Note: Descriptions are shown in the official language in which they were submitted.
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SENSOR FOR Dh`TECT[NG T~IE_EXFIAU_l'lON
OF AN ~DS)RB NT BED
The present invention relates to a sensor for use
in detecting the exhaus-tion of an adsorbent bed and to
adsorbent beds employing such sensors.
Filters and respirators worn over -the face are used
as protection against toxic vapors in many occupations.
At present there is no fast and accwrate way to
determine -the sta-tus o~ a wsed or par-tially used
respirator device. Proposed approaches -to estimating
-the useful life have incl-uded accura-te logging of use
time, periodic breakthrough testing, and color change
indicators. These methods merely estimate the status of
the respirator device and, as is the case with the
breakthrough testing, often result in the exhaustion of
-the filter thereby making the respirator useless. The
ideal solution to this problem consists of a fil-ter
canister which incorporates an indicator or alarm which
si~nals the end of the respirator's useful lifetime.
Ideally, a sensor for detecting the exhaus-tiorl of
an adsorbent bed in a respirator should detect all
vapors the adsorbent is designed to remove. Further,
the sensor is required to operate over -the entire range
of temperatures and pressures which are normaJI~
encountered in adsorben-t use. This means tha-t the
sensor must be capa~le cf operat:irlg in temperatures rom
-65 to 110F and pressures from n.8 to 1.2 atmospheres
for personal protection applications and can be desigrled
for severe pressure, vacuum, humidity
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and temperature conditions present in industrial and
military use. The sensor must also be capable of
enduring all conditions of use for an adaorbent such a~
being attitude in~ensitive, shock and vibration
resistant, storage stable for a~ long as the adsorbent is
stored and the sensor must also last as long in use as
the adsorbent. Finally, the sensor must have a response
time which gives the user suf~ieient warning o~ adsorbPnt
bed exhaustion.
One approach to this problem is to detect the
presence of all possible toxic gases that could emanate
from the adsorbent by using a sensing device which is
sensitive to all the toxic gases which the adsorbent bed
is designed to adsorb. An example of this method can be
seen in U.S. Patent No. 3,902,485 ~Wallace) issued on
September 2, 1975. In this method, spaced electrodes, at
least one of which is coated with a basic nitrogen-
containing polymer of high electrical resistance, project
into an electrical conducting medium such as activated
charcoal in a container. The coated electrode is
connected in series with signalling means which puts out
an audio and~or visual signal. The coating on the
electrode forms an electrically conducting quaternary
ammonium æalt in the presence of selected and
predetermined toxic gases to thereby lower the electr.ical
resistance o~ the polymer coating and aomplete the
eleatrical circuit b0tween electrodee through the
charcoal. Thia activate~ the aignalling means to
generate an alarm signal.
Sensors deaigned for detecting the presence of toxic
gases ~or use in combination with conventional gas filter
breathing apparatus suffer from several drawbacks.
First, these sensors are generally
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relatively ~xpensive when comparsd with the cost o~ the
adsorbent bed. Second, the typical sensors are only
sensitive to a few or several o~ the potentially toxic
gases, i.e. the sensors are somewhak selective in their
response to gases and vapors. Further, sensors detecting
gases in the adsorbent bed output must detect everything
and anything that comes through the bed. This is a
difficult problem since it is nearly impossible to
predict what toxic ga~es the respirator adsorbent and the
user of a respirator may be exposed to. This makes
design of sensors capable of detecting everythin~ that
comes through the bed very difficult and virtually
impossible. In addition, sensors detecting the presence
of toxic gases in the adsorbent bed will have
significantly different reactivity than the adsorbent bed
itself for the same gas or vapor. This will often cause
premature signalling of adsorbent bed exhaustion or, even
more dangerous, the alarm signal will bP generated too
late and toxic gases will pass through the adsorbent bed
to the user.
Accordingly, there is a need in the art for an
improved sensor device which can be used in combination
with an adsorbent bed material to provide a reaL-time
warning of the exhaustion o~ the ad~orbent bed and
thereby prevent human exposure to harm~ul vapors. In
addition to having the appropriate analytiaal re~ponse
described above, the sensor mu~t also be low-cost, low-
power, tiny, stable, rugged and completely reliable.
