Device with exchangeable sealing element for measuring pressure or differential pressure

For measuring pressure or differential pressure, in particular for highly viscous and/or glutinous foodstuffs, provision is made for a device which exhibits a ceramic pressure sensor (1), a housing (2) and a connection element (3) which is connected releasably to the housing (2) and is intended for fastening the device on a wall (6) of a container which contains a measuring medium. Furthermore, two sealing elements (4, 9) which engage around one another coaxially are provided: an outer sealing element (4) which is located between the housing (2) and the pressure sensor (1), on the side facing the measuring medium, and an inner, exchangeable sealing element (9) which is arranged between the pressure sensor (1) and the connection element (3).

FIELD OF THE INVENTION 
The invention relates to a device for measuring pressure or differential 
pressure, having a ceramic pressure sensor. 
DESCRIPTION OF THE PRIOR ART 
DE-A 42 34 290 describes a device for measuring pressure 
having a rotationally symmetrical ceramic pressure sensor, 
having a rotationally symmetrical housing which exhibits an axial bore, 
the diameter of which bore decreases in the direction of the front side 
facing the measuring medium, and 
having a single sealing element which is inserted, flush at the front, 
between the housing and the outer surface of the pressure sensor. 
A disadvantage of such a device is that the sealing element, which is 
constantly in contact with the measuring medium, cannot be exchanged 
without the pressure sensor having to be removed from the housing. After 
the sealing element is exchanged, the device has to be recalibrated. 
However, this exchanging operation is necessary not only for replacement 
purposes, but also for cleaning, in particular when the device is used in 
the food industry. This is because the sealing elements installed in the 
manner described above form potential bacteria traps there. 
Furthermore, DE-A 42 13 857 describes a device for measuring pressure or 
differential pressure, having 
a ceramic pressure sensor, 
a housing, 
a connection element which is connected releasably to the housing and is 
intended for fastening the device on a wall of a container which contains 
a measuring medium, 
the connection element exhibiting a narrow, axial inner bore which has the 
function of a diaphragm seal and by way of which the pressure sensor is in 
contact with the measuring medium, and having 
an exchangeable sealing element which is inserted between the housing, 
connection element and pressure sensor, on the side facing the measuring 
medium, and is clamped in with a defined force by the connection element 
being screwed against a stop. 
A device of this type, however, is not suitable for highly viscous and/or 
glutinous media, since such media block the axial inner bore of the 
connection element and thus impair the pressure transmission. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide a device for measuring pressure 
or differential pressure, which is suitable, in particular, for highly 
viscous and/or glutinous foodstuffs and can be easily cleaned without the 
electrical properties being impaired. 
For this purpose, the invention comprises a device for measuring pressure 
or differential pressure, having 
a ceramic pressure sensor, 
a housing, 
a connection element which is connected releasably to the housing and is 
intended for fastening the device on a wall of a container which contains 
a measuring medium, 
an outer sealing element which is inserted between housing and pressure 
sensor, on the side facing the measuring medium, and 
an exchangeable inner sealing element which is arranged between the 
pressure sensor and connection element, 
which sealing elements engage around one another coaxially. 
According to an advantageous configuration of the invention, the pressure 
sensor exhibits a pressure-sensitive diaphragm, and the housing and the 
connection element are each provided with a central opening through which 
the pressure diaphragm is in contact with the measuring medium. 
According to an advantageous development of the invention, on its side 
facing the measuring medium, the housing terminates with a securing ring 
which projects into the housing interior and exhibits a groove for 
receiving the outer sealing element, the pressure sensor being pressed 
against the outer sealing element. 
According to a further advantageous configuration, the connection element 
engages around the housing on the side facing the measuring medium and 
presses the inner sealing element against a pressure-insensitive border of 
the pressure sensor, the inner sealing element resting against the 
connection element on the side facing the measuring medium, resting 
against the pressure sensor on the side remote from the measuring medium, 
and resting against the housing on the outer side, and being in contact 
with the measuring medium on the inner side. 
A further advantageous configuration of the invention consists in the fact 
that the measuring medium is not in contact with the housing. 
According to a further advantageous configuration of the invention, the 
housing is connected to the connection element in a pressure-tight manner 
via the outer sealing element, and the housing and the connection element 
are screwed to one another. 
Furthermore, the connection element may be connected releasably to the wall 
of the container in a pressure-tight manner and may consist of a material 
which is resistant to the measuring medium, preferably of Hastelloy, 
titanium, tantalum, Monel, nickel or Inconel. 
