DIFFERENTIAL PRESSURE MEASURING SENSOR WITH OVERLOAD PROTECTION

Abstract

A differential pressure transducer with overload protection includes a measuring element body, two separating diaphragms, two overload diaphragms with radially variable material thickness h(r), a differential pressure transducer for converting a pressure difference into an electrical signal, and two hydraulic paths. The overload diaphragms are connected to the measuring element body to form overload chambers and the separating diaphragms are connected to the measuring element body to form separating diaphragm chambers in which one of the overload diaphragms are enclosed. The separating diaphragm chambers are hydraulically connected to the overload chambers below the other separating diaphragm chamber and to the differential pressure transducer via one of the hydraulic paths, which extend at least in sections through the measuring element body. The overload diaphragms have a base surface, which faces a mating surface in an overload chamber against which the overload diaphragms are pre-stressed in the operative state at pressure equilibrium.

Claims

1-16. (canceled)

17. A differential pressure transducer with overload protection, comprising: a measuring element body; a first separating diaphragm; a second separating diaphragm; a first overload diaphragm; a second overload diaphragm; a differential pressure transducer for converting a pressure difference into an electrical signal; a first hydraulic path; and a second hydraulic path; wherein the first overload diaphragm is connected to the measuring element body to form a first overload chamber with variable volume along a circumferential first overload diaphragm edge; wherein the second overload diaphragm is connected to the measuring element body to form a second overload chamber with variable volume along a circumferential second overload diaphragm edge; wherein the first separating diaphragm is connected to the measuring element body to form a first separating diaphragm chamber along a circumferential first separating diaphragm edge, wherein the first overload diaphragm is enclosed between the measuring element body and the first separating diaphragm; wherein the second separating diaphragm is connected to the measuring element body to form a second separating diaphragm chamber along a circumferential second separating diaphragm edge, wherein the second overload diaphragm is enclosed between the measuring element body and the second separating diaphragm; wherein the first separating diaphragm chamber is hydraulically connected to the second overload chamber via the first hydraulic path, which extends at least in sections through the measuring element body; wherein the second separating diaphragm chamber is hydraulically connected to the first overload chamber via the second hydraulic path, which extends at least in sections through the measuring element body; wherein the differential pressure transducer is hydraulically connected to the first separating diaphragm chamber and the second separating diaphragm chamber; wherein the first overload diaphragm has a first base surface, which faces a first mating surface in the first overload chamber; wherein the second overload diaphragm has a second base surface, which faces a second mating surface in the second overload chamber; wherein the first overload diaphragm is pre-stressed against the first mating surface in the operable state at pressure equilibrium, such that the first base surface rests against the first mating surface at least in sections; wherein the second overload diaphragm is pre-stressed against the second mating surface, such that the second base surface rests against the second mating surface at least in sections; wherein the first overload diaphragm has a radially variable first material thickness h(r); and wherein the second overload diaphragm has a radially variable first material thickness h(r).

18. The differential pressure transducer according to claim 17, wherein the first overload diaphragm has a diaphragm bed surface facing the separating diaphragm with a contour k(r) prepared by machining or forming.

19. The differential pressure transducer according to claim 17, wherein the material thickness h(r) of the first overload diaphragm has local extrema in a radial range 0<r<0.9 R, wherein a difference between a local maximum and a local minimum of the material thickness of the overload diaphragm is not less than two, and/or wherein an amount of a difference of a local maximum of the material thickness of the overload diaphragm and a material thickness of the overload diaphragm averaged over the radial range along the radius is not less than one material thickness.

20. The differential pressure transducer according to claim 19, wherein the material thickness h(r) of the overload diaphragm has not less than 4.

