Abstract:
This sensor ( 10 ) generates accurate measuring results, for example with an error in the order of  0.5 % of the measuring value, hat has mimimized production costs as well as a shorter overall length compared to that of conventional sensors. The sensor has two parallel V shaped measuring tubes ( 1, 2 ) each being of one-piece construction. Each tube has a straight inlet portion ( 11, 21 ), a straight outlet portion ( 12, 22 ), an inlet bend ( 13, 23 ) connected with the inlet portion, an outlet bend ( 14, 24 ) connected with the outlet portion, a straight tube portion ( 15, 25 ) connected with the inlet bend, a straight tube portion ( 16, 26 ) connected with the outlet bend, and a vertex bend ( 17, 27 ) connected with the first and second straight tube portions. The inlet portions ( 11, 21 ) are fixed in an inlet manifold ( 18 ) and the outlet portions in an outlet manifold ( 19 ); the manifolds ( 18, 19 ) are mounted in a support frame ( 30 ) which forms part of a housing ( 3 ). An excitation arrangement ( 6 ) causes the measuring tubes ( 1, 2 ) to vibrate as a tuning fork. Interspaced sensor elements ( 7, 8 ) are fixed to the measuring tubes. Mounted in the support frame ( 30 ) is a feedthrough ( 37 ) for a printed-circuit board ( 96 ) having conducting tracks ( 97 ) to which leads ( 63, 64, 73, 74, 83, 84 ) of the excitation system ( 6 ) and of the sensor elements ( 7, 8 ) are connected.

Description:
This application claims the benefit of provisional application No. 60/205,983. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a mass flow rate/density/viscosity sensor working on the Coriolis principle—herein-after referred to as a Coriolis sensor for short—and comprising two bent measuring tubes. 
     With such Coriolis sensors, whose measuring tubes, as is well known, are set into vibration, particularly into flexural vibration with or without superposed torsional vibration, it is possible to measure not only the instantaneous mass flow rate of a fluid flowing in a pipe, but also the density of the fluid via the instantaneous vibration frequency of the measuring tubes and the viscosity of the fluid via the power required to maintain the vibrations of the tubes. 
     Since the temperature of the fluid is not constant during operation of the Coriolis sensor, and the density of the fluid, as is well known, is temperature-dependent, the Coriolis sensor is commonly provided with at least one temperature sensor for measuring the temperature of the fluid. For all those measurements, the Coriolis sensor is connected into the pipe in a pressure-tight manner and generally permanently, for example via flanges. 
     U.S. Pat. No. 4,187,721 discloses a Coriolis mass flow rate/density sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a single, U-shaped measuring tube bent in one plane symmetrically with respect to an axis of symmetry, which 
     is of one-piece construction and 
     has a straight inlet portion fixed in a support angle, 
     a straight outlet portion fixed in the support angle, 
     an offset inlet transition portion connected with the inlet portion, 
     an offset outlet transition portion connected with the outlet portion, 
     a first bent portion connected with the inlet transition portion, 
     a second bent portion connected with the outlet transition portion, 
     a straight base portion connecting the first and second bent portions; 
     an excitation system 
     which in operation causes the measuring tube together with an exciter carrier to vibrate as a tuning fork, 
     a first portion of which is fixed to the base portion in the area of the axis of symmetry, and 
     a second portion of which is fixed to the exciter carrier; 
     a first optical sensor, 
     a first portion of which is fixed to the measuring tube at a location 
     where the inlet transition portion passes into the first bent portion, and 
     a second portion of which is fixed to the support angle; and 
     a second optical sensor, 
     a first portion of which is fixed to the measuring tube at a location 
     where the outlet transition portion passes into the second bent portion, and 
     a second portion of which is fixed to the support angle. 
