Seal for a joint or juncture – Seal combined with indicator – sampler – or inspection feature
Reexamination Certificate
1999-03-31
2001-03-27
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal combined with indicator, sampler, or inspection feature
C277S654000, C277S943000, C204S424000, C204S431000
Reexamination Certificate
active
06206377
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a seal arrangement for a sensing element of a gas sensor, in particular for determining the oxygen content in exhaust gases of internal combustion engines, this seal arrangement comprising a packet made up of several seal elements, packed one on top of another, which immobilize the sensing element in a longitudinal bore of a metallic probe housing.
BACKGROUND INFORMATION
What usually serves as the seal arrangement for a sensing element of oxygen probes is a precompressed and presintered powder packing made of steatite, which during assembly is exposed to a high axial pressure and is thereby reshaped. The sealing effect of this principle with respect to exhaust gases is entirely sufficient, but fuel in liquid or vapor form, and water vapor, can diffuse through the seal element and thereby interfere with probe operation.
German Patent No. 195 32 090 describes a seal arrangement which improves the seal effect with respect to fuel vapor by using a so-called “sandwich packing” of seal elements, in which a boron nitride disk placed in the packing decreases permeability for hydrocarbons. This solution is expensive due to the use of boron nitride.
A further alternative would be fusible sealing of the sensing element. The development of this approach requires extremely precise matching of the coefficients of thermal expansion of the materials used, which can lead to problems in large-volume production.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a seal arrangement for a sensing element of a gas sensor, in particular an oxygen sensor for determining the oxygen content in exhaust gases of internal combustion engines, in such a way that a substantial improvement in resistance to gasoline vapor and water vapor, and a decrease in cost as compared to the “sandwich” approach using a boron nitride disk, are achieved.
According to the present invention, the above object is achieved, in a seal arrangement, in that a funnel-shaped diversion element lying in the axial direction of the sensor is integrated into the seal elements packed one above another in such a way that gas permeability between the diversion element and the inner wall of the probe housing is ensured, and that toward the measured-gas end of the sensor, a small gap remains open between the sensing element and the diversion element.
The effectiveness of the funnel-shaped diversion element lies in the fact that the hydrocarbons and water vapor diffusing from the exhaust gas side through a lower dense powder packing encounter the lower end surface of the diversion funnel, and there can for the most part escape into the atmosphere through the small gap formed between the sensing element and the funnel-shaped diversion element and also through a space between the inner protective sleeve and the outer wall of the probe housing. To ensure that the gases cannot get into the reference air space of the probe, the metallic funnel-shaped diversion element as well as the inner and outer protective sleeves must be joined to one another via a sealed full-circumference laser weld seam.
The effectiveness of the funnel-shaped diversion element can be increased by the fact that placed before its outer end face on the measured-gas side is a coarsely porous powder packing, for example made of steatite that has been presintered at temperatures of 750°, whose diffusion resistance is much lower with respect to the dense powder packing adjoining it toward the measured-gas side and with respect to the dense powder packing adjoining it on the connection side.
It is also alternatively possible to use, instead of the coarsely porous powder packing, a ceramic disk having transverse holes or ventilation grooves extending radially on the end face, with which the same effect as with the coarsely porous powder packing can be achieved.
In summary, the advantages of the seal arrangement according to the present invention are as follows:
a substantial improvement in resistance to gasoline vapor and water vapor as compared to the approach commonly used today;
a decrease in cost as compared to the “sandwich” approach using a boron nitride disk;
the fact that despite the additional funnel-shaped diversion element, the number of components is no greater than with the “sandwich” approach; and
the fact that as compared with the fusible glass sealing approach, no new and (in some cases) critical process steps are required.
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patent: 195 32 090 (1997-03-01), None
patent: 94/29710 (1994-12-01), None
Kenyon & Kenyon
Knight Anthony
Pickard Alison K.
Robert & Bosch GmbH
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