Note: Descriptions are shown in the official language in which they were submitted.
. . CA 02323675 2000-10-17
-1_
CLOSED CELL GAS DETECTOR
Field of the Invention:
The invention pertains to gas detectors. More particularly, the
invention pertains to such detectors wherein ambient gas related signals are
generated using single closed cell gas sensors.
Background of the Invention:
Smoke detectors have been recognized as being useful in providing
early warnings of fire conditions. It has also been recognized that there can
be a
benefit in combining different types of sensors into a single detector. For
example,
smoke and carbon monoxide or carbon dioxide sensors have been combined in a
single detector. Such combinations can, depending on configuration and on fire
type, improve detector performance.
Known types of smoke detectors include photoelectric, ionization-type
or projected beam smoke sensors individually or in combination. Various types
of
gas sensingtechnologies including solid-state, electrochemical and absorption
have
been used to implement gas sensors.
Another known gas sensing technology is based on photoacoustic
phenomena. Every gas will absorb light energy. Each gas is also unique in the
spectrum of light that it will absorb.
A microphone diaphragm or another pressure transducer can detect a
pressure wave that results from absorption and convert it into an electrical
signal.
Fig. 1 is a graph illustrating the absorption spectrum of clean air in the
range of
carbon dioxide and water. Carbon dioxide is strongly absorbed in a 4.1 - 4.5
micron range.
To benefit from the photoacoustic effect, a target gas must be held
,, CA 02323675 2000-10-17
-2-
inside a volume that permits a pressure build up due to gas expansion. This is
achieved either by sealing the target gas inside a vessel, a closed cell, or
by
impeding the gas flow into surrounding air with a restrictive membrane. The
vessel
technique is called "closed-cell" photoacoustics and the membrane technique is
called "open-cell" photoacoustics.
Closed-cell photoacoustic designs have the greatest sensitivity to gas
concentrations but they are large, complex and expensive, often costing $5000
or
more. Such instruments are usually bench type equipment intended for
laboratory
usage.
Open-cell designs, on the other hand, can be quite small and low in
cost. They unfortunately have poor sensitivity and drift. They are useful in
applications where signals can be averaged over long periods of time.
A light pulse directed into a gas filled closed-cell (through an optical
window) generates a pressure wave inside that cell. The strength of this
pressure
wave is proportional to the light energy absorbed. Carbon dioxide will absorb
peak
light energy at a wavelength of 4.3 microns.
If light energy at 4.3 microns is removed from this light (by carbon
dioxide in the atmosphere) before it can enter the cell, the resultant
pressure wave
that is generated is proportionally reduced. This is the basic principle
behind
photoacoustic gas detection.
The light beam is designed to pass through a sample chamber before it
is allowed to enter the closed cell. If carbon dioxide is present in the
sample
chamber, it will absorb some of the 4.3 micron energy from this light pulse.
As a
result, less 4.3 micron light energy will enter the carbon dioxide filled
closed cell.
Less carbon dioxide pressure is therefore produced and a lower
electrical output signal results. This output signal will increase with less
carbon
dioxide in the sample chamber and decrease when more carbon dioxide is
present.
For carbon dioxide concentrations between 0 to 2,000 ppm, the ppm to
CA 02323675 2000-10-17
-3-
voltage relationship is fairly linear. At higher concentrations, the signal
attenuation
plot follows an exponential and is governed by Beer's Law.
Known light-based gas detectors on the market today are based on
"open-cell" photoacoustics or non-dispersive infrared (NDIR). The latter is
much
more popular because it represents a well understood technology and favored
because of its lower cost.
An NDIR type sensor is illustrated in Fig. 2. NDIR is also an optical
absorption technology. A light source 49 with reflector 48 is pulsed by an
external
drive circuit. This light enters a sampling chamber 50 enclosed, at least in
part, by
a permeable membrane 51.
Instead of sensing pressure, NDIR uses a pyroelectric element 53 that
is sensitive to changes in heat. Light passing through the sampling chamber 50
will
increase or decrease in energy based on the degree of gas absorption. This
variation in light energy can then be sensed using a pyroelectric element 53.