     JP-A 56-125 622 discloses a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     an omega-shaped measuring tube bent in one plane symmetrically with respect to an axis of symmetry which 
     is of one-piece construction and 
     has a straight inlet portion with an inlet axis lying in said plane, 
     a straight outlet portion with an outlet axis aligned with the inlet axis, 
     an S-shaped inlet bend connected with the inlet portion, 
     an S-shaped outlet bend connected with the outlet portion, and 
     a vertex bend connecting the inlet and outlet bends; 
     an excitation system 
     which in operation causes the measuring tube together with an exciter carrier to vibrate as a tuning fork, 
     a first portion of which is fixed to the vertex bend in the area of the axis of synunetry, and 
     a second portion of which is fixed to the exciter carrier; 
     a bar-shaped sensor carrier 
     which extends perpendicular to the axis of symmetry, 
     a first end of which is fixed to the measuring tube at a location where the inlet bend passes into the vertex bend, and 
     a second end of which is fixed to the measuring tube at a location where the outlet bend passes into the vertex bend; and 
     a strain-gage bridge disposed as a sensor arrangement on the sensor carrier. 
     U.S. Pat. No. 4,127,028 discloses a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a first U-shaped measuring tube bent in a first plane symmetrically with respect to a first axis of symmetry; 
     a second U-shaped measuring tube bent in a second plane symmetrically with respect to a second axis of syimnetry, 
     which measuring tubes are arranged parallel to each other, are of one-piece construction, and are connected in series in terms of fluid flow, and 
     each of which measuring tubes has 
     a straight inlet portion, 
     a straight outlet portion, 
     an S-shaped inlet bend connected with the inlet portion, 
     an S-shaped outlet bend connected with the outlet portion, 
     a first straight tube portion connected with the inlet bend, 
     a second straight tube portion connected with the outlet bend, and 
     a semicircular base bend connected with the first and second straight tube portions, 
     which inlet and outlet portions extend through a fixed member, 
     with the distance between the inlet and outlet portions of each measuring tube being less than the distance between the first and second straight tube portions of the respective measuring tube; 
     an excitation system 
     which during operation causes the measuring tubes to vibrate as a tuning fork, 
     a first portion of which is fixed to the semicircular base bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and 
     a second portion of which is fixed to the semicircular base bend of the second measuring tube in the area of the axis of symmetry of the second measuring tube; 
     a first optical sensor, 
     a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations 
     where the respective first straight tube portion passes into the respective semicircular base bend; and 
     a second optical sensor, 
     a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations 
     where the respective second straight tube portion passes into the respective semicircular base bend. 
     U.S. Pat. No. 4,622,858 discloses a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a first straight measuring tube; 
     a second straight measuring tube, 
     which measuring tubes are arranged parallel to each other, 
     are of one-piece construction, and 
     are connected in parallel in terms of fluid flow by means of an inlet manifold and an outlet manifold; 
     a driving mechanism 
     which in operation vibrates the measuring tubes as a tuning fork, 
     a first portion of which is fixed to the first measuring tube midway between the inlet manifold and the outlet manifold, and 
     a second portion of which is fixed to the second measuring tube midway between the inlet manifold and the outlet manifold; 
     a first electrodynamic sensor, 
     a first portion of which is fixed to the first measuring tube midway between the driving mechanism and the inlet manifold, and a second portion of which is fixed to the second measuring tube midway between the driving mechanism and the inlet manifold; and 
     a second electrodynamic sensor, 
     a first portion of which is fixed to the first measuring tube midway between the driving mechanism and the outlet manifold, and a second portion of which is fixed to the second measuring tube midway between the driving mechanism and the outlet manifold. 
     U.S. Pat. No. 6,006,609 discloses a Coriolis mass flow rate/density/viscosity sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a single straight measuring tube of one-piece construction 
     which is provided with a cantilever at its midpoint, and 
     an inlet end and an outlet end of which are mounted in a support frame which is disposed in a housing; 
     an excitation arrangement 
     which in operation sets the measuring tube into flexural vibrations and into torsional vibrations equal in frequency to the flexural vibrations, and 
     first portions of which are fixed to the cantilever and second portions of which are fixed to the support frame; 
     a first sensor, 
     a first and a second portion of which are fixed to the measuring tube and the support frame, respectively, approximately midway between the inlet end and the cantilever; and 
     a second sensor, 
     a first and a second portion of which are fixed to the measuring tube and the support frame, respectively, approximately midway between the outlet end and the cantilever. 
