Abstract:
An electromagnetic flowmeter includes: a measurement pipe to which a magnetic pole core is fixed while having a gap between a tip end portion of the magnetic pole core and the measurement pipe; a lining member for covering an inner wall surface of the measurement pipe and the gap; and a locking portion for locking the lining member, the locking portion being provided in a vicinity of the gap.

Description:
This application claims foreign priorities based on Japanese Patent Application No. 2005-206359, filed Jul. 15, 2005, and Japanese Patent Application No. 2006-024341, filed Feb. 1, 2006, the contents of which are incorporated herein by reference in their entireties. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electromagnetic flowmeter that includes a measurement pipe wherein a magnetic pole core is arranged and a lining member that covers the inner wall face of the measurement pipe, and relates particularly to an electromagnetic flowmeter that can effectively lock the lining member. 
   2. Description of the Related Art 
     FIG. 13  is a vertical cross sectional view of the structure of a measurement pipe for an electromagnetic flowmeter having a small diameter in a first related art.  FIG. 14  is a transverse, cross sectional view of the measurement pipe in  FIG. 13 . 
   In  FIGS. 13 and 14 , a cylindrical measurement pipe  2  made of stainless steel (for example) has flange portions  1 A and  1 B at each end respectively, and centrally formed insertion holes  3 A and  3 B. Magnetic pole cores  4 A and  4 B having cylindrical shapes, for example, are inserted into the insertion holes  3 A and  3 B and are securely welded to outer ends  5 A and  5 B of the insertion holes  3 A and  3 B. 
   The magnetic pole cores  4 A and  4 B are disposed so that gaps  6 A and  6 B are formed near their distal ends when they are inserted into the insertion holes  3 A and  3 B in the measurement pipe  2 , while distal ends  7 A and  7 B are maintained at like positions relative to an inner wall face  8  of the measurement pipe  2 . 
   A lining composed, for example, of a fluoroplastic is applied to the inner wall face- 8  of the measurement pipe  2 , the inner distal faces of the magnetic pole cores  4 A and  4 B, and the gaps  6 A and  6 B to provide a lining member  9 . 
   Further, insertion holes  10 A and  10 B, into which detection electrodes (not shown) are to be inserted, are formed on a line that connects the magnetic pole cores  4 A and  4 B and in a direction perpendicular to the center line of the measurement pipe  2 . 
   Further, cylindrical electrode attachment portions  11 A and  11 B for fixing the detection electrodes, are securely welded to the outer wall of the center portion of the measurement pipe  2  perpendicular to the magnetic pole cores  4 A and  4 B. 
     FIG. 15  is a vertical, cross sectional view of the structure of the measurement pipe of an electromagnetic flowmeter having a small diameter in a second related art.  FIG. 16  is a transverse cross sectional view of the measurement pipe in  FIG. 15 . 
   In  FIGS. 15 and 16 , a measurement pipe  12  made of stainless steel, for example, is a cylindrical spool pipe that has flange portions  11 A and  11 B at its two ends and centrally formed insertion holes  13 A and  13 B. Magnetic pole cores  14 A and  14 B having cylindrical shapes, for example, are inserted into the insertion holes  13 A and  13 B and are securely welded to outer ends  15 A and  15 B of the insertion holes  13 A and  13 B. 
   When the magnetic pole cores  14 A and  14 B are inserted into the insertion holes  13 A and  13 B in the measurement pipe  12 , distal ends  17 A and  17 B are maintained in the same plane as an inner wall face  18  of the measurement pipe  12 . 
   Coil bobbins  113 A and  113 B, around which coils  114 A and  114 B are wound, are fitted over the magnetic pole cores  14 A and  14 B. 
   A lining made, for example, of a fluoroplastic is applied to the inner wall face  18  of the measurement pipe  12  and the distal ends  17 A and  17 B of the magnetic pole cores  14 A and  14 B, so as to provide a lining member  19 . 
   Further, insertion holes  110 A and  110 B, into which detection electrodes (not shown) are to be inserted, are formed on a line that connects the magnetic pole cores  14 A and  14 B and in a direction perpendicular to the center line of the measurement pipe  12 . 
   Furthermore, cylindrical electrode attachment portions  111 A and  111 B, for fixing the detection electrodes, are securely welded to the outer wall of the center portion of the measurement pipe  12  perpendicular to the magnetic pole cores  14 A and  14 B. 
   First and second electrodes  112 A and  112 B are located next to the electrode attachment portions  111 A and  111 B, so that their electrodes are exposed through the lining member  19  and face the interior of the measurement pipe  12 . 
   A first signal line  118 A, extending from the first electrode  112 A, is passed through the magnetic pole core  14 B and is twisted together with a second signal line  118 B on the second electrode  112 B side. In order to twist the first signal line  118 A and the second signal line  118 B within the shortest distance possible, this structure is designed so that only the first signal line  118 A is passed through the magnetic pole core  14 B. 