The present invention relates to a sensor u~e~ul in
detecting the exhaustion of an adsorbent bed. The
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sensor includes a vapor sensitive medium having a
response to the vapors ad orbed by the adsorbent bed
which is sub~tantially the same as the response o the
adsorbent to the vapors adsorbed. ~he sensor also
includes a means for monitoring a property of the vapor
sensitive medium which is related to the response of the
vapor sensitive medium to the vapors adsorbed by the
adsorbent.
In a second embodiment, the present invention
relates to an adsorbent bed safety alarm system for
detecting and signalling the exhaustion of an adsorbent
bed. The alarm system includes a sensor means and a
means for generating an alarm signal. The sensor means
includes a vapor sensitive medium having a response to
the vapors adsorbed by the adsorbent bed which is
substantially the same as the response of the adsorbent
to the vapor~ being adsorbed. The sensor means also
includes a means for monitoring a property of the vapor
sensitive medium that i5 related to the response o~ the
vapor sensitive medium to the vapors adsorbed by the
adsorbent. The means for yenerating an alarm signal is
responsive to a change in the property monitored by the
means for monitoring.
In a third embodimentl the present invention relates
to an apparatus for use in adsorbinc~ harm~ul or
undesirable vapors that signals the exhau~tion of the
adsorbent material to prevent ~low oE harm~ul or
undesirable vapors through the apparatus. The apparatus
inaludes a housing having an inlet means and outlet
means. It also has an adsorbent bed housed within the
housing. Located in the adsorbent bed is a sensor means.
The sensor means includes a vapor sensitive medium having
a response to the vapors adsorbed by the adsorbent bed
which is substantially
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the same as the response of the adsorbent to the vapors
bein~ adsor~ed. The ~ensor means also inaludes a means
for monitoring a property of the vapor sensitive medium
which is related to this r~sponse of the adsorhent.
Finally, the apparatus includes a means for generating an
alarm signal. This means for generating an alarrn signal
is responsive to a change in the property of the vapor
sensitive material monitored by the monitoring means.
It is the primary object of the present invention to
provide a low-power, low-cost, reliable sensor that can
be incorporated into respirator devices to provide a
real-time warning of the exhaustion of the adsorbent bed.
It is a further object of the present invention to
provide a sensor for use in adsorbent beds having
sub~tantially the same response of the adsorbent material
to the vapors being adsorbed. This means the sensor
responds to substantially all of the vapors that the
adsorbent is designed to remove.
It is a still further object of the present
invention to provide a sensor for use in an adsorbent bed
alarm system which minimizes the number of false alaxms
and non-occurrence of alarms which should have occurred.
It i9 a still further object of the present
invention to provide a sensor for use in an adsorbent bed
alarm syskem which responds to only the materials which
the adsoxbent bed is designed to adsorh.
The~e and other objects of the preserlt invention
will be apparent to one of ordinary skill in the axt ~rom
the detailed description o~ the invention which
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follows. While the descriptions reEer to a personal
protective device and a carbonaceous adsorbent, the
principle can be easily applied to fixed-bed adsorbent
for purifying and deodorizing air and other ad~orbents
chromatographic stationary phase~.
In the drawings:
FIG. 1 is a front view in perspective of a man
wearing a chemical filter breathing apparatus with alarm
system in accordance with the present invention.
FIG. 2 is a cross-sectional view of sensor in
accordance with the present invention having inter-
digitated electrodes and an adsorbent coating thereon.
FIG. 3 is a plan view of an alternate embodiment of
a sensor in accordance with the present invention,
including circuitry.
FIG. 4 is a cross-sectional view of a filter
cartridge outfitted with an alarm system in accordance
with the present invention.
FIG. 5a is a graph of a breakthrough test for
benzene with the sensor located at a depth of 25% from
the inlet of the ad~orbent bed.
FIG. 5b i8 a graph of a breakthrough test for
benæene with the sensor located at a depth of 50% from
the inlet of the ad~orbent bed.
FIG. 5c i6 a graph of a breakthrough test for
benzene with the een~or located at a depth of 75% from
the inlek of the ad~orbent bed~
FIG. 6a is a graph o~ a breakthrough te~t ~or
benzene with the sensor located at a depth of 25% from
the inlet of a humidified adsorhent bed.
