Abstract
A metal pressure measuring cell for absolute pressure sensing is described, including a metal base body with a membrane and a support body. The membrane includes a first surface and a second surface. The support body includes a cavity which is transversely delimited by an inner surface of the support body and axially delimited at a first side by the first surface of the membrane and open at a second side opposite to the first side to form a trough-shaped chamber for accommodating a measurement medium. The pressure measuring cell further includes a cap mounted on the base body and covering the second surface of the membrane such that a hermetically closed pressure reference volume is formed between the cap and the second surface of the membrane, wherein the cap is made of metal.
Claims
1. A metal pressure measuring cell for absolute pressure sensing, comprising a metal base body a membrane and a support body, the membrane comprising a first surface and a second surface, the support body comprising a cavity which is transversely delimited by an inner surface of the support body and axially delimited at a first side by the first surface of the membrane and open at a second side opposite to the first side to form a trough-shaped chamber for accommodating a measurement medium, the pressure measuring cell further comprising a cap mounted on the base body and covering the second surface of the membrane such that a hermetically closed pressure reference volume is formed between the cap and the second surface of the membrane, wherein the cap is made of metal.
2. The pressure measuring cell according to claim 1, wherein the cap has an inner transverse area which is equal to the area of the membrane or larger than the area of the membrane.
3. The pressure measuring cell according to claim 1, wherein the coefficients of expansion of the base body and the cap are essentially equal.
4. The pressure measuring cell according to claim 1, wherein the base body and/or the cap are made of a duplex stainless, a ferritic or an austenitic steel.
5. The pressure measuring cell according to one of the claim 1, wherein the cap has a circular cross-section.
6. The pressure measuring cell according to claim 1, wherein the cap comprises a transverse cover portion, a side wall and a flange transversely adjoining the side wall, wherein the cap is mounted on the base body by the flange.
7. The pressure measuring cell according to claim 1, wherein the cap is mounted on the base body by soldering.
8. The pressure measuring cell according to claim 7, comprising a solderable metallic layer arranged between the base body and the cap and surrounding the second surface of the membrane.
9. The pressure measuring cell according to claim 8, wherein the solderable metallic layer is made of AgPd.
10. The pressure measuring cell according to claim 8, comprising an intermediate insulating layer arranged between the base body and the metallic layer and surrounding the second surface of the membrane.
11. The pressure measuring cell according to claim 1, wherein the pressure in the reference volume is below 20 mbar.
12. The pressure measuring cell according to claim 1, wherein the inner surface of the support body is shaped such that a transverse diameter (D) of the trough-shaped chamber monotonously decreases from the second side of the cavity towards the first side of the cavity.
13. The pressure measuring cell according to claim 12, wherein the inner surface of the support body adjoins the first surface of the membrane with a slope.
14. The pressure measuring cell according to claim 12, wherein the inner surface of the support body comprises one or more conical profile sections.
15. The pressure measuring cell according to claim 12, wherein the inner surface of the support body comprises a conical profile extending from the second side of the cavity to the first side of the cavity.
16. The pressure measuring cell according to claim 12, wherein the inner surface of the support body comprises one or more concave sections.
17. The pressure measuring cell according to claim 12, wherein the inner surface of the support body comprises one or more convex sections.
18. The pressure measuring cell according to claim 12, wherein the inner surface of the support body comprises a parabolic profile extending from the second side of the cavity to the first side of the cavity.
19. The pressure measuring cell according to claim 1, wherein the trough-shaped chamber is an empty space configured to solely accommodate the measurement medium.
20. The pressure measuring cell according to claim 1, wherein the membrane and the support body are formed as an integral part such that the trough-shaped chamber configured to accommodate the measurement medium is formed by the integral part.
21. A method of manufacturing the pressure measuring cell according to claim 1, comprising the steps of: providing a base body made of a metal with a membrane and a support body; arranging an intermediate insulating layer on the base body to surround the membrane; forming a solderable metallic layer on the intermediate insulating layer to surround the membrane; mounting a cap made of a metal on the base body by soldering a mounting portion of the cap, onto the solderable metallic layer.
22. The method according to claim 21, wherein the cap is manufactured by punching and stamping of a sheet metal.
23. The method according to claim 21, wherein the mounting portion of the cap is soldered onto the metallic layer by vacuum soldering.