   Disclosed in JP-A-2004-354279 is a technique whereby in order to prevent deformation of the lining, a groove is formed in the inner wall near the end face of a measurement pipe separate from the center of a measurement pipe, and a lining member is locked by the groove. 
   Disclosed in JP-A-2002-048612 is a technique whereby, in order to prevent deformation of the lining, a cylindrical locking plate wherein multiple holes are formed is arranged inside a lining member to lock the lining member. 
   Also, refer to JP-A-2004-294176. 
   However, the following problems are encountered with the first related small-diameter electromagnetic flowmeter shown in  FIGS. 13 and 14 . 
   Since the lining member  9  provided for the inner wall of the stainless steel measurement pipe  2  is formed of a fluoroplastic, the lining member  9  is not secured to the measurement pipe  2 . However, since the lining member  9  is also deposited in the gaps  6 A and  6 B surrounding the magnetic pole cores  4 A and  4 B, movement of the measurement pipe  2  in the axial direction due to changes in the temperature is limited, but the measurement pipe  2  can still be moved in the radial direction. Therefore, using this structure, adequate locking effects are not obtained that can prevent deformations of the lining member  9  in the radial direction that are due to temperature fluctuations and pressure changes. 
   Therefore, for a measurement pipe having a small diameter of 15 mm or less, the ratio of deformations in the lining member  9  caused by temperature or pressure changes relative to the internal diameter is increased, and errors increase. Especially, deformation of the lining member  9  near the detection electrodes provides a great effect because the rate at which electromotive force is conducted to the detection electrodes is increased. 
   In order to prevent deformation of the lining member, in JP A-2004-354279, a structure is disclosed wherein a groove is formed in the inner wall of the measurement pipe near the end face to lock the lining member. However, with this structure, the lining near the detection electrodes that greatly influences measurement accuracy can not be secured. 
   Furthermore, for the second related small-diameter electromagnetic flowmeter shown in  FIGS. 15 and 16 , since the internal diameter of the spool pipe (the measurement pipe) is small, the insertion of a lining locking punch plate is difficult. Therefore, the internal lining diameter tends to change as the temperature of a fluid fluctuates, and the measurement accuracy is greatly affected by the change in the temperature of the fluid. 
   In addition, a rise in magnetic flux density is slowed due to an eddy current that is generated at a magnetic pole and affects the frequency property of the magnetic circuit. Further, since the first signal line penetrates a magnetic pole core, differential noise equivalent to a linkage dimension is generated only along the first signal line. When differential noise in the magnetic pole core is converged slowly and converging of the differential noise is still not satisfactory during signal sampling, differential noise is retained in the first signal. Further, since there is a difference in the amount of noise between the first and the second signals, the shifting distance at the zero point may be increased. 
   In JP-A-2002-048612, a structure is disclosed wherein the cylindrical locking plate, in which multiple holes are formed, is inserted inside the lining member. For this structure, however, there is no space in the small-diameter electromagnetic flowmeter for the insertion of a cylindrical locking plate, and this structure can not be provided for a small-diameter electromagnetic flowmeter. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above circumstances, and provides an electromagnetic flowmeter having a small diameter for which a lining member, located near detection electrodes and has a high rate of conduction of an electromotive force, can be effectively locked without having to insert a cylindrical locking plate, which is a difficult task when a small-diameter electromagnetic flowmeter is employed. 
   Another objective of the invention is to provide a structure that reduces an affect produced by an eddy current that is generated inside a magnetic pole, that improves the frequency, and that reduces the affect differential noise has on a first signal line. 
   In some implementations, an electromagnetic flowmeter of the invention comprising: 
   a measurement pipe to which a magnetic pole core is fixed while having a gap between a tip end portion of the magnetic pole core and the measurement pipe; 
   a lining member for covering an inner wall surface of the measurement pipe and the gap; and 
   a locking portion for locking the lining member, the locking portion being provided in a vicinity of the gap. 
   In the electromagnetic flowmeter of the invention, the locking portion includes an outer end portion of an insertion hole in the measurement pipe to which the magnetic pole core is inserted, the outer end portion being formed in a shape (tapered-shape) that extends in a direction outward from the magnetic pole core. 
   In the electromagnetic flowmeter of the invention, the locking portion includes an outer end portion of an insertion hole in the measurement pipe to which the magnetic pole core is inserted, the outer end portion being extended in a direction outward from the magnetic pole core and being formed of a groove (dovetail groove) that cuts into an inner wall side of the measurement pipe. 
   In the electromagnetic flowmeter of the invention, the locking portion further includes a groove (dovetail groove) which is formed in the magnetic pole core so as to narrow a part of the magnetic pole core, the groove being formed in a shape (tapered-shape) that extends towards an inner side of the magnetic pole core, and being opposed to the outer end portion of the insertion hole. 
   In the electromagnetic flowmeter of the invention, the locking portion includes a through-hole opening to either side faces of the magnetic pole core, the side faces being adjacent to the gap around the magnetic pole core. 