FIG. 6b iæ a graph o~ a breakthrough test for
benzene with the sensor located at a depth of 75% from
the inlet of a humidified adsorbent bed.
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The chemical filter breathing apparatus portion of
Fig. 1 is of conventional chin type such as type GMP
produced by Mine Safety Appliances Company of Pittsburg,
Pennsylvania. Beaause of it~ limited capacity, thi~
device is recommended by Mine Safety Appliances Company
for respiratory protection against toxic gases and vapors
in concentrations not in excess of 0.5~ by volume. This
breathing apparatus is illustrat~d in conjunction with an
alarm system in accordance with the present invention.
It includes a filter cartridge 11 consisting of an oval-
shaped body 12, bottom closure wall 13 and top closure
wall 14 which are provided for closing the open lower and
upper ends of body 12. sottom closure wall 13 is
provided with a screen opening 16 which serves as the
inlet to filter cartridge 11 and is sealable when filter
cartridge 11 is not in use. In a conventional breathing
apparatus, a filter (not shown) is mounted in the bottom
of filter cartridge 11 for filtering particulate material
such as toxic dust and the like. A pipe 17 provides gas
communication between an opening in top wall 14 and face
mask 18 and serves as the outlet of filter cartridge 11.
Body 12 defines an open gas passageway through filter
cartridge 11 80 that the respiratory tract of the wearer
is in communication wikh air from the environment after
filtering through filter cartrldge 11. Body 12 aleo
includes an alarm housing 15 located thereon whiah house~
the circuitxy for the alarm sy~tem.
Referring now to Fig. 2 there is shown a vapor
~ensor 20 which includes a substrate 21, a first
electrode 22 and a second electrode 23. Coated over
electrodes 22 and 23 is a vapor sensitive medium 24~
The first electrode 22 and ~econd electrode Z3 are
interdigitated in this embodiment. Interdigitization of
the electrodes is the preferred electrode configuration.
Referring now to Fig. 3 there is ~hown an alarm
system in accordance with the present invention. The
alarm system includes a sensor portion 30 and circuitry
31~ Sensor portion 30 is made up of a substrate 32
having on its surface a first electrode 33, a first
reference electrode 35, a second electrode 34 and a
second reference electrode 36. Also on the surface of
substrate 32 i5 a passive coating 37 and an active
coating 38 which are both made up of vapor sensitive
material. The passive coating 37 is in contact with, and
adheres to the second electrode 34 and the second
reference electrode 36. The active coating 38 is in
contact with, and adheres to the first electrode 33 and
the first re~erence electrode 35. In this embodiment,
the electrodes 33, 34 and the reference electrodes 35, 36
are identical and can be interchanged appropriately.
The circuitry 31 includes a lithium battery 40, a
first current limiting resistor 41 and a second current
limiting re~istor 42. The first terminal 43 of the
lithium batter~ 40 i9 connected by lines 44 and 45 to
first electrode 33 and seaond electrode 34 re~pectively.
The second terminal 46 of the lithium battery 40 :i~
connected ko ~irst resistor 41 and second re~istor 42 by
line~ 47 and 48 respectively. Wire 49 ~onnect~ first
resistor 41 to first reference electrode 35 and wire 50
connects second resistor 42 to second reference electrode
36. Wire 49 is al~o connected at junction 51 to
connection 53 which leads
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to LCD alarm 55. Wire 50 is also connected at junction
52 by connection 54 which leads to LCD alarm 55.
ConnPctions may also be made to the other half o~ each o~
the electrodes 33, 34, 35, 36 sin~e they are also
resistive vapor-sensitive elements and they can be used
in more elaborate circuitry as redundant measurement~
that add reliability, stability a~d dynamic range to the
device. The LCD alarm contains all of the required
components to signal an imbalance or change in the
sensor/bridge circuit. Of course, this is only one
example of the many ways that can be chosen to measure
and display the changes that occur in the vapor-sensitive
element. Any circuit that can make the appropriate
electrical measurement and satisfy the field requirement
can be used with the sensor element.