If a specific bandpass filter 52 is placed in front of the pyroelectric
element 53, the response can be tailored to match specific ranges or windows
of
wavelengths. For example, a carbon dioxide filter located in front of the
element
53 results in a carbon dioxide gas sensor. NDIR detectors, however, are not as
sensitive as photoacoustic detectors due to their inherently poorer signal to
noise
ratio.
High performance photoacoustic instrumentation incorporates a dual
beam, closed cell structure with ratiometric processing. This technology
offers up
to 15 times more sensitivity than NDIR based gas sensors especially in the
presence
of interference gasses. Lab instruments with photoacoustic technology are
routinely used to benchmark performance of other gas detecting products.
Fig. 3 illustrates a dual beam photoacoustic detector for sensing carbon
dioxide, for example. A common light source 33 illuminates two separate
optical
paths A and B. This design contains four chambers.
CA 02323675 2000-10-17
-4-
Light from source 33 is collected by a common reflector 32 and directly
illuminates two chambers 36 and 37. Chamber 37 is the sample gas chamber
having an inlet 44 and an outlet 45. A gas to be sensed flows into and through
chamber 37.
Chamber 36 is sealed and filled with nitrogen. It serves as a reference
chamber. Nitrogen does not absorb any light in the wavelength region of
interest.
Light energy from source 33 enters both chambers 36 and 37 before
entering carbon dioxide filled chambers 40 and 41. Optical filters 46, 47, 38,
39
are all carbon dioxide wavelengths selective with respect to incoming radiant
energy.
Carbon dioxide in chamber 37 will absorb energy from the light
entering chamber 37. The degree of absorption is a function of the carbon
dioxide
density in chamber 37. The remaining light energy then enters detector chamber
41.
The nitrogen in chamber 36 does not absorb the incoming radiant
energy. That incident light then enters chamber 40. It is then converted in
chambers 40, 41 to pressure waves via gas expansion.
The capacitive membrane 42 flexes in the direction of chamber 41 or
40 depending on the vector of this differential pressure. This flexing varies
the
membrane capacitance which can be sensed by an external AC circuit. The peak
amplitude sensed is inversely proportional to the amount of carbon dioxide in
sample chamber 37. A chopper wheel 34 breaks the light beam up into pulses to
permit AC amplification and synchronization.
Photoacoustic pressure signals will normally be the same for optical
paths A and B with no carbon dioxide present in the sample cell 37. Any carbon
dioxide in the sample cell will cause signal attenuation at 4.3 microns since
4.3
micron energy will be absorbed from the light entering chamber A.
As a result of the absorption, the corresponding sensing chamber 41
CA 02323675 2000-10-17
-5-
will experience a lower pressure while the pressure in reference chamber 40
remains unchanged. This pressure imbalance forces the membrane to flex into
chamber 41. The resultant amplifier output will now show an increase in PPM of
carbon dioxide present in sample chamber 37. This technology is complicated
and
involves integration of many sensitive components.
Even though the dual beam closed cell photoacoustic detector of Fig.
3 is very sensitive; it suffers from many drawbacks that limit its
applicability.
Among other problems, such detectors configured for a single gas can cost more
than $10,000. As a group, they are large, heavy, and require special filaments
plus
accurate temperature control. They are also sensitive to vibration, needs
forced-air
cooling and often incorporate a moving chopper wheel assembly. Synchronization
is critical for proper operation and this instrument must be "zeroed" before
every
analysis. These maj or drawbacks have restricted such units to laboratory or
manual
applications.
Because of such drawbacks, NDIR-type sensors have gained popularity
in gas detection. They tend to be simpler in structure. NDIR, however, has its
own
challenges. The major commercial obstacles are speed of response, calibration
and
high cost.
Much of the cost is focused on the bandpass filters) and infrared
detectors) assemblies. Since NDIR technology senses the energy loss in a light
beam through a narrow bandpass filter, the quality of this filter is extremely
important for performance. The same is true for the pyroelectric infrared
detector
used. These are the two primary factors that keep NDIR costs high.