24. The method according to claim 21, wherein the cap is made of a duplex stainless steel, a ferritic or an austenitic steel.
25. A pressure transducer configured to measure pressure of a measurement medium with a density anomaly, comprising a pressure measuring cell according to claim 1.
26. The pressure transducer according to claim 25, wherein the trough-shaped chamber is an empty space for accommodating solely the measurement medium.
27. A dosing unit for dosing an exhaust gas reduction medium, comprising a pressure transducer according to claim 25.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0112] The terms Fig., Figs., Figure, and Figures are used interchangeably in the specification to refer to the corresponding figures in the drawings.
[0113] The present invention will be explained in more detail, by way of exemplary embodiments, with reference to the schematic drawings, in which:
[0114] FIG. 1 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view;
[0115] FIG. 2 shows an illustration of an embodiment of a pressure measuring cell in a perspective view before mounting of the cap;
[0116] FIG. 3 shows the pressure measuring cell of FIG. 2 after mounting of the cap;
[0117] FIG. 4 shows illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising two conical profile sections;
[0118] FIG. 5 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising a conical profile;
[0119] FIG. 6 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising two conical profile sections and a cylindrical profile section;
[0120] FIG. 7 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising a concave parabolic profile;
[0121] FIG. 8 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising a cylindrical profile section and a concave profile section;
[0122] FIG. 9 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising a convex parabolic profile;
[0123] FIG. 10 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising two convex profile sections;
[0124] FIG. 11 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a cavity comprising a convex profile section and a conical profile section;
[0125] FIG. 12a shows an illustration of an embodiment of a pressure transducer in a vertical cut view;
[0126] FIG. 12b shows an illustration of a further embodiment of a pressure transducer in a vertical cut view;
[0127] FIG. 13 shows an illustration of an embodiment of a dosing unit in a vertical cut view;
[0128] FIG. 14 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view where a pin is arranged in the trough-shaped chamber;
[0129] FIG. 15 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a liner insert;
[0130] FIG. 16 shows an illustration of an embodiment of a pressure measuring cell in a vertical cut view with a liner insert.
DETAILED DESCRIPTION
[0131] FIG. 1 shows an illustration of an embodiment of a metal pressure measuring cell 1 comprising a metal base body 11 with a membrane 12 and a support body 13. The membrane 12 comprises a first surface 14 and a second surface 15. The support body 13 comprises a cavity 16 which is transversely delimited by an inner surface 17 of the support body 13 and axially delimited at a first side 18 by the first surface 14 of the membrane 12 and open at a second side 19 opposite to the first side 18, such that a trough-shaped chamber 16 is formed. The trough-shaped chamber 16 accommodates a measurement medium, such as a diesel exhaust fluid. The first surface 14 is facing towards the measurement medium and the second surface 15 is facing away from the measurement medium. The inner surface 17 of the support body 13 forms a side wall surface of the cavity 16.
[0132] The pressure measuring cell 1 further comprises a cap 20 mounted on the base body 11 and covering the second surface 15 of the membrane 12. A hermetically closed pressure reference volume 21 is formed between the cap 20 and the second surface 15 of the membrane 12. The cap 20 comprises a transverse cover portion 22, a side wall 23 and a flange 24 transversely adjoining the side wall 23. The cap 20 is mounted on the base body 13 by the flange 24 by vacuum soft soldering. A base insulating layer 27 is arranged on the base body 11, covering also the second surface 15 of the membrane 12. The base insulating layer 27 extends over a major portion of the top surface of the base body 11. Pressure measuring components 28, as symbolized by a solid line, are applied onto the base insulating layer 27. An intermediate insulating layer 26 is mounted on the base insulating layer 27 carrying the pressure measuring components 28 so as to be arranged between the base body 13 and a solderable metallic layer 25 made of AgPd. The solderable metallic layer 25 mounted on the intermediate insulating layer 26 is arranged between the base body 11 and the cap 20, wherein the cap 20 is soldered onto the solderable metallic layer 25. The insulating layer 26 and the metallic layer 25 have an annular shape with a central opening which is flush with the second surface 15 of the membrane 12. The cap 20 and the base body 11 are made of a duplex stainless, ferritic or an austenitic steel. The transverse area of the pressure reference volume 21 is larger than the area of the second surface 15 of the membrane 12. The pressure in the pressure reference volume 21 is smaller than 1 mbar. The cap 20 and the transverse cover portion 22 have a circular shape. The cavity 16 has a cylindrical profile. However, other profiles of the cavity 16 are possible, as shown in the present disclosure. The cap 20 is manufactured from a sheet metal by punching and stamping.
