12
21
avoided. Where the ratio is significantly above unity, subtraction of channel
A
from channel B would still result in an alarm. Although it is true that the
subtraction process could reduce the output of the monitor for certain types
of
smoke, the fact that unwanted alarms from dust sources can be avoided, permits
the monitor to be operated at higher sensitivity than would otherwise be the
case.
Furthermore, the results are considered consistent with published data
showing that for many fuels, the first particles released by pyrolysis are
comparatively small. Therefore, the type of monitor used here can provide the
earliest warning of pyrolysis.
CIRCUIT DESCRIPTION
Figure 10 illustrates schematically as a block diagram one form of the
present invention useful for detecting smoke. The circuitry drives a pair of
light
projectors 1 and 2, each projector having different characteristics of
wavelength
(colour) and/or polarisation. Each projector is driven independently to
provide a
pulse of light of short duration (for example 0.4mS), alternately at intervals
of
(say) 150mS and 350mS. This enables an update of the air quality twice per
second, being a high sampling refresh rate commensurate with low power
consumption.
Some of the light scattered off airborne particles that pass through the
monitor chamber 3, is received by a photovoltaic cell (not shown) within a
receiver
module 4. This signal is amplified in the receiver module 4 and passed to a
main
amplifier 5 with gain control 6. The amplified signal is then passed to a
discriminator (comprising a pair of synchronous detectors 7, 8 and a pair of
buffered sample-and-hold circuits 9, 10), which separates the signals derived
from the two respective projectors, into two channels, channel A represented
by
numeral 9 and channel B represented by numeral 10. The two channels provide
information about the type of particles in the air. Channel A is particularly
responsive to dust particles, while channel B is predominantly sensitive to
smoke
but has some sensitivity to dust. This is because dust and smoke particles
each
cover a wide range of sizes, which can overlap to some extent. Therefore in
subsequent circuitry, the dust reading of channel A is subtracted from the
smoke
reading of channel B by virtue of adder 11, resulting in a signal that in
essence
provides an indication of the smoke density alone.
CA 02598926 2007-09-12
22
This smoke density signal is applied to threshold sensing circuitry 12 that
operates a series of three lamps and relays 13 in response to the level of
fire
danger that is detected. These lamps and relays are for example denoted Al
(Alert, or level 1), A2 (Action, or level 2) and A3 (Fire, or level 3).
Typically these
three alarm levels indicate smoke densities approximately equivalent to 0.03,
0.06 and 0.10 lolm obscuration, although the monitor could be calibrated to
other
settings, and it would be understood that the signals and settings may be
configured appropriate to the particular application of the present invention.
In addition, a direct output 14 from the A channel is used to indicate when
dust levels are high, independent of the smoke density level. This may also
assist in testing, commissioning and demonstrations. This output also
indicates
when the monitor is in the process of discriminating against dust.
An additional lamp and relay 13 may be configured as a "fail-safe" circuit
applied to adder 11, to provide a fault alarm in the event that the monitor is
not
functioning properly with adequate sensitivity. An analog output from adder 11
may also be provided for remote processing of fault and alarm annunciation.
Alternatively, analog outputs may be provided from each of channels A and B to
permit remote signature analysis, and processing of fault and alarm
annunciation
A clock generator 15 may provide appropriate timing signals as is required,
and a power supply section 16 may reticulate power to all parts of the circuit
at
appropriate voltages.
It is necessary that the output signal from the discriminator channels do not
saturate when very high levels of smoke or dust are encountered. Such
saturation would lose information about the relative signal levels produced by
the
two projectors, thereby overwhelming the discrimination function. Firstly, the
amplifier is provided with a large "headroom" such that full-scale operation
is
achieved at a signal level (say) half that of saturation. Secondly, an
automatic
gain control is provided. The DC output voltages from the discriminator
channels
are fed back to a gain control device to ensure that saturation levels cannot
be
reached.
