Patent Publication Number: US-2018045545-A1

Title: Magneto-inductive flow measuring device with reduced electrical current draw

Description:
The invention relates to a magneto-inductive flow measuring device for measuring the flow of a flowable medium as well as to a method for determining flow in a measuring tube. 
     In process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, mass flow measuring devices, pressure- and temperature measuring devices, etc., which, as sensors, register the corresponding process variables, fill level, flow, pressure, and temperature, respectively. 
     A large number of such field devices are manufactured and sold by the firm, Endress+Hauser. 
     Especially for measuring flow through a measuring tube, a large number of different measuring principles are applied. An important measuring principle is magneto-inductive flow measurement. Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, thus proportional to the volume flow. 
     For the performing the measuring, as a rule, an alternating magnetic field is applied, which is produced by means of a coil system. The electrical current consumption of the magneto-inductive flow measuring device, is, in such case, principally caused by the electrical current draw of the coils for producing the alternating magnetic field. 
     It is an object of the invention to provide a magneto-inductive flow measuring device, which has a lessened electrical current consumption. 
     This object is achieved by the features set forth in claims  1  and  17 . 
     Advantageous further developments of the invention are given in the dependent claims. 
     A magneto-inductive flow measuring device corresponding to forms of embodiment of the invention serves for measuring flow of a flowable medium. 
     The flow measuring device includes a measuring tube, a pair of coils, which are arranged opposite one another on the measuring tube and which are designed to produce an alternating magnetic field, which can be turned on and off, and which extends essentially transversely to the longitudinal axis of the measuring tube, as well as a pair of permanent magnets, which are arranged opposite one another on the measuring tube and which are designed to produce a permanent magnetic field, which extends essentially transversely to the longitudinal axis of the measuring tube. Moreover, the flow measuring device includes one or more pairs of measuring electrodes arranged opposite one another on the measuring tube, of which one pair of measuring electrodes is designed, in the case of turned off alternating magnetic field, to tap a measurement voltage induced by the permanent magnetic field, and an evaluation unit, which is designed, in the case of turned off alternating magnetic field, to monitor the measurement voltage induced by the permanent magnetic field, at least in the case of a predefined change of the measurement voltage, to turn the alternating magnetic field on and, by means of the alternating magnetic field, to determine a measured value for the flow. 
     The flow measuring device corresponding to forms of embodiment of the present invention includes a pair of permanent magnets, which are arranged opposite one another on the measuring tube. These permanent magnets are designed to produce a permanent magnetic field throughout the cross section of the measuring tube. When the medium flows with a certain flow velocity through the measuring tube, also in the case of turned off alternating magnetic field, this permanent magnetic field induces a measurement voltage in the direction perpendicular to the magnetic field. This induced measurement voltage depends on the flow velocity of the medium, so that, based on the measurement voltage induced by the permanent magnet field, flow as a function of time can be followed. Thus, even in the case of turned off alternating magnetic field, it is possible to detect changes in the flow. 
     When, in this way, a change in the flow is detected, the alternating magnetic field can be turned on for a certain time span for performing an exact flow measurement. With the help of the alternating magnetic field, then an exact flow measurement is performed. The alternating magnetic field is only turned on, when, as a result of a change in the flow, a new measured value for the flow is required. Otherwise, the alternating magnetic field remains turned off. 
     The alternating magnetic field is thus not continually turned on, but, instead, only from time to time. So long as the alternating magnetic field is turned off, only very little electrical current is consumed. Averaged over time, this significantly lessens the electrical current draw of the flow measuring device, and, averaged over time, the flow measuring device consumes significantly less power. Nevertheless, a sufficiently exact monitoring of the flow can be assured. 
    
    
     
       The invention will now be explained in greater detail based on examples of embodiments illustrated in the drawing, the figures of which show as follows: 
         FIG. 1A  a magneto-inductive flow measuring device, in the case of which permanent magnets are arranged within the coils; 
         FIG. 1B  a magneto-inductive flow measuring device, in the case of which the permanent magnets are placed above or below the coil cores; 
         FIG. 1C  another option for implementing the coil system of a magneto-inductive flow measuring device; 
         FIG. 2A  a first measurement setup for determining flow in the measuring tube; 
         FIG. 2B  a second measurement setup for determining flow in the measuring tube; 
         FIG. 3A  an analog evaluation unit for evaluating the measurement voltage; 
         FIG. 3B  an evaluation unit with digital signal processing for evaluating the measurement voltage; 
         FIG. 4A  electrical current through the coils as a function of time; 
         FIG. 4B  measurement voltage U E  induced by the alternating magnetic field as a function of time; 
         FIG. 5A  another geometric arrangement of the coils and permanent magnets of the magneto-inductive flow measuring device; 
         FIG. 5B  a particular geometric arrangement, in the case of which the permanent magnets are oriented perpendicularly to the coils; and 
         FIG. 6  another geometric arrangement, in the case of which the coils and the permanent magnets are arranged in cross-sectional planes of the measuring tube spaced from one another. 