A "state-of the-art" NDIR carbon dioxide detector design recently
introduced into the market incorporates an integrated reference. To combat
inaccuracy and component aging problems over time, a ratiometric technique
(vs.
single value readings) is used to determine gas signal attenuation. To perform
this
math, however, a reference signal is needed that does not respond to the
target gas.
CA 02323675 2000-10-17
-6-
This can be accomplished by integrating a tunable filter into the sampling
chamber.
Using a 50% duty cycle, the filter is constantly switched electrically
between carbon dioxide and non-carbon dioxide wavelengths. The ratio of these
two signals is then used to determine carbon dioxide concentration.
In another commercially available design, two pyroelectric detectors
are used each with its own filter, one to pass carbon dioxide and the other
not. The
ratio of these two signals is then used to determine gas concentration. Both
of the
above units sell for over $250 in volume quantities (single unit pricing is
around
$450). This cost is much lower than the detector of Fig. 3 but it is still too
high for
lower priced markets.
Photoacoustic technology, on the other hand, uses the gas itself as the
sensor. The type of gas in the detection chamber is used to detect the
presence of
the same type of gas in the sample chamber. This perfect match of absorption
signature at all wavelengths (and therefore rejection of interferences) is the
reason
1 S why a photoacoustic approach offers much higher sensitivities than an ND1R
approach.
One known "open-cell" photoacoustic carbon dioxide gas sensor is on
the market. An "open-cell" design illustrated in Fig. 4 includes a single
sample
chamber 30 having three components. A permeable filter membrane 29 allows gas
to diffuse into the chamber but prevents dirt ingress.
A microphone 31 is integrated into the body 28 to sense gas pressure
signals. A bandpass filter 27 allows selected external light energy to enter
the cell.
Implemented as a carbon dioxide detector, pulses of light enter the
chamber through the 4.3 micron bandpass filter 27. This light is absorbed by
the
concentration of carbon dioxide inside the chamber that diffuses through
membrane
29 from the outside air.
The energy absorbed by the carbon dioxide in chamber 30 at the instant
of the light flash is transformed into a pressure wave that is detected by the
CA 02323675 2000-10-17
-7-
microphone 31. The amplitude of this signal is proportional to the gas
concentration inside the chamber.
Diffusion membrane 29 introduces a time delay between carbon dioxide
concentrations inside the chamber and carbon dioxide concentrations in the
air.
This delay can be up to 1 S minutes for large carbon dioxide swings from 300
ppm
to 2,000 ppm.
Problems with "open-cell" designs are known. They often require long
integration times to overcome poor signal to noise ratios. They also require a
bright light source (and therefore more power) to excite low concentrations of
carbon dioxide or other selected gas in air. Since there is no reference
signal, the
detector is prone to temperature sensitivities and component drifts over time.
An improvement over the design of fig. 4 was disclosed recently. This
detector is illustrated in Fig. 5. A light source 17 and reflector 16
illuminate
sample chamber 21 and reference chamber 25 through bandpass filters 19 and 18,
respectively. Microphones 23, 24 are each located in a respective chamber.
Gas from the outside air permeates into sample chamber 21 through a
membrane 20. The pressure waves developed are detected by microphone 23 and
ratiometrically compared to output signals from microphone 24 to cancel out
component effects. Unfortunately this design exhibits microphone imbalances
that
can vary up to 70%, poor signal to noise ratios, and long time constants.
Notwithstanding the various known types of gas detectors, there still
continues to be a need for lower cost, higher reliability and less complex
detectors
than now known. Preferably such detectors would require relative low power,
exhibit relatively high sensitivity and an improved signal-to-noise ratios
than
comparably priced gas detectors.
Summary of the Invention:
A single closed chamber photoacoustic gas detector includes an
optically transparent, closed chamber filled with a type of a gas to be
sensed. The
CA 02323675 2000-10-17
_g_
closed chamber is located at least adjacent to a portion of a sensing region
into
which gas to be sensed flows.
A sensing source of radiant energy, which could be a laser diode which
emits light having a wavelength which is known to be absorbed by the gas to be
sensed, injects radiant energy into the sensing region. In other embodiments,
incandescent or gas discharge sources could be used.
