Note: Descriptions are shown in the official language in which they were submitted.
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1 49,907I
IMPROVED SOLI~ ELECTROLYTE GAS SENSING APPARATUS
BACKGROUND OF THE lNV~ O~
The ion conductivity of numerous solid electro-
lyte materials has resulted in the application of solid
electrolyte electrochemical cells for numerous gas measur-
ing applica~ions. Solid electrolyte compositions havebeen incorporated in gas sensing apparatus to measure
oxygen, combustibles, pollutants, i.e, SO2, CO2, etc. and
have provided a basis for product ranging from medical
instrumentation to industrial process and stack gas ana-
lyzers. A solid electrolyte electrochemical cell developsan electrlcal signal, current or voltage, on the ~asis o~
ion conductivity which is a function of the content of a
particular gas species of interest within a monitored gas
environment. The application of a solid electrolyte
electrochemical cell exhibiting oxygen ion conductivity to
monitor the cxygen content of a gas environment is des-
cribed in detail in U.S. Reissue Patent 28,79? issued
April 27, 1976 which is assigned to the assignee of the
present invention. The solid electrolyte electrochemical
cell, which consis~s of an oxygen ion conductive solid
electrolyte having a sensing electrode disposed on one
surface and a reference selector disposed on an
~ 49,907I
opposite surface, develops an EMF signal in accordance
with the Nernst equation in response to a difference in
the oxygen partial pressure b~tween the sensing and refer-
ence electrodes. In order to elimLna~e the oxygen par~ial
pressure at the reference electrode as a ~ariable in the
equation, a stable or known oxygen re~erence environment
is maintained in contact with the reference elec~rode.
This reference enviro~ment is isola-ted, through appro-
priate sealing means, from ~he measured gas environment
contacting the sensing electrode. The same basic mode of
operation of a solid electrolyte electrochemical cell as
employed for the measurement of combustibles is described
in U.S. Patent 4,158,166 issued June 12, 1979, which is
assigned to the assignee of the present invention. In
this embodiment, the electrochemical cell is operated
in a pumping mode to introduce oxygen from the reference
source to combustibly react with combustible constitutents
at th~ sensing electrode with the cell current developed
as a result of the ion conductivity providing an indication
of the combustibles content of a measured gas environment.
The application of solid electrolyte electrochemical cells
employing a stable reference gas environment for develop-
ing an EMF signal indicative of pollutants such as S0~,
C02, N02, etc. is described in detail in issued Canadian
Patents 1,002,599 and 1,040,264 whlch are assigned to
the assignee of the present invention.
The sealing, or isolation, of the measured gas
environment from the reference gas environment poses
significant practical problems in developing a trouble-
free gas-sensing apparatus due to the fragile nature of
the ceramic materia]. comprising the solid electrolyte
element. This problem is further compo~mded when develop
ing a high temperature solid electrolyte gas-sensing
apparatus, particularly when it is intended for monitoring
the gas constituents of an industrial corrosi~e environ-
ment.
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3 4g,907I
Typically, the commercial products presently
available addressed this problem in one of two ways. In a
first approach, the electrolyte material is limited to a
relatively small disc-shaped solid electrolyte element
having electrodes disposed on opposite surfaces thereof
with the electrochemical cell then being sealed within a
relatively strong mechanical tubular housing by an appro-
priate bonding techniques, i.e., brazing. This minimizes
the chances for mechanical fracture of the ceramic mater-
ial but poses potential gas leaks through the seal. Asecond approach is to utilize a closed end solid electro-
lyte tube with the electrodes being disposed on opposite
surfaces of the closed end. The tube extends into a
relatively cool and friendly environment before a transi-
tion is made through a flange or an adapter. This ap-
proach essentially eliminates the seal problem but exposes
the fragile ceramic material to mechanical damage.
Another drawback encountered in presently avail~
able commercial products is the combination of rigid and
flexible tubing employed to introduce the reference gas to-
the electrochemical cell and to exhaust the spent reference
gas from the sensor apparatus. The flexible tubing is
subject to gas leaks, particularly where mated to the
rigid tubing, and to thermally induced damage.
It is an object of this invention to provide a
gas sensing apparatus which vents the reference gas from
the sensing apparatus into the process gas region thus
eliminating the flexible tubing and hermetic seals, here-
tofore common in commercially available products.
