CA1106447A - Probe for an electrochemical oxygen measurement pickup - Google Patents

Abstract

F-6449 .

PROBE FOR AN ELECTROCHEMICAL OXYGEN MEASUREMENT PICKUP

Abstract of the Disclosure A probe for an electrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte with electrodes and electrical contact points connected by conduc-tors to the electrodes, in which the probe has an electro-chemically active region provided with the electrodes and an electrochemically passive region provided with the contact points as well as with their electrical connections to the electrodes. This minimizes or avoids false readings resulting from temperature gradient and changes in chemical equilibrium.

Description

) Background of the Invention Field o~ the Invention ~ . ~ . . I

This invention relates to a probe for an electrochemical oxygen measurement pickup having a solid, oxygen ion-conducting electrolyte with electrodes, and contact points at ~he end, which are connected to the electrodes in an electron-conducting manner.

Description of the Prior Art Probes (sensor elements) ~or oxygen measurement pickups often consist o~ an ion-conducting solid-electrolyte tube which is closed on one end and ~he inner and outer surfaces o~ which .are provlded with electrodesO These electrodes extend up to the open end o~ the solid~electrolyte tube and ~orm there the electrical contact po1nts, from which the electric probe voltage .
i3 taken and i~ conducted to external terminals. If ~uch .
measuring pickups are used, there is danger of false measure- ¦
ments, as the solid~electrolyte tube which usually protrude~
: : . kransYersely :Lnto the hot gas stream has a temperature grad.ient from its tip to its end and therefore has different ion conductivity. As a result~ a voltage which decréases from the probe tip toward the end is dellvered and the resultant electric voltage whlGh is taken o.~f a~ the contack points 1s thereby inaccurate and~cannot be related unequivocally to a defi.nite oxygen content of the gas to be measuredO

If, ln addition3 the elect.rode exposed to the gas to be measured ) mùst be catalytically effective and must bring the gas compo-nents into chemical equilibriumg then the chemical equ~librium ,", ,,,,,, I
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110644S' is adjusted differently along the probe in accordance with the temperature gradient. From this, dif~erent gas concentrations result and therefore, different electric electrode voltages (mixed potentials~,which additionally falsify the electrical signal o~
the probe.

Summary of the Invention i An object of the invention is to provide a probe for oxygen measure-! ment pickups of the type mentioned which will deliver a more ac-curate voltage measurement, i.e. a measurement substantially free -¦ 10 of the errors resulting from temperature gradient and variances in chemical equilibrium.
I With the foregoing and other objects in view, there is provided in accordance with the i~vention a probe for an e~ectrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte i with electrodes and electrical contact points connected to the elec-trodes, the combination therewith o~ an electrochemically active region o~ the probe which has the electrodes and the oxygen ion-conducting solid electrolyte for passage of oxygen ions through the solid electrolyte, and an electrochemically passive region of the probe which carries the contact points as well as their electrical connections to the electrodes, and wherein the passive region has a solid electrolyte on which the contact,points and electrical con-nections are arrangedS with the contact points spaced from each other as well as the electrical connections to the electrodes spaced from each other to require ions moving from one contact point to another and one connection to another to travel a long path, thereby reduc-¦ ing an interfering current9 and wherein the active and passive

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i j~ 106447 . regions are separated by an insulating layer ~hich does not con-duct ions or electrons.
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Other features which are considered as characteristic for the invention are set forth in the appended claims.

.. Although the invention is illustrated and descrlbed herein as embodied in a probe for an electrochemical oxygen measurement .. : plckup, it is nevertheless not intended to be.:limited to the details shown, since various modifications may be made therein . without departing from the spirit of the invention and within the scope and range of equivalents of the clai~.s.
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--` 1106447 Brief ~escription of ~he Drawlngs ~he invention, however~ together with additional ob~ects and advantage~ thereof will be best understood from the ~ollowing descrlptlon when read in connection with the accom-panying drawings, in whlch:

FIG. 1 shows an axlal longitudinal cross section through a tubular probe~ closed on one side, ln accordance with the lnvention3 and 10 FIGo 2 is a cross section through the tubular probe along : line ~ o~ ~a~ l; and . ' ~XC7 ~ 3 ls a cross sec~ion through the tubular probe along line ~ III of FIG l; and FIG. 4 ls a varlant of the tubular probe shown in FXG~ l; and FIGo 5 i~ an axlal longltudinal section of another varlant of the tubular probe æhown in FI~. l; and ~XG~ 6 ls the ~u~ular probe o~ FIG. 5 with another design of the lnsula~lng body.

