Note: Descriptions are shown in the official language in which they were submitted.
This is a ~ vls~ona~of parent application No. 234,646,
filed on August 29, 1975.
The present invention relates to a method for detecting
and measuring the activity of a gas by means of a solid state
sensing element and a device to carry out this method. In parti-
cular, the invention concerns a method and electrochemical means
useful in detecting an anhydride or gases containing anhydrides or
related compounds in air or in oxygen-bearing gas.
The invention is remarkable in that it permits a quanti-
tative determination of the activity of anhydrides and related com-
pounds which pollute surrounding air, such detection being reali- -~- -;
zed by means of a solid state sensor supplying an oxy-anion of the
corresponding anhydride.
Hitherto, research directed to the atmospheric pollution
measurement has been oriented so as to replace so called first gene-
ration monitors which mainly use liquid scrubbers to sample the air.
Among the atmospheric pollutants, sulfur bearing compounds
and nitrogen oxydes are considered as the most harmful agents to
people and among the most agressive to materials. ~ecause of the
wide concentration range that usually exists between industrial
stack gases and ambient air, it is rather difficult to build a sen- -;
sor able to directly monitor the ambient air and other polluting
sources of high concentration. A SO2 activity measuring device
based on a concentration cell principle is described in U.S. Patent ;
No. 3,718,546 issued on February 27 1973 to Salzono et al. This
device uses fused salts as electrolytes, thus cumbersome and rather ~ `
difficult to transport. In addition, a great stability is required
in the flow of gases to obtain realistic measurements.
Several detecting arrangements are also described in
Canadian Patent No. 1,002,599 issued on December 28, 1976, to
M. Gauthier and A.M. Chamberland.
A prime purpose of the present invention consists in pro-
- ,, . , : ~
43Z4
viding an improved means for the detection of anhydrides or relati-
ve compounds mixed with an oxygenated gas. `~
In particular, the present invention is useful for quan-
titavely determinating extremely small and high concentrations of
sulfur bearing compounds in(3as phases by means of an element in contactwith
clppro~riate electx~es and containin~ an oxy-anion of the anhydride to be detect~d.
It is also a particularity of the invention to provide
an activity measurement solid state sensor for carbon dioxyde and `
carbon bearing compounds converted into CO2 in the sensor under
operating conditions. -
The electrochemical principle of the present invention
is also applicable to the sensin~ of NO2 in air or in oxygenated
gases, by means of a nitrate element in contact with appropriate electrodes.
Another object of the invention is to provide a solid
state detecting device the operation of which is independent of
; the flow of the gas used as a reference source or of the gas the
; concentration of which is to be measured.
A further object of the present invention resides in pro- ;~
viding a solid state electrode as a reference source thereby eli-
minating the inconveniences inherent to the use of gas as referen-
ce sources, which substantially increases the miniaturization of
the sensing device and thus decreases its cumbersomeness.
An additional object of the invention consists in providing
a detecting device which may be mounted in a relatively small, com-
pact and portable apparatus readily usable on a site where parti-
cular gases or vapors bearing carbon, nitrogen or related compounds
have to be analyzed. `~
Another object of the invention is to provide a detector ~ -
for anhydrides which is easy to build, has a relatively long use-
ful life and a good calibration stability.
A further object of the invention consists in providing
a detecting device able to detect without interference sulfur
- 2 - `~
'
1()~4324
bearing compounds mixed with other vapors or gases. Similarly, it
is possible to detect without interference the presence of CO2 or
carbon bearing gases or of NO2 in presence of other gases
or vapors.
A further object of the invention resides in an improved
detector wherein the electro-motive force detected signal repre- -
sents the logarithm of the concentration of the anhydride and is
linear.
In preferred ~diments of the invention, there are provided particu-
lar arrangements ofinlet and outlet conduits leading the gas in con- ~ -
tact with the detecting element in order to minimize any dead vo-
lume of gas therein and to optimize the response time of the detec-
tor. These preferred arrangements greatly enhance miniaturisation
of the detector and its operating modes.