One advantage of the invention consists in the fact that the device 
exhibits a largely planar front surface, and there are no constrictions 
through which the measuring medium is directed. 
A further advantage of the invention consists in the fact that the device 
makes it possible for the sealing element which is in contact with the 
measuring medium to be exchanged without recalibration of the device being 
necessary and without there being any need for components which have the 
function of diaphragm seals. 
A further advantage of the invention is that, by virtue of the releasably 
connected connecting element, the device can be used as a universal 
component for a large number of process connections, and that it is 
possible to avoid expensive stock keeping with a large number of 
exchangeable parts which are necessary on account of hygiene regulations 
and/or due to heavy soiling.

DETAILED DESCRIPTION OF THE INVENTION 
In FIG. 1, the device for measuring pressure or differential pressure 
comprises three main elements: a pressure sensor 1, a housing 2 and a 
connection element 3 which is connected releasably to the housing. Said 
connection element serves to fasten the device on a wall 6 of a container 
which contains a measuring medium. An outer sealing element 4 is located 
between the housing 2 and the pressure sensor 1, on the side facing the 
measuring medium. An exchangeable inner sealing element 9 is located 
between the pressure sensor 1 and the connection element 3. The two 
sealing elements 4, 9 engage around one another coaxially. 
The pressure sensor 1 is, for example, a conventional capacitive 
cylindrical pressure measuring cell which comprises a diaphragm 11 and a 
substrate 12, these being held at a defined distance apart from one 
another by a bonding material, e.g. an active brazing material, and being 
connected to one another in a hermetically sealed manner. The inner 
surfaces of the diaphragm 11 and of the substrate 12, which inner surfaces 
are coated with electrode material, form at least one measuring capacitor, 
the capacitance of which depends on the bending of the diaphragm 11 and is 
thus a measure of the pressure acting on the diaphragm 11. 
The diaphragm 11 may consist of ceramic, oxide ceramic, quartz, sapphire or 
a crystalline material. The substrate 12 preferably consists of a material 
which is very similar to the material of the diaphragm 11, or at least has 
a comparable thermal expansion coefficient. 
On the side remote from the measuring medium, the pressure measuring cell 
exhibits an electronic circuit 13, which converts the capacitance of the 
measuring capacitor into a pressure-dependent electric signal and makes it 
available, via electrical connection lines 14, for further processing 
and/or display. 
However, other types of pressure sensors, e.g. pressure sensors which work 
with strain gauges, may also be used for the invention. 
The pressure sensor 1 is introduced into the rotationally symmetrical 
housing 2 such that the pressure-sensitive diaphragm 11 faces the 
measuring medium. The electrical connection lines 14 run in the housing 
interior, on the rear side, remote from the measuring medium, of the 
pressure sensor 1. 
FIG. 2 shows the housing 2 as an individual part. The internal diameter of 
a cylindrical interior 21 of the housing 2 is constant and corresponds to 
the external diameter of the pressure sensor 1. The housing 2 and the 
connection element 3 each exhibit a central opening 23, 31 through which 
the diaphragm 11 is in contact with the measuring medium. The central 
opening 31 of the connection element 3 is designed such that virtually the 
entire diaphragm 11 of the pressure sensor 1 is in connection with the 
measuring medium. This ensures good utilization of the sensitivity of the 
pressure sensor 1 and also means that the device is easy to clean. 
The cross-sectional surface area of the opening 31 increases in the 
direction of the side facing the measuring medium since the border of the 
opening 31 describes a quarter-circle 32 in longitudinal section, see FIG. 
1. That surface of the connection element 3 which faces the measuring 
medium is in alignment with the container wall 6. The connection element 3 
extends in front of the border region of the pressure sensor 1 only as far 
as is necessary for fastening the inner sealing element 9. Due to this 
design of the connection element 3, in which it is virtually flush at the 
front with the pressure sensor 1, the device lends itself well to 
cleaning. 
On the side facing the measuring medium, the housing 2 is designed as a 
securing ring 22 which extends radially into the interior 21 of the 
housing 2 and has an annular-cylindrical collar 221. The diameter of the 
central, circular opening 23 of the housing 2 is equal to the internal 
diameter of the annular-cylindrical collar 221 of the securing ring 22. 
In the interior 21 of the housing 2, on the side facing the pressure 
sensor, the securing ring 22 exhibits a groove 24 of, for example, 
rectangular cross section, the external diameter of which groove 
corresponds to the diameter of the interior 21. The outer sealing element 
4 is located in said groove 24. Said sealing element is, for example, an 
O-ring consisting of an elastomer and has the function of protecting the 
electronic circuit 13, arranged on the rear side of the pressure sensor 1, 
against soiling and/or moisture. 