GAIN CONTROL
Referring to Figure 11, the mid-band gain of an operational amplifier is
determined by the ratio of the feedback resistor to the input resistor. In the
case
CA 02598926 2007-09-12
23
of IC3a in Figure 11, the voltage gain is R4/R3 and in the case of IC3b it is
R6/R5. The high frequency breakpoints are determined by C4=R4 and C6=R6,
while the low frequency breakpoints are determined by C1=(R1//R2), C3=R3 and
C5=R5. The amplifiers are DC coupled and the DC bias is set by R1 and R2.
The gain control device IC4 typically comprises an LDR (light dependent
resistor) and an LED (light emitting diode) closely coupled in a light-tight
box. The
LDR provides an adjustable resistance, the value of which is determined by the
current delivered through the LED which is controlled externally by R7. With
no
current through R7, the LDR resistance is effectively infinite, and at
currents of 10
to 20mA, the resistance falls within the region of lOkQ to 100kS2. Normally
this
LDR would be connected across either R4 or R6. This has the disadvantage that
in operation, it raises the high frequency breakpoint (either C40R4 or C69R6),
thereby upsetting the desired frequency response and phase characteristics of
the amplifier. Moreover, it has been found that this arrangement produces
insufficient dynamic range of gain control.
Because the two-stage circuit is non-inverting to the amplified signal, it is
possible to connect the LDR from the output of the second stage (IC3b) to the
input of the first stage (IC3a). This greatly increases the effective dynamic
range
available. Moreover, neither of the breakpoints C4=R4 and C69R6 is affected
when IC4 comes into operation.
The current driving R7 is derived from the sample-and-hold voltage signals
(high-going-low) of channel A and channel B, via zener diodes D5 and D6, to
ensure that the gain control action does not come into effect until the signal
levels
are significantly large.
Importantly, the characteristic of the LDR, LED and zener diode
combination is neither abrupt nor linear. It is non-linear, with the effect of
providing a logarithmic gain function. An abrupt change in gain could cause
instability or erratic behaviour, because a high signal level would cause a
sudden
reduction in gain, which would cause a sudden reduction in the output, which
would in turn reduce the drive to IC4, causing the gain to rise again. In
turn, this
could cause alarm output relays to chatter. The non-linear design allows for a
CA 02598926 2007-09-12
24
small increase in output as the input rises to high levels, and provides a
wide
dynamic range of control.
The normal full-scale sensitivity of the monitor, corresponding to the
highest alarm threshold ("fire"), is equivalent to 0.1 %/m obscuration, with
intermediate alarm thresholds available below this level. By using this
logarithmic
characteristic it is possible instead to arrange the alarm output thresholds,
so that
the higher levels of alarm can be in the non-linear region. By this means,
adequate resolution to provide a first level alarm ("alert") at very low smoke
densities such as 0.01 %/m can be provided, while the highest level of alarm
could
be raised to 1%/m, 10%/m or even higher.
CHAMBER OPTICS
Figure 12 shows the ray diagrams of the projectors, which operate at
differing wavelengths and/or polarisations. For clarity, the exemplar rays are
shown according to their position at the centre 1201, left or right extremity
1202 of
the beam. In practice these beams are operated for a short pulse duration,
alternately. It can be seen that the beams are formed by the lensed projector
body 1203, 1204 and confined by irises 1205, 1206, so as to pass through the
central, monitoring region or zone 1207 of the chamber. If smoke or dust is
passing through this zone 1207, a small proportion of the beam energy is
scattered off those particles in many directions. Some of this energy is
scattered
in the direction of a primary receiving iris 1208, and thence to a lens 1209
which
focuses the energy onto a photocell within a receiver module 1210. Note that
intermediate irises in this pathway have been avoided because stray light
reflected off chamber features and thereby coming from inappropriate
directions,
may reflect off these intermediate irises and into the lens.
Then the direct beams 1201, 1202 pass into an absorbing gallery 1211
where multiple reflections off the highly absorptive walls 1212, dissipate the
light
energy. The gallery is designed to direct the multiple reflections toward the
far
end of the gallery 1213, so that many reflections occur before any remnant
light
could emerge. The combination of this absorption and the geometry of the
primary iris in relation to the chamber and the beam irises, avoids swamping
the
light scattered off smoke or dust particles, by remnants of the originating
beams.