Referrirlg to Fig. 4 there is shown filter cartridge
11 having a body 12, a bottom closure wall 13, a top
closure wall 14, alarm housing 15, screen opening 16, and
pipe 17. Inside the body 12 is a filter 19 made up of an
adsorbent material sllch as charcoal. Shown inside the
alarm housing 15 is circuitry 31 which is connected to
sensor portion 30 by lines 49 and 50. Sensor portion 3n
is located at a point closer to the pipe 17 (outlet) of
the filter cartridge 11 than to the ~creen opening 16
(inlet) in this figure.
In the simplest embodiment of the pre6ent lnventio~
the upper sensor 20 ~hown in Fig. 2 includes a substrate
21 which may be fabricated from an inert material. On
the surface of the substrate 21 are a ~irst electrode 22
and a second electrode 23 which are preferably
interdigitated. These electrodes may be fabricated from
any suitable conductive material of
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which gold, platinum, silver, or carbon are examples of
preferred materials. Electrodes 22, 23 are formed on the
surface o~ substrate 21 by any conventional mekhod such
as vapor deposition. It is desirable to place electrodes
22, 23 close together and to provide long electrode edges
in order to minimize effects due to the non-uniformity of
the coating of vapor sensitive medium 24 on electrodes
~2, 23.
Vapor sensitive medium 24 preXerably covexs the
entire surface of both first electrode 22 and second
electrode 23. Vapor sensitive medium 24 is selected such
that it has a response to the vapors being adsorbed by
the filter 19 which is substantially the same as the
response of the adsorbent in the filter 19 to the vapors
being adsorbed, The most preferred embodiment of the
present invention employs a vapor sensitive medium 24
which is the same material as that used as the adsorbent
in the filter 19, In this manner, it is assured that the
response of the vapor sensitive medium 24 to the vapoxs
is the same as the response of the ad~orbent to vapors.
It is not necessary that the vapor sensitive medium 24 be
of typical electrolytic materials. Ratherl the important
factor is that the vapor sensitive medium 24 exhibits a
ahange in resistance or other measurable property upon
contact with the vapors which the adsorbent is designed
to adsorb. Thus, some examples of adsorbents whiah are
u~e~ul as tha vapor ~ensitive medium 24 in the present
invention axe silica, silica gel, alumina, a molecular
sieve, drying agents and the like. The vapor sensitive
medium 24 i~ generally from about 5.0 x 10-~ am to about
0.1 cm in thickness and preferably from 5.0 x 10-6 cm to
1.0 x lo-2 cm
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thick. ~owever, -the vapor sen~itive medium 24 may he
thicker or thinner depending upon the sensitivlty and
responge time desired of the sensor.
It i~ particularly important in the present
invention to obtain a response to the vapors being
adsorbed which is substantially the same as the response
of the adsorbent in the ~ilter 19 to the vapors heing
adsorbed. By substantially the same response it is meant
that the vapor sensitive medium 24 will exhibit a change
in a measurable property, such as resistance, impedance,
capacitance, weight, temperature, photo-propertie~, heat
flow, piezoelectricity, pyroelectricity or a measurable
property that can be easily sensed by the sensor (vapor-
sensitive device) when it is exposed to the same
conditions under which adsorption of vapors by the
adsorbent will occur. The change in the measurable
property must be, in some way, proportional to the amount
of adsorption by the adsorbent under those conditions.
In the simplest case the same material is used as both
adsorbent and vapor sensitive medium 24. Upon exposure
to vapors under equivalent conditions both the adsorbent
and vapor sensiti~e medium 24 will adsorb the vapors at
the same rate and thus hoth exhibit the same response~
namely a response to the vapors that i~ proportional to
the extent of ad~orption. Then it i~ a simple matter to
measure a property o~ the vapor ~ensitive medium 24 which
change~ upon vapor adsorption and thence to calibr~te khe
ahange~ in the mea~ured property with variou~ vapor
concentration~ to obtain a reliable indicator of how much
adsorption is occurring at the location o~ the vapor
sensitive medium 24 in the adsorbent bed.
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This, in combinakion with other ~aators discus3ed below,
can be used to detPrmine the threshold level (amount of
change in the measurable property~ at which the alarm
will be triggered by the sensor.
