Differential pressure sensor

Technical Field

[0001] The present invention relates to the field of pressure sensors. In particular, it relates to differential pressure sensors incorporating protection from damage caused by overpressure. State of the art

[0002] US 5,596,148 discloses a differential pressure sensor of the general type indicated above. This sensor comprises a central diaphragm situated in a central chamber formed between two housing sections, and supporting a differential pressure sensing element. Each side of the diaphragm is in fluid communication with a respective chamber defined between a respective separating membrane and a respective housing section, and the corresponding fluid communication channels are filled with oil so as to transmit pressure. Pressure within a pre-chamber adjacent to each separating membrane is applied to the corresponding membrane and is thus transmitted to the central diaphragm via the oil, exposing each side of the differential pressure sensing element to the respective pressures. The difference in the pressure in each pre-chamber can thereby be measured by the pressure sensing element. Furthermore, these changes in pressure cause the central membrane to be displaced in function of the pressure difference. Electrical connections are formed between the pressure sensor and the outside of the construction, so as to permit an external electronic device to be connected thereto while retaining fluid tightness of the assembly. To this end, a connector element is provided in a bore having an axis perpendicular to the plane of the central membrane, and a lateral opening is provided for the exit of the connecting ribbon or cable. All of the membranes are arranged to abut part of the housing in case of overpressure, so that they are supported and cannot burst or split. [0003] Nevertheless, the construction proposed in this document requires complex machining and assembly. Furthermore, this construction requires complex sealing arrangements to assure fluid tightness.

[0004] US 4,135,048 discloses another differential pressure sensor, globally similar to that of US 5,596,148 above. Again, this sensor comprises two housing elements between which is formed a cavity in which is situated a central diaphragm. Although the construction of the sensor is rationalized and easier to produce than that of US 5,596,148, it is still not optimized for easy production since it only comprises two housing elements, the larger of which is still subject to significant machining. Furthermore, since this larger housing element is directly associated with both the central diaphragm and one of the outer membranes, handling during manufacture may be delicate and thus difficult.

[0005] An object of the invention is thus to at least partially overcome the above- mentioned disadvantages of the prior art.

Disclosure of the invention

[0006] This object is attained by a differential pressure sensor comprising:

- a central housing element comprising a longitudinal opening passing therethrough and a lateral opening adjoining the longitudinal opening at an angle thereto and supporting a connector element disposed therein;

- a first outer housing element situated on a first side of the central housing element and comprising a first outer membrane on an outer face thereof;

- a second outer housing element situated on a second side of the central housing element and comprising a second outer membrane on an outer face thereof,

- a differential membrane supporting directly or indirectly a differential pressure sensing element, i.e. a differential pressure transducer, electrically connected to said connector element, the differential membrane being situated in a membrane cavity formed between the central housing element and the second outer housing element, a first side of the differential membrane being in fluid communication with said first outer membrane, a second side of the differential membrane being in fluid communication with said second outer membrane.

[0007] This three-piece construction provides excellent access for mounting the various components and for making the required electrical connections, simplifying construction and assembly.

[0008] Advantageously, the differential pressure sensor further comprises an absolute pressure sensing element, i.e. an absolute pressure transducer of any convenient type, disposed on said connector element and in fluid communication with said longitudinal opening and in electrical communication with the connector element. This permits the absolute pressure of fluid within the differential pressure sensor to be measured, for calibration and correction purposes.

[0009] Advantageously, an intermediate element is disposed in said longitudinal opening. This element reduces the volume of pressure-transmitting hydraulic oil required to fill the inside of the differential pressure sensor. The intermediate element may comprises a plurality of grooves adapted to permit the passage of fluid and/or an electrical connector connecting said differential pressure sensing element to said connector element. Furthermore, the first outer housing element may comprise a cylindrical protrusion extending into the longitudinal cavity in the central housing element, this latter comprising a shoulder. The intermediate element can thus be supported between the cylindrical protrusion and the shoulder, without requiring any further parts or physical features for its retention.

[0010] Advantageously, the first outer housing element and/or the second outer housing element comprise(s) a support surface against which respectively the first outer membrane and/or the second outer membrane can abut in the case of overpressure, thereby preventing damage to the membrane in question in the event of such an overpressure. Furthermore, at least one of the outer membranes comprises surface features, and the corresponding support surface comprising surface features complementary to said surface features of the corresponding outer membrane. This provides great flexibility to the membrane, and permits it to be securely supported in the case of overpressure. Advantageously, these surface features comprise circular undulations.

[001 1] Advantageously, at least one of said first outer housing element and said second outer housing element is attached to an annular protrusion protruding from respectively the first outer housing element and/or the second outer housing element.