The above-mentioned objects of the present invention are
actually achieved through a sensor for detecting the activity of
gaseous anhydrides in an oxygen-bearing gas, comprising a solid
state electrolyte element having oxy-anions of the element forming
the anhydride to be detected, said electrolyte element being cons-
tituted of two parts, one part being an alkali metal salt or analkali-earth metal salt containing said oxy-anions and the other
part being an oxygen ion-bearing electrolyte; a reference electrode
being in contact with said another part of the electrolyte element,
a detection electrode remote from said reference electrode and in
contact with said one part of the electrolyte element; said electrolyte
element and detection electrode being arranged such that they are
free to come into contact with said gaseous anhydride to be detect-
ed, said reference and detection electrodes being arranged such
that a difference of potential occurs therebetween when a sample
of said anhydride to be detected is contacted with said detection
electrode and with said electrolyte element; heating means for
heating said electrolyte element to a temperature such that a
:' ~
~0443Z~
logarithmic variation in the concentration of the anhydride to be
detected causes a proportional and substantially linear variation
in said difference of potential, said temperature being below the
fusion temperature of said electrolyte element; and a potentiome- -
tric measurement device connected to said electrodes for measuring
the activity of said anhydride to be detected by measuring said
difference of potential.
The present invention also relates to a method of detect-
ing gaseous anhydrides in an oxygen-bearing gas.
As non-limitative examples, the following sensing ele-
ments constituting the detector in accordance with the present in-
vention are able to detect a particular anhydride:
1) a sintered electrolyte sensor containing oxy-anions of
sulfur is advantageously used for measuring the activity of com~
pounds such as S03, S02, H2S, CH3S, COS or other sulfur bearing
compounds, in particular, which may be transformed into the cor-
responding anhydride. -- -
2) a sintered electrolyte sensor containing oxy-anions of
carbon, permits to determine the amount of C02, in particular, or ~-
other carbon bearing compounds in air or in mixtures of gases
containing oxygen.
3) a sintered electrolyte sensor containing oxy-anions of
nitrogen is advantageously used for measuring, in particular, the
presence of nitrogen dioxyde in air or in oxygen-bearing gases. ~ ~-
The above and other objects will become apparent through
the followingdescription of preferred embodiments given with refe-
rence to the accompanying drawings, wherein ~-
Figure 1 schematically illustrates a sensor described in
the above-men*ioned Canadian patent, using a standard gas mixture
30 as a reference; -
.. - . . ~ . .
104~3Z4
Figure 2 shows a sensor using the vapor pressure result-
ing of the thermo-decomposition of a metal salt of the anhydride
to be detected, in order to fix the thermodynamical partial pres-
sure of a reference anhydride;
Figure 3 shows another sensor described in the above Cana-
dian patent using a solid ~etal electrode for establishing a reference potential;
Figure 4 shows a sensor in accordance with an embodiment
of the present invention wherein the sensing element is formed of
two juxtaposed compounds, one being an oxy-anion bearing compound,
at the anhydride measuring electrode, and the other ccntaining an
oxygen-bearing electrolyte at the reference electrode, the later
electrode generating a known potential when exposed to air or to
oxygen having a given partial pressure.
` Figure 5 shows a graphic relating to the electromotive
force experimentally obtained for various concentrations of SO2 -
Air, NO2 ~ Air, CO2 - Air, COS - Air and H2S - Air mixtures. The
sensor illustrated in figure 3 was used to compile those results.
Figure 6 shows an arrangement to compensate for the
oxygen partial pressure variations. The oxygen partial pressure
in the gaseous sample is measured by means of a known oxygen sensor
whereas the anhydride concentration is measured by a sensor in
accordance with this invention. The emf produced by both sensors
are electronically corrected and substracted in order to produce
an oxygen compensated signal of the anhydride concentration in the
sampled gas;
Figure 7 shows graphs obtained from experiments carried
out with the arrangement illustrated in figure 6;
Figure 8 shows another arrangement to compensate oxygen
partial pressure variations in a gaseous sample. Compensation is
achieved by in]ection of a fixed amount of oxygen rich gas into
the stream of a sample gas entering the detector in order to in-
crease its oxygen partial pressure to a nearly constant level.