The present invention takes a different approach
than the prior art to detexmine the poînt of exhaustion
of the adsorbent material in an adsorbent bed. In fact,
because the present invention contemplates u~ing the
adsorbent material as the vapor sensing material, the
present invention detects the state of the adsorbent
surface instead of the individual gases or gas mixtures
coming through the bed, as is done in the prior art. For
example, a vapor sensitive medium 24 of charcoal can be
employed inside a charcoal adsorbent bedO As long as the
charcoal adsorbent surface is active, vapor sensitive
medium 24 in the bed remains substantially unaffected by
the vapors because they do not reach the embedded sensor
since the vapors are adsorbed by the fresh adsorbent. As
exhaustion of the charcoal adsorbent bed approaches,
vapor contaminants will reach the surface of he vapor
sensitive medium 24 and vapor sensitive medium 24 will
become contaminated. This contamination by the vapor
proceeds in the same manner as vapor adsorption by the
charcoal adsorbent bed. This design requires that, those
vapors that ad~orb and contaminate the adsorbent will
adsorb and contaminate the vapor sensitive medium 24 r
and, most importantly, gase~ that do not ad~orb (CEl4 on
charcoal) in the adsorbent do not contaminate the vapor
~ensitive medium 24. Thus, the sensor detects the "state
o~ the adsorbent". The adsorption of vapor by vapor
sensitive medium 24 causes a change in the
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properties of vapor sengitive medium 24 on khe vapo.r
sensor 20 which is analogous to the change in prop~rties
in the adsorbent resulting from vapor adsorption. Thi~
change in properties is detected by the circuitry 31.
The circuitry 31 is preEerably fabricated to detect a
change in a single, easily detected property of vapor
sensitive medium 24 which property is altered as a
~unction of vapor adsorption by vapor sensitive medium
24. Once the change in the detected property exceeds a
preset threshold level it sets off an alarm which
indicates the approaching exhaustion of the adsorbent
bed.
Two extremely important advantages are obtained by
employing a vapor sensitive medium 24 which exhibits
substantially the same response to the vapors as the
adsorbent used in the adsorbent bed. Fir t, the vapor
sensitive medium 24 will have a low cost relative to the
adsorbent bed (often its cost is the same as the
adsorbent). Thus, the addition of an alarm system in
accordance with the present invention will not make
respiratory filter cartridges pxohibitively expensive or
large. In comparison, the use of standard ~as sensors
known in the art as the vapor sen~or will generally add
more than $200 to the cost o~ re~piratory filter
cartridge thus makiny the cartridge prohibitively
~xpensive. Size i5 also important and the device of the
present invention can be ~ery small and on the order o~
micron-siæed, iE required.
The second advankage is extremely important. The
use o~ a vapor ~en~itive medium 24 whlah is the same
material as the adsorbent o~ the adsorbent bed or whiah
exhibits substantially the same response to the vapors as
the adsorbent ~ed will ensure the same
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response to each and every vapor present in the filter
cartridcle for -the vapor sensitive medium 24 as ~or the
aclsorben-t becl. Accorclingly, the vapor s~ensor 20 will
only responcl to the vapors that t:he adsorben-t becl
adsorbs. Thus, the vapor sensor 20 does not yive false
alarms as a result of exposure to vapors not interac-tive
with the adsor~ent. In acldition, the vapor sensor 20
will respond to all vapors which are active wi-th -the
adsorbent. This makes the vapor sensor 20 of the
present invention highly reliable for detec-ting the
exhaus-tion oE an adsorbent bed. Reliability is an
important feature of a sensor of this type since a
sensor failure may cause human exposure to toxic vapors.
The coating of vapor sensitive medium 24 rnay be
prepared by dissolving a silicone rubber adhesive in
methylene chloride and suspending the vapor sensitive
material in this solution. The suspension is then
applied to the surface of the substrate 21, first
electrode 22 and second elec-trode 23 using a proprie-tary
spin-coating technique for the preparation of micro-
sensors which is disclosed in United States Pa-tent
4,795,54~. Thick film coatings and carbonaceous
coatings are also feasible by processes such as silk
screening, sputtering, painting and chemical or physica:l
deposi-tion processes.