CA 02598926 2007-09-12
The rays 1214 indicate the region made sensitive to the photocell by the
receiving lens and primary iris. It can be seen that this sensitive region is
focused
within the monitoring zone 1207 but the photocell 1210 retains sensitivity
along
the optical axis beyond the zone. This extended sensitivity is confined by an
5 absorbing region 1215 at the far end of the chamber. The design intent is to
ensure that negligible light energy from the projectors 1203, 1204 can fall
upon
this absorbing region, which would tend to swamp the light scattered off
particles.
This unwanted light primarily arises from reflections off the projector irises
1205,
1206. A combination of shading this absorbing region, and reflecting stray
light
10 away from this area, minimises this swamping light. In addition, the walls
of the
absorbing region are preferably coloured black to absorb incident light.
Figure 13 illustrates typical, unwanted rays arising from reflections off the
projector irises 1205, 1206, which are prevented from reaching the central
absorbing region 1215. This diagram also includes unwanted rays 1216 that
15 pass through the primary iris 1217 and are absorbed within the receiver
gallery
1218. In addition, unwanted rays 1219 that reflect off the primary iris 1217,
are
shown to focus off-axis from the photocell within the receiver module 1210 and
are avoided by means of a photocell iris within the receiver module 1210
(shown
as 1401 in Figure 14).
20 The combination of all these methods serves to avoid swamping the light
scattered off airborne particles. The difficulty of this task can be
appreciated from
the fact that the scattered light intensity is typically 100 million times
lower than
the projector light
Referring again to Figure 12, the brightness within the central cone of light
25 1202 from the projector is regarded as the first order of brightness within
the
chamber. This bright light is directed towards the absorber gallery 1211,
along
which it is efficiently absorbed after multiple reflections. Outside of this
central
cone angle is a second order of brightness 1220 caused by the optics of the
projector and reflections off the projector iris. Therefore the whole of the
projector
iris area must be regarded as bright in many directions. Accordingly the
projector
iris must be shaded from view by the receiver or lens iris, which is achieved
by
positioning of the primary iris 1217. To achieve this shading, the chamber
geometry is set by a line 1221 (shown dashed in Figure 13) from the outermost
CA 02598926 2007-09-12
26
extremity of the projector iris 1205, 1206, to the innermost extremity of the
primary iris 1217, to the outermost extremity of the lens iris 1222. This is
considered a defining geometry given that an objective of an embodiment of the
invention is to produce a monitor of minimum practicable size and the highest
possible sensitivity.
Being outside of the central projector cone 1202, the primary iris 1217 is
exposed to light of second order brightness 1220 from the projector iris 1203,
1204. Therefore the primary iris 1217 will reflect light of third order
brightness
1219, in many directions. Note that in this discussion, an "order of
brightness"
does not necessarily mean a factor of ten. Given that black surfaces can
absorb
99% of the incident light, reflecting only 1% which is yet further reduced by
dispersion due to non-specular reflection, then an order of brightness
reduction
can be a factor of 1000 or more. Accordingly a third order of brightness is
not a
precise measure, but provides a relative indication. A small proportion of
this
third order brightness light 1219 will be reflected towards the lens iris 1208
and
lens 1209. As shown in Figure 14, the lens 1209 will focus this unwanted light
1219, off-axis from the receiver cell 1210, to be stopped by the receiver iris
1401.
The use of a biconvex lens, a relatively long focal length and a wide primary
iris,
enable unwanted rays (off-axis) reflected from the primary iris 1217 fall to
the side
of the receiver cell 1210 and can be attenuated by the receiver iris 1401.
It was expected that relatively accurate control of the focusing of the lens
was necessary in order to control the separation of the unwanted light from
the
wanted light. An aspheric lens 1501 (as shown in Figure 15) of relatively
short
focal length was proposed. Such a lens provides accurate control of focusing
across the whole face of the receiver cell, avoiding spherical aberration and
forming an image of photographic quality. Figure 15 illustrates the operation
of
such a lens 1501 in focusing scattered light received from particles detected
in
the monitoring zone 1207 (Figure 12). Figure 15 also illustrates the placement
of
the lens 1501 relative to the primary iris 1217, and the cell 1210. Figure 16
shows that with such an aspheric lens, however, some of the unwanted light
reflected off the primary iris falls upon the cell. This would swamp the
wanted
signal.