     
    
    
     Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement and are known from a large number of publications. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, and is thus proportional to the volume flow. If the density of the medium is known, the mass flow in the pipeline, or in the measuring tube, as the case may be, can be determined. The measurement voltage is usually tapped via a measuring electrode pair, which is arranged relative to the coordinate along the measuring tube axis in the region of maximum magnetic field strength and where, thus, the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; however, also magneto-inductive flow measuring devices with contactless, capacitively coupling electrodes are known. 
     The measuring tube can be manufactured, in such case, either from an electrically conductive, non-magnetic material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, then it must be lined with a liner of an electrically insulating material in the region coming in contact with the medium. The liner is composed, depending on temperature and medium, for example, of a thermoplastic, a thermosetting or an elastomeric, synthetic material. Known, however, are also magneto-inductive flow measuring devices with a ceramic lining. 
     An electrode can be subdivided essentially into an electrode head, which comes at least partially in contact with a medium, which flows through the measuring tube, and an electrode shaft, which is contained almost completely in the wall of the measuring tube. 
     The electrodes are, besides the magnet system, central components of a magneto-inductive flow measuring device. In the case of the embodiment and arrangement of the electrodes, it is desirable that they can be assembled as simply as possible into the measuring tube and that subsequently in measurement operation no sealing problems occur; moreover, the electrodes should be distinguished by a sensitive and simultaneously low-disturbance measurement signal registration. 
     Besides the measuring electrodes, which serve for tapping a measurement signal, often additional electrodes are installed in the measuring tube in the form of reference- or grounding electrodes, which serve to measure an electrical reference potential or to detect a partially filled measuring tube or to register the temperature of the medium by means of an installed temperature detector. 
     As a rule, the magnet system of a magneto-inductive flow measuring device includes a coil pair, which is designed to produce an alternating magnetic field, which extends through the total cross section of the measuring tube. For producing the alternating magnetic field, the coils are fed by a clocked, direct current, which changes direction, for example, with a frequency of 8 Hz, or 16 Hz. 
     The continuous electrical current flow through the coil pair of the magnet system leads to an accordingly high power consumption in the case of magneto-inductive flow measuring devices. In such case, the power consumption depends especially on the tube cross section, wherein, in the case of greater tube cross sections, a higher power is required for producing the alternating magnetic field than in the case of lesser tube cross sections. In general, there is in the case of magneto-inductive flow measuring devices a need to lessen the electrical current draw. Especially in the case of two-conductor-field devices and in the case of battery operated field devices, a lessening of the electrical current draw would be of interest. 
     In the case of two-conductor field devices, both the power supply as well as also the measured value transmission occur via one pair of connection lines. Since in the case of many two-conductor field devices the measured values are transmitted in the form electrical current values, the field device must frequently operate for longer time periods with a comparatively low electrical current. 
     In the case of battery operated field devices, the supply of the field device occurs via an internal battery. Battery operated field devices are frequently used in poorly accessible locations and are, as a rule, not connected to a fieldbus. In order to enable a longer battery service life, also in this case a lessening of the power consumption of the flow measuring device would be desirable. 
     For lessening the electrical current draw of magneto-inductive flow measuring devices, it is proposed to utilize for monitoring the flow a permanent magnetic field produced by permanent magnet, in which case no electrical current is consumed for producing the field, and to turn the coil system of the flow measuring device responsible for the actual electrical current draw on only from time to time. 
       FIG. 1A  shows a magneto-inductive flow measuring device, which works according to this principle. For producing the alternating magnetic field required for the flow measurement, a coil system is provided. The coil system includes a first coil  101  and a first pole shoe  102  arranged above the measuring tube  100  as well as a second coil  103  and a second pole shoe  104  arranged below the measuring tube  100 . The two coils  101 ,  103  are designed to produce an alternating magnetic field oriented perpendicularly to the flow direction  105  of the medium. The pole shoes  102 ,  104  are so embodied that the magnetic field produced by the coils  101 ,  103  obeys a desired mathematical function, which assures an as linear as possible measurement behavior in the case of different flow profiles. 
