Pressure measuring arrangement for measuring diffusive fluids or gases with a ceramic pressure measuring cell

Abstract

The invention relates to a pressure measuring arrangement for measuring a diffusive fluid or gas with a ceramic pressure measuring cell (16), wherein the pressure measuring cell (16) has a ceramic base body (20) and a ceramic measuring diaphragm (22), wherein the pressure measuring cell (20) is arranged directly on a metal process connection (12) by means of a ceramic adapter (30) in such a way that the process pressure is applied directly to the measuring diaphragm (22), and wherein the base body (20), the measuring diaphragm (22), the ceramic adapter (30) and the process connection (12) are each connected to one another in a materially bonded and diffusion-tight manner.

Description

[0001] The invention relates to a pressure measuring arrangement for measuring diffusive fluids or gases with a ceramic pressure measuring cell according to claim 1.

[0002] Pressure measurement systems with pressure sensors are known in a wide variety of designs from the state of the art. Pressure sensors convert pressure into an electrical signal, which can then be further processed. Pressure sensors are differentiated according to the underlying measuring principle, the materials oriented toward the process, and whether absolute or relative pressures are measured.

[0003] One of the measuring principles is based on a change in capacitance, whereby a change in pressure causes a deformation of a diaphragm, resulting in a change in capacitance. In resistive pressure sensors, the deformation of a diaphragm is detected using strain gauges, for example, and the pressure is determined from a change in the resistance of the strain gauges. Piezoresistive pressure sensors use strain gauges with piezoresistive properties.

[0004] The distinction between the materials oriented towards the process, i.e. the materials that come into contact with the process environment and the process media, is usually made between metallic and ceramic pressure measuring cells, with the former having a metallic and the latter a ceramic diaphragm on the side facing the process.

[0005] Whether absolute or relative pressures can be measured usually depends on whether a second pressure, e.g. ambient pressure, is applied to the back of the membrane or whether the back of the membrane is evacuated.

[0006] The present invention relates to a pressure measuring arrangement with a ceramic pressure measuring cell. These typically comprise a ceramic base body and a ceramic diaphragm spaced apart from it. The diaphragm faces the process and deforms or bulges depending on the applied pressure. The degree of deformation or bulging can be measured (capacitively or resistively), and the pressure in the container can be determined from this. Using a diaphragm, pressure changes in a container can be detected particularly reliably.

[0007] Ceramic pressure measuring cells cover a wide range of applications, both in terms of pressure, temperature and media.

[0008] The underlying object of the invention is to provide a pressure measuring arrangement with a ceramic pressure measuring cell, which is suitable for the pressure measurement of diffusive fluids, such as in particular gaseous hydrogen.

[0009] The object is achieved according to the invention with the features of claim 1. Further practical embodiments and advantages are described in connection with the dependent claims.

[0010] The invention relates to a pressure measuring arrangement for measuring the pressure of a diffusive fluid or gas, and in particular for measuring the pressure of gaseous hydrogen. The pressure measuring arrangement has a ceramic pressure measuring cell. The ceramic pressure measuring cell comprises a ceramic base body and a ceramic measuring diaphragm. In particular, the measuring diaphragm is integrally connected to the base body. In particular, the measuring diaphragm is fastened to the base body in its edge regions by means of a hard solder, glass solder, or a glass seam, and is thus arranged at a distance from the base body. A cavity is formed between the base body and the measuring diaphragm, in which a reference pressure is established. The cavity can be evacuated, so that an absolute pressure can be measured using the pressure measuring cell.Alternatively, another reference pressure can be set in the cavity, for example the ambient pressure, so that the pressure in the container can be determined relative to the environment.

[0011] As described above, the membrane flexes depending on the applied pressure. The flexion can be measured capacitively, resistively, or piezoelectrically in a conventional manner, and the pressure can be determined from this.

[0012] According to the invention, the pressure measuring cell is arranged directly on a metal process connection by means of a ceramic adapter, so that the process pressure is applied directly to the measuring diaphragm. The process connection forms the mechanical interface between the field device and the process environment, in particular the container.

[0013] By applying pressure directly to the measuring diaphragm—and in particular by eliminating the need for a pressure seal—the pressure measuring cell exhibits particularly high efficiency and reliability. Diffusion of fluids through the metal diaphragm, which is typically found in pressure seals, is also effectively prevented.

[0014] The base body, the measuring diaphragm, the ceramic adapter, and the process connection are each bonded together in a material-tight and diffusion-tight manner. This means that the base body is bonded to the measuring diaphragm in a material-tight and diffusion-tight manner, the measuring diaphragm is bonded to the ceramic adapter in a material-tight and diffusion-tight manner, and the ceramic adapter is bonded to the process connection in a material-tight and diffusion-tight manner.