   In some implementations, an electromagnetic flowmeter of the invention comprising: 
   a measurement pipe to which a magnetic pole core is fixed; 
   a lining member for covering an inner wall surface of the measurement pipe; and 
   a locking portion for locking the lining member, the locking portion being formed by providing a through-hole inside the magnetic pole core along an axis of the magnetic pole core so that the through-hole connects with the lining member, and filling the through-hole with a lining resin of a same type as that of the lining member. 
   In the electromagnetic flowmeter of the invention, the through-hole is provided in a central position of the magnetic pole. 
   In the electromagnetic flowmeter of the invention, the locking portion includes a step that is formed in the through-hole. 
   In the electromagnetic flowmeter of the invention, the step in the through-hole is formed so that a diameter of one end portion of the through-hole on the lining member side is smaller than that of the other part of the through-hole. 
   In the electromagnetic flowmeter of the invention, apart of the through-hole on the lining member side is filled with the lining resin and the other part of the through-hole is filled with a mixture of an insulating resin and a soft magnetic metal powder. 
   In the electromagnetic flowmeter of the invention, the through-hole is filled with the lining resin at least to a position where the step is formed. 
   In some implementations, an electromagnetic flowmeter of the invention comprising: 
   a measurement pipe; 
   a pair of magnetic pole cores which are disposed in a direction perpendicular to an axial direction of the measurement pipe and are opposed to each other; 
   a pair of detection electrodes which are disposed in a direction perpendicular to the pair of magnetic pole cores and are opposed to each other, with each pole of the pair of detection electrodes facing an interior of the measurement pipe; 
   a lining member that covers an inner wall surface of the measurement pipe; 
   a locking portion for locking the lining member; 
   a first signal line extending from one of the detection electrodes; and 
   a second signal line extending from the other detection electrode, 
   wherein the first signal line passes through one of the magnetic pole cores so as to make bundles of the first signal line and the second signal line. 
   In the electromagnetic flowmeter of the invention, the locking portion is formed by providing a through-hole inside each of the magnetic pole cores along an axis of the magnetic pole core so that the through-hole connects with the lining member, and filling the through-hole with a lining resin of a same type as that of the lining member. 
   In the electromagnetic flowmeter of the invention, the through-hole is provided in a central position of the magnetic pole. 
   In the electromagnetic flowmeter of the invention, the locking portion includes a step that is formed in the through-hole. 
   In the electromagnetic flowmeter of the invention, the step in the through-hole is formed so that a diameter of one end portion of the through-hole on the lining member side is smaller than that of the other part of the through-hole. 
   In the electromagnetic flowmeter of the invention, a part of the through-hole on the lining member side is filled with the lining resin and the other part of the through-hole is filled with a mixture of an insulating resin and a soft magnetic metal powder. 
   In the electromagnetic flowmeter of the invention, the through-hole is filled with the lining resin at least to a position where the step is formed. 
   According to the electromagnetic flowmeter of the invention described above, the following effects can be obtained. 
   According to the invention, the locking portion for locking the lining member is provided near each of the gaps around the magnetic pole cores, and the gaps are located near the detection electrodes that have a great affect on electromotive force. The lining member can be effectively held in both the axial direction and in the radial direction of the measurement pipe. Thus, a small-diameter electromagnetic flowmeter can be provided that is little affected by temperature changes or fluid conditions under pressure. 
   Further, according to the invention, the insertion holes in the measurement pipe into which the magnetic pole cores are inserted are provided as tapered portions that spread outward, and serve as locking portions. Thus, since the lining member enters these tapered portions, movement of the lining member in the radial direction of the measurement pipe can be prevented. 
   Furthermore, according to the invention, the outer ends of the insertion holes in the measurement pipe into which the magnetic pole cores are inserted extend outward, so that the locking portions are formed in and engage dovetailed grooves that are cut into the inner wall of the measurement pipe. Thus, the lining member engages more deeply with these dovetailed grooves, and movement of the lining member can more effectively be prevented. 
   In addition, according to the invention, the outer ends of the insertion holes in the measurement pipe into which the magnetic pole cores are inserted are provided as tapered portions that spread outward. In the magnetic pole cores, tapered portions are formed that narrow inwardly that are opposite of the outward spreading tapered outer ends. Thus, these tapered-portions form V-shaped, dovetailed grooves to provide locking portions. Therefore, the lining member can be effectively held in the radial direction of the measurement pipe. 
   Moreover, according to the invention, through holes are formed in the side faces of the magnetic pole cores that face the gaps surrounding the magnetic pole cores, and these also serve as locking portions. Therefore, through the bridge locking effects provided by the lining member material that is used to fill in the through holes, excellent effects can be obtained that prevent the lining member from being deformed in the radial direction of the measurement pipe. 
   Also according to the invention, the locking portion communicates with the through holes formed in the magnetic pole cores, and the same type of lining resin as is used for the lining member is used to fill in the through holes. Thus, the locking effects can be obtained that are provided by the lining resin, and the affect on the measurement accuracy caused by changes in the fluid temperature can be reduced. 