CA 02598926 2007-09-12
27
Turning again to Figure 12, a relatively thick biconvex lens (having two
convex faces) is used, and which is shown in more detail in Figures 14 and 17.
As shown in Figure 14, because the unwanted light 1219 arrives from off-axis
directions, the spherical aberration of this type of lens 1402 helps to
increase the
separation of the two sets of light rays. This separation is further assisted,
by
using a relatively long focal length (and it has been found that the
separation is
proportional to focal length). In Figure 17, it can be seen that the use of
the
biconvex lens 1402 is made possible because it is not required to form an
accurate photographic image at the receiver cell 1210 - it is only necessary
to
collect light, so the point of focus is not so important as the light ray
paths
involved. In this way, the geometry of the receiver cell 1210 and the lens
1402 is
arranged preferably so that a relatively maximum amount of scattered light
from
detected particles is able to fall onto the receiver cell (as shown in the
drawings
where the cell 1210 is illuminated with light substantially all over the cell
1210
surface) whilst unwanted light is either blocked from the cell by receiver
iris 1401
as described above, or allowed to pass to the side of the cell.
FLUID DYNAMICS
The design of the chamber from a fluid dynamics standpoint is quite
important. One embodiment of the invention includes a miniature duct probe to
gather a continuous, small but representative sample of the air passing
through a
ventilation duct, for example a probe as disclosed in co-pending US patent
application 2003/0011770, also by the present inventor.
Referring to Figure 13, the fluid, such as air, sampled from the environment
is drawn into the chamber of the present invention via inlet 1301, passes
through
the detection chamber and monitoring zone 1207 (Figure 12) and exits via
outlet
1302. It is possible to use a relatively large filter 1303 that can
efficiently remove
dust for a long period of service, without incurring significant head loss
(pressure
drop). The preferred type of filter in use is a large-pore, open-cell foam
filter of
large depth. The smallest dust particles that the filter is designed to
remove, are
generally at least 10 times smaller than the average pore size of the filter.
Dust
removal is achieved as a result of Brownian motion (rapid thermal
oscillation), by
which the dust particles react as if they were many times larger than their
physical
size. Dust is removed statistically as the flow passes through the deep
filter, so
CA 02598926 2007-09-12
28
that virtually all the dust considered harmful is removed before the flow
exits the
filter outlet 1314. This has been found to minimise dust build-up (soiling)
within
the chamber beyond, greatly extending the maintenance period. However, the
open structure of the filter avoids a significant problem that has occurred
with
aspirated smoke detectors of the prior art, namely the removal of smoke
particles,
increasingly with time, which reduces sensitivity. Moreover, the filter is of
a type
in which the head loss in the filter does not increase appreciably as the
filter
becomes loaded with dust.
Typically, smoke particles lie in the range 0.01 to 1 micron, whereas
airborne dust particles lie in the range 1 to 100 micron. However there is
some
overlap at the 1 micron boundary because the smallest dust particles in nature
are smaller than the largest possible smoke particles. Therefore it is
inappropriate that the filter should be a perfect dust arrestor. To avoid a
reduction
in sensitivity to smoke, a small fraction of the dust particles must therefore
pass
through the filter, which needs to be accommodated in other ways (as is
disclosed
later).
There are mirror-image diffusers, 1312, 1313 either side of the filter 1303.
The outlet face 1314 of the filter is presented to a diffuser 1313 that
efficiently
recombines the flow, turns the flow through 90 and presents the flow to a
passage 1304. In a preferred embodiment of the invention, this passage narrows
to a cross-sectional area that is still some 5 times larger than the inlet
tube, so the
loss remains very low, but the local air velocity is some 8 times faster than
it is at
the exit face 1314 of the filter.