, . ' , ' :
1.~4~3Z~
Figre 9 schematically illustrates three preferred ar-
rangements of the anhydride detectors. Gas circulation chambers
are formed by a system of small diameter parallel holes, inside a
quartz or alumina rod. One end of this rod is mechanically pressed
against the solid electrolyte in such a way as to seal the gas cham-
bers by thermal deformation of the solid electrolyte. A gas cir-
culating path is achieved between the inlet and the outlet chambers
by perforating the common wall of these chambers in the vicinity
of the solid electrolyte electrode. ~
Figures 9a and 9b show a gas circulating system in which ~ -
a measuring electrode runs through the outlet gas chamber. -
Figure 9c shows a double independent gas circulating
system, each of which being similar to the one shown in figure 9a.
One of the gas conduits is used as the measuring system and the
other as the gas reference system in which a known anhydride con-
centration is maintained.
Figure 10 shows the curve obtained from SO2 concentration
measurements effected by means of K2SO4/ZrO2-CaO arranged as shown
in figure 4 and using gas circulating system of figure 9a. This `
graphic representation shows the evolution of the emf signal at
different SO2 concentrations in function of the time. ~-
Figure l illustrates a sensor described in Canadian Pa-
tent No. 1,002,599. This sensor comprises a detecting element l
constituted of an electrolyte containing an oxy-anion of the gaseous
anhydride to be analyzed. The element 1 is made up of an alkali
metal salt or an alkali-earth metal salt.
That element l is preferably pellet shaped, but, of ;
. . .
course, any other form is also quite acceptable. Each end of the
element l is in contact with an electronically conductive material
2 and 3 such as silver, platinum, gold or other. ~-
The electrolyte element l is tightly inserted into a
tube 4 made o~ alumina so as to hermetically separate a measure
-- 6
iO443Z4
compartment "A" from a reference compartment "B". Each end of the
tube 4 is sealed with any appropriate material.
A sample "C" of the anhydride the concentration of which
is to ~e determined is introduced into the measure compartment "A"
through a conduit 5. Similarly, a corresponding anhydride "D"
of known concentration is introduced into the reference compartment
"B" via a conduit 6. These gas supply conduits 5 and 6 are prefe-
rably disposed axially and at the center of the alumina tube 4 so
as to provide a better contact for each of the gases with the cor-
responding metal surface. The anhydride gases are thereafter
exhausted through outlet tubes 7 and 8 respectively extending from ;
each of the compartments to the outside.
Each of the metal surfaces 2 and 3 are connected to the
terminals of a potentiometric measuring instrument 9, such as a
voltmeter, by means of conductive wires 10 and 11. The measuring
instrument 9 operates to indicate the difference of potential exis- -
ting between the electro-motive forces built-up on each of the
conductive surfaces 2 and 3 when in contact with the sampled gas, -~
and the reference gas, respectively.
In order to increase the sensing capacity of the element 1,
by increasing the ionic conductivity of the electrolyte, the tube
4 is introduced into an electrical oven (not shown). The heating
temperature of the oven is however not to exceed the fusion point
of the electrolyte element.
A modified arrangement of the embodiment shown in figure
1 is presented in figure 2 wherein a block 12 of any metal salt of
the anhydride to be detected is placed inside the then hermetically
closed reference compartment "B". When heated, that metal salt 12
evolves a metal oxyde and an anhydride identical to the one to be
analyzed. For instance, where CO2 is the anhydride fed at "C",
the corresponding metal salt chosen will then be MCO3 which, when
heated, will give MO ~ CO2, the latter defining a partial pressure
~0443;Z~
which will therefore produce a fixed reference potential at the
reference electrode 3. Therefore, the thermo-decomposition of a
salt of the anhydride to be detected sets at the reference electro-
de a stable partial pressure which results in a fixed potential at -
that electrode, thereby allowing detection and measurement of the
anhydrides to be analysed. The arrangement of figure 2 permits to
avoid the reference gas circulating arrangement of figure 1. It
is to be noted that by setting the metal salt block 12 close to -
the reference electrode 3, the concentration of the reference an-
hydride evolved from 12 remains stable, and then the compartment ~,
s does not need to be hermetically closed, but actually is open air.
