Abstract:
A magnetic inductive flowmeter having a measuring distance that is surrounded by a wall; a magnetic field generating device, and; an electrode device, which has at least one electrode on the inside of the wall and an electrode connection on the outside of the wall. The electrode connection is connected to the electrode by a plug-type connection, and the electrode connection fixes the electrode to the wall.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is the US National Stage of International Application No. PCT/EP2004/003019, filed Mar. 22, 2004 and claims the benefit thereof. The International Application claims the benefits of German application No. 10312824.7, filed Mar. 22, 2003, both applications are incorporated by reference herein in their entirety. 
   FIELD OF INVENTION 
   The invention relates to a magnetic inductive flowmeter comprising a measuring distance that is surrounded by a wall, a magnetic field generating device and an electrode device, which has at least one electrode on the inside of the wall and an electrode connection on the outside of the wall. 
   BACKGROUND OF THE INVENTION 
   A flowmeter of this type is known, for example, from U.S. Pat. No. 4,297,897. The measuring distance is normally formed by a tube. Two magnetic coils that generate a flow through the tube perpendicular to the direction of flow are disposed on the outside of the tube. Electrodes that have contact with the fluid are arranged at approximately 90° to the magnetic coils. A voltage between the two electrodes is dependent both on the strength of the magnetic field and on the speed of the fluid flowing through. 
   In the known case, the electrode is screwed into an inner thread of a fitting that is inserted through the wall of the tube. The fitting in turn has an outer thread onto which a nut is threaded. The fitting has an enlarged head on the inside of the wall, so that the wall is clamped between the head and the nut when a washer and a spring are interposed. 
   A further electrode array for a magnetic inductive flowmeter is known from EP 1 217 338 A2, in which the electrode has a shaft, which is routed through the wall of the measuring tube. The shaft has an outer thread onto which a nut is threaded in order to tighten the electrode to the wall. 
   The assembly cost is relatively high in such an embodiment. At least one, but in most cases several, screw connections are needed in order to fix the electrode in the wall of the measuring distance and in order to connect an electrical cable. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to simplify the assembly. 
   This object is achieved by means of a flowmeter of the type described above, in which the electrode connection is connected to the electrode via a plug-type connection and the electrode connection fixes the electrode to the wall. 
   This embodiment has a number of advantages. The assembly is considerably simplified. Screw connections no longer have to be provided between the electrode connection and the electrode. This means that the assembly of the electrode array can be effected largely independently of the person who carries out the assembly. For example, it is not possible for excessive restraining moments to occur. The results of the assembly can therefore be reproduced more readily. 
   The plug-type connection is preferably configured inside the wall. The advantage of this is, firstly, that the electrode per se can be kept relatively small. It no longer needs to be inserted through the wall, but may terminate in the wall by means of an extension. This design is particularly effective in reducing costs if special and therefore expensive materials need to be used for the measurement in a special fluid, because only very little of the expensive material is required. The electrode connection may be formed from a different material because it is no longer in contact with the fluid. In addition, the plug-type connection is protected. Both the electrode and the electrode connection project into the wall, hence are at least partially routed. This makes the connection more stable. 
   The electrode connection is preferably surrounded, at least outside the wall, by a metallic shield. The electrode connection therefore has, so to speak, a protective housing that forms a Faraday cage. This is effective in suppressing spurious radiation from electrical or magnetic fields. This is particularly advantageous in a flowmeter in which the signal voltages that indicate the speed of the fluid flowing through are relatively small. It is possible, for example, for a shielded cable to be fed into a metallic shield of this type, said cable then being connected to the electrode connection inside the metallic shield. This connection may be prefabricated. The final assembly of the electrode in the wall is then easily effected by inserting the electrode into the wall from one side and inserting the electrode connection with its protective housing from the other side. When the plug-type connection is established between the electrode and the electrode connection, the electrode is then firmly held in the wall and the electrode connection is shielded. 
   The electrode preferably has one part of a barb connection, which holds it in position. This makes it possible to increase the forces with which the electrode is held in the wall, over and above simple frictional locking between the electrode and the electrode connection. A barb connection has an edge at the end of a conical surface that can interconnect to a mating component. For example, the edge can be pushed into the material that forms the wall. 
   The barb connection is preferably configured between the electrode and the electrode connection. This means that the wall is not burdened by the connection. In the event of a malfunction, it is possible simply to disconnect the connection and use a new electrode and electrode connection pairing, which can then once again securely form a barb connection because there are no corresponding impressions in the wall of the housing. 
   In this context it is particularly preferable for the barb connection to be designed as a snap connection in which a first connection element, which is designed as a spring, is locked into place behind a second connection element. A snap connection of this type permits positive locking, which enables far greater retaining forces to be used than a simple frictional lock even if the latter is reinforced by a sharp edge. 