     U.S. Pat. No. 5,796,011, particularly in connection with FIG. 5, describes a Coriolis mass flow rate sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a first measuring tube bent in a first plane symmetrically with respect to a first axis of symmetry; 
     a second measuring tube bent in a second plane symmetrically with respect to a second axis of symmetry, 
     which measuring tubes are arranged parallel to each other and are of one-piece construction, and 
     each of which measuring tubes has 
     a straight inlet portion with an inlet axis lying in the first plane and the second plane, respectively, 
     a straight outlet portion with an outlet axis aligned with the inlet axis, 
     an inlet bend connected with the inlet portion, 
     an outlet bend connected with the outlet portion, and 
     a circular-arc-shaped vertex portion of minimum height connected with the inlet bend and outlet bend, 
     which inlet portions and which outlet portions are connected in parallel in terms of fluid flow by means of an inlet manifold and an outlet manifold, respectively, and 
     which manifolds are mounted in a support frame which forms part of a housing; 
     a first node plate rigidly connecting the two measuring tubes at a location 
     where the inlet bend passes into die circular-arc-shaped vertex bend; 
     a second node plate rigidly connecting the two measuring tubes at a location 
     where the outlet bend passes into the circular-arc-shaped vertex bend; 
     an excitation system 
     which in operation causes the measuring tubes to vibrate as a tuning fork, 
     a first portion of which is fixed to the circular-arc-shaped vertex bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and 
     a second portion of which is fixed to the circular-arc-shaped vertex bend of the second measuring tube in the area of the axis of synmtetry of the second measuring tube; 
     a first sensor, 
     a first portion of which is fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations 
     where the respective inlet bend passes into the respective circular-arc-shaped vertex bend; 
     a second sensor, 
     a first portion of which fixed to the first measuring tube and a second portion of which is fixed to the second measuring tube at respective locations 
     where the respective outlet bend passes into the respective circular-arc-shaped vertex bend; 
     a feedthrough mounted in the support frame opposite the circular-arc-shaped vertex bends and containing several electric conductors; and 
     a printed-circuit board attached to the support frame and extending between the inlet manifold and outlet manifold and having conducting tracks 
     via which leads of the excitation system and the sensors are connected to the conductors of the feedthrough. 
     To the above referred ensembles of features of the individual prior-art arrangements it should be added that a straight measuring tube or straight measuring tubes are preferably made of pure titanium, a high-titaniuin alloy, pure zirconium, or a high-zirconium alloy, since, compared with measuring tubes of stainless steel, which is suitable material for straight measuring tubes in principle, shorter overall lengths are obtained, and that a bent measuring tube or bent measuring tubes are preferably made of stainless steel, although titanium or zirconium or their alloys are suitable materials for such tubes as well. 
     The design principle of the Coriolis mass flow rate sensor according to U.S. Pat. No. 5,796,011 permits the use of only such circular-arc vertex bends which have a great radius of curvature, i.e., where the distance between the circular-arc vertex bend and the inlet/outlet axis is minimal as a function of the inside diameter and the wall thickness of the measuring tubes and of a permissible, temperature-range-induced mechanical stress. For distances between the vertex and the inlet/outlet axis that are greater than the minimum distance, however, particularly for distances greater than the minimum distance by an order of magnitude, the design principle of US. Pat. No. 5,796,011 is unsuitable. 
     Therefore, starting from the design principle U.S. Pat. No. 5,796,011, it is an object of the invention to provide a Coriolis mass flow rate/density/viscosity sensor in which the distance between the vertex of the vertex bend and the inlet/outlet axis can be virtually arbitrarily great. At the same time, high measurement accuracy, for example of the order of ±0.5%, is to be achievable, manufacturing costs are to be minimized as compared to those of prior-art mass flow rate sensors, mass flow rate/density sensors, or mass flow rate/density/viscosity sensors, and a shorter overall length is to be made possible. 