In a preferred embodiment, two sensing devices 1305, 1306 can be
mounted, one 1306 at the filter outlet and one 1305 within this narrow region
1304. In this arrangement the sensor 1306 is subject to the relatively very
low
velocity air flow exiting the filter, so that very little cooling of the
sensor takes
place. This sensor 1306 may be further protected from cooling by means of a
shroud 1307. By contrast, sensor 1305 is relatively fully exposed to a
significantly
higher velocity air flow and is therefore subject to substantially more
cooling than
sensor 1306. The two sensors 1305, 1306 are preferably exposed to the same
ambient air temperature. Preferably matched devices having a known
temperature dependence can be utilised, whereby their different rates of
cooling
CA 02598926 2007-09-12
29
caused by the different air flow velocities to which they are exposed, can be
used
to generate a different voltage across each sensor, thereby providing a
measure
of the air velocity in a manner that is largely independent of ambient air
temperature.
The sensors may be of the type disclosed in US 4,781,065, however, the
positioning of the sensors in the present arrangement of the invention is
uniquely
different.
Also, in the present arrangement, the sensors are exposed to airflow after
it has passed through the dust filter 1303, thus soiling is minimised. Soiling
may
interfere with the cooling characteristics of the sensors 1305, 1306, thereby
detracting from the accuracy of the airflow measurement circuitry.
The flow continues into a further diffuser 1308, which is also the light
absorber gallery 1308 for projector 1203 (Figure 12). As the air flow reaches
the
mouth of the absorber gallery 1308, a change of direction is imparted while
its
velocity has slowed to some 25 times less than the velocity at the inlet tube.
Therefore very little loss is incurred in the air flow passing through the
gallery
1308, across the monitoring zone 1207 (Figure 12) and into the second gallery
1309. Because the velocity here is relatively low, any remnant dust particles
that
may be in the air stream, being small in number and size (because of the
filter
1303), have a very low momentum and are not therefore spun-out of suspension
in the fluid by centrifugal forces, thereby minimising the potential for
soiling within
the vicinity of the monitoring zone 1207. In the event that there was a
tendency
for centrifugal separation of dust particles, their direction of momentum
would be
such that these particles would be deflected harmlessly away from the primary
orifice 1217.
The air flow is drawn towards the second absorber gallery 1309 and by
diffuser action is gradually and efficiently accelerated and turned to match
the
exhaust exit 1302. The exhaust air is then efficiently returned to the
sampling
environment, such as a duct, as described in US 4,781,065 noted above.
It has been explained how the air flow passes through a series of stages in
a manner that minimises loss and promotes laminar flow. Accordingly, the
chamber is purged with a fresh sample of air very efficiently and quickly,
with
minimal smoke retention. Despite the low local velocities caused by the large
CA 02598926 2007-09-12
cross-sectional areas, the response of the chamber assembly to changes in
smoke levels has proven to be quite rapid, and suitable for the purpose of
smoke
monitoring alarms.
Because there is very little pressure drop within the monitor of the
5 invention, the absolute pressure anywhere inside the monitor is similar to
that
inside the duct. Because there can be a large pressure differential between
the
inside of the duct and the ambient environment in which the monitor is placed,
the
monitor must maintain a good pressure seal to avoid leakage at any point. The
opportunity for leakage is minimised by the chamber design, which comprises
two
10 similar halves connected by flat, mating flanges 1310. Therefore only one
flat
gasket is required to seal the chamber. In one embodiment, a thick closed-cell
foam gasket is preferred because this can easily adapt to variations in the
chamber flange flatness, overcoming the small amount of bowing and warpage
that may occur with plastic injection mouldings. Areas of the chamber,
15 particularly near the monitoring zone 1207, that are sensitive to the light-
absorbing quality of the chamber walls, are hidden from the gasket by means of
extending small rims 1311 that meet at the centre join of the two chamber
halves.
Actual contact between the two halves of the chamber is preferred only at
these
rims, greatly simplifying the demands upon manufacturing of the flatness of
20 mating parts.
