Note: Descriptions are shown in the official language in which they were submitted.
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CoM~INED ~ R D~NIOE FOR DE~ G T~XIC GAS~S
This invention relates ~o devices for detec~ing
pollutants and more particularly to devices capable of
detecting pollutants at relatively low concentrations in
ambieTlt gases, Elcctrochemical sensing devices, especially
amperometric sensors such as those disclos~d in U.S. Patent
~los. 3,776,832, 4,201,634 ar.d 4,32~,9~7 offer the advantages
of portabiiity, real-tim~ readout, relativeiy low cost, and
fair sensitivity and selectivity to a few specific
pollutants such as CO, ~S, I~IO, ~2' SO2, hydrazine, CCC12,
HCN, or C12. However, these devices are not applicable, at
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present to the detectlon of many pecies that are not
electrochemically active. i~
In detec~ing pdl~lutants inc~ding toxic substances,
sensing devices have usually been limited with respect to
concentration~ of the pollutants partlcularly when Ihe
pollutants are essentially not alectrocheT~cally active or
have an activity d~fficul~ to detect. Below cert~in ~alues
of concentratiGn, e.g., }00 ppm (parts per milllon) o~
benzyl chloride, tetrachloroethylene or the like, presently
available portable d~vices are essentially unresponsive to
the pollutant. Since some pollutants may be extremely
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toxic, it i3 important to d velop devicc for detecting
varlous polluta~ts at low concentrations.
Accordingly~ one object of this invention is a device
for detecting pollutants at low concentrations in gases.
Another object is an electrochemic~l device for detect~ng
pollutants in ambient gases where the pollutants are
essen~ally electrochemically inactive or have an activity
difficult ~o detect. An additional object is a device which
~ay be utilized as a portable instrument to survey an area
10 or as a slte monitor for a wide variety of gases. A further
object is a device which is also capable of detecting
pollutants at higher concentrations, at which level they nay
present an acute toY.icity or flammability hazard.
Briefly, the invention relates to a device for
detecting a pollutant in an a~bient gas and comprises (1)
electrical conversion means having a catalytic surfacc to
che~ically convert the pollutant (e.g., a hydrocarbon) to a
derivatlve product (e.g., carbon monoxide) having a
characteristic electrochemical activity, and (2) electro-
chemicaL sensing means responsive ~o that electrochemicalactivity and providing a signal indicative of the derivativP
product and thereby the original pollutant. In one embodi-
m~nc, the conversion means includes a sensor respGnsive to
higher concen~rations of an elecero~hemically inactiv2
pollutant, but essentially unresponsive to low~r concen~
trations, and an electrochemical sensor responsive to the
derivat~ve product, but essentially unresponsive to the
pollutant at lo~ concesltrations. The conversi~n means may
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comprise a heated filament made of or coated with a noble metal
catalyst, such as Pt, Pd, Ir, Rh, Au, Ag, or an alloy or compound
of one of such metals.
Thus broadly, the invention contemplates a device for
detecting a gas, vapor, chemical pollutant or other component
of interest in a gaseous medium which comprises an electrical
heating element means including a noble metal~containing
catalytic surface arranged for exposure to a sample gaseous
medium including a component of interest for heating the component
for catalytically producing a derivative product of the component
not present in the sample gaseous medium, with the derivative
product having a characteristic electrochemical activity, and
an electrochemical sensing means arranged for exposure to the
derivative product as it leaves the heating element means, with
the electrochemical sensing means being responsive to the electro-
chemical activity of the product, and being capable of producing
a signal indicative of the product and thereby the component
of interest.
In another embodiment the invention provides a device for
detecting a component of an ambient gas which comprises a
conversion means for chemically changing said component to a
derivative product having a characteristic electrochemical
activity, with the conversion means having means for generating
a signal when the component is present in a concentration above
about 0.1%, but essentially unresponsive at concentrations below
about 0.1~, and a sensing means responsive to the electrochemical
activity of the product for concentrations of the component
.~ below about 0.1~ for providing a signal indicative of the product
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and thereby the component.
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Fig. 1 is an exploded view of one embodiment of the
invention.
Fig. 2 is a cross-sectional view of yet another embodi-
ment of the invention.
Fig. 3 is a flow diagram of yet another embodiment of the
invention.
Figs. 4A, 4B & 4C show representative response curves for
benzene from devices constructed according to the invention.