     To attain these objects, the invention provides a Coriolis mass flow rate/density/viscosity sensor designed to be installed in a pipe through which a fluid flows at least temporarily, and comprising: 
     a first measuring tube bent to a V shape in a first plane symmetrically with respect to a first axis of symmetry; 
     a second measuring tube bent to a V shape in a second plane symmetrically with respect to a second axis of symmetry, 
     which measuring tubes are arranged parallel to each other and are each of one-piece construction, and 
     each of which measuring tubes has 
     a straight inlet portion with an inlet axis lying in the first plane and second plane, respectively, 
     a straight outlet portion with an outlet axis lying in the first plane and second plane, respectively, and aligned with the inlet axis, 
     an inlet bend connected with the inlet portion, 
     an outlet bend connected with the outlet portion, 
     a first straight tube portion connected with the inlet bend, 
     a second straight tube portion connected with the outlet bend, and 
     a vertex bend connected with the first and second straight tube portions, 
     which inlet portions are fixed in an inlet manifold, which outlet portions are fixed in an outlet manifold, and 
     which manifolds are mounted in a support frame which forms part of a housing; 
     an excitation arrangement 
     which in operation causes the measuring tubes to vibrate as a tuning fork, 
     a first portion of which is fixed to the vertex bend of the first measuring tube in the area of the axis of symmetry of the first measuring tube, and 
     a second portion of which is fixed to the vertex bend of the second measuring tube in the area of the axis of symmetry of the second measuring tube; 
     a first velocity or displacement sensor, 
     a first portion of which is fixed to the first straight tube portion of the first measuring tube, and 
     a second portion of which is fixed to the first straight tube portion of the second measuring tube; 
     a second velocity or displacement sensor, positioned symmetrically with respect to the axes of symmetry of the measuring tubes, 
     a first portion of which is fixed to the second straight tube portion of the first measuring tube, and a second portion of which is fixed to the second straight tube portion of the second measuring tube; 
     a feedthrough mounted in the support frame opposite the vertex bends and containing several electric conductors; and 
     a printed-circuit board attached to the support frame and extending between the support frame and the vertex bends and having conducting tracks 
     to which leads of the excitation system and of the velocity or displacement sensors are connected. 
     In a preferred embodiment of the invention, the measuring tubes 
     are rigidly connected by a first node plate in the vicinity of a location 
     where the respective inlet portion passes into the respective inlet bend, 
     are rigidly connected by a second node plate in the vicinity of a location 
     where the respective inlet bend passes into the respective first straight tube portion, 
     are rigidly connected by a third node plate in the vicinity of a location 
     where the respective outlet portion passes into the respective outlet bend, and 
     are rigidly connected by a fourth node plate in the vicintiy of a location 
     where the respective outlet bend passes into the respective second straight tube portion. 
     According to a first development of the invention and/or of the above preferred embodiment, electrodynamic velocity sensors are used and the excitation system is of the electrodynamic type. 
     According to a second development of the invention, which can also be used with the above preferred embodiment and/or the first development, 
     the support frame is of one-piece construction and is made of stainless sheet steel of constant width and thickness having a front face and a rear face, comprises: 
     a plane inlet frame portion, which has the inlet manifold welded therein, 
     a plane outlet frame portion, which has the outlet manifold welded therein, 
     a plane feedthrough frame portion connecting the inlet frame portion and outlet frame portion and having the feedthrough mounted therein in a pressure-tight manner, 
     a first plane extension frame portion extending from the inlet frame portion at an angle greater than 90°, 
     a bent vertex frame portion passing into the first extension frame portion, and 
     a second plane extension frame portion extending from the outlet frame portion at said angle and passing into the vertex frame portion; and 
     the support frame is supplemented by a plane front sheet of stainless steel, which is welded to the front, and a plane rear sheet of the same steel, which is welded to the rear face, to form the housing. 
     According to a third development of the invention, which can also be used with the preferred embodiment and/or the first and/or second developments, the feedthrough comprises: 
     a flange attached to the support frame and having a hole; 
     the printed-circuit board, which is passed through a slot formed in the feedthrough frame portion and extends into the flange, with the printed-circuit board and the slot separated by a distance sufficient for electric isolation; 
     a disk of insulating material resting on the feedthrough frame portion and through which the printed-circuit board is passed; and 
     an insulating compound filling a portion of the hole lying above the disk, the insulating compound having a thickness at least equal to the gap length specified for type of protection Ex-d as a function of gap width. 
     One advantage of the invention is that it permits the construction of Coriolis mass flow rate/density/viscosity sensors whose overall length, i.e., the length along the inlet/outlet axis, is considerably shorter than the overall length of the assembly according to U.S. Pat. No. 5,796,011. This is due to, among other things, the V shape of the measuring tube. A compact sensor with the desired measurement accuracy is obtained. 