The inj ected radiant energy passes through the gas in the sensing region
wherein a portion of the energy therein is absorbed by the gas of interest.
The
radiant energy continues into the closed chamber.
A microphone is located within the closed gas chamber. Incoming
radiant energy which has passed through the sensing region and through the
closed
chamber is converted to an acoustic signal therein and produces an electrical
signal
indicative thereof.
A reference source is positioned adjacent to the closed chamber. The
reference source, which could be implemented as a light emitting diode, a
laser
diode or any other type of source with an emission frequency having a
wavelength
in the region of absorption of the gas of interest, injects radiant energy
into the
closed chamber. The inj ected radiant energy in turn produces a reference
electrical
signal at the output of the microphone.
The sensing source and the reference source can be pulsed alternately
at a preselected frequency. A ratio of the two signals could be formed for
purposes
of minimizing component variations and aging effects. Filters responsive to
the
wavelength of interest can be interposed between each of the sources and the
respective adjacent chamber for improved performance.
In yet another aspect, a sensing chamber can be formed of two,
connected, substantially identical housing portions which define an internal
sensing
volume having an ellipsoid profile. The internal walls of the sensing region
can be
made reflective by a deposited reflective metal surface such as a chrome
surface.
CA 02323675 2000-10-17
-9-
A closed gas supply tube is located at a tapered end of the chamber
extending thereunto. The gas supply tube contains a quantity of a gas of
interest
to be sensed. An acoustic transducer or microphone can be located at an end of
the
closed tube, displaced from the sensing region.
A sensing source is carried in the housing at an end of the sensing
region displaced as far as possible from the gas tube. The source can be
triggered
repetitively at a predetermined frequency whereupon it injects pulses of
radiant
energy, of a selected wavelength into the sensing region. The injected pulses
are
directed by the reflective surfaces toward the closed gas chamber at the far
end
thereof. Gas input and output ports can be provided into the sensing region.
The
ports can be provided with appropriate filters to exclude dust, airborne
particulate
matter, insects and gases not of interest.
A reference source can be carried by the housing adj acent the glass tube
for purposes of establishing a reference signal. A ratio can be formed of the
sensed
and reference output signals.
In yet another aspect, instead of a glass chamber, a closed radiant
energy transparent plastic chamber can be used as a container of a suitable
gas of
interest. It will also be understood that optical filters can be located adj
acent to the
sensing source and to improve detector performance.
A multisensor detector combines a smoke signal, from a smoke sensor,
with gas dynamic profile signal, from a gas sensor. In one embodiment, a
carbon
dioxide sensor, such as a photoacoustic-type, generates an output signal. A
degree
of threat parameter can be derived from the gas output signal and combined
with
the signal from the smoke sensor to make a fire determination. In another
aspect,
the signal from the gas sensor can also be combined with the smoke signal and
its
rate of change.
A combination detector includes a common sensing region with a
housing. A smoke sensor, photo or ion-type shares the sensing region with a
single
CA 02323675 2000-10-17
- 10-
sealed gas sensor of a type described above. Signals from the sensors can be
processed locally, remotely or both locally and remotely. The configuration
and
arrangement of the sealed gas cell can be consistent with the form factor of
the
detector's housing.
Numerous other advantages and features of the present invention will
become readily apparent from the following detailed description of the
invention
and the embodiments thereof, from the claims and from the accompanying
drawings.
Brief Description of the Drawings:
Fig. 1 is a graph illustrating an infrared absorption spectrum;
Fig. 2 is a diagram of a prior art non-dispersive infrared gas detector;
Fig. 3 is a diagram of a prior art photoacoustic gas detector;
Fig. 4 is a diagram of a prior art single chamber, open cell,
photoacoustic detector;
Fig. 5 is a diagram of a prior art dual chamber, open cell, photoacoustic
detector;
Fig. 6 is a side sectional view of a closed cell photoacoustic gas detector
in accordance with the present invention;
Fig. 7A is a perspective view of a portion of a housing for a gas
detector of the type illustrated in Fig. 6;
Fig. 7B is a perspective view of another embodiment of the detector of
Fig. 6;
Fig. 8 is a block diagram of another detector in accordance with the
present invention;
Figs. 9A, 9B illustrate different packaging configurations for the
detector of Fig. 8; and
Figs. l0A lOB illustrate different gas detector form factors for
the detector of Fig. 8.