Figure 3 illustrates a variant of the arrangement shown
in figure 1. To the reference gas source "D" of figure 1 is subs-
tituted a solid state reference element, this is an electrode 12. -
Then, the conduits 6 and 8 used for supplying and exhausting the
reference gas from compartment "B" become superfluous and are
eliminated. The use of the solid state electrode 12 which is made
of a metal, is rendered possible owlng to the use of a detecting
element 1'. This element 1' is constituted through the sintering
of a pure electrolyte compound la made of an alkali metal salt or :~
an alkali-earth metal salt, which corresponds to the oxy-anion of
the anhydride to be detected, and a second compound lb made of the
compound la to which a small amount of a metal salt has been added.
The electrode 12 must be formed of a metal corresponding to the
metal salt added by doping or vice-versa. For instance, if K2SO4
is used as compound la, the compound lb will be constituted of
K2SO4 doped with about 1% of Ag2SO4 or of AgCl, provided the elec-
trode 12 is made of silver. The other numeral references indi-
cated in figure 2 represent the same elements as those to which
they refer in figure 1.
Figure 4 illustrates a further embodiment of a sensor
having a solid state reference. In this embodiment, to the oxy-
- 8 -
.' , .
1~:)443Z4
anion bearing compound 1 made of an alkali metal salt or alkali-
earth metal salt is juxtaposed an oxygen-ions bearing electrolyte
compound 13. A stable reference potential is thus produced at the
reference electrode 3 whenever this electrode is exposed to ambient
air or to oxygen, provided the oxygen partial pressure in air is
constant.
As mentioned previously, the sensors shown in figures 1
to 4 may be introduced into an electric oven (not shown) so as to
increase the sensing capacity of the electrolyte element. However,
the temperature of the oven should not go beyond the melting point
temperature of the electrolyte.
It is to be noted that the sensors illustrated in figures
1 to 4 are able to produce potential differences in a range running
from a few millivolts to several hundred of millivolts when a ga-
seous state compound is put into contact with the detecting part
thereof.
Experiments were carried out by means of the arrangements
illustrated in figures 1 to 4 and certain results of which have been
plotted on figure 5, which results will be discussed in connection
with specific examples given hereafter.
Figure 6 shows an arrangement to compensate for any varia-
tions in the partial pressure of the oxygen gas of a gaseous sample
C. Actually, the anhydride detector is influenced both by a varia-
tion in the partial pressure of the anhydride and by a variation in
the partial pressure of the oxygen in the sample. This phenomena
does not interfer in environmental measurements since the oxygen
partial pressure remains constant in air, but such variations are
to be taken into account in stack gas analysis, for example, and
other gases where the oxygen partial pressure fluctuates. To com-
pensate for the oxygen partial pressure variations in a stack gas C,a portion Cl of this gas is fed to an oxygen sensitive detector
13'a made up of an oxygen ions bearing electrolyte 13a having a
10443Z4
reference electrode 3a and a measuring electrode 2a, these two
electrodes being of any electrically conductive material. Another
portion C2 of the gaseous sample C is forwarded toward a second
detector 13' which is identical to the one shown in figure 4. The -
reference electrodes 3 and 3a of detectors 13' and 13'a, respecti-
vely, are exposed to ambient air. In addition, the two solid
state detectors are placed into the same electrical oven 16 to
achieve uniformity of operating temperature for both of them. The
potentials built-up at each electrode are sent to an analyser 14
which differentiates the signals from both detectors, thereby can-
celling the variation effects of oxygen in the measurement of the
anhydride concentration in the sample C. The sample gas is exhaus-
ted from both detectors through conduits El and E2, respectively,
by means of a pump 15.
Although the arrangement illustrated in figure 6 has been
described above with reference to a detector 13' similar to the one
shown in figure 4, it should be understood that anyone of the anhy-
dride detectors o figures 1 to 3 may as well be used. The use of -
the detector 13' in the arrangement of figure 6 being given by way
of example only. On the other hand, the oxygen detector 13'a may
be of any known type, and the one described in U.S. Patent No.