   The first connection element is preferably configured as a ring with interruptions, which is surrounded by an annular spring. This ring has an inwardly projecting circumferential flange or—in any case—parts that project inward radially, which may be beveled on their upper surface. The annular spring, for example an elastic ring, allows the individual ring sections to be contorted outward radially when the connection is established. However, the sections are then pushed back again by the annular spring when the second connection element is locked in place. 
   The second connection element is preferably supported on the wall of a cylindrical hole. In this case, it is useful if the first connection element can be sprung radially inward. If the second connection element is supported on the wall of the cylindrical hole, this provides a further retention option so that the positioning of the electrode array in the wall is further improved. 
   In this context it is preferable for the second connection element to be configured on the electrode. The electrode is not contorted when the snap connection is established. This means, therefore, that there will be no concerns subsequently about malfunctions being caused by the electrode possibly being unintentionally contorted. 
   Preferably, a spring arrangement that stresses the electrode connection in a direction away from the electrode, acts upon the electrode connection. In this way the electrode is securely pulled against the inside of the wall. If a washer is provided between the electrode and the wall, then the forces of the spring arrangement have the effect of compressing said washer, thus improving the sealing efficiency. Particularly with regard to a snap connection, the retaining forces in the connection between the electrode and the electrode connection are increased. It is virtually impossible to accidentally loosen a snap connection under voltage. 
   In this context it is particularly preferable for the spring arrangement to act upon the shield and to push it against the outside of the wall. This provides a virtually interruption-free shield. The outside of the wall is provided with many layers of shielding foil, for example a copper foil. The spring arrangement therefore creates sufficient contact force between the shield and the shielding foil. 
   In this context it is preferable for the shield with a ring surface to be located outside the spring arrangement on the outside of the wall. The ring surface is relatively narrow and therefore small. This enables a relatively large amount of pressure to be achieved between the shield and the foil on the outside of the wall. 
   It is also advantageous if the electrode connection is in one piece and is connected directly to a signal conductor. This connection may be formed either by the signal conductor being extended in the electrode connection, or by the signal conductor being soldered, welded or glued to the electrode connection, or fastened to it by similar means. Subsequent fastening stages may be dispensed with. The electrical contact is improved. 
   The invention is described in greater detail below on the basis of preferred exemplary embodiments together with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of a magnetic inductive flowmeter, 
       FIG. 2  shows a first embodiment of an electrode array, 
       FIG. 3  shows a second embodiment of an electrode array, 
       FIG. 4  shows a third embodiment of an electrode array, and 
       FIG. 5  shows a fourth embodiment of an electrode array. 
   

   DETAILED DESCRIPTION OF INVENTION 
     FIG. 1  shows a magnetic inductive flowmeter  1  with a measuring distance  2 , through which a fluid may flow perpendicular to the plane of projection. The measuring distance is configured in the interior of a tube  3  with a wall  4 . Two coils  5 ,  6  are connected to a power supply  7 . When the coils  5 ,  6  are supplied with power, they generate a magnetic field that is oriented perpendicular to the direction of flow. Two electrode arrays  8 ,  9  are connected to an analysis device  10 . When the coils  5 ,  6  generate a magnetic field and a fluid flows through the tube  3  perpendicular to the plane of projection, a voltage is created between the electrodes  8 ,  9 . This voltage can be evaluated by the analysis device  10  and enables the mass flow of the fluid to be indicated. 
     FIG. 2  now shows such an electrode array  8  in greater detail. 
   The electrode array  8  has an electrode  11  which is arranged on the inside  13  of the wall  4  with a washer  12  interposed. The electrode  11  has an extension  14 , which is plugged into an aperture  15  in the wall  4 . The radial exterior of the extension  14  rests against the wall of the cylindrical hole  15 . The end of the extension is shaped radially inward, so that a circumferential projection  16  is formed that has a beveled upper surface  17 . The projection  16  thus forms an inner cone. 
   The lower end  19  of an electrode connection  18  is likewise inserted into the cylindrical hole  15 . The electrode connection  18  has sprung fingers  20  at its end, said sprung fingers terminating in projections  21  that point outward radially. The projections  21  have a beveled end face  22 . 
   The projections  21  of the electrode connection  18  grasp the projection  16  of the extension  14  of the electrode  11 . This forms a snap connection between the electrode  11  and the electrode connection  18 . 
   The electrode connection  18  has a shield  23  with a housing  24 . The housing  24  has an outer ring  25 , the end face  26  of which is disposed on the outside  27  of the wall  4 . A copper foil  28  is laminated on the outside  27 . Contact between the outer ring  25  and the copper foil  28  creates an electrically conductive connection between the housing  24  and the copper foil  28 . 