CA 02323675 2000-10-17
-11-
Detailed Description of the Preferred Embodiments:
While this invention is susceptible of embodiment in many different
forms, there are shown in the drawing and will be described herein in detail
specific
embodiments thereof with the understanding that the present disclosure is to
be
considered as an exemplification of the principles of the invention and is not
intended to limit the invention to the specific embodiments illustrated.
Figs. 6 and 7A and B illustrate a photoacoustic sensor in accordance
herewith. A closed glass chamber 11-1, for example a test tube, made from soft
glass that also has good optical and resonant properties when operated at 4.3
microns contains a quantity of a gas to be sensed. The glass is selected to be
sufficiently transparent at the wavelengths, of interest and capable of
containingthe
selected gas, as discussed below.
Alternately, an appropriate plastic could be used instead of glass
without departing from the spirit and scope hereof. In addition, the closed
chamber
can be formed in a variety of shapes.
An electret condenser microphone 11-2 is epoxy sealed or integrated
11-4 into the open end of the tube 11-1 under an atmosphere or more of the gas
to
be detected. Once sealed, the tube 11-1 forms the basis of a one chamber gas
photoacoustic detector. If the tube contains carbon dioxide, that will be the
sensed
gas.
The sample chamber 11-5 enclosing the test tube is shaped with an
ellipsoid profile to maximize light ray travel 11-7 between a sensing source
11-8,
filter 11-10 and the tube 11-2 to increase sensitivity. 'The flash frequency
of source
11-7 is 2.8 Hz, which is the preferred operating frequency for the illustrated
configuration. Those of skill in the art will understand that other
frequencies might
be preferable with other configurations.
With reference to Fig. 7, sample chamber 11-5 can be implemented
with two substantially identical molded top [11-A (not shown)] and bottom 11-B
CA 02323675 2000-10-17
-12-
housing elements that can be sonically welded or heat staked together to form
the
closed sensing volume 11-SC.
When the lower sample chamber section 11-SB is mated to the top half
11-SA outside air will flow into the chamber 11-SC via ports 11-12A and 11-
12B.
A second light source 12-1 and filter assembly 12-2 are positioned adjacent to
the
base of the sensor tube 11-1. This light source 12-1 provides a reference
function
for ratiometric operation.
Sources 11-8 and 12-1 are time division multiplexed with a 10 second
period. Source 11-8 is ON for 10 seconds flashing at 2.8 Hz, source
12-1 is energized, then source 12-1 and so forth. Other periods could be used.
Those of skill will understand that a variety of radiant energy sources
could be used without departing from the spirit and scope of the present
invention.
These include solid state sources, laser diodes or light emitting diodes,
incandescent sources or gas discharge sources. The details of the sources) are
not
a limitation of the present invention.
The closed chamber design is also extremely effective in isolating
external sounds from the microphone. Fundamental vibrations mechanically
transmitted into the assembly can be filtered out by the electronics and
supporting
software. Using this approach, only one microphone and one sampling chamber
are required for ratiometric operation. This structure substantially reduces
design
complexity and associated costs.
With carbon dioxide sealed inside the detection chamber 11-1,
(assuming that to be the gas to be sensed), the gas sample flow rate into the
chamber 11-Sc can be very fast or very slow. It can be optimized to match an
application. It is not necessary to delay the pressure decay in the sampling
chamber
(as required with "open-cell") to permit microphone detection.
The sealed detection chamber 11-1 can also be filled with a blend of
two or more gases for multigas response. In such an application, the reference
CA 02323675 2000-10-17
-13-
signal serves as a common signal for calculating concentrations. Another
sensing
source 11-8a and filter 11-l0a are required for each additional gas. The low
profile
ellipsoid chamber 11-5 accommodates numerous light sources (numerous gases)
using time division multiplexed sensing.