3,400,054 issued on September 3, 1968, to Ruka et al, may, for ~-
instance, be advantageously utilized.
Conclusive results have been obtained with the arrangement
shown in figure 6, specific experimental results for SO2 and CO2
being presented on figure 7. An experiment carried on with CO2 is
further given below in example 6.
Figure 3 shows another arrangement suitable to compensate
variations of the partial pressure of oxygen contained in a gaseous
sample. Compensation is achieved by injecting a predetermined
amount of an oxygen rich gas F into the incoming stream of a sample
gas C, the flo~ of the oxygenated gas F being regulated by means of
'.
-- 10 --
.
~ .. . . ~ ., . .. . .. ", - ., , . . .:
~0443~æ4
a flow-meter 17. Thus, the partial pressure of oxygen is increased
to a nearly constant level, which enables a true determination of
the concentration of the anhydride to be detected by the detector
18, the latter being of the type described in anyone of figures 1
to 4. It is therefore noted that the concentration of oxygen at
the measuring electrode of detector 18 is substantially stable and
proportional to the ratio F/C. A pump 19 controls the flow of the
gas mixture, which flow value may be observed by means of the flow-
meter 20.
Referring to figures 9a, 9b and 9c, there are shown par-
ticular arrangements of the inlet and outlet conduits suitable to
bring the anhydride to be detected and/or the reference gas in
close contact with the corresponding electrode. Although these em-
bodiments may appear quite simple, they have proven to be highly ef-
fective in hermetically sealing the contact points with the surface
of the solid state sensors. As illustrated in figures 9a and 9b,
two substantially parallel channels 22 are pierced in a rod-like
material 21, and thereafter the extreme portion 23 of the rod,
that is the portion facing the measuring electrode 2, is cut off
in order to provide a free gas flow path for sample C between the
two channels. Sealing is effected by heating the electrolyte
element to a temperature in the vicinity of its sintering tempera-
ture and then by pressing the extremity of the rod-shaped material
provided with the opening 23 against the surface of the electrolyte
element so as to slightly embedding the peripherical extremities
thereof into the electrolyte element. A highly hermetical sealing
is thus produced. Although in figures 9a and 9b the particular
conduit arrangement is used in connection with the sensors shown
in flgures 4 and 3, respectively, it is understood that the above-
described sealing method may be readily applied to any other types
of solid state sensors, particularly those illustrated in figures -
1 and 2. In this respect, utilizing the solid state sensor of
-- 11 -- ~
~, .
~.6)443Z4
figure 1, an arrangement of a particular interest, being highly com-
pact, is presented in figure 9c in which a plurality of substan-
tially parallel channels 22' have been pierced through the rod-
like material 21' and openings 23 provided at the rod extremity and
in alignment with the respective measuring electrode 2 and referen- -
ce electrode 3 to bring the sample gas C and the reference gas D
in intimate contact with the corresponding electrodes. It is to
be noted that with such arrangement both electrodes may be set at
the same side of the solid state electrolyte element 1, thereby
greatly increasing the compactness of the detector. A separating
wall 24 prevents the intermixing of the gas sample and the referen~
ce gas, this separating wall being also embedded ïnto the element 1
in accordance with the sealing method mentioned above so as to
sealingly separate the sample gas channel from the reference gas
channel. -
The rod-like material may be of ceramic, alumina, mullite,
or quartz.
The arrangement shown in figure 9 offers several advanta-
ges over hitherto known gas inlet and outlet conduits. Indeed, the
present arrangement allows a direct contact of the gases with the
different electrodes of the solid state electrolyte element and
ensures the complete sealing of the various channels with respect
to the environment and other adjacent channels. Since the diameters ~
of the channels are relatively small, their volume are reduced to -
a strict minimum and the gases, after having contacted the electro-
de, are promptly exhausted. Thus, the system response time, always
influenced by gas dead volumes, is substantially increased and the
true values of the potential corresponding to the concentration of
the sample gas become available for analysis at the very start of
the detection proceedings. Moreover, it is to be noted that addi-
tional hermetically sealed channels may be provided for the leads
connecting the various electrodes to the voltmeter 9, rather than ~ -`
- 12 -
: ' , '. ~ . ~ : . . ' ,
~ ^~
~0443Z4
running through the gas channels, as shown in figure 9c for ins-
tance.