21. The differential pressure transducer according to claim 17, wherein the first overload diaphragm has a first base surface facing the first mating surface and a first diaphragm bed surface facing the first separating diaphragm, wherein the first diaphragm bed surface has a first diaphragm bed contour with an axial coordinate k(r) in order to support the first separating diaphragm in the event of a one-sided overload, wherein in the operable state at pressure equilibrium, for the second derivation of the axial coordinate k(r) of the first diaphragm bed contour according to d.sup.2k/dr.sup.2 and the second derivation of the first material thickness h(r) of the first overload diaphragm according to the radius d.sup.2h/dr.sup.2, the following applies: $\frac{{}_0^{.Math.R}\frac{d^{2}k\left(r\right)}{{\mathrm{dr}}^2}.Math.\frac{d^{2}h\left(r\right)}{{\mathrm{dr}}^2}\mathrm{dr}}{{}_0^{.Math.R}{\left(\frac{d^{2}h\left(r\right)}{{\mathrm{dr}}^2}\right)}^2\mathrm{dr}}=T$ where g is a factor defining a bound for the integration, where 0.9g1, where T is a dimensionless characteristic number that is not less than 0.9.

22. The differential pressure transducer, according to claim 21, wherein the first base surface has an axial coordinate b(r), wherein, when the first overload diaphragm rests against the mating surface, for the second derivation of the axial coordinate b(r) of the base surface according to the radius d.sup.2b/dr.sup.2 and the second derivation of the first material thickness h(r) of the first overload diaphragm is according to the radius d.sup.2h/dr.sup.2, the following applies: $\frac{{}_0^{.Math.R}\frac{d^{2}k\left(r\right)}{{\mathrm{dr}}^2}.Math.\frac{d^{2}b\left(r\right)}{{\mathrm{dr}}^2}\mathrm{dr}}{{}_0^{.Math.R}{\left(\frac{d^{2}h\left(r\right)}{{\mathrm{dr}}^2}\right)}^2\mathrm{dr}}=U$ where U is a dimensionless characteristic number that is not more than 0.1.

23. The differential pressure transducer according to claim 17, wherein the material thickness h(r) of the first overload diaphragm has local extrema in a radial range 0<r<0.9 R, wherein a difference between a local maximum and a local minimum of the material thickness of the overload diaphragm is not less than two material thicknesses of the first separating diaphragm, and/or wherein an amount of a difference of a local maximum of the material thickness of the overload diaphragm and a material thickness of the overload diaphragm averaged over the radial range along the radius is not less than one material thickness of the separating diaphragm.

24. The differential pressure transducer according to claim 17, wherein the first overload diaphragm has a first central region (C) the outer radius r of which is not less than 30% of the radius R of the overload diaphragm, wherein the central region (C) is adjoined by a first transition region (B) which extends over not less than 20% of the radius (R) of the first overload diaphragm, wherein the axial coordinate of the base surface b(r) in the uninstalled equilibrium state of the first overload diaphragm in the first transition region (B) has a larger mean dimensionless slope R/h.sub.max(B).Math.db(r)/dr than the first central region, wherein h.sub.max(B) is the maximum thickness of the overload diaphragm in the transition region (B).

25. The differential pressure transducer according to claim 24, wherein a dimensionless mean slope R/h.sub.max(B).Math.db(r)/dr of the first transition region determined with linear regression in the uninstalled equilibrium state of the first overload diaphragm is not less than 1.5.

26. The differential pressure transducer according to claim 24, wherein the first transition region (B) has a frustoconical shape in the uninstalled equilibrium state of the first overload diaphragm.

27. The differential pressure transducer according to claim 23, wherein a first edge region (A) adjoins the first transition region radially outwardly, wherein the first edge region has, in the uninstalled equilibrium state of the first overload diaphragm a mean slope l/(h R) dz(r)/dr that is no more than one quarter of the mean slope of the transition region.

28. The differential pressure transducer according to claim 17, wherein the first base surface in the uninstalled state of the first overload diaphragm has an equilibrium shape for the radius-dependent axial coordinate g(r) of which there exists an equilibrium approximation function GN(r) determined by linear regression; wherein the first base surface in the installed operable state of the overload diaphragm has a shape for the radius-dependent axial coordinate b(r) of which there exists an operational approximation function BN(r) determined with linear regression; wherein the slope of the operational approximation function BN(r) is not more than three quarters of the slope of the equilibrium approximation function GN(r).

29. The differential pressure transducer according to claim 17, wherein a differential pressure measuring range is specified for the differential pressure transducer, wherein the first overload diaphragm is pre-stressed against the mating surface to such an extent that, at a temperature of 300 K and a differential pressure corresponding to the maximum value of the differential pressure measuring range, a volume stroke of the first overload diaphragm corresponds to no more than 10% of the oil volume contained in the second separating diaphragm chamber at pressure equilibrium and normal pressure.