[0015] The integral and diffusion-tight connection of the individual components to each other—that is, the connection from the base body to the measuring diaphragm, from the measuring diaphragm to the ceramic adapter, and from the ceramic adapter to the process connection—effectively prevents the diffusion of diffusive fluids, such as gaseous hydrogen. In particular, the connection points are elastomer- and polymer-free, meaning that, for example, no sealing rings, or not only sealing rings made of elastomer or polymer, are arranged between the individual components.

[0016] In particular, the pressure measuring arrangement comprises a housing in which the pressure measuring cell is arranged and protected against external influences. The housing is, in particular, firmly connected to the process connection. The housing also contains electronics that serve to control the pressure measuring cell and read the measurement signals.

[0017] The pressure measuring cell is arranged, in particular, on a side of the process connection facing away from the process. Viewed in the axial direction of the pressure measuring arrangement, the process connection and the pressure measuring cell extend one behind the other. Starting from the process-side end, the process connection is arranged first, followed by the ceramic adapter, and then the pressure measuring cell. The pressure measuring cell is arranged further away from the process than the process connection. The pressure measuring cell, which is sensitive to external mechanical stresses, can thus be installed as free from mechanical stress as possible. This low-stress installation is ensured precisely by the ceramic adapter between the measuring cell and the process connection.

[0018] In a practical embodiment, the process connection and the ceramic adapter have aligned through-holes. The process pressure prevailing in the process can spread directly through the through-hole in the process connection and the ceramic adapter to the measuring diaphragm.

[0019] In another practical embodiment, the ceramic adapter is connected to the measuring diaphragm using hard solder and / or glass solder. Connections using hard solder and / or glass solder are well-proven in the state of the art and are particularly pressure- and diffusion-tight, so these joining techniques excellently meet the requirements described above.

[0020] In a practical embodiment, the ceramic adapter has the same dimensions as the base body. In particular, the ceramic adapter has the same height, width, and length as the base body. In particular, the connection of the ceramic adapter to the measuring diaphragm is analogous to the connection of the base body to the measuring diaphragm. In particular, the material-to-material and diffusion-tight connection between the measuring diaphragm and the ceramic adapter is as identical as possible to the dimensions of the material-to-material and diffusion-tight connection between the measuring diaphragm and the base body. Due to this arrangement, which is essentially mirrored to the measuring diaphragm, the measuring diaphragm is clamped evenly on both sides by the base body and the measuring diaphragm, and no or negligible undesired deformation occurs.A particular difference between the ceramic adapter and the base body is that the ceramic adapter has a through-hole so that the process pressure is applied directly to the measuring diaphragm.

[0021] The process connection is also connected to the ceramic adapter, particularly using brazing and / or glass solder. As mentioned above, connections using brazing or glass solder are well-proven in the state of the art and are particularly pressure- and diffusion-tight.

[0022] The direct transition from the metallic process connection to the ceramic adapter can be improved, particularly with regard to temperature transition, if the process connection has a main part and a connecting part, wherein the connecting part is arranged between the main part and the ceramic adapter and is made of an expansion alloy. An expansion alloy is a metallic alloy. There is also direct contact between the metallic process connection and the ceramic adapter. To mitigate the difference in the expansion coefficient, the connecting piece is made, in particular, of a ferritic alloy such as an iron-nickel alloy or an iron-cobalt alloy, which has a low expansion coefficient. In particular, it can be Kovar. ® (Fe-Ni-Co compound) or Vacon ®(Co-Fe alloy). The connecting piece is arranged as a nozzle on the process connection and has the same diameter and wall thickness at the transition to the further process connection.

[0023] The connector and the main part both have a through-hole so that the applied process pressure is directly applied to the measuring diaphragm.

[0024] To ensure the tightest possible transition between the connection piece (made of an expansion alloy) and the main part of the process connection, the process connection and the connection piece are also connected in a material-to-material and diffusion-tight manner, particularly by welding or soldering. Process connections made of different metals can be used in the measuring arrangement, whose thermal expansion coefficients can then be adapted to the ceramic adapter using the connection piece.

[0025] Further practical embodiments and advantages are described below in conjunction with the figure. It shows: Fig. 1 a pressure measuring arrangement in a schematic representation in cross section

[0026] In Fig. 1 shows a pressure measuring arrangement 10. The pressure measuring arrangement 10 includes a process connection 12 for connecting the pressure measuring arrangement 10 to a container (not shown). Furthermore, the pressure measuring arrangement 10 includes a housing 14, in which a ceramic pressure measuring cell 16 and corresponding electronics 18 are arranged.

[0027] The ceramic pressure measuring cell 14 comprises a ceramic base body 20 and a ceramic measuring diaphragm 22. The measuring diaphragm 22 is connected to the base body 20 in a materially bonded and diffusion-tight manner by means of glass solder 24. The glass solder 24 is arranged circumferentially between the measuring diaphragm 22 and the base body 20, so that a cavity 26 is created between the base body 20 and the measuring diaphragm 22.