S The "closed-cell" design produces excellent signal amplitudes. This
means less complexity for support electronics to process the signal. "Closed-
cell"
photoacoustic detection permits the use of wide tolerance filters (11-10)
because
the target gas inside the closed chamber (e. g., carbon dioxide) is a perfect
filter for
the gas of interest (also carbon dioxide). This result is due to perfect
spectrum
matching in absorption profiles made possible only by using the target gas to
sense
itself thereby achieving maximum rejection of other gas species.
Open-cell designs do not have this advantage because interference
gases (such as water in the sample chamber) can easily be excited if filter
parameters are not narrow and tightly controlled. The tighter the
specifications of
the interference filter, however, the higher is the cost. A wide band filter
costs, for
example, $0.20. A tight narrow bandpass filter is $3.00 or more.
In summary, the present photoacoustic sensor eliminates costly
components yet results in a high sensitivity gas detector. It retains many of
the
benefits of a photoacoustic dual beam instrument. It preserves the power of
ratiometric signal processing and removes traditional response-speed
limitations
common to other sensors.
Detector 10 has enough sensitivity to sense weak absorbing gases and
can detect several gases in one instrument with no moving parts. Power
requirements can be as low as 100uA average (24VDC) to support applications in
fire and smoke detection.
The tapered optical chamber design features an in-situ photoacoustic
tube that integrates high signal to noise performance into a low profile
package.
The photoacoustic tube is also an excellent omni-directional heat-radiation
CA 02323675 2000-10-17
- 14-
detector. As a thermal sensor, it can easily detect the high frequency flicker
signature of fires from a distance.
The detector 10 can be used in combination with fire detectors such as
ionization-type, a photoelectric-type smoke detects in the same housing or in
displaced housings.
The gas and smoke signals can be processed to establish the presence
of an alarm condition such as a fire. One form of processing has been
disclosed
and claimed in Tice U.S. Patent application filed April 19 1999, entitled,
System
and Method ofAdjusting Smoothing, Ser. No. 09/294,932, assigned to the
assignee
hereof and incorporated by reference. Other processing can be used without
departing form the spirit and scope of the present invention.
Outputs from a smoke sensor could be combined with a rate of change
output from the gas sensor. The gas sensor output can also be incorporated
into the
housing.
Fig. 7B is an illustration of an embodiment of a detector 10' in
accordance with the detector of Fig. 6. The various components of the detector
10'
carry the designated identification numerals. Radiant energy source 7-1
provides
sampling pulses. Radiant energy source 7-2 provides reference pulses. Detector
10' includes carbon monoxide filters 7-18 to exclude frequencies not highly
absorbed by carbon monoxide.
Fig. 8 is the block diagram of a multiple sensor detector 80 in
accordance herewith. Detector 80 includes a photoacoustic gas sensor,
comparable
to the sensor 10 along with one or more smoke sensors) 80-1. The sensors) 80-1
could be implemented using a variety of known sensing technologies including
photoelectric, ionization-type orprojectedbeam sensing. As illustrated,
sensors 10
and 80-1 are carried by a common housing 82.
Detector 80 further includes control circuitry 80-2 coupled to sensors
10 and 80-1. Dual sensor processing of the type noted above could be performed
CA 02323675 2000-10-17
-15-
using circuitry 80-2 local to the sensors. Alternately, circuitry 80-2 via
input/output circuitry 80-3 could transmit one or more sensor related values
via
either a hardwired or wireless medium to other electrical units in a
respective
system or to a common processing element for further signal processing.
It will also be understood that a plurality of detectors, such as detector
80, could be incorporated into such a system. Such detectors could communicate
directly with one another via the medium. Alternately or in addition to the
system
can incorporate a common control element for carrying out some or all of the
signal
processing.
In apreferred embodiment, control circuitry 80-2 could be implemented
using a programmed processor and executable instructions stored at detector
80.