30. The differential pressure transducer according to claim 17, wherein the first overload diaphragm has a steel of the grade 1.4310 or 1.8159.

31. The differential pressure transducer according to claim 17, wherein the first overload diaphragm has been tempered.

32. The differential pressure transducer according to claim 17, wherein the second overload diaphragm is substantially identical in construction to the first overload diaphragm.

Description

[0026] The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawings. In the figures:

[0027] FIG. 1 shows a schematic illustration of the operating principle of the differential pressure transducer according to the invention;

[0028] FIG. 2a: shows a longitudinal section of an overload diaphragm in the equilibrium position;

[0029] FIG. 2b: shows a longitudinal section of an overload diaphragm positioned above the mating body in the equilibrium position;

[0030] FIG. 2c: shows a longitudinal section of a separating diaphragm chamber and an overload chamber in the operable state at pressure equilibrium, that is, a pressure difference of zero;

[0031] FIG. 2d: shows a longitudinal section of a separating diaphragm chamber and an overload chamber in the event of overload;

[0032] FIG. 3a: shows a diagram of the volume stroke of the overload diaphragm as a function of a pressure difference across the overload diaphragm;

[0033] FIG. 3b: shows a diagram of the maximum mechanical stresses of the overload diaphragm as a function of a pressure difference across the overload diaphragm.

[0034] The schematic illustration in FIG. 1 shows how the various components of an exemplary embodiment of a transducer according to the invention are hydraulically coupled to develop their effect. The differential pressure transducer 100 has a metallic measuring element body 110 to which a first separating diaphragm 130 and a second separating diaphragm 140 are attached with a circumferential joint to form a first and a second separating diaphragm chamber 132, 142, respectively. A first and second overload diaphragms 150, 170, respectively, which are fastened to the measuring element body 110 with a circumferential joint to form first and second overload chambers 152, 172, respectively, are arranged within each of the separating diaphragm chambers 132, 142. The two separating diaphragms 130, 140 have a steel, in particular a stainless steel or spring steel in a material thickness of less than 100 m, for example 30 m, wherein a contour is imprinted on the diaphragms in order to improve the linearity of the relationship of a pressure difference between the two separating diaphragms and the resulting volume stroke of the separating diaphragm. The diameter of the separating diaphragms can be, for example, 25 to 30 mm. As a result, the separating diaphragms are very soft and may be deflected to both sides with small pressure differences depending on their sign.

[0035] The two overload diaphragms 150, 170 have a slightly smaller diameter than the separating diaphragms 130, 140, wherein the effective diameter can be 22 to 26 mm in particular. They also have a greater starting material thickness, depending on the measuring range, for example 200 to 1000 m, in the example 700 m, such that the overload diaphragms 150, 170 are many times stiffer than the separating diaphragms 130, 140, in particular more than 1000 times stiffer. In addition, the overload diaphragms 150, 170, are pre-stressed against the measuring element body 110, such that, firstly, deflection toward the measuring element body 110 is not possible and, secondly, deflection away from the measuring element body 110 does not occur until the pressure difference between the overload chamber and the adjacent separating diaphragm chamber is sufficient to overcome the pre-stressing of the overload diaphragm. The overload diaphragms 150, 170 in each case have a contoured diaphragm bed on their side facing a separating diaphragm 130, 140, in order to support the separating diaphragm 130, 140 in the event of overload.

[0036] The separating diaphragm chambers 130, 140 and the overload chambers 152, 172 are coupled by hydraulic paths, as follows. From the first separating diaphragm chamber 132, a first hydraulic path 200 comprising bores at least in section in the measuring element body 110 extends to the second overload chamber 172. Accordingly, a second hydraulic path 210 comprising bores at least in sections in the measuring element body 110 extends to the second overload chamber 172. Accordingly, a second hydraulic path 210 extends from the second separating diaphragm chamber 142 to the first overload chamber 152.

[0037] The differential pressure transducer 100 further comprises a differential pressure transducer 190, for example a (piezo-) resistive transducer or a capacitive transducer, connected to the first hydraulic path via first and second capillary lines 202, 212 to the two hydraulic paths 202, 212, in order to record a pressure difference between the two hydraulic paths and provide a primary electrical signal representing that pressure difference.

[0038] For the differential pressure transducer 190, a measuring range is defined, which is usually less than a maximum differential pressure that the differential pressure transducer 190 can withstand. The pre-stressing of the overload diaphragms 150, 170 is selected in such a way that they are deflected only negligibly in the event of pressure differences within the measuring range. However, if a pressure difference exceeds the measuring range, the deflection of the overload diaphragm on the lower pressure side starts to absorb volume of the transfer fluid of the high pressure side separating diaphragm chamber, such that the separating diaphragm on the high pressure side comes into contact with the diaphragm bed on the high pressure side, preventing a further increase in the pressure difference at the differential pressure transducer. Thereby, the overload diaphragm is dimensioned such that this state occurs before the strength limits of the differential pressure transducer are reached.

[0039] In the drawing, the separating diaphragms 130, 140 and overload diaphragms 150, 170 are represented by circular arcs. This illustration has nothing to do with their actual shape, since FIG. 1 is concerned only with the functional arrangement of the components of the differential pressure transducer relative to one another.

[0040] FIGS. 2a to 2d show an exemplary embodiment of an overload diaphragm 150 of a differential pressure transducer according to the invention in various installation and operating situations. In FIG. 2a, the free overload diaphragm 150 is shown in the equilibrium state, that is, free from external forces and moments. The overload diaphragm is substantially rotationally symmetrical about the Z axis. It has a radius R of between 11 mm and 14 mm and a maximum material thickness h.sub.max of 0.6 mm to 0.8 mm. The overload diaphragm 150 has a planar central region C and a planar edge region A, wherein a transition region B extends between the two, in particular one that is frustoconical at least in sections. Therefore, the overload diaphragm 150 has a base surface 150 the axial coordinate b(r) of which is constant in the equilibrium state in the central region, for example zero, and which increases uniformly in the transition region B to a value that is approximately 60% to 80% of the maximum material thickness h.sub.max of the overload diaphragm 150. Such value for b(r) is then constant in the edge region A and thus also corresponds to the value b(R) at the radius R. The transition region B begins at a radius R.sub.k, which is approximately half of the radius R. The upper side of the overload diaphragm facing away from the base surface 155 has a diaphragm bed with a (here, wave-shaped) contour k(r), which is prepared by machining or forming. The depth of the contour here is up to slightly more than half the maximum material thickness h.sub.max of the overload diaphragm 150. Accordingly, the minimum material thickness h.sub.min of the overload diaphragm 150 is slightly less than the maximum material thickness MAX. The material thickness h(r) as a function of the radius arises from the difference between the contour k(r) and the axial coordinate of the base surface b(r), that is, h(r)=k(r)b(r). It can be seen that the material thickness h(r) is influenced by the contour k(r) and practically not by b(r). In principle, the elastic characteristic of the diaphragm is thus specified by the course of the coordinate b(r) of the base surface, wherein the contour k(r), which is designed independently of this, serves on the one hand to form the diaphragm bed and on the other hand to model the stresses and stiffness of the overload diaphragm.

[0041] FIG. 2b shows the overload diaphragm 150 over the mating surface 115 of a measuring element body 110. Here, it is substantially important that the mating surface is not planar, but has a maximum in the center and slopes outwardly. Such course of the mating surface corresponds approximately to a bending line of the overload diaphragm 150 if the overload diaphragm 150 rests on a planar base and is clamped against the planar base with the base surface in the edge region. In fact, a pressure acts on the base only in the vicinity of the radius R.sub.k. If the center of the mating surface 115 thus follows exactly the bending line of the base surface during the deformation described above, the overload diaphragm is not yet supported. Only a further increase in the center of the mating surface 115 relieves the overload diaphragm in the vicinity of the radius R.sub.k. Regardless of whether this relief is desirable, it is desirable for the mating surface 115 to approach the bending line b(r), in order to minimize the quantity of transfer fluid in the overload chamber. The mating surface 115 is designed following these considerations.

[0042] FIG. 2c shows a section of the differential pressure transducer (100) according to the invention in the ready-to-operate state at pressure equilibrium. The overload diaphragm 150 is stretched over a mating surface 115 formed according to the above considerations and is fastened in its edge region to the measuring element body 100 by a circumferential weld seam 154, such that an overload chamber 152 is formed between the overload diaphragm 150 and the measuring element body 100, which however is compressed at pressure equilibrium by the pre-stressing of the overload diaphragm 150 down to a residual volume not shown here. A separating diaphragm 130 is arranged above the overload diaphragm 150 and is fixed to the measuring element body 110 by a circumferential weld seam 134, wherein a separating diaphragm chamber 132 is formed between the measuring element body 110 and the separating diaphragm 130. The separating diaphragm 130 is imprinted on the diaphragm bed 153 of the overload diaphragm. The separating diaphragm chamber 132 is filled with a hydraulic transfer fluid and communicates via a first hydraulic path 210 with a second overload chamber 172, which is identical in construction to the first overload chamber 152. The first overload chamber 152 communicates via a second hydraulic path 212 with a second separating diaphragm chamber 142, which is identical in construction to the first separating diaphragm chamber 132.

[0043] Finally, FIG. 2d shows the event of overload if the difference between the pressure in the second separating diaphragm chamber 142 or in the second hydraulic path 212 and the pressure in the first separating diaphragm chamber 132 exceeds a limit value outside a measuring range specified for the differential pressure transducer. In this case, the first overload diaphragm is deflected and takes up a volume of transfer fluid displaced from the second separating diaphragm chamber 142, such that the second separating diaphragm comes into contact with the second overload diaphragm.

[0044] To facilitate reliable deflection of the overload diaphragm 150, it is advantageous if the mating surface 115 only approximately follows the bending line of the base surface 155 at pressure equilibrium, such that a residual amount of transfer fluid still remains in the overload chamber 152 in order to transmit the pressure to the overload diaphragm 150. For support, channels in the form of grooves may be prepared in the mating surface and/or the base surface in order to facilitate distribution of the transfer fluid under the overload diaphragm.

[0045] The diagrams in FIGS. 3a and 3b show results of FEM calculations for the overload diaphragm from FIGS. 2a to 2d. Thereby, the overload diaphragm features a spring steel of grade 1.8159 with an experimentally determined modulus of elasticity of 195 GPa and an R.sub.p02 of 1.3 GPa. The starting material thickness h.sub.max was set at 700 m and the effective diameter of the overload diaphragm was set at 23.6 mm. FIG. 3a shows the volume stroke of the overload diaphragm as a function of a differential pressure between the two sides of the overload diaphragm. Accordingly, the volume stroke is practically negligible up to a differential pressure of approximately 1.8 MPa, and then increases abruptly in the event of overload, reaching approximately 55 l at approximately 3.9 MPa. This is sufficient to absorb the full volume of transfer fluid from the separating diaphragm chamber connected to the overload chamber, in order to thus prevent a further increase in differential pressure and protect the differential pressure transducer. It has been shown that it is advantageous to apply such a one-sided overload pressure to the overload diaphragm one time before the differential pressure transducer is put into operation, for example when the measuring element is filled with transfer fluid, to bring about the maximum required volume stroke. This leads to slight modifications of the characteristic curve of the volume stroke as a function of the differential pressure. Thus, the overload diaphragm is marginally plastically deformed. The solid characteristic curve in FIG. 3a represents such first deflection of the overload diaphragm, while the dashed characteristic curve applies to all further deflections. The effect of such first deflection is clearly shown in FIG. 3b, which shows results of FEM calculations on the maximum Von Mises stresses in the overload diaphragm as a function of differential pressure. In turn, the solid line applies to the first deflection of the overload diaphragm, while the dashed line describes all further deflections. At the first deflection, stress peaks that exceed R.sub.p02 for the material of the overload diaphragm can be seen. This results in minimal plastic deformation of the overload diaphragm upon initial deflection. However, with repeated deflections, the stress peaks are largely eliminated, which is why a constant relationship between volume stroke and differential pressure is thus achieved. This allows the differential pressure transducer to be reliably protected against overloads on one side.