Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1996-08-08
1998-05-26
Tung, T.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204427, 204428, G01N 27407
Patent
active
057559414
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention is based on an electrochemical measuring element, particularly for determining the oxygen content in exhaust gases of internal combustion engines, having a sensor element in the form of a tube that is closed at one end and is inserted into a metallic housing having a seal. Electrochemical measuring elements are configured, for example, to include a sensor element with a so-called finger design. The sensor element is a solid electrolyte body configured as a closed tube and secured in a gas-tight manner inside a metallic housing. The finger sensors are divided into potential-free and potential-bound measuring elements. In potential-bound measuring elements, the conductor path of the outer electrode is contacted to the housing by means of an electrically-conductive sealing ring. In potential-free measuring sensors, each electrode connection is tapped separately, which does not permit electrical contacting with the housing. In both cases, a seal must be produced between the solid electrolyte body and the housing.
In potential-free measuring elements, the gas-tight and benzene-resistant mounting of the sensor element in the metallic housing is associated with special problems. To solve these problems, either a metallic sealing ring or an electrically-insulating, ceramic sealing ring is used. Ceramic sealing rings are not absolutely gas-tight and benzene-resistant. If metallic sealing rings are used, the conductor path extending on the outer surface of the solid electrolyte body, in the region of the sealing ring, must be covered with an electrically-insulating cover layer. A drawback, however, is the occurrence of pressure peaks that originate at the metallic sealing ring and damage the cover layer, thus reducing its insulating effect.
SUMMARY OF THE INVENTION
In contrast, the measuring element of the invention having the characterizing features of the main claim has the advantage that it permits the solid electrolyte body to be mounted in the metallic housing to be gas-tight and benzene-resistant, as well as electrically insulated. The insulating effect of the cover layer is not reduced due to the use of the seal arrangement.
Advantageous modifications of and improvements to the measuring element of the invention are possible with the measures outlined in the dependent claims. A particularly gas-tight and benzene-resistant mounting is achieved with a metallic sleeve which is connected to the solid electrolyte body to form a structural unit, with the metallic sleeve being welded to the metallic housing. The additional use of a pressure-distributing sealing ring reduces pressure peaks onto the seal arrangement. Moreover, resistance to heat corrosion is improved with the additional sealing ring. To avoid cracks in the ceramic sealing ring and/or in the glass sealing, it is also useful for the thermal expansion coefficient of the connected materials to be as close to each other as possible. It could be ascertained, however, that a slightly higher thermal expansion coefficient of the metallic sleeve is not harmful, because the resulting compressive strains can be compensated by the ceramic sealing ring.
BRIEF DESCRIPTION OF THE DRAWING
An embodiment of the invention is illustrated in the drawing and described in detail below.
FIG. 1 shows a cross section through a part of a measuring element of the invention, the part being located on the side of the exhaust gas, and
FIG. 2 shows an enlarged cutout of a seal zone in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The electrochemical measuring element 10 according to FIG. 1, shown in cutout, has a metallic housing 11 having a thread 12 as securing mean for mounting in an exhaust gas pipe, not shown, and a sensor element 13. The housing 11 has a longitudinal bore 15 having a seal seat 16 on the side of the housing. A seal zone 30 shown on an enlarged scale in FIG. 2 forms at the seal seat 16 on the side of the housing, between the sensor element 13 and the housing 11. The sensor element 13
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Robert & Bosch GmbH
Tung T.
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