It will also be understood that the details of the input/output circuitry 80-3
required
for bi-directional communication by the medium are not a limitation of the
present
invention. In yet another embodiment, sensors 10 and 80-1 could be displaced
1 S from one another in separate housings.
Figs. 9A and 9B illustrate two different packaging configurations
wherein a gas detector of the general type illustrated in Fig. 6 can be
incorporated
into a smoke detector having a photoelectric, or an ionization-type sensor or
both.
With reference to Fig. 9A, housing 9A-10 carries the various sensors.
The housing 9A-10 defines an internal region 9A-12 wherein can be carried one
or
more smoke sensors 9A-14.
In addition to the smoke sensor or sensors, housing 9A-10 carries a
radiant energy or light source 9A-16 which projects reflected rays 9A-1 and
direct
rays 9A-2 across a reflective sampling chamber 9A-18 located between source 9A-
16 and a closed sensing tube 9A-20. Housing 9A-10 also defines a plurality of
slots or openings 9A-22 whereupon airborne gases and smoke can flow into and
out
of chambers 9A-14 and 9A-18. In the process of the in-flow, airborne gases
will
absorb selected frequencies of the pulses of light from source 9A-16 for
purposes
CA 02323675 2000-10-17
-16-
of detecting a respective gas, as described above with respect to detector 10
of Fig.
6. In addition, the airborne particulate matter can be sensed by one or more
smoke
detectors in chamber 9A-14.
Detector 9A can also include temperature sensors such as thermistors.
Control circuitry, indicated at 80-2, coupled to the sensors, can be carried
by
housing 9A-10.
The detector 9A could incorporate one or more smoke sensors 9A-14
such as a photoelectric-type sensor, an ionization-type sensor or both in
combination with the gas detector. It will be understood that the exact
configuration of the smoke sensors in the housing 9A-10 is not a limitation of
the
present invention.
Fig. 9B illustrates an alternate configuration of a detector 9B which
includes a housing 9B-1. The housing 9B-1 can be removably attached to a
surface
mounted base 9B-2 and could carry therein one or more smoke sensors.
The housing 9B-1 includes an upper region which incorporates a
plurality of openings 9B-3 to provide for the ingress and egress of airborne
particulate matter, typically smoke, and gases. The members 9B-4 of a
plurality
of radiant energy sources are also positioned in common sensing region 9B-1'.
A photoacoustic tube 9B-5 is centrally located relative to the radiant
energy sources 9B-4. The gas sensor of detector 9B operates in accordance with
the previously discussed principles of detector 10 of Fig. 6. An electronics
packaging 9B-6 can be carried adjacent to the sensing region 9B-1'. An upper
section 9B-7 of the housing 9B-1 can be coated with an internal reflective
surface
such as chrome, or its equivalent to provide for reflection of radiant energy
pulses
from sources 9B-4 into the sensing tube 9B -5.
Detector 9B can also incorporate a plurality of thermistors 9B-8 which
provide a temperature sensing function in addition to the smoke and gas
sensing
function. It will be understood that neither the exact configuration of the
smoke
CA 02323675 2000-10-17
- 17-
sensors nor thermistors 9B-8 are a limitation so the present invention.
Figs. l0A and lOB illustrate alternate form factors for gas detectors
such as the detector 10 of Fig. 6. Configuration l0A illustrates photoacoustic
tube
1 OA-1 oriented perpendicularly to a gas sampling chamber 1 OA-2. Chamber 1 OA-
2
includes a plurality of openings indicated generally at l0A-3 for ingress and
egress
of airborne gases which in turn can be sensed with detector 10A.
A sampling source of radiant energy l0A-4 is located at one end of the
sampling chamber l0A-2. Photoacoustic tube l0A-1, as discussed previously, is
a sealed container of the gas to be sensed which includes at one end a
microphone
and related circuitry l0A-5 as discussed previously.
It will be understood that configuration 1 OA could be incorporated into
a circular or cylindrical smoke detector housing which might include a
sampling
chamber l0A-6 for one or more photoelectric, ionization-type or obscuration-
type
smoke detectors. Thermodetectors could also be incorporated into the detector
10A.
Fig. lOB illustrates an alternate configuration of a detector IOB.