     Furthermore, the design of the housing, which consists essentially of a support frame, a front steel sheet, and a rear steel sheet, contributes to the fact that the Coriolis sensor can be manufactured at very bw cost. Manufacturing costs are also kept low through the use of the printed-circuit board for the feedthrough, since the board provides a simple and low-cost electrical connection between the excitation system and the sensors on the one hand and evaluation electronics on the other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained in more detail with reference to the accompanying drawings, which show a preferred embodiment of the invention. Corresponding components are designated by the same reference numerals throughout the various figures, but reference numerals are repeated in subsequent figures only if this appears appropriate. In the drawings: 
     FIG. 1 is a perspective view showing mechanical details of a Coriolis sensor, with its housing not completed; 
     FIG. 2 is a front view of the Coriolis sensor of FIG. 1, again with its housing not completed, but with additional electrical details; 
     FIG. 3 is a section taken along line A—A of FIG. 2, showing the Coriolis sensor in a plan view, but with the housing completed; and 
     FIG. 4 is a section taken along line B—B of FIG. 2, showing the Coriolis sensor in a side view and again with the housing completed. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     While the invention is susceptible to various modifications and alternative forms, exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular forms desclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     FIG. 1 is a perspective view showing only mechanical details of a Coriolis mass flow rate/density/viscosity sensor, referred to herein as a Coriolis sensor  10  for short, but with its housing not completed in order to more clearly show its internal construction, and FIG. 2 is a corresponding front view with additional electrical details. 
     FIGS. 3 and 4 are sectional views of FIG. 2 with the housing completed. Because of the representation chosen, a perspective FIG. 1 along with front, plan, and side views, in the following the figures are described not one after the other, but together. 
     Coriolis sensor  10  has a first V-shaped measuring tube  1 , which is bent in a first plane symmetrically with respect to a first axis of symmetry. A second V-shaped measuring tube  2  is bent in a second plane symmetrically with with respect to a second axis of symmetry. Measuring tubes  1 ,  2  are arranged parallel to each other, and each of them is of one-piece construction. 
     Measuring tube  1  has a straight inlet portion  11  with an inlet axis lying in the first plane, and a straight outlet portion  12  with an outlet axis lying in the first plane and aligned with the inlet axis; a common axis is thus obtained, which will hereinafter be referred to as an inlet/outlet axis. 
     Measuring tube  2  has a straight inlet portion  21  with an inlet axis lying in the second plane, a straight outlet portion  22  (visible only in FIG. 3) with an outlet axis lying in the second plane and aligned with the inlet axis; this common axis, too, will hereinafter be referred to as an inlet/outlet axis. 
     Measuring tube  1  further has an inlet bend  13  connected with inlet portion  11 , an outlet bend  14  connected with outlet portion  12 , a first straight tube portion  15  connected with inlet bend  13 , a second straight tube portion  16  connected with outlet bend  14 , and a vertex bend  17  connected with the first and second straight tube portions  15 ,  16 . 
     Measuring tube  2  further has an inlet bend  23  connected with inlet portion  21 , and outlet bend  24  (visible only in FIG. 3) connected with outlet portion  22 , a first straight tube portion  25  connected with inlet bend  23 , a second straight tube portion  26  connected with outlet bend  24 , and a vertex bend  27  connected with the straight tube portions  25 ,  26 . In the embodiment shown, the curvature of the axis of vertex bend  17  and that of vertex bend  27  correspond practically to the arc of a circle. 
     Inlet portions  11 ,  21  are fixed in an inlet manifold  18 , and outlet portions  12 ,  22  are fixed in an outlet manifold  19 . These manifolds  18 ,  19  are mounted in a support frame  30 , which forms part of a housing  3  (visible only in FIGS.  3  and  4 ). 
     In the embodiment, measuring tubes  1 ,  2  and manifolds  18 ,  19  are made of stainless steel. Preferably, the stainless steel with the European material number 1.4539, corresponding to the American designation 904 L, is used for measuring tubes  1 ,  2 , and the stainless steel with the European material number 1.4404, corresponding to the American designation 316 L, is used for manifolds  18 ,  19 . 
     Coriolis sensor  10  is designed to be installed in a pipe through which a fluid to be measured flows at least temporarily. To that end, the manufacturer provides inlet and outlet manifolds  18 ,  19  with customized connection means, such as connections with an internal or external thread, flanges, or clamping devices as are commercially available, for example, under the registered trademark Triclamp. 