   The housing  24  also has an inner ring  29 , which is pressed into the outer ring  25 . On the end of the inner ring  29  adjacent to the wall  4 , there is an inward projection  30  on which a spring  31  is supported. The spring  31  acts upon a head  33  via a disk  32 , said head  33  in turn acting on the upper end of the electrode connection  18  and thus forcing the electrode connection  18  away from the electrode  11 . An insulating device  34  is disposed between the head  33  and the electrode connection  18 , said insulating device extending into the cylindrical hole  15 . 
   In the exemplary embodiment, the head  33  is shown to have a circumferential, inward projection in the area of the disk  32 , said projection acting upon a step in the insulating device  34 . The insulating device  34  has an enlarged diameter on which rests a corresponding enlarged diameter of the electrode connection. Fundamentally, all contact surfaces need only be capable of being subjected to pressure. Other embodiments are, however, clearly possible. 
   The head  33  is connected to a shield  35  of a shielded cable  36 . The shielded cable  36  has a core  37  that is connected to the electrode connection  18 , for example by soldering. 
   The upper end of the housing  24  encloses the head  33  as tightly as possible. However, a telescopic movement of the head  33  against the force of the spring  31  is still permitted in the housing  24 . 
   The electrode connection  18 , together with the housing  24 , the spring  31 , the head  33  and the cable  36  may be preassembled, for example by automatic handling devices. 
   To effect assembly of the electrode  11 , the electrode  11  is inserted into the cylindrical hole  15  in the wall  4  from the inside  13 . Since the electrode  11  is arranged with the outside of the extension  14  on the wall of the cylindrical hole  15 , this produces a gentle frictional lock that ensures that the electrode  11  cannot fall out of the cylindrical hole  15 . 
   The electrode connection  18  is inserted into the cylindrical hole from the opposite side, with the head  33  being pushed toward the electrode  11 . If the end face  22  of the projections  21  on the fingers  20  then comes to rest on the upper end  17  of the extension  14 , the fingers  20  are pushed radially inward such that the projections  21  on the end of the fingers  20  can adapt to the projection  16  on the extension  14 . As soon as they have been moved past the extension  14 , they spring back outward radially and become firmly hooked behind the projection  16  of the extension  14 . If the head  33  is not then being pushed further against the force of the spring  31  toward the electrode  11 , then the spring  31  can expand and pushes the electrode connection  18  away from the electrode  11  so that the projections  21  on the fingers  20  hook behind the projection  16  on the extension  14 . This results initially in a positively locked connection between the electrode  11  and the electrode connection  18 . The projections  21  are arranged in this connection with a force on the projection  16  that is sufficient to establish an electrical contact with the requisite degree of reliability. 
   At the same time the washer  12 , as shown, is compressed so that the electrode  11  is also retained in the wall  4 , with a fluid-tight seal. 
   The connection between the electrode  11  and the electrode connection  18  is effected in the middle of the wall  4 . For this purpose the wall  4  may have a section  38  that is slightly thicker. The section  38  may be designed to be level on the outside  27 , so that the end face  26  of the outer ring  25  may also be designed to be level. It is therefore not necessary to observe any specific orientation when the electrode connection  18  is inserted into the electrode  11 . 
   The embodiment in  FIG. 3  essentially corresponds to the embodiment in  FIG. 2 . The same components are therefore labeled with the same reference numbers. Unlike in the configuration according to  FIG. 2 , in the configuration according to  FIG. 3  the electrode is provided with an extension  14 , which has slots  39 , permitting the formation of legs  40  that can spring outward. These legs  40  are retained inward radially by an annular spring  41 , for example an elastic ring. 
   On the end of the legs  40  there is a beveled head  42 , which also protrudes radially inward over the legs  40 . A mating component  43  on the electrode connection  18  may then become hooked behind the heads  42  on the legs  40 . A snap connection can also be established in this way. 
     FIG. 4  shows an embodiment in which a snap connection has been established between the electrode  11  and the electrode connection  18  in practically the same way as in  FIG. 2 . However, in this case a spring  44  is still disposed between the housing  24  and the wall  4 . This is a disk spring so that electrical shielding is also present in the area between the housing  24  and the wall  4 . 
   In the embodiment according to  FIG. 5 , there is a simple plug connection between the electrode  11  and the electrode connection  18 . The electrode  11  may have a sleeve extension  45  for this purpose, into which the electrode connection  18  is inserted, thus resulting an a relatively long contact surface. 
   The housing  24  is connected to the copper foil  28  via a soldered connection. 
   The extension  14  has a projection  47  that protrudes outward radially, the maximum outer diameter of said projection being slightly larger than the inner diameter of the cylindrical hole  15 . When the electrode  11  is pushed into the cylindrical hole  15 , the projection  47  “bites” firmly into the wall  4 . In this embodiment the electrode is retained in the wall  4  by the projection  47 . The housing  24  is positioned via the solder connection  46 . For this reason the frictional locking of the plug connection between the electrode connection  18  and the sleeve extension  45  is sufficient.