Components which are common to detector l0A carry corresponding identification
numerals and were discussed previously.
The above described gas sensors exhibit a variety of performance
advantages. This sealed cell design permits the use of relatively short
radiant
energy pulses to stimulate the sensor and enable the sensing process. Such
radiant
energy pulses tend to be fully absorbed when inj ected into the sealed
photoacoustic,
gas carrying tube. This will produce a highly reproducible pressure wave with
little
or no response time delays. In contradistinction, open cell designs must
balance
response time with the pulse rate of the radiant energy source along with the
sensor's leak rate to achieve a proper response or signal to noise ratio.
Since the gas sensor or the present application is responsive to short
radiant energy pulses, strobe light sources such as Xenon flash tubes, pulsed
light
CA 02323675 2000-10-17
L ,
-18-
emitting diodes or pulsed incandescent bulbs can be used. The advantage of
being
able to pulse the source of radiant energy is that it results in low average
power
consumption. For example, a radiant energy pulse having a duration of ten
milliseconds and requiring a peak current on the order of 70mA, if energized
once
every five seconds, has a resultant average current consumption of the order
of
140uA.
For C02 monitoring, one sample every 10 seconds is sufficient (70uA
average). If suspicious fire activity is sensed, the sampling rate can
increase to
once every 5 seconds or faster. More data per unit time allows for better
detection
sensitivity. Using this energy management technique, average power is greatly
reduced.
The jump into higher sampling rates can be based on the signal profile
of the associated smoke sensor(s). Profiles include, amplitudes, rate of
change of
slope and profile trend shifts.
The sealed tube design permits a high degree of miniaturization. For
gases such as C02, strong absorption requires only about 1 cm of travel inside
the
tube (filled with pure C02) to produce strong signals. This permits both tube
and
air sampling chambers to take on many forms.
In a smoke detector, a 2 inch path length can be used. The path can be
arranged in a shallow "V" or "T" configuration to minimize product size, see
Figs.
10A, B for example.
A SOmm long, lOmm diameter photoacoustic tube can be selected for
C02 is based on convenience and cost. This is a readily available glass tube
used
in other industries. Other shapes for photoacoustic cells, however, can be
pursued
if applications demand custom designs.
Detectors in accordance with the present invention exhibit very fast
response to detected gases, for example, carbon dioxide, yet at the same time
exhibit small size, low cost and require low power levels. The fast response
time
CA 02323675 2000-10-17
- 19-
of the present sensor overcomes deficiencies of some of the known gas sensors
which have a slower response rate. The higher response rate of the present
sensor
is particularly beneficial when combined with profile processing. More
particularly where the gas of interest is carbon dioxide, carbon dioxide
predictive
profiles can be used to improve the reliability of detection of actual fires
as
opposed to conditions which might represent false alarms.
A profile, a relatively short snap shot, over time can be used to estimate
the predictive value of carbon dioxide concentration patterns. A profile which
results from a mufti-factor analysis correlates gas amplitude levels, long and
short
term slopes, and frequency content. As a result, it functions like a
predictive
barometer for a fire threat. It provides a strong confirmation value to the
output
signal of a photoelectric smoke sensor.
For example, using such predictive processing, profiles with strong
carbon dioxide patterns can be characterized into fires or false alarms with
greater
confidence levels than is the case with known processing. This is a result of
the
result of the rapid response characteristics of gas sensors in accordance with
the
present invention. Such sensors have the capability of accurately attracting
large
changes in gas concentration as rapidly as the concentration changes.
The threat factor within a carbon dioxide profile can be used to adjust,
or throttle the alarm impact of additional data. Hence, a carbon dioxide
sensor in
accordance with the present invention which exhibits a fast response
characteristics
makes it possible to closely track rapid changes of the gas to improve
discerning
between nuisance conditions and real fires.
From the foregoing, it will be observed that numerous variations and
modifications may be effected without departing from the spirit and scope of
the
invention. It is to be understood that no limitation with respect to the
specific
apparatus illustrated herein is intended or should be inferred. It is, of
course,
intended to cover by the appended claims all such modifications as fall within
the
-20-
scope of the claims.
