Differential pressure measuring sensor with overload protection

Abstract

The invention relates to a differential pressure measuring sensor (1) which is protected against one-sided overload and has a double-membrane system consisting of two double membranes (4a, 4b) with a separating membrane (5a, 5b) and an overload membrane (6a, 56b). In order to ensure effective overload protection, the two membrane beds (9a, 9b) are designed substantially as planar surfaces for supporting the overload membranes (6a, 6b). In the non-assembled state, each of the two overload membranes (6a, 6b) is curved along a radially symmetrical bending line. The curvature extends substantially analogously but oppositely to the curvature of the bending line of the overload membrane (6a, 6b) when it is pre-loaded during assembly. The invention also relates to a method for producing and assembling a differential pressure measuring sensor (1) according to the invention.

Description

[0001] Differential pressure sensor with overload protection

[0002] The invention relates to a differential pressure sensor for determining the differential pressure between two pressures. The differential pressure sensor according to the invention is preferably used in the field of automation technology. Differential pressure sensors are particularly useful for the continuous measurement of pressure differences in measuring media, e.g., liquids, vapors, gases, and dusts. From the differential pressure, for example, the fill level of a substance in a container or the flow rate of a measuring medium through a pipeline can be determined.

[0003] A silicon chip is typically used as the pressure-sensitive element. To achieve good measurement sensitivity, a differential pressure sensor preferably operates in a range close to a critical pressure limit (nominal pressure). If the critical limit is exceeded, there is a risk that the chip will be destroyed by an uneven overpressure. Since silicon chips, in particular, have a relatively low overload capacity, a differential pressure sensor is usually equipped with overload protection. This protection is preferably designed to minimize any impact on the measurement sensitivity and accuracy of the pressure-sensitive element within the sensor's measuring range.

[0004] Differential pressure sensors typically comprise a measuring body with a first pressure inlet port and a second pressure inlet port, each with a hydraulic path extending to the differential pressure transducer. A differential pressure transducer usually incorporates a deformation element, such as a measuring diaphragm, whose opposing surfaces are each subjected to one of the two pressures, causing the diaphragm to undergo elastic deformation dependent on the difference between the two pressures. The pressure inlet ports of the measuring body are usually closed by flexible metallic diaphragms, forming diaphragm chambers. Each chamber directs the pressure of the medium being measured, acting on one side of the diaphragm, into the chamber and the connected hydraulic path.In measuring instruments with coplanar pressure inlet openings, the pressure inlets are located next to each other on a process connection surface of the measuring instrument body. Besides coplanar differential pressure sensors, there are so-called bipolar differential pressure sensors, in which the pressure inlets point in opposite directions. Overload protection is provided by both a double-deflection overload diaphragm and an overload diaphragm system with two oppositely pre-tensioned overload diaphragms, each of which reacts to a unilateral overload. Examples of this are described in patent literature, e.g., in DD 279 065 A1, DD 287 328 A5, DD 290 716 A5, DE 32 22 620 A1, and US 10,656,039 B2.To reliably implement this principle, it must be ensured that the overload diaphragms are deflected only when pressure differences exceed the measuring range, with the required volume displacement of the overload diaphragm corresponding to the total volume of the separating diaphragm chamber, and with any plastic deformation resulting from the required displacement being negligible. Furthermore, in the event of a one-sided overload on the high-pressure side, the overload diaphragm should support the separating diaphragm when all oil has been displaced from the high-pressure-side separating diaphragm chamber, in order to effectively prevent plastic deformation of the separating diaphragm. For this purpose, the overload diaphragm must conform to the contour of the...

[0005] The separating membrane must have a corresponding contour, prepared, for example, by embossing an overload membrane disc with a die, as is likely the case for the overload membranes according to US 10,656,039 B2. According to US 10,656,039 B2, the resulting wave contour of the overload membrane should also have a larger axial stroke than a coaxial wave contour on the surface of the measuring instrument body, against which the overload membrane partially rests. This requires, firstly, a very complex manufacturing process, and secondly, the various requirements for the overload membrane are hardly compatible.