   Further, according to the invention, since the through holes are centrally formed at positions along the axes of the magnetic pole cores, the lining member can be appropriately bonded to the lining resin used to fill in the through holes. Satisfactory locking effects can thus be obtained. 
   Furthermore, according to the invention, since the resin locking section for locking the lining resin is formed by filling in the through holes, the lining resin bonded to the lining member is stably held in the through holes. 
   In addition, according to the invention, since steps are formed in the through holes as resin locking portions, the lining resin can be held by the steps. The state in which the lining member is held (locked) is in a more stable state. 
   Moreover, according to the invention, since the steps are formed so that the diameters of the holes close to the lining member are small, the lining resin can be held in consonance with the downsizing of the diameter. Therefore, the state in which the lining member is held can be more stable. 
   Also, according to the invention, the lining resin is used to fill in part of each of the through holes, and the insulating resin with which the soft magnetic powder is mixed is used to fill in the remainder of each of the through holes. Since the affect produced by an eddy current generated inside a magnetic pole core is reduced in this manner, the frequency property of the magnetic circuit is improved, and the affect of differential noise on a signal line can be reduced. 
   Further, according to the invention, the lining resin is used to fill in at least to the position of the step formed in each of the through holes. Thus, the amount of lining resin used for filling in can be minimized, and instead, more insulating resin into which soft magnetic powder has been mixed can be used for filling. Accordingly, the affect produced by an eddy current generated inside a magnetic pole core is reduced, so that the frequency property of the magnetic circuit can be improved and the effect of differential noise on a signal line can be reduced. 
   Furthermore, according to the invention, the electromagnetic flowmeter comprises: 
   a measurement pipe; 
   a pair of magnetic pole cores which are disposed in a direction perpendicular to an axial direction of the measurement pipe and are opposed to each other; 
   a pair of detection electrodes which are disposed in a direction perpendicular to the pair of magnetic pole cores and are opposed to each other, with each pole of the pair of detection electrodes facing an interior of the measurement pipe; 
   a lining member that covers an inner wall surface of the measurement pipe; 
   a locking portion for locking-the lining member; 
   a first signal line extending from one of the detection electrodes; and 
   a second signal line extending from the other detection electrode, 
   wherein the first signal line passes through one of the magnetic pole cores so as to make bundles of the first signal line and the second signal line. 
   With this structure, effective holding of the lining resin can be obtained, and the affect on the measurement accuracy of changes in the fluid temperature can be reduced. 
   Also according to the invention, the lining locking portion communicates with the through holes formed in the magnetic pole cores, and the same type of lining resin as is used for the lining member is used to fill in the through holes. Thus, the locking effects can be obtained that are provided by the lining resin, and the affect on the measurement accuracy of changes in the fluid temperature can be reduced. 
   Further, according to the invention, since the through holes are centrally formed at positions along the axes of the magnetic pole cores, the lining member can be appropriately bonded to the lining resin used to fill in the through holes. Thus, satisfactory locking effects can be obtained. 
   Furthermore, according to the invention, since the resin locking portion for locking the lining resin is formed by filling in the through holes, the lining resin bonded to the lining member is stably held in the through holes. 
   In addition, according to the invention, since steps are formed in the through holes as resin locking portion, the lining resin can be held by the steps. Thus the state in which the lining member is held in a more stable state. 
   Moreover, according to the invention, since the steps are formed so that the diameters of the holes close to the lining member are small, the lining resin can be held in consonance with the downsizing of the diameter. Therefore, the state in which the lining member is held can be more stable. 
   Also, according to the invention, the lining resin is used to fill in part of each of the through holes, and the insulating resin with which the soft magnetic powder is mixed is used to fill in the remainder of each of the through holes. Since the affect produced by an eddy current generated inside a magnetic pole core is reduced in this manner, the frequency property of the magnetic circuit is improved, and the affect of differential noise on a signal line can be reduced. 
   Further, according to the invention, the lining resin is used to fill in at least to the position of the step formed in each of the through holes. Thus, the amount of lining resin used for filling in can be minimized, and instead, more insulating resin into which soft magnetic powder has been mixed can be used for filling. Accordingly, the affect produced by an eddy current generated inside a magnetic pole core is reduced, so that the frequency property of the magnetic circuit can be improved and the effect of differential noise on a signal line can be reduced. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a vertical, cross-sectional view of the structure of the essential portion of a measurement pipe according to a first embodiment of the present invention. 
       FIG. 1B  is a detailed diagram showing a portion A in  FIG. 1A . 
       FIG. 2  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 1A . 
       FIG. 3A  is a vertical, cross-sectional view of the structure of the essential portion of a measurement pipe according to a second embodiment of the present invention. 
       FIG. 3B  is a detailed diagram showing a portion B in FIG.  3 A. 
       FIG. 4  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 3A . 
       FIG. 5A  is a vertical, cross-sectional view of the structure of the essential portion of a measurement pipe according to a third embodiment of the present invention. 
       FIG. 5B  is a detailed diagram showing a portion C in  FIG. 5A . 