[0133] FIG. 2 shows an illustration of an embodiment of a metal pressure measuring cell 1 in a perspective view before mounting of the cap. The metal base body 11 comprising the membrane 12 and the support body 13 exhibits a circularly symmetric shape (except for e.g. electrical structures such as connection pins or small deviating structures of the base body such as recesses etc.). An intermediate insulating layer 26 is arranged on the base body 11. In particular, the intermediate insulating layer 26 is mounted on a base insulating layer 27 carrying pressure measuring components. The intermediate insulating layer 26 has an annular shape surrounding the second surface 15 of the membrane 12. On the intermediate insulating layer 26, there is mounted a solderable metallic layer 25, onto which the cap is to be soldered. The metallic layer 25 has an annular shape surrounding the second surface 15 of the membrane 12. Four connection pins 28 for electrically connecting to pressure measuring components are arranged on the base body 11 of the pressure measuring cell 1.
[0134] FIG. 3 shows the pressure measuring cell 1 of FIG. 2 after mounting of the cap 20. The cap 20 is mounted on the base body 11 by vacuum soft soldering of the flange 24 onto the solderable metallic layer 25 shown in FIG. 2. The metallic layer 25 of FIG. 2 is therefore arranged between the base body 11 and the cap 20, in particular between the intermediate insulating layer 26 and the flange 24. The cap 20 has a circular transverse cross section and comprises a circular transverse cover portion 22.
[0135] FIG. 4 shows an illustration of an embodiment of a metal pressure measuring cell 1.0 comprising a membrane 1.1 with a first surface 1.11 and a second surface 1.12. The pressure measuring cell 1.0 is made of a duplex stainless, a ferritic or an austenitic steel. The pressure measuring cell 1.0 further comprises a support body 1.2 with a cavity 1.21 which is transversely delimited by an inner surface 1.213 of the support body 1.2. The inner surface 1.213 of the support body 1.2 therefore forms a side wall surface of the cavity 1.21. The cavity 1.21 is axially delimited at a first side 1.211 by the first surface 1.11 of the membrane 1.1 and open at a second side 1.212 opposite to the first side 1.211. The cavity 1.21 therefore forms a trough-shaped chamber 1.21 which accommodates a measurement medium, such as a diesel exhaust fluid. The ratio of the transverse diameter of the membrane 1.1 to the axial height of the trough-shaped chamber 1.21 is about 1:1. The first surface 1.11 of the membrane 1.1 is facing towards the measurement medium and the second surface 1.12 of the membrane 1.1 is facing away from the measurement medium.
[0136] As can be recognized in FIG. 4, the transverse diameter D of the trough-shaped chamber 1.21 at the second side 1.212 of the cavity 1.21 is larger than the transverse diameter D of the trough-shaped chamber 1.21 at the first side 1.211 of the cavity 1.21. For the shown pressure measuring cell 1, the transverse diameter D strictly monotonously decreases from the second side 1.212 of the cavity 1.21 towards the first side 1.211 of the cavity 1.21. The transverse diameter D at different axial heights of the pressure measuring cell 1 is measured in a common vertical plane oriented perpendicular to the membrane 1.1. In the shown example, the common vertical plane coincides with the plane of drawing.
[0137] The inner surface 1.213 of the support body 1.2 or the cavity 1.21, respectively, comprises a first conical profile section adjacent to the membrane 1.1 and extending over about half of the axial length of the trough-shaped chamber 1.21. The first conical profile section corresponds to a cone (or a frustocone) with an apex angle di. The inner surface 1.213 of the support body 1.2 or the cavity 1.21, respectively, further comprises a second conical profile section adjoining the first conical profile section and extending towards the second side 1.212 of the cavity 1.21, corresponding to a cone with a larger apex angle .sub.2 than the cone of the first conical profile section. The first and second conical profile sections furthermore extend around the circumference of the trough-shaped chamber 1.21 which exhibits circular symmetry with respect to the axial direction of the pressure measuring cell 1. The axial direction of the pressure measuring cell 1 is perpendicular to the plane of the membrane 1.1.