     Like measuring tubes  1 ,  2 , support frame  30  is of one-piece construction. It was made from a flat bar of high-grade steel and of constant width and thickness by suitably bending the bar and welding its ends, see the joint  33 , and it has a front face  31  and a rear face  32  (visible only in FIGS.  3  and  4 ). 
     Support frame  30  comprises a plane inlet frame portion  34 , in which inlet manifold  18  is fixed by welding, and a plane outlet frame portion  35 , in which outlet manifold  19  is fixed by welding, see in FIG. 2 the portions  18  and  19  protruding from support frame  30 , with associated welds  18 ′ and  19 ′, respectively. 
     Support frame  30  further comprises a plane feedthrough frame portion  36 , which connects inlet frame portion  34  and outlet frame portion  35 , and in which a feedthrough  37  (visible only in FIG. 4) is fixed in a pressure-tight manner. Feedthrough frame portion  36  forms respective right angles with inlet and outlet frame portions  34 ,  35 . 
     Support frame  30  further comprises a first plane extension portion  38 , which extends from inlet frame portion  34  at an angle greater than 90°, in the embodiment approximately 120°. Support frame  30  finally comprises a bent vertex portion  39 , which passes into extension portion  38 , and a second plane extension portion  40 , which extends from outlet frame portion  35  at the above-mentioned angle and passes into vertex portion  39 . 
     Support frame  30  is supplemented by a plane front sheet  41  of stainless steel welded to front face  31  and a preferably plane rear sheet  42  of the same steel welded to rear face  32  to form the housing  3 , so that the latter is pressure-tight. Front and rear sheets  41 ,  42  can only be seen in FIGS. 3 and 4. In the embodiment, the steel preferably used for housing  3  is the stainless steel with the European material number 1.4301, which corresponds to the American designation  304 . 
     The preferably plane front and rear sheets  41 ,  42  result in a higher stiffness of housing  3  under compressive stress in the direction of the inlet/outlet axis than if these sheets were provided with longitudinal crimps. Measuring tubes  1 ,  2  are rigidly connected by a first node plate  51  in the vicinity of a location where the respective inlet portion  11 ,  21  passes into the respective inlet bend  13 ,  23 , and by a second mode plate  52  in the vicinity of a location where the respective inlet bend  13 ,  23  passes into the respective first straight tube portion  15 ,  25 . 
     Furthermore, measuring tubes  1 ,  2  are rigidly connected by a third node plate  53  in the vicinity of a location where the respective outlet portion  12 ,  22  passes into the respective outlet bend  14 ,  24 , and by a fourth node plate  54  in the vicinity of a location where the respective outlet bend  14 ,  24  passes into the respective second straight tube portion  16 ,  26 . 
     The four node plates  51 ,  52 ,  53 ,  54  are preferably thin plates of stainless steel, particularly of the same steel as that used for housing  3 . These plates are provided with holes whose diameters correspond to the outside diameters of measuring tubes  1 ,  2 , and with slots, so that they can be first clamped onto and then brazed to measuring tubes  1 ,  2 , with the slots being brazed together as well, so that the plates are seated on measuring tubes  1 ,  2  unslotted as node plates. 
     In operation, an excitation system  6  vibrates measuring tubes  1 ,  2  as a tuning fork at a frequency equal or close to the mechanical resonance frequency of the vibrating system formed by measuring tubes  1 ,  2 . This vibration frequency, as is well known, is dependent on the density of the fluid flowing through measuring tubes  1 ,  2 . Therefore, the density of the fluid can be determined from the vibration frequency. 
     A first portion  61  of excitation system  6  is fixed to vertex bend  17  of measuring tube  1  in the area of the above-mentioned axis of symmetry of this tube, and a second portion  62  of excitation system  6  is fixed to vertex bend  27  of measuring tube  2  in the area of the above-mentioned axis of symmetry of this tube, see FIG.  4 . 
     In the embodiment shown in the figures, excitation system  6  is an electrodynamic shaker, i.e., portion  61  is a coil and portion  62  a permanent magnet that cooperates with the coil by riding therein. 