     The direction of the alternating magnetic field produced by the coils  101 ,  103  is shown in  FIG. 1A  by the double arrow  106 . Movement of charge carriers of the medium perpendicularly to the magnetic field induces a measurement voltage U E , which can be tapped via the two measuring electrodes  107 ,  108 . In this regard, the two measuring electrodes  107 ,  108  are arranged essentially perpendicularly to the flow direction  105  of the medium and perpendicularly to the direction of the alternating magnetic field. The measurement voltage U E  tapped on the measuring electrodes  107 ,  108  is directly proportional to the flow velocity v of the medium. The faster the medium in the measuring tube  100  flows, the higher is the voltage U E  tappable on the measuring electrodes  107 ,  108 . 
     In the case of the flow measuring device shown in  FIG. 1A , supplementally to the coils  101 ,  103 , two permanent magnets  109 ,  110  are arranged above and below the measuring tube  100 . The first permanent magnet  109  is arranged in the interior of the coil  101  above the measuring tube  100 , and the second permanent magnet  110  is arranged in the interior of the coil  103  below the measuring tube  100 . The two permanent magnets  109 ,  110  produce throughout the cross section of the measuring tube  100  a permanent magnetic field, which extends in the direction of the arrow  111 . When the two coils  101 ,  103  are turned off and no alternating magnetic field is being produced, the two permanent magnets  109 ,  110  are nevertheless producing a permanent magnetic field. This permanent magnetic field also induces in the flowing medium in the case of turned off alternating magnetic field a measurement voltage U E , which can be tapped on the measuring electrodes  107 ,  108 . The measurement voltage U E  depends, in such case, on the flow velocity of the medium. 
     The measurement voltage induced by the permanent magnetic field U E  is influenced by electrochemical potential influences and is, consequently, not suited for an exact determination of the absolute flow value. However, the measurement voltage U E  induced by the two permanent magnet  109 ,  110  is quite well suited for monitoring flow is a function of time and for detecting significant changes of the flow. When such a significant change of the flow is detected, the alternating magnetic field produced by the coils  101  and  103  is turned on for a certain time span, in order to perform an exact measuring of the changed flow. 
     In contrast with solutions of the state of the art, the coils  101 ,  103  are thus not continually turned on, but, instead, are only activated from time to time, for example, when, as a result of a flow change, a new determination of the flow is required. The coils  101 ,  103  are thus only turned on during certain time spans. In this way, the average power consumption of the magneto-inductive flow measuring device can be significantly decreased. In this way, magneto-inductive flow measuring devices can be built, which have a significantly lessened electrical current requirement. 
       FIG. 1B  shows a further magneto-inductive flow measuring device, wherein the coil system of the flow measuring device shown in  FIG. 1B  differs from the coil system shown in  FIG. 1A . In  FIG. 1B , features, which are equal or similar to the features already shown in  FIG. 1A , are provided with the same reference characters as in  FIG. 1A , so that subsequently only differences will be explored and reference is otherwise taken to the description of  FIG. 1A . 
     In contrast to  FIG. 1A , coil cores  112 ,  113  are arranged within the coils  101 ,  103 . The presence of these coil cores  112 ,  113  in the interior of the coils  101 ,  103  causes the alternating magnetic field to be strengthened and led to the pole shoes  102 ,  104 . The first permanent magnet  114  is arranged above the coil core  112  and adjoins the coil core  112 . The second permanent magnet  115  is arranged below the coil core  113  and adjoins the coil core  113 . Although the permanent magnets  114 ,  115  are arranged, in each case, on the ends of the coil cores  112 ,  113  facing away from the measuring tube  100 , the permanent magnetic field produced by them is strong enough to pass through the interior of the measuring tube  100  with a uniform permanent magnetic field. 
       FIG. 1C  shows another option for embodiment of the coil system, wherein here only the first coil  101  is shown. Arranged in the interior of the first coil  101  is a coil core  116 , which, however, only partially fills the interior of the coil. The rest of the coil interior is filled by a permanent magnet  117 , which adjoins the coil core  116  and partially replaces the coil core  116 . In the case of the solution shown in  FIG. 1C , the permanent magnet  117  ends flush with the first coil  101 . Alternatively thereto the permanent magnet  117  also could extend out above the first coil  101 . 