[0138] Due to the first conical profile section, the inner surface 1.213 of the support body 1.2 adjoins the first surface 1.12 of the membrane 1.1 with a slope. Furthermore, the first and second conical profile sections represent linearly slanted sections of the inner surface 1.213 of the support body 1.2 exhibiting two different slopes with respect to the plane of the membrane 1.1, as the conical profile sections are only curved in transverse direction and linearly slanted in vertical direction. The person skilled in the art furthermore understands that small curvatures as e.g. recognizable at the transition from the first surface 1.11 of the membrane 1.1 to the first conical profile section, due to for example manufacturing imperfections are not to be understood as concave or convex profile sections. The linearly slanted section adjoining the first surface 1.11 of the membrane therefore to be understood disregarding such small 1.1 is curvatures. Similarly, small chamfers, e.g., at the first or second side of the cavity without substantial effect on frost protection shall not be understood as separate conical profile sections. The different apex angles mentioned above translate into the slope of the linearly slanted section at the second side 1.212 of the cavity 1.21 being smaller than the slope of the linearly slanted section adjacent to the membrane 1.1.
[0139] The pressure measuring cell 1.0 further comprises a cap 1.20 mounted onto the support body 1.2 and covering the second surface 1.12 of the membrane 1.1. A hermetically closed pressure reference volume 1.201 is formed between the cap 1.20 and the second surface 1.12 of the membrane 1.1. The cap 1.20 is made of a duplex stainless, a ferritic or an austenitic steel.
[0140] FIG. 5 shows a further embodiment of a metal pressure measuring cell 1. The pressure measuring cell 1 is similar to the pressure measuring cell 1 shown in FIG. 4, with the difference that the inner surface 1.213 comprises a conical profile extending from the second side 1.212 of the cavity 1.21 to the first side 1.211 of the cavity 1.21 and that the apex angle of the cone to which the conical profile corresponds is larger than the apex angle of the cone of the first conical profile section shown in FIG. 4. Due to the larger apex angle, the trough-shaped chamber 1.21 of the pressure measuring cell 1 exhibits a more efficient guiding away of the forces from the membrane arising from freezing of the measurement medium and a larger measuring volume compared to the trough-shaped chamber 1.21 of the pressure measuring cell 1 shown in FIG. 4. The pressure measuring cell 1 also comprises a cap 1.20 and a pressure reference volume 1.201.
[0141] FIG. 6 shows a further embodiment of a metal pressure measuring cell 2 where the inner surface 2.213 of the cavity 2.21 or the support body 2.2, respectively, comprises a first conical profile section adjacent to the first surface 2.11 of the membrane 2.1 and a cylindrical profile section adjoining the first conical profile section. The pressure measuring cell 2 comprises a cap 2.20 and a pressure reference volume 2.201. The pressure measuring cell 2 is made of a duplex stainless, a ferritic or an austenitic steel. The cylindrical profile section and the first conical profile section adjoin to each other forming a step 2.214 such that a transverse annular surface area is formed. The inner surface 2.213 of the support body 2.2 comprises a second conical profile section arranged between the cylindrical profile section and the second side 2.212 of the cavity 2.21. The first and second conical profile sections correspond to a cone with the same apex angle , which has the advantage of easier manufacturability. However, the apex angles may also differ from one another depending on the desired freezing characteristics. FIG. 7 shows a further embodiment of a metal pressure measuring cell 3 with a cap 3.20 and a pressure reference volume 3.201. The inner surface 3.213 of the support body 3.2 or the cavity 3.21, respectively, comprises a parabolic profile extending from the second side 3.212 of the cavity 3.21 to the first side of the cavity 3.21. Due to the parabolic profile, the inner surface 3.213 adjoins the first surface 3.11 of the membrane 3.1 with a slope. The parabolic profile represents a concave profile (section) of the inner surface 3.213 of the support body 3.2 extending around the circumference of the trough-shaped chamber 3.21 (or the cavity 3.21, respectively) and from the second side 3.212 to the first side 3.211 of the cavity 3.21. The parabolic profile exhibits a shape of a frustum paraboloid due to the membrane 3.1 transversely intersecting the parabolic profile. The curvature of the parabolic profile at the second side 3.212 of the cavity 3.21 is smaller than the curvature at the first side 3.211 of the cavity 3.21. In comparing the curvatures, the vertical curvatures shall be considered, as shown in FIG. 7. Guiding the forces arising from freezing away from the membrane therefore occurs predominantly in the vicinity of the membrane 3.1 in the region of the first side 3.211 of the cavity 3.21.
