Electricity: electrical systems and devices – Electrostatic capacitors – Variable
Reexamination Certificate
1995-03-14
2001-06-26
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Variable
Reexamination Certificate
active
06252759
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to capacitive pressure sensors and capacitive differential pressure sensors.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,388,668 describes a capacitive pressure sensor comprising:
a substrate made of ceramic material,
on the one surface of which
a first metallic electrode is disposed in direct contact with the substrate;
a diaphragm made of ceramic material,
which is permanently joined and pressure-sealed to the substrate by means of an original glass frit acting as a spacer holding the diaphragm and the substrate at a defined distance from each other to form a cavity, and
which on the surface facing the substrate is provided with a second metallic electrode; and
leads
which extend inwards to the electrodes in the region of the spacer.
Further, U.S. Pat. No. 4,531,415 describes a capacitive differential pressure sensor, comprising:
a substrate made of ceramic material,
whose surfaces facing away each other—the first and second surfaces—support
a first metallic electrode and a third metallic electrode, respectively, each being in direct contact with the substrate;
a first and second diaphragm made of ceramic material,
which are permanently joined and pressure-sealed to the first and second surfaces of the substrate, respectively, by means of an original glass frit acting as a spacer holding the diaphragms and the substrate at defined distances from each other to form a first and second cavity, respectively, and
which on each cavity side are provided with a second and a fourth metallic electrode, respectively; and
a hole connecting the two cavities and leading through the substrate and outwards,
the two cavities and the hole being filled with oil.
With the two types of pressure sensors mentioned, the prevention of short-circuits between the electrodes facing each other is important. Such short-circuits may be caused by material particles coming between the electrodes during manufacture and not being discovered there, or during operation by pressures which are higher than the pressure admissible for a concrete type of pressure sensor and which thus lead to contact between the electrodes, or through parts of the electrodes detaching themselves from the diaphragm and/or substrate and contacting each other again.
According to U.S. Pat. No. 4,531,415 already mentioned, an attempt was made to solve the short-circuit problem by fixing to the substrate some parts of insulating material protruding from the electrodes as projections, so that in case of an inadmissible high pressure the electrodes of the diaphragm can be intercepted by the projections and thus cannot touch the electrodes of the substrate.
SUMMARY OF THE INVENTION
This design, however, only eliminates the second cause of a short-circuit described above, while the two other causes are not precluded. The invention therefore serves for a general solution to the short-circuit problem.
To this end, the invention consists on the one hand of a capacitive pressure sensor, comprising:
a substrate made of ceramic material,
on one surface of which
a first metallic electrode is disposed and
a glass layer formed from an original glass frit is disposed at least on the first metallic electrode;
a diaphragm made of ceramic material,
which is permanently joined and pressure-sealed to the substrate by means of the original glass frit acting as a spacer holding the diaphragm and the substrate at a defined distance from each other to form a cavity, and
which on the cavity side is provided with a second metallic electrode; and
leads
which extend inwards and/or through the diaphragm to the electrodes in the region of the spacer.
The invention consists on the other hand of a capacitive differential pressure sensor, comprising:
a substrate made of ceramic material,
whose surfaces facing away each other—the first and second surfaces—support
a first metallic electrode and a third metallic electrode, respectively,
a first glass layer formed from an original glass frit at least on the first metallic electrode,
a second glass layer formed from the original glass frit at least on the third metallic electrode;
a first and a second diaphragm made of ceramic material,
which are permanently joined and pressure-sealed to the first and second surfaces of the substrate, respectively, by means of the original glass frit acting as a spacer holding the diaphragms and the substrate at defined distances from each other to form a first and a second cavity, respectively, and
which on each cavity side are provided with a second and a fourth metallic electrode, respectively;
leads,
which extend inwards and/or through the first and the second diaphragm, respectively, or through both diaphragms to the respective electrodes; and
a hole connecting the two cavities, leading at least through the substrate and outwards,
the two cavities and the hole being filled with oil.
In accordance with a development of the pressure sensor of the invention, a thin insulating layer made of the ceramic material of the substrate or of a high-temperature glass is disposed on the glass layer covering the first electrode, the diaphragm being permanently joined to the insulating layer via the spacer.
In accordance with a development of the differential pressure sensor of the invention, thin insulating layers made of the ceramic material of the substrate or of a high-temperature glass are disposed on the glass layers covering the first and the third electrodes, respectively, the respective diaphragms being permanently joined to the respective insulating layers by means of the respective spacers.
The glass layer disposed on the electrode of the substrate ensures that no short-circuits whatsoever can occur between the electrodes of the substrate and of the diaphragm, since one of the electrodes is completely covered with a coating of insulating material.
Furthermore, the electrodes of the substrate are protected substantially completely against any moisture that can enter the cavity of the pressure sensor first referred to above when said sensor is not used as an absolute pressure sensor with evacuated cavity but as a reference pressure sensor with a cavity which is connected to the environment by means of an aperture, cf. U.S. Pat. No. 5,079,953, or which may be contained as a water content in the oil filling in the differential pressure sensor referred to above in second position. This moisture protection increases the long-term stability of the pressure sensor and/or the differential pressure sensor.
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patent: 4531415 (1985-07-01), Orlowski et al.
patent: 5186054 (1993-02-01), Sekimura
patent: 5349492 (1994-09-01), Kimura et al.
patent: 5375034 (1994-12-01), Foyt et al.
patent: 5400489 (1995-03-01), Hegner et al.
patent: 3137219 (1983-04-01), None
patent: 40 04 275 A1 (1991-08-01), None
patent: 0 472 108 A2 (1992-02-01), None
patent: 2101331 (1983-01-01), None
Gill Norbert
Lange Jurgen
Bose McKinney & Evans LLP
Endress + Hauser GmbH + Co.
Nguyen Chau N.
Reichard Dean A.
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