       FIG. 2A  illustrates the measuring principle. Plotted along the vertical axis is the measurement voltage U E , which is caused by the permanent magnetic field and which can be tapped on the measuring electrodes  105 ,  106  in the case of turned off alternating magnetic field. Plotted along the horizontal axis is the time t. The curve  200  shows the measurement voltage U E  tappable on the measuring electrodes  105 ,  106  as a function of time. During the time interval  201 , the measurement voltage U E  changes only slightly, so that no exact measuring is initiated. Only at the point in time  202  is a significant rise of the measurement voltage U E  detected. Accordingly, during the thereon following time span  203 , the alternating magnetic field is turned on, and a more exactly measured value determined for the flow. During the following time interval  204 , the changes of the tapped measurement voltage U E  are again relatively small. Therefore, during the time interval  204 , no exact measuring of the flow value is initiated. 
     In the case of the measuring illustrated in  FIG. 2A , an exact measuring of the flow is initiated only when a significant change of the tapped measurement voltage U E  is detected, thus, for example, at the point in time  202 . In addition to these measurements, which are initiated in the case of significant changes of the flow, exact measurements of the flow can also be performed in regular time intervals ΔT. 
     Such a measurement procedure is shown in  FIG. 2B . The curve  205  shows measurement voltage U E  as a function of time. In regular time intervals ΔT, an exact determining of flow is performed, thus at the points in time  206 ,  207 ,  208 . For performing the exact measurements, the alternating magnetic field is turned on during the time intervals  209 ,  210 ,  211 . Moreover, at the point in time  212 , a significant rise of the measurement voltage U E  is detected, and, consequently, during the thereon following time interval  213 , the alternating magnetic field is likewise turned on. On the whole, the alternating magnetic field is thus only turned on during the time intervals  209 ,  210 ,  211 ,  213 . For the rest of the time, the alternating magnetic field remains turned off, which leads to a considerable sinking of the average electrical current consumption. In this way, magneto-inductive flow measuring devices can be built, which have a clearly lessened electrical current requirement. This is especially advantageous for two-conductor field devices and battery driven field devices. 
       FIG. 3A  shows an evaluating circuit for monitoring the measurement voltage induced by the permanent magnetic field. The signal voltages S 1  and S 2  tappable on the two measuring electrodes  107 ,  108  are fed to a difference amplifier  300 , which subtracts the two signal voltages S 1  and S 2  from one another. The so obtained difference signal  301  is sent through a filter  302 . 
     The filter  302  serves mainly for filtering out disturbance signals. The filtered signal  303  is fed to a differentiator  304 , which forms the derivative of the filtered signal  303 . Based on the derivative, it can be detected how strongly the filtered signal  303  changes per unit time. The derivative signal  305  is fed to a comparator  30 , where it is compared with a limit value  307 , which is provided by a reference value unit  308 . So long as the derivative signal  305  lies below the limit value  307 , no new flow measurement is initiated, and the magneto-inductive measuring system  310  remains turned off. As soon, however, as the derivative signal  305  exceeds the limit value  307 , the comparator  306  produces a switch-on signal  309  (a so-called “wake-up signal”), which turns the magneto-inductive measuring system  310  on for producing the alternating magnetic field and a new flow measurement is initiated using the alternating magnetic field. 
     The limit value  307  is provided by the reference value unit  308 . In such case, it is advantageous that the limit value  307  produced by the reference value unit  308  be adapted dynamically as a function of the required accuracy of the flow measurement. Conforming the limit value establishes how frequently a new determination of the flow value is performed. When a comparatively high limit value is set, an exact measuring the flow is initiated only in the case of relatively strong changes of the flow, and the measurements then occur relatively infrequently. When the limit value is, in contrast, selected relatively low, then the limit value is frequently exceeded, and, accordingly, an exact measuring of the flow value is initiated frequently. By adjusting the limit value, the accuracy, with which the flow is tracked, can be established dynamically. 
     In such case, it can be provided that the limit value  307  of the reference value unit  308  is set from the magneto-inductive measuring system  310  with the assistance of a control signal  311 . When the flow measurements occur too frequently, the limit value  307  is increased by the magneto-inductive measuring system  310 . When the flow measurements occur too infrequently, the limit value  307  is reduced. 
       FIG. 3B  shows another evaluation unit, in the case of which the measurement voltage induced by the permanent magnetic field is evaluated by means of digital signal processing. The signal voltages S 1  and S 2  tappable on the two measuring electrodes  107 ,  108  are fed to a difference amplifier  312 , which subtracts the two signal voltages S 1  and S 2  from one another. The so obtained difference signal  313  is then fed to an analog/digital converter  314 , which converts the analog signal into a sequence of digital, sampled values. These digital, sample values are then fed to a microprocessor  315  (or to a digital signal processor), which ascertains how strongly the measurement voltage U E  changes per unit time. When the voltage change ΔU E  per unit time Δt exceeds, for example, a predetermined limit value, an exact measuring of the flow is initiated. 