The diaphragm 11 rests with its outer border on the housing seal 4 located 
in the groove 24. There is a narrow gap 223 between the diaphragm 11 and a 
shoulder 25 of the collar 221. On the side remote from the measuring 
medium, a threaded ring 5 which has an external thread and is screwed into 
an internal thread 26 of the interior 21 of the housing 2 presses the 
pressure sensor 1 against the securing ring 22 and the outer sealing 
element 4, with the result that the pressure sensor 1 rests there in a 
pressure-tight manner. The outer sealing element 4 cannot be exchanged. 
This arrangement has the advantage that the pressure-sensor characteristic 
curve is constant over large temperature ranges since the pressure sensor 
i is clamped in in a defined manner. In this form, comprising a pressure 
sensor 1, housing 2, outer sealing element 4, threaded ring 5, electronic 
circuit 13 and the electrical connection lines 14, a pressure-measuring 
device which is inherently fully functional is already provided. 
For fixing the inner sealing element 9, the connection element 3 engages 
around the collar 221 of the securing ring 22 such that, in the border 
region of the pressure sensor 1, there is a gap 10 between the pressure 
diaphragm 11 and connection element 3. The sealing element 9 rests against 
the connection element 3 on the side facing the measuring medium, rests 
against the pressure sensor i on the side remote from the measuring medium 
and rests against the housing 2 on the outer side. The sealing element 9 
is in contact with the measuring medium on the inner side. The sealing 
element 9 consists of a material which is resistant to the measuring 
medium. Depending on the medium, this is, for example a VITRON (a 
synthetic rubber composition) O-ring, a KALREZ seal or a (a synthetic 
rubber composition) seal sheathed in polytetrafluoroethylene. The housing 
2 is not in contact with the measuring medium. On the outer side and on 
the front side, facing the measuring medium, of the housing 2, the 
connection element 3 fully encloses the securing ring 22. 
For pressure-tight, releasable fastening of the connection element 3 on the 
housing 2, the latter exhibits, level with that surface of the pressure 
sensor 1 which is remote from the measuring medium, an offset ring 27 
having at least two bores 28, by way of which the housing 2 is connected 
to the connecting element 3 by means of screws 8. Consequently, the 
housing 2 rests, with the offset ring 27, in a sealed manner on the 
connection element 3, and the latter, at the lower housing border, rests 
in a sealed manner on the securing ring 22, on the side opposite the 
groove 24. This ensures that the gap 10, in which the inner sealing 
element 9 is located, always has the same dimensions and that the inner 
sealing element 9 is exposed to the same contact-pressure force in each 
installation. 
Apart from the pressure-sensitive diaphragm 11 of the pressure sensor 1, 
the connection element 3 and the inner sealing element 9 are the only 
constituent parts of the device which come into contact with the measuring 
medium. It is only these two parts which therefore have to be resistant to 
the measuring medium. In the case of particularly aggressive measuring 
media, the material of the connection element 3 is a cost-intensive 
special material, e.g. titanium, tantalum, nickel, MONEL (a corrosion 
resistant nickel alloy), INCONEL (a corrosion resistant nickel alloy), or 
HASTELLOY, (a corrosion resistant nickel alloy). In contrast, the material 
of the housing 2 may be a comparatively cost-effective standard material, 
e.g. aluminum or steel. 
The connection element 3 is fastened on the container wall 6 by means of a 
process connection 7. In FIG. 1, the process connection 7 is only shown 
schematically. While the inner geometry of the connection element 3 is 
identical for all applications, there are a large number of variants for 
the outer geometry of the connection element 3, for example the design as 
a flange which is screwed to a mating flange at the measuring location, 
the design as an external thread which is screwed into a corresponding 
opening, or the design as a swivel part which is welded into a container 
opening 6. Further releasable or non-releasable process connections 7 are 
known to the person skilled in the art. 
The connection element 3 thus fulfills two tasks. On the one hand, it 
ensures that the sealing element 9 is clamped in in a reproducible manner, 
i.e. such that the pressure-sensor characteristic curve does not change, 
and, on the other hand, it has the function of an adaptor, by means of 
which the actual pressure-sensor arrangement, i.e. the pressure sensor 1, 
the housing 2, the outer sealing element 4, the threaded ring 5, the 
electronic circuit 13 and the electrical connection lines 14, is fastened 
as a universal component onto one of a large number of possible process 
connections on the container wall 6.