The device of the invention is suitable for use in detecting
at least one of a variety of pollutants or otherwise hazardous
gases or vapors in ambient gas. These pollutants commonly include
various organic compositions such as benzene, benzyl chloride,
toluene, methane, tetrachloroethylene, tetrahydrofuran, cyclo-
hexane and the like. It is particularly useful for detecting
the presence of a pollutant such as benzyl chloride or benzene
at low concentrations in the order of about 1-100 ppm where
some sensing devices are inoperative. In addition, the device
may be constructed of components permitting its use as a portable
instrument capable of on-site detection of a pollutant and in
some instances fixed site analysis of the general concentration
of the pollutant.
The device includes in combination, electrical heating
means having a noble metal exposed surface for chemically
changing the pollutant to a derivative product having a
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characteri6tic electrochemical activity and a sensing ~l~ans
responsive to the electrochemical acti~ity of the product
and including signal means providing a signal indicative o,
the product ana thereby the polluL~n~. The derivative
product results from the chemical change ~n the pollutant
which may occur rrom the oxidation or other process on the
pollutal~t to either form electrochemical activ$ty or chan~e
the existing activity of the pollutant. A particularly
useful combination for the dev~ce includes a noble metal
heating means which also operates as a sensor providing a
signal at concentrations of the pollutant above about
O.l-l.OZ while providing the derivative product over the
overall concentration range r^or detectlon by the
electrochemical sensing means. The presence or absence of
~he signal from the noble metal heating means in co~bination
with the signal from the electrochemical sensing ~czns may
be used in determining the presence or absence of a
pollutant and its general concentration range.
The deriva~ive product from the pollutant exhibits a
characteristic electrochemical activity which may be
detected by ~he sensing means at levels as low as about one
ppm. These products may be the oxides of carbo~, sulfur,
nit~o~en and the like having electrochemical activity and
other compositions indicative of partifil oxidation vr
dccomposition of ~he pollu~ant.
The device of Fig. 1 includes the catalytic heating
means as illustrated by a hot-wire sensor arranged ~o
receive a sample of the ambient gas and senslng ~cans
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illustrated by an electzochemical sensor arranged to rcceive
the ~erivative product. The conversion means may include a
signal means fo~ providing a signal apart from the signal
fro~ the sensing means. rl'i,e ~ re sensor includes a
heating means. and a ca~alyst such as one or more of the
oxidation catalysts. In general, hot-wire sensors and
catalysts based on Pt, Pd, Rh and ~u are particularly
useful. Also useful may be catalysts cor.prising Ir or Ag,
or various noble metal allovs, such as Pd-Ag, Pt^Rh, Pt-Ir
or Au-A~, or a compound, espec~ially an oxide, of o~e of said
metals. The hot wires or filamPnts ~ay be either made of a
pure noble metal or, preferably, be coated on a suitable
baser metal or alloy. The catalysts may also be dispersed
on a support such as C, Si or alù~.ina. As the s2mple is
exposed to the hot catalyst, it is chemically changed and
pre~erably oxidized or deco~posed to at least one derivative
product which may be detec~ed by th~ electrochemical sensor.
~ le electrochemical sensor i8 responsive to low levels
of the derivative product and provide~ a signal indicati~e
of the product and thereby the pollutant. While the
electroche~ical sensor is responsive to some electrv-
che~ic~lly active substances at these low l~vels~ its
response to compounds such as benzyl chloride and the like
which are not primarily characterized by electroche~ical
activ~y tends to be so limited that i~ ~ay not be used for
detection unless aided by one of the above-mentioned
oxidation catalysts.
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Sultably, the electroche~ical sensor m~y be a
con tant-potential amperometric sensor. As the derivative
product is sensed, a signal is generated indicative of the
electrochemlcal ~r~.~vit~ the "working electrod4" of the
sensor. In general, the ho~ wire sensor representin~ the
conversion means and the ampero~etric chemical sensor
representing the sensing means are sufficiently small that
they may be bu~lt ineo a single unit where the derivstive
product from the hot wire sensor may interact with the
electrochemical sensor within the same unit. Preferably,
the sa~ple is introduced into a sample channelin~
arrangement by which the sample is first introduced to the
hot-wire sensor. The derivative product from the hot-wire
sensor is then routed to the electrochemicai sensor either
directly, but in a con~rolled manner, so as to enhance the
acti~ity of said Qensor by raising its temperature to a
preferred value or through a baffle arrangement or other
isolation syste~ 80 that the electrochemical sensor need not
be directly expos~d to the heater of the hot-w~re sensor.