We will hereinafter give some examples of experiments
carried out by means of the arrangements illustrated in the above-
described figures with particular reference to the graphs shown
in figures 5, 7 and 10 on which experimental results have been
quoted.
EXAMPLE 1:
A series of tests were conducted using the arrangement
of figures 1 and 2 to monitor the amount of SO2 in air. Various
S2 ~ Air mixtures were prepared and tested. In a concentration
range running from 0.1 to 20,000 ppm, it was established that a
linear relationship existed between the logarithm of the SO2 con
tent of the sampled gas and the electro-motive force recorded on
the voltmeter.
EXAMPLE 2:
A series of tests were conducted using the arrangement
of figure 3 for samples such as SO2, H2S, CH3S and COS in air.
A pellet similar to that shown in figure 3 was used as
a sensor. The pellet was of pure K2SO4 in contact with a platinum
electrode and of X2SO4 doped with 1% of Ag2SO4 in contact with a
silver electrode.
Each series of tests of a sulfur-bearing molecule in
air at different concentrations gives a linear relationship bet- -
ween the log of the concentration of the sulfur-bearing compound
and the electro-motive force recorded on the voltmeter. The re-
sults obtained from samples of SO2 - Air, H2S - Air and COS - Air ~
are presented on figure 5. ~ -
It was also demonstrated that the presence of a high
concentration of NO2 in a tested sample does not affect the mea-
surement of SO2.
. . ~
The detector of figure 3 was kept into operation for more
1~)4~3Z9~ -than 5 weeks and proved to be stable and the results reproducible.
EXAMPLE 3:
Another series of tests were effectuated by using the
arrangement of figure 3 for determining the amount of CO2, CO, COS, -
HCHO, CH30H, (CH3)2CO, CH4, C2H6, C2H4 and C2H2 in air samples.
The sensing element used for those tests consisted of - -
pure K2CO3 near a platinum electrode and of K2CO3 doped with 1% of
Ag2SO4 near a silver electrode. The results obtained with different ~--
mixtures CO2 - Air are summarized on figure 5 and demonstrate well
the linearity of the detected values.
EXAMPLE 4:
A series of tests were also performed with NO2 - Air, ;~
using a nitrate pellet. Each pellet was of pure Ba(NO3)2
near a platinum electrode and of Ba(NO3)2 doped with 1% of
AgCl near a silver electrode. The operating temperature of
the system was in the range of 450C.
~ The results obtained from different mixtures of nitro-
; gen oxyde in the air indicate that the mixtures NO2 - Air gave
rise to a linear emf signal through the pellet of nitrate. Those
results with mixtures of nitrogen dioxyde-Air are reproduced on
figure 5.
EXAMPLE 5:
A series of tests were performed with the arrangement
of figure 9a with various SO2 - Air samples.
A sensing element similar to the one shown on figure 4
was used as a sensor. The sensor was made of pure K2SO4 in con-
tact with a (ZrO2) 85(CaO) 15 electrolyte, the latter being in con-
tact with a platinum electrode in air.
The series of tests obtained with the detector of figu-
re 4, when using the oxygen of air as a reference, gave a linearrelationship between the log of the concentration of SO2 and the
electro-motive force recorded on the voltmeter.
- 14 -
10443Z4
The anhydride detector of figure 4 has proven to be
highly stable and reproductible for a continuous operation of more
than a month.
EXAMPLE 6:
As mentioned above with reference to figure 6, the anhy-
dride detector is influenced both by a variation in the partial pres-
sure of the anhydride and of the oxygen in the sample. This pheno-
mena does not however interfer in environmental measurement since
the oxygèn partial pressure is constant in air, but it has to be
taken into account in stack gas analysis and other gases where
the oxygen partial pressure may fluctuate.