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Referring now to the alternate embodiment shown in
Fig. 3, which is the preferred embodiment of the present
inventiion,it has been determined that the ideal sen~or
is a relatively high impedance device having four
electrode~. This consists of two pair~ of two
electrodes. One pair oF electrodes is an active sensor
area and the other is passivated and provides temperature
compensation for the sensor device if required by the
application. More particularly, all four electrodes 33,
34, 35, 36 may be substantially identic~l. Electrodes 33
and 35 are covered with an active coating 38 o~ vapor
sensitive medium 24. The active coating is preferably a
material having a response to the vapors being adsorbed
which os substantially imilar to the response of the
adsorbent bed to the vapors being adsorbed. Electrodes
34 and 36 are coated with a passive coating 37 which
responds identically to active coating 38 except that it
does not respond to the vapor being adsorbed by the
adsorbent bed. ~owever, pas~ive coating 37 respond~ to
temperature variations in a manner which i~ substantially
identical to the response of active coating 38 o
temperature variations. In this manner, a convenient
method of signal correction for temperature effects i~
obtained. A~ a result, a ~en~or o~ this typ0 is aapable
of operating o~er a large temperature range. ~ecause it
is similar in performance to the ad~orbent, it ~an be
used over any temperature or pre~sure (or other
condition~ euah as relative humidit~f eta.) range a~ khe
adsorbent it~elf. The particular geometry shown in ~ig.
3 also enhances the thermal ~tability of the sensor.
Referring now to circuitry 31 shown in Fig. ~ it
in~ludes first current limiting resistor 41 and second
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current limiting resistor 42 having resistances of R1 and
R2 respectively. Passive coatin~ 37 and active aoa~ing
38 have resistances Rp and Ra respectively. Analysis of
circuitry 31 shows that as long as the sum of resistance~
Rl + Ra does not change with respect to the sum o~
re~istances R2 + Rb then the signal to LED alarm 55 will
remain constant. However~ when the sum of resistances R1
+ Rp then the signal to LED al rm 55 will vary. The LED
or LCD alarm 55 wil~ generate an alarm signal in response
to a variance in the signal being inputted to LED or LCD
alarm 5S. Accordingly, the present system will detect
either an increase or a decrease in the resistance due to
adsorption on the active coating 38 and the LCD alarm 55
will go off in either situation. This is an important
feature since, in some cases, vapor adsorption onto the
active coating 38 may cause a reduction in the resistance
o~ the active coating 38 wher~as adsorption of other
vapors may cause an increase in the resistance o~ the
same active coating 380 In either case, it is important
that the LCD alarm 55 be activated since any adsorption
onto the surface of the active coating 38 is an
indication of the oncoming exhaustion of the adsorbent
bed. The dual sensor/bridge circuit approach of Fig. 3
is preferable to the sensor of Fig. 2 since
fundamentally, mea~urement o~ null in a bridge circuit is
a more sensitive method to measure changes in output
~ignals, the temperature compensation is likely to be
more exact with a pair o~ identical sensors, the
cixcuitry ~or aompensat:ion is quite simple using a bridge
cirauit and the device will be less expensive,
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more uni~orm and more repeatable performance i3 obtained
if the ~our electrodes on the chip are identical. Also,
le~s proces ing o~ the chip is required. The addikional
measurement of impedances (complex and ~imple parts) can
be accomplished between electrodes and are redundant.
This redundancy is not required for operation nor
utilized herein for simplicity. But in cases where it is
advantageous to have several measurements of the same
property (or several properties)~ the device illustrated
herein can accommodate such applications.
An important consideration when employing an alarm
device in accordance with the present invention is where
in the filter 19 to locate the sensor portion 30. The
location of the sensor portion 30 in the filtex 19 will
depend on several variables which include humidity r
temperature, response time, the concentrakion profile of
the adsorbent bed during exposure to vapor and the
selected threshold level at which the alarm will be
triggered to indicate oncoming exhaustion of the
adsorbent bed. Each of these factors will have a bearing
on the optimum locakion of the sensor portion 30 in the
filter 19.
Generally, the location of the sensor portion 30 in
the filter 19 is decided based on the sensitivity of the
sensor portion 30 in combination with the margin of
safety desired. The le88 sensitive the sensor poxtion 30
is, the closer to the inlet of the filter 19 it must ~e
placed. In contrast, a very sen~itive and reliable
sen~or may be placed near the outlet of the filter 19.
The ideal sensor location can be affected by changing the
selected thre~hold level of the alarm.