SUMMARY OF THE INVENTION
There is disclosed herein with reference to the
accompanying drawings a technique for minimixing the
adverse affects of seal leaks and solid electrolyte mater-
ial. fracture in solid electrolyte electrochemical cell gas
measuring apparatus. In the prior art gas analyzer con-
figurations employing a flowing reference gas environment
the flowing reference gas is exhausted from the electro-
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4 49,907I
chemical cell gas sensor at a location remo-te from ~he
solid electrolyte electrochemlcal cell. It has been
determined experimentally that lf the ;~lowing reference
gas is vented from the solid el~trolyte el~ctrochemical
cell assembly to the measured gas environment at a loca-
tion which is close to the electrochemi.cal cell, the gas
pressure across the solid electrolyte electrochemical cell
will be essentially equalized. Tha equalization of the
gas pressure across the electrochemical cell minimizes the
likelihood of leakage between the gas environments con
tacting the sensing and reference electrodes of the elec-
trochemical cell and eliminates potential back pressures
which could result in'mechanical fracturing of the solid
electrolyte element. This equalization of total gas
pressures on either side of the solid electrolyte element
further eliminates the requirement for adjusting the
pressure of the reference gas environment to approximate
that of the measu~ed ~as environment in order to assure
the ,validity of the gas measuring electrical signal dev
eloped by the solid electrolyte electrochemical cell.
Thiæ disclosed technique essentially eliminates the need
for employing a long fragile closed and solid electrolyte
tube in one gas apparatus design approach, and the dif-
ficult high temperature sealing requirements of the com-
bination of a solid electrolyte cell and a mechanicalsupporting tube.
While the novel reference gas vent technique has
particular application to gas sensors employing solid
electrolyte electrochemical cells, the concept is also
applicable to electrochemical cell gas analyzers using a
gel or liquid electrolyte.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more readily apparent
from the following exemplary description in connection
with the accompanying drawings:
Figure 1 is a sectional schematic illustration
of an in-situ gas sensing device employing the invention;
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Figure 2 is a sectional schematic lllustration
of an externally mounted gas sensing device employing the
invention; and
Figure 3 is a sectionecl ~chemat.ic :illustration
of a high temperature probe-type gas sensing device, all
according to the teachings of this inVention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a typical commercially available solid elec-
trolyte gas analyzer using a flowing reference gas, the
reference gas is supplied to the reference electrode of
the solid electrolyte electrochemical cell and subsequent-
ly vented to an atmosphere remote from the process gas
environment being measured by the solid electrolyte gas
analyæer. It has been determined experimentally, that if
the reference gas is vented, or discharged, to the pro
cess, or monitored, gas environment at a location rela-
tively close to the solid electrolyte electrochemical
cell, the pressure of the reference gas contacting the
reference electrode of the solid electrolyte electro-
chemical cell can be maintained essentially equal to thepressure of the process gas contacting the sensing elec-
trode. This is accomplished regardless o variations in
the process gas pressure. This technique not only elimin-
ates the error during voltage mode operation in the elec-
trochemical cell signal caused by process gas pressurevariations, but also reduces the pressure drop across the
ceramic solid electrolyte sensor sufficiently to minimize
the gas leakage between the process gas and the reference
gas environments in both the voltage mode and the current
mode. The reference gas vent is located so as not to
alter the process gas in the vicinity of the sensing
electrode of the electrochemical cell. Similarly, if the
gas analyzer provides for introducing test or calibration
gases to the sensing electrode of the electrochemical cell
it is necessary to locate the reference gas vent so as to
avoid mixing the vented reference gas with the calibration
or test gas. Typical embodiments of solid electrolyte gas
6 49,~07I
analyzers incorporating the novel reference vent technique
are described below wlth reerence to the illustrations of
Figs. 1 and 2.
In the industrial probe-type solicl electrolyte
gas analyzer 10 illustrated in E'ig~ 1, a disc-shaped solid
electrolyte electrochemical concentration cell 20 is
sealed within a support ~ube 30 via a seal 31. The sup-
port tube 30, which may be typically a metal member,
passes through a first bulk head member 40 and a second
bulk head 50. The second bulk head 50 includes a tubular
member 52 which supports an annular porous dust seal 54
which contacts an outer tubular shield member 42 extending
from the bulk head 40. The porous seal 54 reduces the
passage of particulate material from the process gas
environment PG into the gas analyzer assembly 10. The
tubular protec-tive shield 42 is secured within an opening
of the wall W of the process gas enclosure which may
typically be a stack in an industrial environment. The
solid electrolyte electrochemical cell 20, as described in
the above-referenced patents, consists of an ion conduc-
tive solid electrolyte element 22 having a reference
electrodé 24 and a sensing electrode 26 disposed on oppo-
site surfaces thereof. The composition of the solid
electrolyte member 22 is selected so as to render the cell
20 responsive to a particular gas constituent of interest
in the process, or monitored gas, gas environment PG. The
gas constituent of interest may be oxygen, a combustibles
constituent, a pollutant constituent, etc. A known or
stable reference gas environm~nt RG is maintained in
contact with the reference electrode 24 by flowing a
reference gas at a controlled rate from a remote reference
gas source RS through an inlet tube member 60. The elec-
trical signal developed by the electrochemlcal cell 20 in
response to the partial pressure of the gas constituent o
interest of the process gas environment PG is monitored by
a remote measuring circuit MC connected to the electrodes
24 and 26 via electrical leads 62 and 64. In the event
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in-situ calibration of the electrochemicaL cell 20 is
required, a test gas inlet tube 65 is provided for supply-
ing a calibration gas within the tubular member 52 from
test gas source TS or contactinc3 the sen~ing electrode
26. The gas sensiny assembly described thus far is indi-
cative of commercially available gas analyzer probe assem-
blies. The commercially available analyzers vent the
reference gas to the atmosphere A, remote from the process
gas environment PG. In the embodiment of Figure 1 a
reference gas vent 70, illustrated as an aperture in the
support tube 30, provides for the venting of the reference
gas from the re~erence gas environment RG through the
a~nular chamber 74 defined as the space between the tubu-
lar members 30 and 42, through the porous seal 54 and into
the process gas environment PG as indicated by the arrows.