[0142] FIG. 8 shows a further embodiment of a metal pressure measuring cell 4 with a cap 4.20 and a pressure reference volume 4.201. The inner surface 4.213 of the support body 4.2 comprises a cylindrical profile section adjoining the first surface 4.11 of the membrane 4.1. The inner surface 4.213 of the support body 4.2 therefore adjoins the first surface 4.11 of the membrane 4.1 perpendicularly. The cylindrical profile section extends over the circumference of the trough-shaped chamber 4.21. A concave profile section adjoins the cylindrical profile section by forming a step 4.214. The concave profile section extends over the circumference of the trough-shaped chamber and from the cylindrical profile section to the second side 4.212 of the cavity 4.21. The concave profile section extends over about three quarters of the axial height of the trough-shaped chamber 4.21 wherein the cylindrical profile section extends over about one quarter of the axial height of the trough-shaped chamber 4.21. While a particular partition is shown in present FIG. 8, it is clear that other partitions between the cylindrical profile section and the concave profile section, as disclosed above, are also possible.
[0143] FIG. 9 shows a further embodiment of a metal pressure measuring cell 5 with a cap 5.20 and a pressure reference volume 5.201. The inner surface 5.213 of the support body 5.2 comprises a parabolic profile extending from the second side 5.212 to the first side 5.211 of the cavity 5.21 and over the circumference of the trough-shaped chamber 5.21 (or the cavity 5.21, respectively). Compared to the embodiment shown in FIG. 7, the parabolic profile is convex. The curvature of the parabolic profile at the second side 5.212 of the cavity 5.21 is larger than the curvature of the parabolic profile at the first side 5.211 of the cavity 5.21. Guiding the forces away from the membrane therefore occurs predominantly in the region of the second side 5.212 of the cavity 5.21. The inner surface 5.213 of the support body 5.2 adjoins the first surface 5.11 of the membrane 5.1 with a large slope or almost perpendicularly.
[0144] FIG. 10 shows a further embodiment of a metal pressure measuring cell 6 with a cap 6.20 and a pressure reference volume 6.201. The inner surface 6.213 of the support body 6.2 comprises two convex profile sections adjoining to one another. A first convex profile section adjoins the membrane 6.1 with a slope and extends over the circumference of the trough-shaped chamber 6.21. A second convex profile section adjoins the first convex profile section by forming a step 6.214 and extends from the from the first convex profile section to the second end 6.212 of the cavity 6.21. The second convex profile section also extends around the circumference of the trough-shaped chamber 6.21. The second convex profile section exhibits a smaller curvature than the first convex profile section. The inner surface 6.213 of the support body 6.2 is therefore steeper at the second convex profile section than at the first convex profile section. The second convex profile section in turn exhibits a larger curvature at the second side 6.212 of the cavity than at the step where the first and second convex profile sections adjoin to one another. The first convex profile section extends over about one third of the axial height of the trough-shaped chamber 6.21 and the second convex profile section extends over about two thirds of the axial height of the trough-shaped chamber 6.21. While a particular partition is shown in present FIG. 10, it is clear that other partitions between the two convex profile sections, as disclosed above, are also possible.
[0145] FIG. 11 shows a further embodiment of a metal pressure measuring cell 7 with a cap 6.20 and a pressure reference volume 6.201. The pressure measuring cell 7 is similar to the pressure measuring cell 6 shown in FIG. 10 with the difference that instead of the second convex profile section, a conical profile section adjoins the first convex profile section. The inner surface 7.213 of the support body 7.2 thus comprises a convex profile section adjoining the first surface 7.11 of the membrane 7.1 and a conical profile section adjoining the convex profile section by forming a step. The conical profile section extends from the convex profile section to the second side 7.212 of the cavity 7.21. Both the convex profile section and the conical profile section extend over the circumference of the trough-shaped chamber 7.21.