       FIGS. 4A and 4B  show how an exact measurement of the flow is performed with the assistance of the alternating magnetic field produced by the coils  101 ,  103 . Such flow measurements are performed during the time interval  203  in  FIG. 2A  and during the time intervals  209 ,  210 ,  211 ,  213  in  FIG. 2B . For producing the alternating magnetic field, the two coils  101 ,  103  are fed with a clocked, direct current, which is plotted in  FIG. 4A  as a function of time. Plotted along the vertical axis is the electrical current I through the two coils  101 ,  103 , and plotted along the horizontal axis is the time t. It can be seen that the clocked, direct current changes its polarity with a predetermined frequency (for example, 8 Hz, or 16 Hz). As a result of this alternating electrical current flow through the coils  101 ,  103 , the coils  101 ,  103  produce an alternating magnetic field, whose direction continually changes corresponding to the direction of the electrical current flow shown in  FIG. 4A . During the measurement interval  400  shown in  FIG. 4A , the magnetic field changes direction, for example, a total of eight times. 
     As a result of the alternating magnetic field and the movement of the charge carriers of the flowing medium, a measurement voltage U E  is induced in the direction transverse to the measuring tube  100  and can be tapped on the two measuring electrodes  107 ,  108 . This measurement voltage U E  is plotted in  FIG. 4B  as a function of time.  FIG. 4B  shows that the measurement voltage U E  changes with the frequency of the alternating magnetic field. So long as the magnetic field is oriented in a first direction, thus, for example, upwardly, an induced voltage  402  of positive sign is superimposed on the dashed voltage offset  401 , and one obtains a first measurement voltage value  403 . As soon as the magnetic field changes direction and is oriented now in reverse direction, for example, downwardly, an induced voltage  404  of negative sign is superimposed on the voltage offset  401 , and one obtains a second measurement voltage value  405 . By ascertaining the difference between the first measurement voltage value  403  and the second measurement voltage value  405 , one obtains a difference voltage ΔU E , which is no longer dependent on the voltage offset  401 . This difference voltage ΔU E  depends only on the magnitude B of the alternating magnetic field: 
       Δ U   E   =k·B·D·v,  
 
     wherein k is a proportionality constant, D his the diameter of the measuring tube, v is the flow velocity of the medium and B the magnitude of the alternating magnetic field. 
     Through use of the alternating magnetic field, thus, the influence of the voltage offset  401  can be eliminated. The voltage offset  401  depends decisively on the electrochemical potential of the two measuring electrodes  107 ,  108 , which can change in the course of time and is subject to a permanent drift. Moreover, the voltage offset  401  is also a result of the induced voltage contribution brought about by the permanent magnetic field produced by the two permanent magnets  109 ,  110 . The magnetic field component produced by the two permanent magnets  109 ,  110  does not disturb the exact determining of flow velocity v and of the flow, because this permanent magnetic field contributes only to the voltage offset  401 , which is, in any event, eliminated by the difference forming. Thus, an exact determining of flow velocity v can be performed by measuring with the alternating magnetic field. In this way, the flow can be determined with high accuracy. 
     In the case of the flow measuring device shown in  FIG. 1 , the coils  101 ,  103  and the permanent magnets  109 ,  110  are oriented along the same axis. However, other geometric arrangements are possible.  FIGS. 5A, 5B and 6  show possible alternative arrangements for the coils and the permanent magnets. 
       FIG. 5A  shows a first coil  501  arranged above the measuring tube  500 , and a second coil  502  arranged below the measuring tube  500 . The alternating magnetic field produced by the two coils  501 ,  502  is illustrated by the double arrow  503 . The two measuring electrodes  504 ,  505  are arranged perpendicularly to the flow direction of the medium and perpendicularly to the axis  506  fixed by the coils  501 ,  502 . With the alternating magnetic field turned on, there can be tapped on the two measuring electrodes  504 ,  505  a measurement voltage U E1 , which enables an exact determining of the current flow value. 