       FIG. 6  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 5A . 
       FIG. 7A  is a vertical, cross-sectional view of the structure of the essential portion of a measurement pipe according to a fourth embodiment of the present invention. 
       FIG. 7B  is a detailed diagram showing a portion D in  FIG. 7A . 
       FIG. 8  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 7A . 
       FIG. 9  is a diagram showing the structure of the essential portion of a measurement pipe according to a fifth embodiment of the invention. 
       FIG. 10  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 9 . 
       FIG. 11  is a diagram showing the structure of the essential portion of a measurement pipe according to a sixth embodiment of the invention. 
       FIG. 12  is a transverse, cross-sectional view of the center portion of the measurement pipe shown in  FIG. 11 . 
       FIG. 13  is a vertical, cross-sectional view of the structure of the measurement pipe of a first related electromagnetic flowmeter that has a small diameter. 
       FIG. 14  is a transverse, cross-sectional view of the measurement pipe shown in  FIG. 13 . 
       FIG. 15  is a vertical, cross-sectional view of the structure of the measurement pipe of a second related electromagnetic flowmeter that has a small diameter. 
       FIG. 16  is a transverse, cross-sectional view of the measurement pipe shown in  FIG. 15 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will now be described in detail while referring to the accompanying drawings. 
   First Embodiment 
     FIG. 1A  is a vertical cross-sectional view of the structure of the essential portion of a measurement pipe according to a first embodiment of the invention.  FIG. 1B  is a detailed diagram showing a portion A in  FIG. 1A .  FIG. 2  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIGS. 1A and 1B . 
   In  FIGS. 1A ,  1 B and  2 , flange portions  20 A and  20 B are formed at the respective ends of a cylindrical measurement pipe  21  made, for example, of stainless steel. Insertion holes  22 A and  22 B are formed opposite each other, in the center portion of the pipe shaft of the measurement pipe  21 . 
   While predetermined gaps  24 A and  24 B are maintained relative to the insertion holes  22 A and  22 B, magnetic pole cores  23 A and  23 B which have (for example) a cylindrical shape are inserted into the insertion holes  22 A and  22 B and are securely welded to the outer ends of the insertion holes  22 A and  22 B. 
   These outer ends are formed so that the insertion holes  22 A and  22 B are tapered and spread outward, and tapered portions  25 A and  25 B serve as stoppers SP 1 A and SP 1 B for locking a lining member. 
   A lining member  26  made of a fluoroplastic is deposited by lining, on the inner wall of the measurement pipe  21 , the distal ends of the magnetic pole cores  23 A and  23 B and inside the gaps  24 A and  24 B and the tapered portions  25 A and  25 B. 
   In order to provide the lining member  26  inside the gaps  24 A and  24 B and the tapered portions  25 A and  25 B, resin molding need only be performed by applying a fluoroplastic, together with the formation of the internal diameter. 
   Further, in a direction perpendicular to the center line that connects the centers of the magnetic pole cores  23 A and  23 B, insertion holes  27 A and  27 B, used to insert detection electrodes (not shown), are formed through the lining member  26 . 
   Further, cylindrical electrode attachment portions  28 A and  28 B for fixing the detection electrodes are welded to the outer wall in the center portion of the measurement pipe  21 , perpendicular to the magnetic pole cores  23 A and  23 B. 
   Second Embodiment 
     FIG. 3A  is a vertical cross-sectional view of the structure of the essential portion of a measurement pipe according to a second embodiment of the invention.  FIG. 3B  is a detailed diagram showing a portion B in  FIG. 3A .  FIG. 4  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIGS. 3A and 3B . 
   In  FIGS. 3A ,  3 B and  4 , flange portions  30 A and  30 B are formed at the respective ends of a cylindrical measurement pipe  31  made, for example, of stainless steel. Insertion holes  32 A and  32 B are formed opposite each other in the center portion of the pipe shaft of the measurement pipe  31 . 
   While predetermined gaps  34 A and  34 B are maintained relative to the insertion holes  32 A and  32 B, magnetic pole cores  33 A and  33 B, which have, for example, a cylindrical shape, are inserted into the insertion holes  32 A and  32 B and are securely welded to the outer ends of the insertion holes  32 A and  32 B. 
   The outer ends of the insertion holes  32 A and  32 B of the measurement pipe  31  into which the magnetic pole cores  33 A and  33 B are inserted extended outwards, and cut into the inner wall of the measurement pipe  31 . As a result, these outer ends are formed like dovetailed grooves  35 A and  35 B, and have substantially N shapes in cross section, in the axial direction of the insertion holes  32 A and  32 B. In this manner, stoppers SP 2 A and SP 2 B are provided. 
   A lining member  36  made of a fluoroplastic is deposited by lining, on the inner wall of the measurement pipe  31 , the distal ends of the magnetic pole cores  33 A and  33 B and inside the gaps  34 A and  34 B and the dovetailed grooves  35 A and  35 B. 