With CO2, for example, the above experimental observation
may be translated by the following electrode reaction:
C2 t 1/2 2 ~ 2e ~-CO3
which explains the form of the signal obtained at the electrode:
signal = ~o ~ 4F log PO2 ~ 4F log PCO2
E = the signal of the anhydride detector in volts
Eo - a constant fixed by the composition of the reference electrode
R = gas constant
T = temperature in K
F -= Faraday number
pO2=oxygen partial pressure
pCO2 =partial pressure of the carbon dioxyde in the sample '
On the other hand, it is known that the oxygen partial
pressure can be measured with a zirconia stabilized oxygen detector
which produces a following signal:
E = E ' ~ RT g P 2
where Epo = Signal proportional to PO2 -~
E ' = a constant characterized by the material of the re-
ference electrode.
The arrangement of figure 6 was used in order to verify
the proposed mechanism of oxygen compensation for the CO2 detector.
- 15 -
- : :
i~)44~4
The experimental results of figure 7, confirm such a mechanism
and indicate that the electronic compensation of the signal from
the oxygen detector by the signal from the CO2 detector leads to a
resulting signal which is independent of the concentration varia-
tions of the oxygen contained in the analysed gas sample.
With CO2, compensation is obtained by directly opposing
the signals from the two detectors. This same signal opposition
principle may be applied in connection with any other anhydride
detectors.
In certain cases, the oxygen detector signal has to be
modified before being opposed to the signal from the anhydride de-
tector. For example, in compensating for the signal from a SO2
detector, it has been experimentally observed that the signal from
this latter detector may be compensated by the signal from the
oxygen detector if the oxygen signal is multiplied by two (2)
before opposing it to the SO2 signal. This fact can be mathemati- `~
cally demonstrated by introducing the equilibrium constant of the
reaction (S02 ~ 1/2 2 ~-' SO3) in the calculation of the signal from
,~,! .
the SO2 detector in the appropriate low concentration range of
oxygen ( ~ 20% 2)
EXAMPLE 7:
Another series of tests was made in order to demonstrate
the performance and the reliability of the particular arrangement
of figure 9a. The solid state detector used was a sensing element
as described on figure 4 and in example 5. Sealing through thermal
deformation was easily obtained at temperatures near the sintering
temperature of the element. The curve shown in figure lO clearly
shows the short response time obtained wlth that arrangement since
90% of the signal is available in about 30 seconds from the start of
the measurement. Such performances are mainly due to the short
time spent by the gas at the measuring electrode, to the small
dead volumes of ~as in the inlet and outlet channels and to the
- 16 -
1044324
reduced adsorption surface.
EXAMP LE 8:
-
Moreover, in order to demonstrate that the illustrated
geometrical forms of the sensor are not limitative and that any
form respecting the electrochemical principle of the sensor can be
used and that multiple combination of the different preferred em-
bodiments described abo~e may be used to achieve a multi-function
detector, tests were performed on a combination detector.
A combination detector madeup of a sulfur anhydride
detector, a carbon anhydride detector and an oxygen detector suita-
ble to compensate for the oxygen variations in this sample gas (as
mentioned in example 6 above) was built up. An oxygen ion elec-
trolyte tube (one end closed), the electrolyte being for instance
stabilized zirconia, is interiorly platinized in order to provide
a common reference electrode with air (PO2 = O.21 atm). A portion
of outside surface of the tube is covered with a K2SO4 electrolyte "
and another portion with a K2CO3 electrolyte. These electrolyte
layers are obtained by wetting the tube with the molten electrolyte
or by any powder deposition technique, and are then sintered. ~;
Then, two platinum wires are turned around respectively -
both electrolyte layers covering the tube outside surface to cons- ~ ~ ~
titute the measuring electrodes for the two anhydrides to be detec- ~ ~ -
ted whereas a third platinum wire is simply wound around the zir~
conia tube to constitute the measuring electrode used to compensate
for oxygen partial pressure variations.
Therefore, when samples containing SO2 and CO2 are passed ~-
over this tube, emf signals identical to those shown in figure 7
are obtained, which signals are proportional to the anhydride con-
centrations when the oxygen partial pressure varies.
- 17 -
'.