The location of the vent 70 assures the passage of the
reference gas into the process gas environment PG without
diluting or interfering with the gas composition of the
process gas environment or the test gas at the sensing
electrode 26 of the electrochemical cell 20. The ventiny
of the reference gas from a vent location 70 in close
proximity to the electrochemical cell 20 into the pro-
cessed gas environment PG achieves the benefits of gas
pressure equalization across ths solid electrolyte elec-
trochemical cell 20 for a predetermined reference gas flowrate. The reference gas flow rate is set such that the
difference in gas pressure between the reference gas
environment RG and process gas environment PG across the
cell 20 corresponds to the pressure drop established by
the reference gas vent path, i.e., vent 70. The gas
pressure o~ the ref~rence gas environment RG is thus
maintained essentially equal to the gas pressure of the
processed, or monitored, gas environment. Further, in the
embodiment illustrated in Fig. 1, the pressure drop across
vent 70 created by the reference gas flow rate results in
the reference gas functioning as a purge gas to prevent
corrosive gases of the process yas environment PG from
entering-this area of the assembly lO.
8 49,9071
While the embodiment of Fig. 1 illustrates a yas
analyzer probe assembly for the in-situ measUrement of the
gas constituent of a process yac enVironment, the gas
analyzer 100 of Fig. 2 is mounted outslcle the process gas
enVironment PG. It responds to a sample of the proce~s
gas drawn throuyh a sample tube ST to provide a measure-
ment of the par-tial pressure of a gas constituent of
interest of the process gas PG. ~ combination of a disc-
shaped solid electrolyte electrochemical cell 20 sealed
within a tubular support member 30, comparable to that
described above with reference to Fig. 1, is sealed within
a tubular housing 110 through the use of a seal member
111. A pump P draws a sample of the process gas into the
sample gas flow tube ST. An inlet tube 112, which pro-
vides gas flow communication between the sample gas flow
tube ST and the internal volume of the housing 110 exposes
a sensing electrode 26 of the electrochemical cell 20 to a
sampl~ of the process gas ~nvironment PG. The sample gas
is then drawn from the internal volume of the housing 110
through the output tube 114 back into the sample flow tube
ST. A flowing re~erence gas is brought into contact with
the reference electrode 24 of the electrochemical cell 20
via a reference gas inlet tube 120 which enters the sup-
port.tube 30 through a gas seal 32. The reference gas
flow is supported by the action of the pump P which draws
the reerence gas from the support tube 30 through a
reference gas.exhaust tube 150 and introduces the exhaust
reference gas into the sample gas flow tube ST at a loca-
tior~ slightly downstream from the sample gas inlet tube
112. The coupling of the exhaust reference gas from the
reference gas environment of the cell 20 to the sample gas
flow tube ST at a location sliyhtly downstream from the
sample gas inlet tube 112 avoids mixing of the reference
gas with the sample gas being measured by the cell 20. It
is however sufficiently close to the inlet tube 112 so as
to achieve the desired gas pressure equalization across
the electrochemical cell 20 to realize the objectives
9 49,907I
defined above. The conventional need for a leak-tight
seal between the sensing electrode surface of the electro-
chemical cell 20 and the refer~nce electrode surface of
the electrochemïcal cell 20 is minimized throuyh the use
of the reference gas vent technique illustrated in Eigs. 1
and 2 due to the fact that the total gas pressure differ-
ence across the electrochemical cell 20 is held to a value
e~ual to the pressure drop through the reference gas vent
path as determined by controlling the reference gas flow
rate. -
An industrial probe-type solid electrolyte gas
analyzer according to this invention which is particularly
well suited for high temperature applications is generally
i~dicated by the reference character 210 in Figure 3. By
high temperature application, it is understood that the
environment to which the solid electrolyte electrochemical
concentration cell 212 is exposed can be up to about
3000F. As previously indicated, severe operational and
structural problems are encountered in such a hostile
environment. The embodiment described in connection with
Figure 3 not only eliminates the error caused by process
gas pressure variations but also reduces the pressure drop
across the sensor cell 212 when operated in the voltage
mode. As a result, it is not necessary to provide a leak
free seal between the cell 212 and the cell supporting
member 214. The elimination of the raquirement of such
seals is particularly beneficial in high temperature
operations. Since different materials are utilized in the
sensor construction it has been extremely difficult to
provide a seal or bond capable of withstanding ths mechani-
cal stress resulting from dif~ering thermal expansion
rates.