   In order to provide the lining member  36  inside the gaps  34 A and  34 B and the dovetailed grooves  35 A and  35 B, resin molding need only be performed by applying a fluoroplastic, together with the formation of the internal diameter. 
   Further, in a direction perpendicular to the center line that connects the centers of the magnetic pole cores  33 A and  33 B, insertion holes  37 A and  37 B used to insert detection electrodes (not shown) are formed through the lining member  36 . 
   Further, cylindrical electrode attachment portions  38 A and  38 B for fixing the detection electrodes are welded to the outer wall in the center portion of the measurement pipe  31 , perpendicular to the magnetic pole cores  33 A and  33 B. 
   Third Embodiment 
     FIG. 5A  is a vertical cross-sectional view of the structure of the essential portion of a measurement pipe according to a third embodiment of the invention.  FIG. 5B  is a detailed diagram showing a portion C in  FIG. 5A .  FIG. 6  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIGS. 5A and 5B . 
   In  FIGS. 5A ,  5 B and  6 , flange portions  40 A and  40 B are formed at the respective ends of a cylindrical measurement pipe  41  made, for example, of stainless steel. Insertion holes  42 A and  42 B are formed opposite each other in the center portion of the pipe shaft of the measurement pipe  41 . 
   While predetermined gaps  44 A and  44 B are maintained relative to the insertion holes  42 A and  42 B, magnetic pole cores  43 A and  43 B, which have, for example, a cylindrical shape, are inserted into the insertion holes  42 A and  42 B and are securely welded to the outer ends of the insertion holes  42 A and  42 B. 
   The outer ends of the insertion holes  42 A and  42 B of the measurement pipe  41  into which the magnetic pole cores  43 A and  43 B are inserted are formed with tapered portions TA 1  and TB 1  that spread outwards. Likewise, for the magnetic pole cores  43 A and  43 B, tapered portions TA 2  and TB 2  that narrow inwardly are formed opposite the tapered portions TA 1  and TB 1 . Then, for the insertion holes  42 A and  42 B, these tapered portions are employed as dovetailed grooves  45 A and  45 B whose cross sections extend outward, like a cone. In this manner, stoppers SP 3 A and SP 3 B are provided. 
   A lining member  46  made of a fluoroplastic is deposited by lining, on the inner wall of the measurement pipe  41 , the distal ends of the magnetic pole cores  43 A and  43 B and inside the gaps  44 A and  44 B and the dovetailed grooves  45 A and  45 B. 
   In order to provide the lining member  46  inside the gaps  44 A and  44 B and the dovetailed grooves  45 A and  45 B, resin molding need only be performed by applying a fluoroplastic, together with the formation of the internal diameter. 
   Further, in a direction perpendicular to the center line that connects the centers of the magnetic pole cores  43 A and  43 B, insertion holes  47 A and  47 B used to insert detection electrodes (not shown) are formed through the lining member  46 . 
   Further, cylindrical electrode attachment portions  48 A and  48 B for fixing the detection electrodes are welded to the outer wall in the center portion of the measurement pipe  41 , perpendicular to the magnetic pole cores  43 A and  43 B. 
   Fourth Embodiment 
     FIG. 7A  is a vertical cross-sectional view of the structure of the essential portion of a measurement pipe according to a fourth embodiment of the invention.  FIG. 7B  is a detailed diagram showing a portion D in  FIG. 7A .  FIG. 8  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIGS. 7A and 7B . 
   In  FIGS. 7A ,  7 B and  8 , flange portions  50 A and  50 B are formed at the respective ends of a cylindrical measurement pipe  51  made, for example, of stainless steel. Insertion holes  52 A and  52 B are formed opposite each other in the center portion of the pipe shaft of the measurement pipe  51 . 
   While predetermined gaps  54 A and  54 B are maintained relative to the insertion holes  52 A and  52 B, magnetic pole cores  53 A and  53 B, which have, for example, a cylindrical shape, are inserted into the insertion holes  52 A and  52 B and are securely welded to the outer ends of the insertion holes  52 A and  52 B. 
   The outer ends of the insertion holes  52 A and  52 B are tapered so that the insertion holes  52 A and  52 B spread outwards. Tapered portions  25 A and  25 B serve as first stoppers SP 1 A and SP 1 B, which lock a lining member as shown in  FIGS. 1 and 2 . Further, through holes  55 A and  55 B are formed in the side faces of the magnetic pole cores  53 A and  53 B that face the gaps  54 A and  54 B. Thus, these components constitute second stoppers SP 4 A and SP 4 B. 
   A lining member  56  made of a fluoroplastic is deposited by lining, on the inner wall of the measurement pipe  51 , the distal ends of the magnetic pole cores  53 A and  53 B, inside the gaps  54 A and  54 B and the tapered portions  25 A and  25 B, and the through holes  55 A and  55 B. 
   In order to provide the lining member  56  inside, for example, the gaps  54 A and  54 B, the tapered portions  25 A and  25 B and the through holes  55 A and  55 B, resin molding need only be performed by applying a fluoroplastic, together with the formation of the internal diameter. 