[0006] From DE 82 04 310 U1, a differential pressure measuring device is known, comprising a central receiving body and a measuring chamber located therein, subdivided by a pressure sensor device protected against pressure overload. The sub-chambers of the measuring chamber are each connected via a connecting channel to pre-chambers, which are each formed by a separating membrane and the receiving body. Near each separating membrane, an additional membrane is attached to the receiving body in a pressure-transmitting connection with the separating membrane and rests against the receiving body under preload. The additional membranes are pressed against the receiving body by at least one compression spring in each of the additional chambers.

[0007] A comparable solution is also described in DD 230 930 A1. Here, each of the two additional diaphragms is pressed against the receiving body by at least one compression spring in the additional chambers. The spring force is dimensioned such that the additional diaphragms are held in their initial position during normal measuring operation. In the known solution, the diaphragm is indeed pre-tensioned in the installed state; however, before the pre-tension is applied, the diaphragm contour is either flat or curved in the same direction as during operation under pressure: If the diaphragm contour is convexly curved due to the pre-tensioning, then the deflection during operation will consequently also be convexly curved. This prevents optimal utilization of the displaceable oil volume. Furthermore, the diaphragm bed has a non-planar contour that is adapted to a non-trivial curvature of the diaphragm. Therefore, the contour must be specially manufactured.This introduces tolerance uncertainties regarding the contact points of the membrane and therefore also its performance.

[0008] In the differential pressure gauge with overload protection disclosed in EP 86 737 B1, there is no compression spring between the diaphragm bed and the auxiliary diaphragm, but rather a cast body. After the auxiliary diaphragm is mounted, this cast body is introduced into the space between the auxiliary diaphragm and the diaphragm bed as molten metal. Specifically, the liquid is a molten metal of a low-melting-point metal. Additionally, a thin sealing membrane is arranged between each cast body and its associated auxiliary diaphragm; this sealing membrane is attached to the receiving body within the auxiliary diaphragm.

[0009] Aside from the complicated design and elaborate manufacturing, the curvature of the auxiliary diaphragm, responsible for the preload, also points in the same direction as the deflection of the auxiliary diaphragm under overload conditions. Again, optimal utilization of the movable oil volume cannot be achieved.

[0010] A coplanar double-diaphragm system with overload protection is disclosed in DE 10 2020 121 585 A1. During measurement, the two overload diaphragms are positively engaged with the measuring mechanism. At least one hydraulic channel is provided in the diaphragm beds and / or in the corresponding back surfaces of the overload diaphragms. The deflection of the overload diaphragms is prevented up to a predetermined value due to their preload, or is so small that it can be disregarded. Here, too, the preload is designed so that it only activates outside the measuring range of the differential pressure sensor.

[0011] The preload of the overload diaphragms ensures that deflection of the overload diaphragm only occurs when a critical overpressure arises at one of the double diaphragms, which would otherwise risk destroying the pressure-sensitive element. For example, as soon as a critical overpressure occurs at the second separating diaphragm, the second separating diaphragm is moved against the second overload diaphragm until it is in contact with the overload diaphragm. If the preload of the first overload diaphragm is exceeded, it is deflected, and the transfer fluid pushed out of the second pressure chamber is transferred via the coupled connecting capillaries into the first.

[0012] The additional pressure chamber is displaced. This process ends when the hydraulic fluid has been shifted from the high-pressure side to the low-pressure side.

[0013] Another solution using pre-tensioned overload diaphragms in a coplanar double diaphragm system is also known from WO 2022 037 859 A1. The deflection of the overload diaphragms is forcibly prevented up to a predetermined value due to their pre-tension, and this value is designed such that the overload diaphragm is not activated within the measuring range of the pressure sensor.

[0014] German patent DE 10 2004 006 383 A1 describes a differential pressure sensor with an overload diaphragm and hydraulic pressure transmission to a measuring element via a transmission fluid. The differential pressure sensor has a hydraulic body containing an overload chamber with an overload diaphragm that divides the chamber into a high-pressure and a low-pressure half-chamber. The high-pressure half-chamber communicates via a first hydraulic path extending between a first pressure transmitter with a separating diaphragm over a diaphragm bed and the high-pressure side of the pressure measuring cell. The low-pressure half-chamber communicates via a second hydraulic path extending between a second pressure transmitter with a separating diaphragm over a diaphragm bed and the low-pressure side of the measuring element. The low-pressure chamber has a convexly shaped diaphragm bed.

[0015] In the event of overpressure, the transmission fluid is completely forced out of the pressure transmitter, and the diaphragm rests against the diaphragm bed. The transmission fluid, displaced into the relevant hydraulic path by the overload pressure, causes the overload diaphragm to deflect in order to accommodate the additional volume and relieve the overload pressure. The spring stiffness of the overload diaphragm depends on the operating temperature range, the possible system pressure, the volume of the hydraulic transmission fluid, and the overload factor of the sensing element. The overload factor defines by what factor the overload pressure can exceed the measuring range before the diaphragm engages and no further pressure increase occurs at the pressure-sensitive sensing element. The higher the overload factor of the sensing element, the stiffer the overload diaphragm can be.The stiffer the overload diaphragm, the faster the measuring cell reacts to pressure fluctuations. A comparable differential pressure sensor as described above is disclosed in DE 103 34284 A1. However, in this case, the overload diaphragm is optimally adjusted to the asymmetrical burst values ​​of the measuring element; that is, the maximum spring stiffness of the overload diaphragm, and thus a minimum hydraulic capacity of the overload diaphragm, is ensured depending on the direction of travel. In other words, the overload diaphragm exhibits asymmetrical stiffness against high-pressure and low-pressure overloads.

[0016] WO 2022 122 411 A1 also discloses a differential pressure sensor with overload protection, in which the first and second overload diaphragms are pre-tensioned against the first and second counter-surfaces, respectively, when the sensor is in operational condition at pressure equilibrium (i.e., a pressure difference of zero), such that the first and second base surfaces are in contact with the first and second counter-surfaces, respectively, at least partially. This pre-tensioning of the overload diaphragms is achieved by a radially variable material thickness. Furthermore, it is provided that both overload diaphragms have a diaphragm bed surface facing the respective separating diaphragm with a predetermined contour. The contour is prepared by machining or forming. The contoured diaphragm bed of the overload diaphragms serves to support the adjacent separating diaphragm in the event of an overload.

[0017] Minimizing the required oil volume can be achieved, for example, by using a double diaphragm system instead of a system with a center diaphragm; by using overload diaphragms that are pre-tensioned so that no oil volume needs to be moved in the measuring range.

[0018] In general, practical solutions aim to keep the volume of oil used in the pressure sensor as small as possible. This is important for several reasons:

[0019] Oil changes its volume with temperature changes or with static pressure, i.e., pressure acting equally on both sides of the differential pressure sensor. This causes the diaphragms to shift. Since the diaphragms possess a certain stiffness, a change in oil volume results in a different pressure being exerted on the oil in the hydraulic system. This leads to a distortion of the measured pressure values ​​or non-linear ("disturbed") behavior of the differential pressure sensor, which must be corrected computationally. If it is possible to use diaphragms that exhibit zero stiffness within a specific volume range, minimizing the oil volume reduces the size of the volume range in which the diaphragms must have zero stiffness.

[0020] In the event of an overload, the oil volume located between a process diaphragm and an overload diaphragm must be completely displaced and absorbed by the diaphragms on the opposite side. The smaller the oil volume, the lower the requirements for the diaphragms on the opposite side to absorb the displaced oil volume without damage.

[0021] The more oil is present, the more oil needs to be displaced during operation when the pressure changes. With large oil volumes, this leads to longer response times, especially when pressure transmitters with long supply lines are used. However, a minimum oil volume is also necessary to allow the oil volume within the sensor's measuring range to "breathe" sufficiently, i.e., to expand and contract without causing damage to any of the process diaphragms.

[0022] In summary, the state-of-the-art solutions exhibit at least one of the following disadvantages:

[0023] The overload membrane is not pre-tensioned, so a significant volume of oil is displaced within the measuring range.

[0024] The residual oil volume that remains under the overload membrane without pressure is not negligible.

[0025] Although the diaphragm is pre-tensioned in its installed state, its contour is either flat or curved in the same direction as during pressure-loaded measurement operation before the pre-tensioning is applied. Therefore, optimal utilization of the movable oil volume cannot be achieved.

[0026] The membrane bed has a non-planar contour adapted to the non-trivial curvature of the overload membrane, requiring custom manufacturing of the contour. Furthermore, this solution introduces tolerance uncertainties regarding the contact points of the overload membrane on the membrane bed, which in turn impairs its performance.

[0027] The invention is based on the objective of proposing a differential pressure sensor with overload protection, which has optimized overload diaphragms. Furthermore, a method for manufacturing a corresponding overload diaphragm is proposed. This objective is achieved by the differential pressure sensor according to claim 1 of the invention.

[0028] The differential pressure sensor for determining the differential pressure of two pressures comprises a measuring element body and a transducer chamber, wherein a differential pressure measuring cell with a pressure-sensitive element is arranged in the transducer chamber. A double-diaphragm system with two double diaphragms is located on the measuring element body. Each of the two double diaphragms consists of a separating diaphragm and an overload diaphragm arranged downstream of the separating diaphragm in the direction of the pressure effect. A first pressure chamber is formed between the first separating diaphragm and the first overload diaphragm, and a first overload chamber is formed between the first overload diaphragm and the measuring element body. A second pressure chamber is formed between the second separating diaphragm and the second overload diaphragm, and a second overload chamber is formed between the second overload diaphragm and the measuring element body.The first pressure chamber is assigned a first connecting capillary, and the second pressure chamber a second connecting capillary, through which the pressures are hydraulically transmitted to the pressure-sensitive element. Furthermore, the first overload chamber is assigned a first auxiliary capillary, and the second overload chamber a second auxiliary capillary. To protect against unilateral overpressure, the first connecting capillary is hydraulically coupled to the second auxiliary capillary, and the second connecting capillary is coupled to the first auxiliary capillary. The two overload diaphragms are prestressed such that, during normal measurement operation, their base surfaces facing the measuring element body are in at least partial contact with a corresponding diaphragm bed of the measuring element body. They only lift away from the diaphragm bed of the measuring element body when a predetermined critical pressure limit is exceeded.The two membrane beds essentially have a flat surface. Each of the two overload membranes exhibits a radially symmetrical deflection curve in its unmounted state, the curvature of which is essentially analogous (the same) but contrary (opposite) to the deflection curve of the overload membrane when it is prestressed after installation.

[0029] The solution according to the invention has the following advantages:

[0030] • As long as no one-sided overpressure occurs at one of the separating membranes, the overload membranes rest with their base surfaces against the flat surface of the measuring instrument body. The enclosed oil volume is approximately zero.

[0031] • The overload diaphragms displace virtually no oil volume during normal operation of the differential pressure sensor. • Because the overload diaphragms are convexly curved in the unmounted state – and not concave or flat as in prior art – and curve in the opposite direction when a one-sided overpressure occurs, a higher oil capacity can be achieved.

[0032] • The contact surfaces with the measuring body are flat, which simplifies manufacturing and minimizes error tolerances.

[0033] A further development of the solution according to the invention provides that the first overload diaphragm is connected to the measuring instrument body along a circumferential first overload diaphragm edge, forming the first overload chamber; likewise, the second overload diaphragm is connected to the measuring instrument body along a circumferential second overload diaphragm edge, forming the second overload chamber. Furthermore, the first separating diaphragm is connected to the measuring instrument body along a circumferential first separating diaphragm edge, forming the first pressure chamber, whereby the first overload diaphragm is arranged between the measuring instrument body and the first separating diaphragm. The same applies to the second separating diaphragm: it is connected to the measuring instrument body along a circumferential second separating diaphragm edge, forming the second pressure chamber, whereby the second overload diaphragm is also arranged between the measuring instrument body and the second separating diaphragm.

[0034] Furthermore, in connection with the differential pressure sensor according to the invention, it is provided that the base surface of the two overload membranes, which face the flat membrane bed in the measuring body, have a substantially flat surface.

[0035] According to one embodiment, the surface of each of the two overload membranes, which faces the corresponding separating membrane in the assembled state, has a contour. Preferably, the contour is a radially symmetrical waveform. The waveform is necessary, or at least advantageous, so that the separating membrane, which is embossed on the overload membrane, has a wavy contour. This is relevant for the performance of the separating membrane. In principle, the overload membrane and the separating membrane can also be designed with non-radially symmetrical contours. However, a non-radially symmetrical overload membrane may be more complex to manufacture.

[0036] The double diaphragm system of the differential pressure sensor according to the invention can be configured as either a coplanar double diaphragm system or a bipolar double diaphragm system. Both configurations are well known in the art. The method for manufacturing an overload diaphragm for a differential pressure sensor, as described above, is characterized by the following process steps: a disk-shaped – usually circular – overload diaphragm is formed radially symmetrically convex or concave such that it has a defined deflection curve; during assembly, the overload diaphragm is prestressed in the edge region by a force acting in the opposite direction to the curvature until the flat base surface of the overload diaphragm rests at least approximately against the flat diaphragm bed of the measuring body; under this prestress, the edge region of the overload diaphragm is connected to the measuring body.In particular, the connection is made via a welding or soldering process.

[0037] A further development of the inventive method provides that the deflection curve of the convex or concave shaped overload membrane is dimensioned such that the flat base surface of the disc-shaped overload membrane, after assembly on the measuring body, lies as far as possible against the flat membrane bed of the measuring body under a defined prestress.

[0038] Furthermore, in a further development of the inventive method, it is proposed that the preload with which the convex or concave overload diaphragm rests against the measuring element body is dimensioned such that the overload diaphragm rests against the diaphragm bed during normal measuring operation of the differential pressure sensor and only lifts away from the diaphragm bed when an overpressure occurs that would destroy the pressure-sensitive element. The overload protection is only activated when a critical limit pressure for the sensitive measuring element is reached or exceeded.

[0039] The invention is explained in more detail with reference to the following figures. They show:

[0040] Fig. 1: a schematic representation of a differential pressure sensor according to the invention a) with bipolar arrangement of the two double diaphragms b) with coplanar arrangement of the two double diaphragms

[0041] Fig. 2: Schematic representation of an overload membrane according to the invention in its original state before the curvature is applied, a) in perspective view b) in sectional view, Fig. 3: A schematic representation of the individual process steps for mounting a radially symmetrical overload membrane with a curved bending line according to the invention on the measuring instrument body a) overload membrane with bending line before mounting on the measuring instrument body, b) the overload membrane prestressed according to the invention during normal measuring operation, c) the overload membrane prestressed according to the invention when a critical overpressure pk occurs,

[0042] Fig. 4: Schematic representations of two possibilities for imprinting the bending curve onto the overload membrane: a) the original overload membrane with central support from below and force applied at the edge from above during the imprinting of the desired displacement to the curvature according to the bending curve in a perspective view; b) the overload membrane in cross-section with the imprinted displacement to the curvature according to the bending curve when applying the method shown in Fig. 4a; c) the overload membrane in cross-section with the inversely imprinted displacement to the curvature according to the bending curve, with central support from above and force applied at the edge from below.

[0043] Fig. 5: a pV diagram illustrating the operation of the overload membrane according to the invention.

[0044] Fig. 1 shows a schematic representation of two embodiments of the differential pressure sensor 1 according to the invention for determining the pressure difference between two pressures p1, p2. In Fig. 1a, the two double diaphragms 4a, 4b are arranged bipolarly; in Fig. 1b, a coplanar arrangement of the two double diaphragms 4a, 4b of the differential pressure sensor 1 is sketched.

[0045] The differential pressure sensor 1 comprises a measuring body 2 (shown only partially) and a transducer chamber 3. A differential pressure measuring cell 12 with a pressure-sensitive element 18 is arranged in the transducer chamber 3. A double diaphragm system with two double diaphragms 4a, 4b is provided on the measuring body 2, wherein each of the two double diaphragms 4a, 4b consists of a separating diaphragm 5a, 5b and an overload diaphragm 6a, 6b arranged downstream of the separating diaphragm 5a, 5b in the direction of the pressure effect. A first pressure chamber 7a is formed between the first separating diaphragm 5a and the first overload diaphragm 6a, and a first overload chamber 8a is formed between the first overload diaphragm 6a and the measuring body 2. Likewise, a second pressure chamber 7b is formed between the second separating membrane 5b and the second overload membrane 6b, and a second overload chamber 8b is formed between the second overload membrane 6b and the measuring body 2.

[0046] The first pressure chamber 7a is assigned a first connecting capillary 10a, and the second pressure chamber 7b a second connecting capillary 10b, via which the pressures p1 and p2 are hydraulically transmitted to the pressure-sensitive element 13 during normal measurement operation. The first overload chamber 8a is assigned a first auxiliary capillary 11a, and the second overload chamber 8b a second auxiliary capillary 11b. To protect against unilateral overpressure, the first connecting capillary 10a is hydraulically coupled to the second auxiliary capillary 11b, and the second connecting capillary 10b is coupled to the first auxiliary capillary 11a.To ensure that the overload protection only activates when a critical overpressure occurs on one side that could damage the pressure-sensitive element, the overload diaphragms 6a, 6b are pre-tensioned such that, during normal measuring operation, their base surfaces 14a, 14b facing the measuring body 2 are at least largely in contact with a corresponding diaphragm bed 9a, 9b of the measuring body 2. Only when a critical limit pressure acts on one of the two separating diaphragms 5a; 5b does one of the overload diaphragms 6a; 6b lift off the diaphragm bed 9a, 9b of the measuring body 2; the displaced volume of hydraulic fluid 13 is absorbed in the resulting overload chamber 8a; 8b.

[0047] Figure 2 shows a schematic representation of an overload membrane 6 according to the invention in its original state – before the radially symmetrical curvature is applied according to a defined bending curve. The perspective view in Figure 2a shows only part of the overload membrane 6, while the sectional view in Figure 2b shows the entire overload membrane 6. The base surface 14 of the overload membrane 6, which rests against the flat membrane bed 9 of the measuring body 2 in the assembled state, is flat. The surface 17 of the overload membrane 6, which faces the separating membrane 5 in the assembled state, has a contour. In the case shown, the contour of the surface 17 is a radially symmetrical waveform that begins in a central region Z of the overload membrane 6 and ends before the edge region 15 of the overload membrane 6. The axis of symmetry runs through the center Z of the separating membrane 6.The deflection curve of the overload membrane 6 is determined when it is supported only in the central area Z and pulled down radially symmetrically at the edge. This constitutes the first process step for manufacturing the overload membrane 6 according to the invention.

[0048] Figure 3 schematically and successively illustrates further process steps for mounting the disc-shaped, radially symmetrical overload membrane 6 on the measuring body 2. Figure 3a shows the circular separating membrane with a convexly curved deflection curve. The deflection curve is determined by supporting the overload membrane centrally and loading it at the edge. The deflection curve is analyzed numerically using the FEM (Finite Element Method).

[0049] The deflection curve here is convex or "negatively" curved, meaning it curves upwards or in the opposite direction to the determined deflection curve. The distance between the upwardly curved edge of the overload membrane 6 and the corresponding edge region of the flat membrane bed 9 is given by the distance x. The radius of curvature of the overload membrane 6 is dimensioned such that, after assembly with a predetermined preload, the overload membrane 6 comes into close contact with the membrane bed 9. Since the unpreloaded overload membrane 6 follows the opposite deflection curve, the preload ensures that the base surface 14 of the preloaded overload membrane 6 is again approximately flat and thus lies almost flush on the flat surface of the flat membrane bed 9. Finally, the functionality of the solution is preferably investigated using simulation.

[0050] Fig. 3b shows the overload diaphragm 6 after its installation on the measuring body 2. The required preload, with which the overload diaphragm 6 rests against the flat diaphragm bed 9, is achieved by pressing the separating diaphragm 6 down in its edge region 15, e.g., with a hold-down device, under the influence of a defined force and connecting it to the corresponding region of the diaphragm bed 9. The connection is preferably made by a welding or soldering process. The separating diaphragm 5 and the pressure chamber 7 are also clearly visible in Fig. 3b.

[0051] Figure 3c illustrates the behavior of the overload diaphragm 6, prestressed according to the invention, when a critical overpressure pk occurs. If a critical overpressure pk occurs at the first double diaphragm 4 (not shown), the overload diaphragm 6 of the second double diaphragm 4 is lifted from the diaphragm bed 9, and the overload chamber 8 can—to protect the pressure-sensitive element 18—accommodate the volume of hydraulic fluid 13 displaced from the pressure chamber 8 of the first double diaphragm. The advantages of the solution according to the invention compared to the prior art solutions have already been described in detail above. A repetition is omitted here.

[0052] Figure 4 schematically illustrates two variants for determining the radially symmetrical deflection curve on the overload membrane 6. In the first variant (Fig. 4a, Fig. 4b), the overload membrane 6 is supported centrally from below, the central area rests on a solid surface, and a force is applied from above to the circular outer area until the overload membrane exhibits the desired curvature of the deflection curve. In this configuration, the deflection curve is determined by supporting the overload membrane 6 centrally and pulling or pushing it down at the outer area. The correspondingly convexly curved overload membrane 6 is also shown in Fig. 3. In Figure 4, the force applied centrally, as well as the force applied at the outer area, is visualized by the arrows pointing in the corresponding directions.

[0053] A second variant (essentially the reverse of the first) is shown in Fig. 4c. Here, the radially symmetrical deflection curve is determined by supporting the central area of ​​the overload membrane 6 from above while pulling the outer area upwards. Alternatively, the outer area of ​​the overload membrane 6 is supported from below, and the central area is pressed downwards. These two methods for determining the deflection curve are equivalent and result in the same curvature of the overload membrane.

[0054] If the overload membrane 6 has a wave shape on one side – as is the case in an advantageous embodiment of the overload membrane 6 – this generally leads to slightly different bending lines in the two previously described manufacturing variants. However, the difference in the determined bending lines is so small that they can be used in conjunction with the invention.

[0055] Figure 5 shows the pV characteristic curve for the deflection of the overload diaphragm 6 for an exemplary solution. The so-called dead volume – i.e., the volume of hydraulic fluid 13 in the overload chamber 8 at p=0 – is only 0 ml of hydraulic fluid 13 for a separating diaphragm 6 with a diameter of 25.4 mm. The volume V displaced within the measuring range of the differential pressure sensor 1 is also as small as possible. It is determined by the volume change / g between p=0 and p=pMeasuring. The smaller the displaced volume V within the measuring range, the flatter the slope of the characteristic curve in this range.

[0056] The vertical line marks the pressure pk at which the overload chamber 8 has absorbed the displaced oil volume (here 55 pil). A minimum oil volume is required so that the oil volume in the measuring range of the differential pressure sensor 1 can "breathe" sufficiently, i.e., expand and contract within the required temperature range and for all required static (i.e., bilateral) pressures without either of the overload diaphragms becoming compressed. Reference numeral list

[0057] 1 Differential pressure sensor

[0058] 2 measuring instrument bodies

[0059] 3 converter chamber

[0060] 4 double membrane

[0061] 5 Separation membrane

[0062] 6 Overload membrane

[0063] 7 Pressure chamber

[0064] 8 Overload chamber

[0065] 9 Membrane bed

[0066] 10 connecting capillaries

[0067] 11 auxiliary capillaries

[0068] 12 Differential pressure measuring cell

[0069] 13 Hydraulic fluid

[0070] 14 Base area

[0071] 15 Overload membrane edge

[0072] 16 Separation membrane edge

[0073] 17 Contoured surface of the overload membrane

[0074] 18 pressure-sensitive elements

Claims

Patent claims 1. Differential pressure sensor (1) for determining the differential pressure of two pressures (p1, p2) comprising a measuring body (2) and a transducer chamber (3), wherein a differential pressure measuring cell (12) with a pressure-sensitive element (18) is arranged in the transducer chamber (3) and wherein a double diaphragm system with two double diaphragms (4a, 4b) is provided on the measuring body (2), wherein the two double diaphragms (4a, 4b) each consist of a separating diaphragm (5a, 5b) and an overload diaphragm (6a, 6b) arranged in the direction of the pressure effect behind the separating diaphragm (5a, 5b), wherein a first pressure chamber (7a) is formed between the first separating diaphragm (5a) and the first overload diaphragm (6a) and a first overload chamber (8a) is formed between the first overload diaphragm (6a) and the measuring body (2),wherein a second pressure chamber (7b) is formed between the second separating membrane (5b) and the second overload membrane (6b) and a second overload chamber (8b) is formed between the second overload membrane (6b) and the measuring body (2), wherein a first connecting capillary (10a) is assigned to the first pressure chamber (7a) and a second connecting capillary (10b) is assigned to the second pressure chamber (7b), via which the pressures (p1 , p2) are hydraulically transmitted to the pressure-sensitive element (13), wherein a first auxiliary capillary (11a) is assigned to the first overload chamber (8a) and a second auxiliary capillary (11b) is assigned to the second overload chamber (8b), wherein - to protect against a one-sided overpressure - the first connecting capillary (10a) is hydraulically connected to the second auxiliary capillary (11b) and the second connecting capillary (10b) to the first auxiliary capillary (11a) coupled, wherein the overload membranes (6a, 6b) are pre-tensioned in such a way thatthat in normal measuring operation they each bear at least largely against a corresponding membrane bed (9a, 9b) of the measuring body (2) with a base surface (14a, 14b) facing the measuring body (2) and only lift off from the membrane bed (9a, 9b) of the measuring body (2) when a predetermined critical limit pressure (pk) is exceeded, characterized in that the two membrane beds (9a, 9b) are essentially designed as planar surfaces, and that each of the two overload membranes (6a, 6b) has a radially symmetrical deflection curve in the unmounted state, the curvature of which is essentially analogous to, but contrary to, the deflection curve of the overload membrane (6a, 6b) when it is prestressed during assembly.