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Another important ~actor in determining the location
of the sensor portion 30 in the ~ilter 19 i8 the
breakthrough concentration curve of the adsorbent bed
during vapor exposure. Generally, harmful vapors will
enter the filter 19 through the screen opening 16 and
spread out through the adsorbent material. As the vapors
spread out they will be adsorbed and a concentration
profile will be formed. Usually the highest
concentrations will be detected at the point closest to
the screen opening 16 and the concentration level will
decrease as one moves away from the screen opening 16 in
the filter 19. Since it is desirable to prevent any
toxic vapors from reaching the pipe 17 the sensor portion
30 must be located such that the selected level
concentration which sets off the alarm is reached at the
location of sensor portion 30 prior to the time when
toxic vapors reach the pipe 17. In this manner passage
of toxic vapors into pipe 17 is prevented.
The concentration profile may be altered by humidity
since humidity often reduce~ the efficiency of an
adsorbent bed. Because of this reduced efficiency of the
adsorbent bed there will be a gradual increase in the
concentration o the vapor passing through the adsor~ent
bed rather than a sharp concentration front moving
through the adsorbent bed as occurs when the adsorbent
bed is functioning at peak e~iciency. Thu~, ~or u9e in
high humidity, adjustments may be required.
q'he invention i~ urther illustrated, but is not
intended to be limited by, the following examples.
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Example 1
A micro~ensor in accordance with the present
invention was ~abricated b~ ~irst depo iting
interdigitated gold electrodes on a silicon dioxide
substrate and then pin coating the ~lectrode arrays with
a solution of silicone caulk, carbon and methylene
chloride to deposit a thin film ~f vapor sensitive
material on the electrodes. This carbon microsensor
device operated a~ a chemiresistor.
Then the microsensor was connected to the al~rm
circuitry and placed in an adsorbent bed. The adsorbent
bed was exposed to toxic gases and the resistance of the
microsensor was monitored. At the point when the gases
reached the microsensor a large change in its resistance
was noted and the alarm went off indicating that the
threshold level had been exceeded.
Example 2
Referring to Gig~. Sa, 5b and 5c, there are shown
three di~ferent breakthrough curves for charcoal
adsorbent bed~ exposed to benzene. The curves wPre
determined by using two sen ors. Sensor 1 is a sensor in
accordance with the present invention and it monitors the
concentration at different depths in the adsorbent bed,
while sensor 2, a commercial SAW device, monitors the
exit concentxation of the adsorbent bed. Sensor 1
employed a vapor ~ensitive medium of a thin coating o~
aharcoal and was fabricated by the method o~ Example 1.
A concentration of 1% benzene vapor in dry air is fed to
the adsorbent bed. The arrow on the graph indicates a
concentration of 200 ppm ~threshold level) at the bed
exit.
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From these graphs the threshold level for a given
placement of the sensor in the adsorben-t bed can be
determined. For instance, for a sensor at 50% depth a
threshold concentration setting of 0.4% will trigger the
alarm at the poin~ where the exit concentration i~ 200
ppm.
Example 3
Referring to Figs. 6a and 6b, there are shown two
different breakthrough curves for charcoal adsorbent beds
exposed to benzene. The curves were determined by using
two sensors~ Sensor 1 monitors the concentration at
different depths in the adsorbent bed, while sensor 2
monitoxs the exit concentration of the ad~orbent bed.
Sensor 1 is a sensor in accordance with the present
invention employing a vapor sensitive medium of charcoal
and was fabricated by the method of ~xample 1. Sensor 2
is a commercially available SAW device. A concentration
of 1% benzene vapor in humidified air is fed to the
adsorbent bed to show the effects of humidity on the
breakthrough curve. The arrow on the graph indicates a
concentration of 200 ppm (threshold level) at the bed
exit.
From these graphs the threshold level for a given
plaaement of the sensor in the ad~orbent bed can be
determined. It ~hould be noted that the sen~or respondY
more gradually in humidified air and a lower ovexall
re~pon~e i~ genexated,
The foregoing de~cription of embodiments o the
invention ha~ been presented for purposes of illustration
and de~cription. It is not intended to be exhaustive or
to limit the invention to the preci8e forms disclosed,
and many modifications and variations will be obvivus to
one of ordinary skill in the art in light of the above
teachings. Accordingly, the scope of the invention is to
be determined by the claims appended hereto.
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