   Further, in a direction perpendicular to the center line that connects the centers of the magnetic pole cores  53 A and  53 B, insertion holes  57 A and  57 B used to insert detection electrodes (not shown) are formed through the lining member  56 . 
   Further, cylindrical electrode attachment portions  58 A and  58 B for fixing the detection electrodes are welded to the outer wall in the center portion of the measurement pipe  51 , perpendicular to the magnetic pole cores  53 A and  53 B. 
   When the tapered portions  25 A and  25 B that serve as the stoppers SP 1 A to SP 4 B, the dovetailed grooves  35 A and  35 B, the dovetailed grooves  45 A and  45 B and the through holes  55 A and  55 B described above are appropriately employed, great effects for the locking of the lining member can be obtained by the interaction of these components. 
   Fifth Embodiment 
     FIG. 9  is a diagram showing the structure of the essential portion of a measurement pipe according to a fifth embodiment of the invention.  FIG. 10  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIG. 9 . 
   In  FIGS. 9 and 10 , flange portions  60 A and  60 B are formed at the respective ends of a cylindrical measurement pipe  61  made, for example, of stainless steel. Insertion holes  62 A and  62 B are formed opposite each other in the center portion of the pipe shaft of the measurement pipe  61 . 
   Magnetic pole cores  63 A and  63 B, which have (for example) a cylindrical shape are inserted into the insertion holes  62 A and  62 B and are securely welded to the outer ends of the insertion holes  62 A and  62 B. 
   The magnetic pole cores  63 A and  63 B are held so that when they are inserted into the insertion holes  62 A and  62 B of the measurement pipe  61 , distal ends  67 A and  67 B are located on the same plane as an inner wall face  68  of the measurement pipe  61 . Coil bobbins  114 A and  114 B, around which coils  113 A and  113 B are wound, are fitted over the magnetic pole cores  63 A and  63 B. 
   Furthermore, through holes  115 A and  115 B are formed in the axial direction at the center positions of the magnetic pole cores  63 A and  63 B. Lining resins  119 A and  119 B, which are the same type as a lining member  66 , are used to fill in the through holes  115 A and  115 B, and serve as a stopper for locking a lining member  66 . In the through holes  115 A and  115 B, steps  116 A and  116 B are formed on the lining member  66  side. That is, the diameters of the through holes  115 A and  115 B near the lining member  66  are reduced, while the diameters on the other side are increased, so that the steps  116 A and  116 B are provided. 
   When the lining resins  119 A and  119 B are used to fill in the through holes  115 A and  115 B having the steps  116 A and  116 B, the lining resins  119 A and  119 B can be held at the positions of the steps  116 A and  116 B. Therefore, effective locking of the lining resins  119 A and  119 B can be obtained, and the affect on the measurement accuracy caused by changes in the fluid temperature can be reduced. 
   Further, since the through holes  115 A and  115 B are formed at the center positions in the magnetic pole cores  63 A and  63 B, the occurrence of eddy currents in the center of the magnetic pole cores  63 A and  63 B can be prevented, and the frequency property of the magnetic circuit can be improved. 
   As described above, since resin molding is performed with formation of the internal diameter of the measurement pipe  61 , the lining member  66  that includes the stopper can be formed on the inner wall of the measurement pipe  61  and in the through holes  115 A and  115 B of the magnetic pole cores  63 A and  63 B. 
   Further, in the direction perpendicular to the center line that connects the centers of the magnetic pole cores  63 A and  63 B, electrode insertion holes  117 A and  117 B, into which detection electrodes (first and second electrodes  112 A and  112 B) are to be inserted, are formed through the lining member  66 . 
   In addition, cylindrical electrode attachment portions  111 A and  111 B for fixing the first and the second electrodes  112 A and  112 B, are welded to the outer wall of the center portion of the measurement pipe  61 , which is perpendicular to the magnetic pole cores  63 A and  63 B. 
   The first and the second electrodes  112 A and  112 B are arranged in the electrode attachment portions  111 A and  111 B, so that these electrodes are exposed through the lining member  66 , facing the interior of the measurement pipe  61 . 
   A first signal line  118 A, extending from the first electrode  112 A, is passed through the magnetic pole core  63 B. The first signal line  118 A and a second signal line  118 B are twisted together on the second electrode  112 B side. 
   Since the first and the second signal lines  118 A and  118 B are twisted together at the shortest distance possible, only the first signal line  118 A is passed through the magnetic pole core  63 B. 
   According to the above described structure, in order to obtain the shortest distance, only the first signal line  118 A is passed through the magnetic pole core  631  and led to the second signal line  118 B side. Thus, an eddy current generated in the magnetic pole core  63 B is affected and differential noise tends to occur. However, since the hollow through hole  115 B is formed in the magnetic pole core  63 B, an eddy current does not occur in the center of the magnetic pole core  63 B (or  63 A), and accordingly, differential noise generated along the first signal line  118 A is reduced. 