Detailed Descrlption of the Inventlon rrhe probe has a~ least one electrochemically act~ve region provlded wlkh ~he electrodes and an electrochemicall~ passlve, or at least largely pass~ve region which is provided with the contact points as well as wlth their electrical connectlons to the electrodes. The active region can therefore be chosen independently of the total length of the probe and be adapted .

3 -~ 1106447 specially for the intended application. Here, different temperatures of the passive part are without, or at least without appreciable effect on the voltage delivered by the probe.

A minimum of structural means is required if both regions of the probe have a solid electrolyte, on which the contact points as well as the connections are arranged with a large mutual dlstance for forming the passive region. Although in this embodiment, the contact points as well as the connec-tlons are arranged on the solid electrolyte, their effect asactlve electrodes is very small, as the path which is very long as compared to the active regionJ for the ions in the solld electrolyte of the passive reglon results in a high lnternal resistance and thus ~n a small delivered current, whlch hardly causes interference. The active region effect~
the voltage delivered by the probe. The passive region is wlthout appre¢iable effect on the voltage delivered by the probe.

In~order to reduce the inf1uenoe of the p-ssive part further, 20~ ~ an~1nsu1ating 1ayer which does not conduct ions or electrons is inserted between the two reglons~ This eliminates part- ;
1cular1y~the e~fects of the junctions from the electrodes to the~connections. The influence of the passive part on the measurement is p~actically completely eliminated by formlng the pas6ive r~eglon with an lnsulating part, preferably of ceramic, ~which does not condu~t electrons or ~onsO

The connections consi6t advantageou91y of conduotor runs. I~
the~probe is of tubular design,~ it is advisable to arrange the .

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- llU6~47 I conductor runs approximately d~ametrically. For a good compromise between the requirement of an active region as large as possible and the requirement of a measurement sub~
stantially una~fected by influence of temperature gradient and chemical reaction, it is advisable to have the passive region o~ the probe d~sposed in that part of the probe which is to extend in a probe housing.

~n the drawings, identical parts in the individual figures have the same reference symbols .

Referring to FIG. 1, the probe comprises a ~olid-electrolyte ~ube 1~ with a closed left and an open right end~ Zirconium dioxide serves as the oxygen ion-cond~cting solid electrolyte~
In ~he actlve region 11 o~ the probe which is provided ~or delivering the measurement signal and which extends from the tip o~ the solid-electrolyte tube 10 in the axial direction, the en~lre outer surface 1s pre~erably provided wi~h an outer electrode 13 and the entlre inside surface with an inner electrode 14. In some cases it may also be sufficient to make one or both electrodes as mutually opposite strips. The active region 11 is designed as to its electrode areas and the electro-lyte $hickness so that at the operatlng temperature a sufficient-lg low in~ernal electric resistance is obtained, for instance, between 10 and 10,000 K Q, and preferably 10 to 100 K~J. The passive reg~on 12 formed w~th the same ~olid electrolyte carrles an outer connection 15 connected to the outer electrode ~ 13 and an inner connection 16 connected to the inner electrode `~ ) 14, the ends of which ~orm the contact points 17 and 18.
~ , . , The connections are electron~conducting and, as can be seen clearly ~rom FI~. 3, consist Or conductor runs, the width ... . . .
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`` 11064g~7 J of which is between 1 and 10 mm, depending on the probe diameter, but preferably between 2 and 4 mm. It is important .
here that the path for the ions from the one to the othe~ :
connection through the solid-electrolyte tube is very long, as thereby the internal electric resistance is increased considerably and the passive region 12 cannot there~ore deliver an appreciably interferlng current and thus, for all practical purposes~ cannot falsify the measurement voltage~ The connec-tions 15 and 16 may be made of any electron-conducting, .
corrosion-resistant material but preferably, they consist of :
extensions of the respective electrodes~ In order to ensure good contact with the potential leads whlch go on from there, the conduc~ors can be w~dened at the contact pointsO

The con~kruc~ion o~ the probe according to FIG. 4 is similar to that of the probe of FIG. 1. The difference between the probe o~ FIG. 1 and the probe of FIG. 4 consists merely 1 .
the ~olid electrolyte o~ the active region 11 belng separated ~rom the solid electrolyte of the passive region 12 by an interposed circular insulating layer 19 (FIG. 4~. Th~s measure creates a clean separation of the two regions The insulating layer consists preferably of gas-tlght, sintered-glass ceramic which does not conduct ions or electrons, such ~: as ceramlc o~ the ~ollowing compositionO Sl023 35 to 50~0;
: Mg0, 50 to 30%; and A1203, 15 to 20~ The insulating layer .
is connected by a sintering or fusing process to the two parts of the tube~ which~ inciden~ally, may also have different .
outside diameters; as shown in FIG. 6. .

The probe shown in FIG. 5 corresponds generally to the probe - 6 - ~

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44'7 FIG. 1. The probe shown ln ~IG. 5 corresponds generally ¦
to the probe of FIG. 1. The difference between the probes is that the passive region 12 is built up on an insulating part Or tubular shape 20 tFIGo 5) which i9 fastened to the solid-electrolyte tube 10, and the outside and inside diameter of the insulatlng part 20 corresponds approximately to the diameters of the solid-electrolyte tube 10. As this insulating part 20 consists of a material that does not conduct ions or electrons9 there is practically no danger that the measurement result is lnfluenced in any way by the passive region. Thls would not b.e the case if the connections 15 and 1~ did not consist of narrow c~nductor runs but were simply formed by an extension o~ the electrodes 13 and 14 covering the entire inside and outside surfaces. The insulating par~ 20 preferably conslsts of magnesium silicate Mg2(SiO4), magneslum-aluminum spinel MgO-A1203 or the sintered-glass ~eramlc mentioned above.
. The electrolyte tu~e is fastened to the insulating part either by directl~ ~lntering together the two parts or by inserting a connectlng layer of glass solder or sintered-glas~ ceramicO

20 me basic de~ign of the probe shown in FIG. 6 corresponds to that of FIG. 5. However, the insulating part 20.of FIG, 6 ls made ~n the shape of a circular f'lange, the outside diameter -of which is larger than the corresponding diameter of the solld-electrolyte tube 10. In order to center this tube with respect to the flange~ it ls inserted into a step~ e expansion ~ of the ~lange opening and fastened there. The free end of the : ~lange has likewise an inner step for forming the shoulder 21, on which the connections 15 and 16 end and ~orm the contact points 17 and 18. To this end~ the outer connection 15 ls brought on the flange on the outside up to the shoulder 21, ..... . . ..

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as can be seen in ~IG. 65 and the inner connectlon 15 runs on :
the inner wall of the flange up to the shoulder 21. However, ~ would also be possible to also arrange the connection 15 on the inside wall o~ the flange approximately diametrically oppo-site to the connection 16 and to let it end on the shoulder 21.
To.this end it is necessary to bring the connection 15 at the ~oin~ of the tube and the flange from the ou~side to the inside, which can be done easily particularly if a connecting layer is interposed at this pointO ln order to avoid shor~ circuits, ¦~
the inner electrode 14 should not quite exkend to the end of the ~olid-electrolyte tube 10 at this ~eedthrough point The inside diameters of the solid-electrolyte tube and the ~lange are .
approximately equal and the ratio Or the outslde dlame~ers i~
about 1 : 1~5 to lo 2. The choice of the material ~or the ~lange may be the same as those made in connection with FIG. 5.

For determining the size and length relations of the active to ~he passive region, the rule generally applies that the active region is made only large enough so that a suff~ciently low internal re~istance o~ the probe is provided. The remaining part o~ the probe is~made pas~ive. The size will vary depending I on the given measurement problem. As a minimum, howeve~, at least the reglon of the probe which is intended to be inserted in a probe houslng, should be deslgned aa passlve.

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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a probe for an electrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte with electrodes and electrical contact points connected to the electrodes, the combination therewith of an electrochemically active region of the probe which has the electrodes and the oxygen ion-conduction solid electrolyte for passage of oxygen ions through the solid electro-lyte, and an electrochemically passive region of the probe which carries the contact points as well as their electrical connections to the electrodes, and wherein the passive region has a solid electrolyte on which the contact points and electrical connections are arranged, with the contact points spaced from each other as well as the electrical connections to the electrodes spaced from each other to require ions moving from one contact point to another and one connection to another to travel a long path, thereby re-ducing an interfering current, and wherein the active and passive regions are separated by an insulating layer which does not conduct ions or electrons.

Claim 2. Probe according to claim 1, wherein the insulating layer consists substantially of magnesium silicate Mg2(SiO4), magnesium-aluminum spinel MgO.A12O3 or sintered glass ceramic with approxi-mately the composition: SiO2, 35 to 50%; MgO, 50 to 30%; and A12O3, 15 to 20%.