[0012] Advantageously, the second outer housing element comprises an overhead cavity situated such that said differential pressure sensing element is at least partially situated therein, and can move therein.

Brief description of the drawings

[0013] Further details of the invention will become apparent in the following description, which makes reference to the accompanying drawings, which show:

- Figure 1 : a perspective exploded diagram of a differential pressure sensor according to the invention; and

- Figure 2: a cross-section in the plane A-A, B-B of figure 1 , the differential pressure sensor being in an assembled state. Embodiment of the invention

[0014] Figures 1 and 2 illustrate respectively an exploded view and a cross-sectional view in the plane defined by lines A-A and B-B on figure 1 of a differential pressure sensor 1 according to the invention, that is to say a plane in which are situated the axis of longitudinal opening 5 and the axis of lateral opening 6.

[0015] Differential pressure sensor 1 comprises a central housing element 3 of generally cylindrical form, which comprises a longitudinal opening 5, likewise of generally cylindrical form, which in the illustrated embodiment is coaxial with central housing element 3. Central housing element 3 further comprises a lateral opening 6 extending laterally from a side wall of longitudinal opening

5 to the outside of central housing element 3, and arranged substantially perpendicular to the axis of longitudinal opening 5. A shoulder 6a is provided at the interior end of lateral opening 6, to which is bonded a connector element 8 provided with electrical contacts 8a for connecting the sensor 1 to a further device. This bonding may be welding or similar. The electrical contacts 8a extend parallel to the axis of the lateral opening 6. Connector element 8 further comprises an absolute pressure sensing element 10 arranged on a side of connector element 8 opposite to that with the electrical contacts 8a, i.e. on the inside of the differential pressure sensor 1 and in fluid communication with the longitudinal opening 5. Absolute pressure sensor 10 thus serves to measure the absolute pressure of the oil filling the interior of the longitudinal cavity 5, and is in electrical connection with corresponding electrical contacts 8a. Absolute pressure sensing element 10 may be of any convenient type, such as piezoelectric or capacitive, and serves for calibration and for compensation of static pressure effects and temperature- related effects.

[0016] On a first side 3a of central housing element 3, which in the illustration is the lower side, is a first outer housing element 7, and is attached thereto in a fluid-tight manner, e.g. by welding. First outer housing element 7 comprises, protruding from its outer face 7a, an annular protrusion 9 which serves as a support for a first outer membrane 1 1. First outer membrane 1 1 is attached to annular protrusion 9 by welding, as is generally known. In use, first outer membrane 1 1 will be exposed to a certain pressure, e.g. in a cavity, pipe or similar, as is generally known.

[0017] As illustrated, first outer membrane 1 1 is of circularly-corrugated form, although a simple, flat membrane is also possible. The portion of the outer face 7a of first outer housing element 7 which is surrounded by annular protrusion 9 constitutes a first support surface 12 which comprises circular undulations or corrugations 13 of complimentary shape to the angularly- symmetric circular corrugations of first outer membrane 1 1. These grooves are arranged such that, in the case in which first outer membrane 1 1 is exposed to an overpressure, it enters into contact with said portion of the outer face 7a of first outer housing element 7, the grooves serving to intimately support the first outer membrane 1 1 and prevent it tearing or breaking. [0018] First outer housing element 7 also comprises a passageway 15 leading from the volume defined by the first outer membrane 1 1 and the annular protrusion 9 to the longitudinal opening 5 of the central housing element 3. This passageway 15 ensures that the first outer membrane 1 1 is in fluid communication with the differential pressure sensing element 47, i.e. a differential pressure transducer. Although this passageway 15 is illustrated as an axial passageway, this does not have to be the case, and the passageway may be offset from the central axis of the device, and/or may be formed at an angle thereto. Indeed, multiple passageways are possible. Furthermore, a port 17 is provided at an angle to passageway 15, to permit introduction of hydraulic oil (e.g. silicone oil) for transmitting pressure. As in the prior art, in use, the internal cavities of the differential pressure sensor 1 are filled with this hydraulic oil, so that pressure is transmitted from each outer membrane 1 1 , 29 to the differential pressure sensing element 47. Port 17 is closed e.g. by a ball valve (not illustrated) or any other convenient means, as is generally known. Finally, first outer housing element 7 comprises a cylindrical protrusion 19 situated on its interior face 7b (i.e. the face opposite to face 7a on which annular protrusion 9 is situated), which serves to support intermediate element 21 , in cooperation with a shoulder 23 provided on central housing element 3.