[0028] If a process pressure acts on the measuring diaphragm 22, the measuring diaphragm 22 is deformed. The deformation can be determined in a known manner and a process pressure can be derived from it.

[0029] The electronics 18 are arranged on the rear side of the ceramic pressure cell 16 facing away from the process, with individual electrical connections 28 being led to the rear through the housing 14.

[0030] The measuring membrane 22 is connected to a ceramic adapter 30 on the side facing away from the ceramic base body 20. The ceramic adapter 30 is identical in its external dimensions, i.e., its width, length, and height, to the dimensions of the base body 20.

[0031] The ceramic adapter 30 is connected to the measuring membrane 22 in a material-to-material and diffusion-tight manner by means of glass solder 24. The material-to-material and diffusion-tight connection 24 between the measuring membrane 22 and the ceramic adapter 30 is identical in height, width, and length to the dimensions of the material-to-material and diffusion-tight connection 24 between the measuring membrane 22 and the base body 24.

[0032] A through-opening 32 is formed in the ceramic adapter 30.

[0033] The ceramic adapter 30 is connected to the metallic process connection 12. The process connection 12 has a main part 34 and a connecting piece 36. The transition between the metallic process connection 12 and the ceramic adapter 30 is designed such that the connecting piece 36 is arranged between the main part 34 of the process connection 12 and the ceramic adapter 30. The connecting piece 36 is made of an expansion alloy.

[0034] The connecting piece 36 is connected to the ceramic adapter 30 in a material-to-material and diffusion-tight manner by means of brazing alloy 38. The connecting piece 36 is welded to the main part 34 of the process connection at a connection point 40.

[0035] The main part 34 of the process connection 12, the connection piece 36, the ceramic adapter 30 and the pressure measuring cell 16 are arranged one behind the other in the axial direction, with the process connection 12 being located at the end of the pressure measuring arrangement 10 facing the process.

[0036] The main part 34 of the process connection 12, the connecting piece 36 and the ceramic adapter 30 have through openings 44, 42, 36 that are aligned with one another, so that the process pressure from the process is applied directly to the ceramic measuring diaphragm 22.

[0037] The main part 34 of the process connection 12, the connecting piece 36, the ceramic adapter 30, the measuring diaphragm 22, and the base body 20 are connected to the respective adjacent components in a material-to-material and diffusion-tight manner. Overall, the pressure measuring assembly 10 is elastomer- and polymer-free. List of reference symbols 10 Pressure measuring arrangement 12 Process connection 14 housings 16 ceramic pressure measuring cell 18 Electronics 20 basic bodies 22 measuring membrane 24 glass solder 26 cavity 28 electrical connection 30 ceramic adapters 32 through hole (ceramic adapter) 34 Main part 36 connecting part 38 brazing alloy 40 connection point 42 Through opening (connecting part) 44 Through opening (main part)

Claims

[1] Pressure measuring arrangement for measuring a diffusive fluid or gas with a ceramic pressure measuring cell (16), wherein the pressure measuring cell (16) has a ceramic base body (20) and a ceramic measuring diaphragm (22), wherein the pressure measuring cell (20) is arranged directly on a process connection (12) made of metal by means of a ceramic adapter (30) in such a way that the process pressure is applied directly to the measuring diaphragm (22), and wherein the base body (20), the measuring diaphragm (22), the ceramic adapter (30) and the process connection (12) are each connected to one another in a materially bonded and diffusion-tight manner. [2] Pressure measuring arrangement according to the preceding claim, characterized by that the pressure measuring cell (16) is arranged on a side of the process connection (12) facing away from the process. [3] Pressure measuring arrangement according to one of the preceding claims, characterized bythat the process connection (12) and the ceramic adapter (30) have through openings (44, 32) that are aligned with one another. [4] Pressure measuring arrangement according to one of the preceding claims, characterized by that the ceramic adapter (30) is connected to the measuring membrane (22) by means of hard solder and / or glass solder (24). [5] Pressure measuring arrangement according to one of the preceding claims, characterized by that the ceramic adapter (30) has the same dimensions as the base body (20). [6] Pressure measuring arrangement according to one of the preceding claims, characterized by that the process connection (12) is connected to the ceramic adapter (30) by means of hard solder (38) and / or glass solder. [7] Pressure measuring arrangement according to one of the preceding claims, characterized byin that the process connection (12) has a main part (34) and a connecting part (36), wherein the connecting part (36) is arranged between the main part (34) and the ceramic adapter (30) and consists of an expansion alloy. [8] Pressure measuring arrangement according to the preceding claim, characterized by that the connecting part (36) is welded or soldered to the main part (24) of the process connection (12).

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