[0146] The pressure measuring cells as shown in FIGS. 4-16 may comprise a base insulating layer, an intermediate insulating layer and a solderable metal layer, as described for example in connection with FIG. 1, although not explicitly shown in the FIGS. 4-16.
[0147] FIG. 12a shows an embodiment of a pressure transducer 100 comprising an embodiment of a pressure measuring cell 8. The pressure measuring cell 8 corresponds to the embodiment shown in FIG. 5 and comprises a trough-shaped chamber with a conical profile.
[0148] FIG. 12b shows an embodiment of a pressure transducer 100 comprising an embodiment of a pressure measuring cell 8. Again, the pressure measuring cell 8 corresponds to the embodiment shown in FIG. 5 and comprises a trough-shaped chamber with a conical profile. Different to the embodiment of a pressure transducer shown in FIG. 12a, the pressure transducer 100 comprises a bellow 101 as a compensation element to further improve the frost protection by enabling a adaptable size of the measurement volume.
[0149] FIG. 13 shows an embodiment of a dosing unit 1000 for dosing an exhaust gas reduction medium comprising the pressure transducer 100 of FIG. 12b.
[0150] FIG. 14 shows a further embodiment of a pressure measuring cell 9 with a cap 9.20 where a pin 9.2 is at least partially arranged in the trough-shaped chamber 9.21. The pin 9.3 has a conical shape. A portion of ice of a measurement medium freezing from the second side 9.212 of the cavity 9.21 may become wedged between the pin 9.3 and the inner surface 9.213 of the support body 9.2 and thereby be spatially fixed at a site remote from the first surface 9.11 of the membrane 9.1.
[0151] FIG. 15 shows a further embodiment of a pressure measuring cell 10 with a cap 10.20. The inner surface 10.213 of the support body profile. A liner insert 10.215 is 10.2 comprises a conical arranged in the cavity 10.21 to cover the inner surface 10.213 of the support body 10.2 forming the side wall of the cavity 10.21. The liner insert 10.215 comprises ribs 10.216 which engage with corresponding recesses in the inner surface 10.213 of the support body 10.2 for secure mounting of the liner insert 10.215. The liner insert 10.215 further comprises a flange 10.217 which abuts on an outer transverse surface of the support body 10.2 at the second side 10.212 of the cavity 10.21. The liner insert 10.215 has a conical shape and forms a side wall of the trough-shaped chamber 10.22. The liner insert 10.215 is open at the upper end in order to leave the first surface 10.11 of the membrane 10.1 open. The liner insert 10.215 is made of a urea-resistant elastomer and has a lower roughness than the inner surface 10.213 of the support body 10.2.
[0152] FIG. 16 shows a further embodiment of a pressure measuring cell 11.0 with a liner insert 11.215 and a cap 11.20. The inner surface 11.213 of the support body 11.2 comprises a conical profile, similar to the embodiment shown in FIG. 15. A liner insert 11.215 is arranged in the cavity 11.21 to cover the inner surface 11.213 of the support body 11.2 forming the side wall of the cavity 11.21. The liner insert 11.215 comprises outer ribs 11.216 which abut on the even inner surface 11.215 of the support body 11.2 such that buffer chambers 11.218 filled with air are arranged between the liner insert 11.215 and the inner surface 11.213 of the support body 11.2. The outer ribs 11.216 therefore serve as spacer elements for generating the buffer chambers 11.218. In case of freezing of the measurement medium, the buffer chambers 11.218 may be compressed such that the increase in measurement volume can be compensated for. The liner insert 11.215 further comprises a flange 11.217 which abuts on an outer transverse surface of the support body 11.2 at the second side 11.212 of the cavity 11.21. The liner insert 11.215 has a conical shape and forms a side wall of the trough-shaped chamber 11.22.
[0153] Further, the liner insert 11.215 also covers the first surface 11.11 of the membrane 11.1 in order to prevent the measurement medium, such as a urea-water solution to creep into the buffer chambers 11.218. The liner insert 11.215 is made of a urea-resistant plastics with a sufficient rigidity to withstand the fluid pressure of the measurement medium before freezing. Therefore, the liner insert 11.215 preferably exhibits a larger rigidity than the liner insert 10.215 shown in FIG. 15. Furthermore, the liner insert 11.215 preferably has a lower roughness than the inner surface 11.213 of the support body 11.2.