     Excitation system  6  is supplied with AC power from a driver circuit (not shown), which may, for instance, be a PLL circuit that continuously adjusts the instantaneous resonance frequency of the vibrating system of measuring tubes  1 ,  2 . Such a PLL circuit is disclosed in U.S. Pat. No. 4,801,897, the disclosure of which is hereby incorporated by reference. 
     A first velocity or displacement sensor  7  and a second velocity or displacement sensor  8 , which are mounted on measuring tubes  1 ,  2  symmetrically with respect to the aforementioned axes of symmetry, produce measurement signals from which the mass flow rate, the density, and, if desired, the viscosity of the fluid can be determined. 
     A first portion  71  of velocity or displacement sensor  7  is fixed to the straight portion  15  of measuring tube  1 , and a second portion  72  is fixed to the straight portion  25  of measuring tube  2 , see FIG. 3. A first portion  81  of velocity or displacement sensor  8  is fixed to the straight portion  16  of measuring tube  1 , and a second portion  82  is fixed to the straight portion  26  of measuring tube  2 , see FIG.  3 . 
     In the embodiment shown in the figures, velocity or displacement sensors  7 ,  8  are preferably electrodynamic velocity sensors; thus, each of portions  71 ,  81  is a coil, and each of portion  72 ,  82  is a permanent magnet that can ride in the associated coil. 
     As already briefly mentioned above, feedthrough  37 , which contains several electric conductors, is mounted in support frame  30  opposite vertex bends  17 ,  27 , and thus opposite vertex frame portion  39 , particularly in a pressure-tight manner. To that end, a flange  90  is attached to support frame  30 ; preferably, flange  90  is welded to support frame  30 . Flange  90  has a hole  91 , so that feedthrough  37  is accessible from outside housing  3 . 
     Feedthrough  37  comprises a printed-circuit board  96 , which is fastened to support frame  30  by means of an angled support plate  95  and which extends between support frame  30  and the vertex bends toward the latter. Printedcircuit board  96  has conducting tracks formed thereon, cf. conducting track  97 , which are only visible in FIG.  2 . 
     Connected to respective ones of these conducting tracks are leads  63 ,  64  of excitation system  6 , leads  73 ,  74  of velocity sensor  7 , leads  83 ,  84  of velocity sensor  8 , and leads  93 ,  94  of a temperature sensor  9 , which are thus also connectect to the individual conductors ot feedthrough  37 . Leads  63 ,  64 ,  73 ,  74 ,  83 ,  84 ,  93 ,  94  can only be seen in FIG.  2 . In addition, a conducting track SN to ground is provided on the printed-circuit board, which is mechanically and, thus, electrically attached to the metallic support plate  95 . 
     In the embodiment shown, temperature sensor  9  (visible only in FIGS. 2 and 3) is attached to outlet bend  14  of measuring tube  1 , for instance with adhesive, and is preferably a platinum resistance element. As mentioned above, it serves to measure the current temperature of the fluid. Temperature sensor  9  may also be positioned at any other suitable location of measuring tubes  1 ,  2 . 
     Feedthrough  37  further comprises a slot  361  formed in feedthrough frame portion  36 , through which the printed-circuit board  96  is passed and extends into flange  90 , with a distance sufficient for electrical isolation being maintained between printed circuit board  96  and slot  361 . 
     Furthermore, printed-circuit board  96  is passed through a disk  362  of insulating material resting on feedthrough frame portion  36 . An insulating compound  363  completely fills a portion of hole  91  lying above disk  362 , and may also have penetrated into the space between printed-circuit board  96  and the internal wall of slot  363 . 
     The thickness of insulating compound  363  in the direction of the open end of hole  91  is at least equal to the gap length required for type of protection Ex-d according to European Standard EN 50014 and EN 50018 as a function of gap width, the disclosures of which are hereby incorporated by reference. These standards correspond to comparable standards of other countries. 
     As Coriolis sensor  10  has to be equipped with associated control and evaluation electronics to obtain an operational Coriolis mass flow rate/density/viscosity meter, a housing (not shown) for those control and evaluation electronics or a terminal arrangement (not shown) for a cable running to a control and evaluation electronics housing remote from the Coriolis sensor is screwed to flange  90 . 
     While the invention has been illustrated and described in detail in the drawing and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it beeing understood that only exemplary embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.