2. Differential pressure sensor according to claim 1, characterized in that, that the first overload membrane (6a) is connected to the measuring body (2) along a circumferential first overload membrane edge (15a), forming the first overload chamber (8a); that the second overload membrane (6b) is connected to the measuring body (2) along a circumferential second overload membrane edge (15b), forming the second overload chamber (8b); that the first separating membrane (5a) is connected to the measuring body (2) along a circumferential first separating membrane edge (16a), forming the first pressure chamber (7a); that the first overload membrane (6a) is arranged between the measuring body (2) and the first separating membrane (5a); that the second separating membrane (5a) is connected to the measuring body (2) along a circumferential second separating membrane edge (16b), forming the second pressure chamber (7b); and that the second overload membrane (6b) is arranged between the measuring body (2) and the second separating membrane (5b). is.

3. Differential pressure sensor according to claim 1 or 2, characterized in that the base surface (14a; 14b) of the overload diaphragm (6a; 6b), which faces the flat diaphragm bed (9a; 9b) in the measuring body (2), is designed to be flat.

4. Differential pressure sensor according to claim 1, 2 or 3, characterized in that the surface (17a; 17b) of the overload diaphragm (6a; 6b), which in the assembled state faces the separating diaphragm (5a; 5b), has a contour, wherein the contour is preferably a radially symmetric waveform.

5. Differential pressure sensor according to one or more of the preceding claims, characterized in that the double diaphragm system, which consists of two double diaphragms (4a, 4b), is designed as a coplanar double diaphragm system.

6. Differential pressure sensor according to one or more of the preceding claims, characterized in that the double diaphragm system, which consists of two double diaphragms (4a, 4b), is designed as a bipolar double diaphragm system.

7. Method for manufacturing an overload diaphragm (6a; 6b) for a differential pressure sensor (1) as described in one of claims 1-4, characterized in that a disk-shaped - preferably circular - overload diaphragm (6a; 6b) is formed in such a radially symmetric convex or concave manner that it has a defined deflection line, that the overload diaphragm (6a, 6b) is prestressed during assembly on the measuring body (2) by a force acting in the opposite direction to the curvature in the edge region (15a; 15b) until the flat base surface 14a; 14b) of the overload diaphragm (6a; 6b) at least approximately abuts the flat diaphragm bed (9a; 9b) of the measuring body (2), and that the edge region (15a; 15b) of the overload diaphragm (6a; 6b) is connected to the measuring body (2).

8. Method according to claim 7, characterized in that the deflection curve of the convex or concave shaped overload membrane (6a; 6b) is dimensioned such that the disk-shaped overload membrane (6a; 6b) after assembly on the measuring body (2) is substantially in contact with the membrane bed (9a; 9b) of the measuring body (2) under a defined preload.

9. Method according to claim 7 or 8, characterized in that the preload with which the convex or concave shaped overload membrane (6a; 6b) is positioned on the measuring body (2) in such a way that the overload diaphragm (6a; 6b) rests against the diaphragm bed (9a; 9b) during normal measuring operation of the differential pressure sensor (1) and only lifts off from the diaphragm bed (9a; 9b) when a one-sided overpressure (pk) occurs.

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