An effective, thermal impact resistive probe
structure for high temperature operations is provided
through the use of the reference gas vent 216 and the
packing gland 218 features of this invention. The gas
analyzer 210 includes a housing portion 220 which secures
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49,907I
the probe 210 to wal] "W" of the process gas enclosure
The housing portion 220 ls preerably stainless steel. A
pressurized reference yas chamber 222 exterlds frorn one end
of the housing 220 and the sensor cell structure 212 and
5 associated structural members project from the other end
of the housing. The support member 214 which is preerably
an aluminum tube is secured in the housing 220 by securing
means such as ceramic cemented joints 224. An interna
ceramic joint 226 supports and seals a reference gas inle-t
10 tube 228 within the support member 214. The tubular cell
structure 212 is moun-ted at the okher end of the support
member 214 by the packing gland 218. The gland 218 and
the cemented joints 226 define the reference gas RG zone
of the gas analyzer 210. The yland 218 is a material
15 characterized by both its stability at high kemperatures
as well as its flexi~ility in absorbing the aforedescribed
thermally induced mechanical stress between the sensor
ceLl and the support member. A preferred material from
which the gland 218 is formed i9 saffil wool that is
20 packed between the support member 214 and the cell struc-
ture 212. Powdered æirconium oxide or aluminum oxide is
poured into the wool packing and fused. The gland 218 is
substantially resistant to gas flow therethrough. An
outer tube 230 protects the cell 212 from the erosive
25 effect of particulate in the process gas and defines a
test gas conduit 232 circumferentially disposed about the
cell 212. A test gas inlet tube 234 feeds a calibration
gas to the conduit 232 from a -test gas source TS.
The reference gas is provided from a pressurized
3() source RG through regulator 236 where reference gas pres-
sure is maintained at a predetermined level. The reference
gas RG enters the chamber 222 where it passes through
inlet tube 228 prior to contacting the cell 212. After
contacting the cell 212 the reference gas circulates from
35 the sensor end of the analyzer back toward the housing 220
as indicated by the several arrows. The reference gas is
then vented to the process gas environment PG through an
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aperture 238 in the support member 214 and outlet ~40 of
the housing 220. The reference qas is main~ained at a
10w rate such that the yas pressure difference between
t~e refere~ce gas environment and ~he monitored gas en-
vironment across the ceLl is substantially equal to thepressure drop across the reerence gas vent means. As can
be seen in Figure 3, the ceramic joint securing means 224
isolates the reference gas vent 216 from the test gas
conduit 232. Moreover, the isolation of the test gas
conduit 232 from both the reference gas supply RG and the
reference gas vent 216 eliminates the heretofore common
practice of using flexible tubing mate~ to rigid tubing as
feeder lines for the various gas supplies disposed at one
end of the gas analyzer. The separate reference gas vent
216 and test gas conduit 232 make it practicable to utilize
the chamber 222 as a reference gas feed means which sub-
stantially encloses one end of the analyzer, and to provide
a simplified ceramic joint arrangement as at 224 and 226.
The location of the vent 216 assures the passage
of the reference gas into the process gas environment
without diluting or interfering with the gas composition
of the process gas environment or the test gas. Addi-
tionally, the vPnt 216 maintains the pressure difference
across the sensor cell 212 at very low values regardless
of variations in process gas pressure. As a result, the
gas analyzer 210, operating in the voltage mode, can
accurately measure the percentage of a particular consti-
tuent of interest in the process gas under all process gas
pressure conditions without correcting for static pressure
changes.
The high temperature gas analyzer 210 which
utilizes the reference gas venting techni~ue and the
unigue packing gland 218 which separates the process gas
from the reference gas, permits the use of a relatively
short sensor cell 12 mounted within a support member made
of sturdy, thermally shock resistant material without the
requirement of a hermetic seal. Moreover, the gas analyzer
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210 utilizes a reference gas ~eed chamber 222 which in
combination with the reference gas vent 216 eliminates the
use of flexible tubing and simplifies sensor apparatus
structure.
What has been described is a reference gas
venting technique for industrial gas analyzers of both the
probe-type and the externally mounted type. Additionally,
the present reference gas venting technique together with
a sensor-supporting packing gland permit high ~emperature
in-situ process gas analysis.