In Fig. l, an exploded view of the invention is
illustrated showing a path including a baffle arrangenent to
isolate ~he electrochemical sensor fro~ the heater of the
hot-wire sensor and thereby reduce the possibility o~ da~age
to the electrochemical sensor.
The inventive device includes electrical heating m~ans
with a noble metal surface exposed to the gas for heating
the ga3 and catalyzing the combustion o~ the pollutant eo
provide a derivative product having a characteristic
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electrochemical activity and electrochemlcAl sensing means
responsive to that activity for providing a signal
indicative of that activlty. As illustrated in Fig. 1, the
heating means is pr~,~ided by a filament 10 composed of a
heating element core 12 ana a noble metal s~rface 14 mounted
in a reaction chamber 16 of block 15. A sensing ele~ent 18
is mounted in chamber ~0 of block 19 ror detecting the
electrochemical activlty of the product derived fro~ the
combustion of the pollutant. A reference electrode , ~nd a
counter electrode 21 are also provided. Blocks 15 and 19
are joined by block 23 with chamber 24. Inlet 26, outlet ~8
and cha~bers 16 nd 24 provide channeling means for expo~ing
the gas to the heating filament 10 and the derivative
product to the sensing element 18`. Closure`member 30
provides sealing of chamber 20. A plug~able openir.~ 32
above chamber 20 permits introduction of electrolyte it~to
the electrochemical sensor.
In ano~her e~bodiment of the in~ention, shown in
Fig. 2, a diffusion-type electrochemical sensor is used to
detect concentrations of 0-2.4% me~hane in air by
pree~:posing any ambient gas diffusing or convectin~ towards
said sensor to an iridium-coat~d filam~nt hea~ed to a
te~perature of 300-600C and pre~crably 400-500C. l'he
filament i8 ~ade preferably of~ metal or alloy of
relatively high resistivity having a coefficien~ of ther~.al
expansion close to that of iridium, e.g., commercial grade
ti anium. By ad~usting the filametlt length and
cross-section t~ yield a reslstance of about ~,000 ohmQ and
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temperature of 400-500C when heatcd by a current of about
10 mill~ampcres, it beco~.es possible to achieve a low-power
low-curren~-drain methane detector suitable for
in~rinsically safe mine-monitoring applications.
In Fig~ 2 the device 36 includes hea~ing filament 3~
mounted in housing 37 wlth electrochemical sensLng elenent
40 hidden from direct exposure to filament 38 by barrier 42
of glass wool or other suitable porous material. In Fig. 2,
inLet 52, vent 46, and chanrlels 48, 49 and S0 provide
channeling of the gAs and derivaeive product. A sou~ce of
electrical current is provided by leads 54 and 56 passing
through a ceramic plug 44 to fila~ent 38.
The heating of the iridium-coated filament of Fi~. 2 is
prefera~ly governed by a temperature con~rolier, such as
that shown in ~he block diagra~ of Fig. 3. To further
reduce the heating power requirc~lents, ehe filament may be
heated in intermittent pulses, e.g., in pulses of O.S to 3
secondsi duration every 10 seconds. In general, the
response time Oc the device is less than about 20 seconds. ~
The heating pulses are ~ufficient to bring the peak filam~ne
temperature up to the range of 400-S00C.
It i~ noteworthy ehat electrochemical sensars when~used
by themselves such as that used in the de~rice o~ Fig. ~, do
not u~ually respont to methane even in concentrstions as
high as lOOZ. Yet the signals obtained wieh the heatec
iridium filament permit easy measurement o~ methane
concentrations a~ lou as 0.05: or less.
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A~ an illustration of the general f low diagram
associa~ed with the detection process, F~g. 3 ~llustrates an
arrange~ent where the derivative product may be
au~omatically routed through a multipath solenoid valve or
other diversion means to either the electroche~ical sensor
or removed fro~ the device before expo~ure ~o the sensor
depending on the concentration levels. In this flow
arrangement, ~he signal from the hot-wire sensor may serve
to indicate the presence Or the pollutant at a concentration
above about 0.5-l.OZ and may be used to direct the flow of
derivatLve product away from the device to avoid the
possible effects oF an excessive concentration of cert~in
pollutants on the electrochemical sensor.
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~ en no signal fro~ the hot-wire sensor'is proYided,
the flow pattern will be used to direct the deri~ative
product to the electrochemical sensor and $ts s'gn 1 wiIl
provide an indication of the presence of the derivative
product and thereby the pollutznt. With a combination of
signal6, the presence of the pollutant and it~ conc~r.tration
may be determined.