     Additionally arranged on the measuring tube  500  at mutually opposite positions are the two permanent magnets  507 ,  508 , which produce in the cross section of the measuring tube  500  a permanent magnetic field, whose direction is illustrated by the arrow  509 . The axis  510  fixed by the two permanent magnets  507 ,  508  is oriented offset by an angle α from the axis  506  fixed by the coils  501 ,  502 . The angle α should not be selected too small, because the pole shoes arranged outside the coils  501 ,  502  take up a certain space. For example, the angle α could be selected to equal 45°. 
     In contrast to the solution shown in  FIG. 1 , in the case of which a single measuring electrode pair was sufficient, in the case of the flow measuring device shown in  FIG. 5A , a second pair of measuring electrodes  511 ,  512  is provided, in order to be able to tap the measurement voltage U E2  induced by the permanent magnets  507 ,  508 . The two measuring electrodes  511 ,  512  are arranged perpendicularly to the flow direction of the medium and perpendicularly to the axis  510  fixed by the two permanent magnets  507 ,  508 . The voltage U E2  tappable on the two measuring electrodes  511 ,  512  permits a permanent monitoring of the flow. When a more exactly measured value of the flow is required, the coil system is activated for a short time, in order to produce the alternating magnetic field required for the exact flow measurement. 
       FIG. 5B  shows a special example, in the case of which the two coils  513 ,  514  are arranged above and below a measuring tube  515  and in the case of which the two permanent magnets  516 ,  517  are arranged perpendicularly to the direction fixed by the two coils  513 ,  514 . When the two coils  513 ,  514  are supplied with a clocked, direct current, they produce an alternating magnetic field. The voltage U E1  induced by the alternating magnetic field can be tapped on the two measuring electrodes  518 ,  519  arranged perpendicularly to the coils  513 ,  514 . The two measuring electrodes  518 ,  519  can extend, for example, through bores in the permanent magnet  516 ,  517 , into the interior of the measuring tube  515 . The evaluation of the voltage U E1  induced by the alternating magnetic field measurement enables an exact determining of the current flow value. 
     The two permanent magnets  516 ,  517  are arranged in  FIG. 5B  perpendicularly to the two coils  513 ,  514  and produce throughout the cross section of the measuring tube  515  a permanent magnetic field. As a result of this permanent magnetic field, there is induced perpendicularly to the two permanent magnets  516 ,  517  a measurement voltage U E2 , which can be tapped by the two measuring electrodes  520 ,  521 . These two measuring electrodes  520 ,  521  extend through the coils  513 ,  514  into the interior of the measuring tube  515 . Based on the measurement voltage U E2 , the flow in the measuring tube  515  can be permanently monitored, wherein, when required, the alternating magnetic field is turned on and an exact flow measurement initiated using the alternating magnetic field. 
       FIG. 6  shows another geometric arrangement of coils and permanent magnets in a magneto-inductive flow measuring device. The coil system of the flow measuring device includes a first coil  600 , which is arranged above the measuring tube  601 , as well as a second coil  602 , which is arranged below the measuring tube  601 . The two coils  600 ,  602  are designed to produce an alternating magnetic field throughout the cross section of the measuring tube  601 . Arranged perpendicularly to the axis  603  fixed by the two coils  600 ,  602  is a pair of measuring electrodes  604 ,  605 , which are designed to tap the measurement voltage U E1  induced by the alternating magnetic field. The evaluation of this measurement voltage U E1  caused by the alternating magnetic field permits an exact determining of flow through the measuring tube  601 . 
     In the case of the previously shown solutions in  FIG. 1 ,  FIG. 5A ,  FIG. 5B , coils and permanent magnets were, in each case, arranged in the same cross sectional plane of the measuring tube. In contrast, in  FIG. 6 , the coils  600 ,  602  and the measuring electrodes  604 ,  605  are arranged in a first cross sectional plane  608 , while the permanent magnets  606 ,  607  are arranged in a second cross sectional plane  609  spaced therefrom. For tapping the voltage U E2  induced by the permanent magnets  606 ,  607 , there is provided in the second cross sectional plane  609  a second pair of measuring electrodes  612 ,  613 , which is arranged perpendicularly to the axis  611  fixed by the permanent magnets  606 ,  607 . 
     The second cross sectional plane  609  is arranged a certain distance  610  from the first cross sectional plane  608 . The axis  611  fixed by the two permanent magnets  606 ,  607  can be oriented at any angle α relative to the axis  603 . The voltage U E2  tappable on the two measuring electrodes  612 ,  613  enables a continuous monitoring of the flow through the measuring tube  601 . Only in the case of significant changes of the flow, or supplementally also in regular time intervals, is an exact measuring of the flow using the alternating magnetic field initiated.