[0019] On a second end face 3b of the central housing element 3 is attached e.g. by welding a second outer housing element 25. Analogously to the first exterior housing element 7, second outer housing element 25 comprises on its outer face 25a an annular protrusion 27 supporting a second outer membrane 29, as well as a second support surface 32 comprising corresponding grooves 31. Furthermore, second outer housing element comprises an analogous passageway 33 leading to a substantially cylindrical overhead cavity 34, and a corresponding port 35, with the analogous function to port 17. This passageway 33 likewise ensures that the second outer membrane 29 is in fluid communication with the differential pressure sensing element 47 (see below). In use, second outer membrane 29 will be exposed to a certain pressure, e.g. in a cavity, pipe or similar, as is generally known. [0020] Each of the second face 3b of the central housing element 3 and the interior face 25b of the second outer housing element 25 comprise generally conically-shaped indentations 37, 39 respectively, which define a membrane cavity 41. It should be noted that indentations 37, 39 and overhead cavity 34 as illustrated here are coaxial with the central axis of the differential pressure sensor 1 , however this does not strictly have to be the case.

[0021] Supported between the second end face 3b of the central housing element 3 and the interior face 25b of second outer housing element 25 is a differential membrane 43. Differential membrane 43 comprises a central opening 43a in which is attached a sensing element support 45, which supports differential pressure sensing element 47 on one side thereof. Differential pressure sensing element 47 may be of any convenient type such as piezoelectric or capacitive, and the skilled person knows how to arrange it so as to measure differential pressure. A passageway 45a is provided through sensing element support 45 so that differential pressure sensing element 47 is in fluid connection with both outer membranes 1 1 , 29, and is thus arranged to measure the differential pressure to which the two outer membranes 1 1 , 29 are exposed. In response to changes in differential pressure across differential membrane 43 above a certain predetermined threshold, differential membrane 43 flexes within membrane cavity 41 , and the sensing element support 45 and the differential pressure sensing element 47 move axially, the sensing element support 45 being supported and/or guided by the side walls of the overhead cavity 34. In case of differential overpressure, that is to say an overpressure applied to one or other of the outer membranes 1 1 , 29, differential membrane 43 moves towards one or other of the surfaces of the conical-shaped indentations 37 and 39, which limit displacement of the differential membrane 43. This movement permits sufficient hydraulic oil to flow such that the respective outer membrane 1 1 , 29 exposed to the higher pressure can abut against the corresponding support surface 12, 32 and be adequately supported thereupon.

[0022] Differential pressure sensing element 47 is connected to contact elements 8a via a ribbon film cable 49, which passes from the sensing element support 45 to the connector element 8 on the inside of the longitudinal cavity 5. To this end, intermediate element 21 is provided with a longitudinal groove 51 to allow ribbon film cable 49 to pass, and also to permit hydraulic oil to move freely around intermediate element 21 from passageway 15 to the sensing element support 45. Furthermore, two radial grooves 53 permit fluid access from passageways 15 and 33 to longitudinal groove 51. It should be noted that the primary purpose of intermediate element 21 is to take up space in longitudinal opening 5, and thereby to reduce the quantity of hydraulic oil required to fill the interior of the differential pressure sensor 1.

[0023] Assembly of the differential pressure sensor 1 may proceed in several orders, non-limiting examples of which are as follows:

[0024] Differential membrane 43 is assembled with the sensing element support 45 and differential pressure sensing element 47, and is clamped between second outer housing element 25 and central housing element 3, which are then welded together. Second outer membrane 29 may be attached to the second outer housing element 25 at any convenient point before or after this step.

[0025] Connector element 8 is assembled to central housing element 3, and is welded or glued thereto. Naturally, this step can take place prior to the step of clamping the differential membrane.

[0026] Ribbon film cable is then electrically connected to the sensing element support 45 and to the connector element 8, e.g. by soldering, for which step the relatively large longitudinal opening 5 in the central housing element 3 provides excellent access for an appropriate tool.

[0027] Subsequently, the intermediate element 21 is inserted into the longitudinal opening 5, and the first outer housing element 7 is placed on the central housing element 3, holding the intermediate element in place. First outer housing element 7 is then welded to the central housing element 3. First outer membrane 1 1 can be attached to the first outer housing element 7 at any convenient moment. [0028] The interior cavities of the differential pressure sensor 1 are then filled with hydraulic oil via respective ports 17, 35. This can take place either before or after installation of the differential pressure sensor 1 in a systenn in which it is to take measurements

[0029] Although the invention has been described in terms of a particular embodiment, it is not limited thereto. Variations therefrom are possible without departing from the scope of the invention as defined in the appended claims.