The foregoing description has been discussed with the use of a duct probe
in mind, however in other embodiment(s) of the invention, the probe may be
replaced with other means to capture a sample of the fluid, such as air, to be
monitored. This other means (disclosed in US 4,781,065) may be a venturi
25 device within a small-bore pipe such as 20mm diameter. This pipe may be
connected to an aspirating pump or fan (aspirator), placed either upstream or
downstream of the venturi. If placed downstream, then a plurality of monitors
may be connected to a single aspirator. Upstream of each monitor, the small-
bore pipe may extend throughout a fire zone. The sampling pipes may be
30 configured as a network or branches extending into areas or zones where
fluid is
to be monitored or detected. Each said pipe may contain branches. Each said
pipe and branch may contain a number of small holes so that air in the
vicinity of
each hole is drawn into the pipe. The contribution of air samples from all
such
CA 02598926 2007-09-12
31
holes is then drawn intermiftently or relatively continuously towards the
venturi.
The venturi is designed so that a proportion of the air within the pipe is
drawn
through the monitor so that the presence of smoke or dust is sensed, before
the
monitor air flow is returned to the pipe. All the air is then drawn to the
aspirator
and exhausted.
Note that it is preferred that either in the case of the duct probe or the
venturi, only a proportion of the available air passes through the monitor.
This
proportion or sample of the air contains smoke and/or dust at the same density
as
the main flow. However, by carefully minimising the flow through the monitor,
the
rate of dust buildup in the dust filter can be minimised, thereby maximising
the
maintenance interval without affecting the sensitivity of the monitor.
In a further alternative embodiment of the present invention, instead of the
venturi it would be possible to connect the monitor directly to a small-bore
tube
such as 5mm internal diameter. This would be suitable for running short
distances such as several metres. In this case, the entire air flow would pass
through the monitor, but the flow rate would be low and therefore the
maintenance interval would not necessarily be affected. To achieve rapid
response times with small-bore tubes over long distances the pressure drop
would be very high, necessitating an aspirator of high pressure and energy
consumption.
MONITOR MOUNTING
Referring to Figure 18, the monitor 1801, for example a monitor according
to the present invention may be mounted to a flat-sided, circular or other
shaped
surface, such as a duct 1802 by means of mounting tabs 1803. The monitor
1801 may be secured by screws for example, or other suitable means (not
shown). In mounting the monitor, the tabs 1803 are simply bent until the tabs
match the surface of where the monitor is to be fixed. For example, in
mounting
to a duct, the tabs are bent till they sit upon or match the surface of the
duct, as
illustrated in Figure 18. This duct may be as small as 200mm (8 inch)
diameter.
The tabs 1803 may be formed integral with the housing of the monitor 1801, in
which case, a slot (not shown) formed in the housing may define the tabs and
enable bending of the tabs without skewing, so as to sit firmly on a surface
of a
duct or other mounting surface.
CA 02598926 2007-09-12
32
While this invention has been described in connection with specific
embodiments thereof, it will be understood that it is capable of further
modification(s). This application is intended to cover any variations uses or
adaptations of the invention following in general, the principles of the
invention
and including such departures from the present disclosure as come within known
or customary practice within the art to which the invention pertains and as
may be
applied to the essential features hereinbefore set forth.
As the present invention may be embodied in several forms without
departing from the spirit of the essential characteristics of the invention,
it should
be understood that the above described embodiments are not to limit the
present
invention unless otherwise specified, but rather should be construed broadly
within the spirit and scope of the invention as defined in the appended
claims.
Various modifications and equivalent arrangements are intended to be included
within the spirit and scope of the invention and appended claims. Therefore,
the
specific embodiments are to be understood to be illustrative of the many ways
in
which the principles of the present invention may be practiced. In the
following
claims, means-plus-function clauses are intended to cover structures as
performing the defined function and not only structural equivalents, but also
equivalent structures. For example, although a nail and a screw may not be
structural equivalents in that a nail employs a cylindrical surface to secure
wooden parts together, whereas a screw employs a helical surface to secure
wooden parts together, in the environment of fastening wooden parts, a nail
and a
screw are equivalent structures.
"Comprises/comprising" when used in this specification is taken to specify
the presence of stated features, integers, steps or components but does not
preclude the presence or addition of one or more other features, integers,
steps,
components or groups thereof."