~ e general flow pat~ern of the gas and~derivative
product are illustrated i~ Fig. 3. As illustrated, a gas~
sample is ad~itted via l~ne 60 to conversion cha~ber 62
having filament 64 whose temperature i9 controlled by
co~txoller 6S. S~gnal indicating means is provided by
indica~or 66 to provlde any signal a~ailable from
filament 64~ .he derivative produc~ i~ channeled via line~
68, vaIve 70 and line 72 to the electrochemlcal sen60r 74
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having a signal ~n~icator 76. Pump 78 provides the suction
and detenmines the flow rate within the device. Controller
65 and valve 70 are governed by a microprocessor (computer/
controller) 79 which receives the signals from indicator 66
and 76.
As an illustration of the representative performance o~
the a~ove-disclosed embodiments, Fig. 4 provides re~ponse
curves for benzene. In general, b~nzene i5 not readily
detected on ele~trochemical sensors at ppm levels and is
further essentially undetectable by a hot-wire sensor at ppm
levels. However, by combining the ~wo different sensors, it
is possible to produce signals indicative of the presence Or
benzene at a concentra~ion of less than 100 pp~ level ir,
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air. While ~enzene at such levels~is not detectable by
hot-wire or heated semiconductor sensors, the heated
catslytic surface of such sensors or other noble
metal-co&ted filaments converts the benzene to one or more
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oxides of carbon or degradation products which are then
detected by the electrochemical s~nsor.
In Fig. 4A, the heated filAment was made of a finc
platinum wire (0.08 millimeter in diameter) si~lilar eo th~t
used in hot-wire fla~able gas sensors, heated to a
temperature of about 1000C. The t.wo humps star~ing at
about 1.5 D.nutes and 4 minutes correspond to e~posure to
samples of 200 ppm and 50 ppm of benzene, respectiveiy.~ ~n
Fig, 4B, a commercial ~in:oxide semiconductor sensor, he~ted ;~
to about 300C, was used as a conversion mean~, The three
humps st~rting at aboue 1 minute, 6 minutes, and 10 ~inutes~
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correspond to samples of 20~ ppm, 50 ppm, and 200 ppm of
benzene, respectively. In Fig. 4C, the conversion device
consisted of a fine (0.08 milli~eter ~n dia~eter) gold
filament heated to 950~50C, and the hu~p star~ing at about
0.6 minute W85 due ~o a sample of 200 pp~ of benzene. The
same electrochemical sensor, co~prising a platinu~ black
sensing electrode at a potential of about 1.1 volt relative
to the standard reversible hydrogen electrode, was used in
all three cases. A comparison of the ordinates of ~igs. 4A,
B, and C shows that the heated ~in oxide yields an
approximately three to four times higher response than the
platinum filament, but that the gold filament yields a
: ~ three-fold higher response-~than the tin oxide and a ten
times higher rèsponse than the platinum.
Other representative compounds which do not appear to
be detected by either type of sensor ~ndependently but are :
by the two sensors in combination at concentrations of about
100 pp~ or less are benzyl chloride and tetrachloroethyiene.
The following example is provided or illustrative
purposes and is not intended to be restrictive as to the
~cope of the invention~
E~U~PLE I
A ample of alr containing~200 ppm of tetrachloro~ :~
ethylene was tested in an apparatus having a catalytic bead :
sensor available from Rexnord Corporation of Sunnyvale,
Cali~ornia-and a Pt-catalyzed CO fil~mcnt available fr~m
Energetics Science of ~aw~horne, New York. : The sample was~
tested with each element operatin~ separately, Observed
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cuxrents from each sensor operating separately were indis-
tinguishable from norma} noise levels. In another test, a
si~ilarly constructed electrochemical sensor ~n combination
with a Pt-catalyzed CO filament wa~ used for an air sample
containing only 20 ppm tetrachloroethylene. A signal of
about 0.3 microamps was observed for the combina~ion of
sensors a~ compared to a value o~ less than 0.2 (noise
level) microamps without the filament.
As de~cribed above, the invention provides a useful
device for detecting a gas, vapor, chemical pollutant or
other component in a gaseous mcdium and is capabie by a
combination of a catalytic heating elen~nt and an
electrochemical scnsor of de~ecting the component at low
concentraticns. -- -
The foregoing description of embodiments of theii~vention has been presented for purposes of illustration
and description. It is not intended to be exhau~tive or to
li~it the invention to the precise form disclosed, and
obviously many modifications and variations are possible in
light of the above teaching.
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