   Sixth Embodiment 
     FIG. 11  is a diagram showing the structure of the essential portion of a measurement pipe according to a sixth embodiment of the invention.  FIG. 12  is a transverse, cross-sectional view of the center portion of the measurement pipe in  FIG. 11 . 
   In  FIGS. 11 and 12 , flange portions  70 A and  70 B are formed at the respective ends of a cylindrical measurement pipe  71  made, for example, of stainless steel Insertion holes  72 A and  72 B are formed opposite each other in the center portion of the pipe shaft of the measurement pipe  71 . 
   Magnetic pole cores  73 A and  73 B, which have, for example, a cylindrical shape, are inserted into the insertion holes  72 A and  72 B and are securely welded to the outer ends of the insertion holes  72 A and  72 B. 
   The magnetic pole cores  73 A and  73 B are held so that when they are inserted into the insertion holes  72 A and  72 B of the measurement pipe  71 , distal ends  77 A and  77 B are located on the same plane as an inner wall face  78  of the measurement pipe  71 . Coil bobbins  124 A and  124 B around which coils  123 A and  123 B are wound, are fitted over the magnetic pole cores  73 A and  73 B. 
   At the center positions in the magnetic pole cores  73 A and  73 B, through holes  125 A and  125 B are formed in the axial direction. For the through holes  125 A and  125 B, lining resins  129 A and  129 B are used to partially fill them near the lining member  76 . Soft magnetic metal powder resin mixtures  130 A and  130 B, such as insulating silicon resins or epoxy resins into which soft magnetic metal powder has been mixed are used to fill in the remaining portions. Since a mixture that includes soft magnetic metal powder is used, the magnetic flux densities of the magnetic pole cores  73 A and  73 B can be increased. Eddy currents occur in the soft magnetic metal resin mixtures  130 A and  130 B; however, the magnitudes of the eddy currents generated in the individual mixtures are small, and since the soft magnetic metal is surrounded by insulating resin, the eddy currents converge quickly and have very little affect. 
   Furthermore, the soft magnetic metal powder resin mixtures  130 A and  130 B may not only be used to fill in part of the through holes, but since they provide adhesion effects, they may also be employed for the fixing of a sheet core  131 . 
   In the through holes  125 A and  125 B, steps  126 A and  126 B are formed near the lining member  76 . That is, the diameters of the through holes  125 A and  125 B near the lining member  76  are reduced, and the diameters on the other side are increased, so that the steps  126 A and  126 B are provided. 
   When the lining resins  129 A and  129 B are used to fill in the through holes  125 A and  125 B up to the positions of the steps  126 A and  126 B from the lining member  76  side, the lining resins  129 A and  129 B can be held at the positions of the steps  126 A and  126 B. Therefore, effective locking of the lining resins  129 A and  129 B can be obtained, and the affect on the measurement accuracy caused by changes in the fluid temperature can be reduced. 
   Further, since the through holes  125 A and  125 B are formed at the center positions in the magnetic pole cores  73 A and  73 B, the occurrence of eddy currents in the center of the magnetic pole cores  73 A and  73 B can be prevented, and the frequency property of the magnetic circuit can be improved. 
   As described above, when a fluoroplastic is deposited by lining, on the inner wall of the measurement pipe  71  and in part of the through holes  125 A and  125 B of the magnetic pole cores  73 A and  73 B, the thus deposited lining member  76  can be held by the lining resins  129 A and  129 B that are used to fill in the through holes  125 A and  125 B. 
   Further, in the direction perpendicular to the center line that connects the centers of the magnetic pole cores  73 A and  73 B, electrode insertion holes  127 A and  127 B, into which detection electrodes (first and second electrodes  122 A and  122 B) are to be inserted, are formed through the lining member  76 . 
   In addition, cylindrical electrode attachment portions  121 A and  121 B for fixing the first and the second electrodes  122 A and  122 B are welded to the outer wall of the center portion of the measurement pipe  71 , which is perpendicular to the magnetic pole cores  73 A and  73 B. 
   The first and the second electrodes  122 A and  122 B are arranged in the electrode attachment portions  121 A and  121 B so that these electrodes are exposed through the lining member  76 , facing the interior of the measurement pipe  71 . 
   A first signal line  128 A, extending from the first electrode  122 A, is passed through the magnetic pole core  73 B. The first signal line  128 A and a second signal line  128 B are twisted together on the second electrode  122 B side. 
   Since the first and the second signal lines  128 A and  128 B are twisted together at the shortest distance possible, only the first signal line  128 A is passed through the magnetic pole core  73 B. 
   According to the above described structure, in order to obtain the shortest distance, only the first signal line  128 A is passed through the magnetic pole core  73 B and led to the second signal line  128 B side. Thus, an eddy current generated in the magnetic pole core  73 B is affected and differential noise tends to occur. However, since the hollow through hole  125 B is formed in the magnetic pole core  73 B, an eddy current does not occur in the center of the magnetic pole core  73 B. Accordingly, differential noise generated along the first signal line  128 A is reduced. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the described preferred embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents.