Abstract
A magnetic-inductive flow meter includes: a measuring tube for conducting a flowable medium, the measuring tube having a wall; at least three measuring electrodes arranged in the wall to form a galvanic contact with the flowing medium; a magnetic field-generating device for generating a magnetic field that passes through the medium; a measuring circuit designed to ascertain at least one first measurement variable, wherein measured values of the first measurement variable are ascertained at a first measuring electrode pair; and an analysis circuit designed to ascertain a Reynolds number and/or a kinematic viscosity value of the medium in the measuring tube using measured values for the first measurement variable and a second measurement variable, which differs from the first measurement variable, the measured values of the second measurement variable being ascertained at a second measuring electrode pair.
Claims
1-11. (canceled)
12. A magnetic-inductive flow meter, comprising: a measuring tube adapted to conduct a flowable medium and including a wall; at least three measuring electrodes disposed in the wall such that each measuring electrode is in galvanic contact with the flowing medium; at least one magnetic-field-generating device configured to generate a magnetic field that passes through the measuring tube; a measuring circuit configured to determine at least one measurement variable, wherein measured values of a first measurement variable of the at least one measurement variable are determined at a first measuring electrode pair, which includes two measuring electrodes of the at least three measuring electrodes; and an analysis circuit configured to determine a Reynolds number and/or a kinematic viscosity value of the medium in the measuring tube using measured values of the first measurement variable and a second measurement variable, which differs from the first measurement variable, wherein measured values of the second measurement variable are determined at a second measuring electrode pair or at a third measuring electrode relative to a reference potential.
13. The magnetic-inductive flow meter of claim 12, wherein the at least three measuring electrodes are arranged in the measuring tube such that, in a test measurement, quotients of current measured values of the first and second measurement variables correspond bijectively to the Reynolds number (Re) of the flowing medium in the measuring tube, at least within a Reynolds-number range of 1,000≤Re≤1,000,000.
14. The magnetic-inductive flow meter of claim 13, wherein the quotients of current measured values of the first and second measurement variables correspond bijectively to the Reynolds number (Re) of the flowing medium in the measuring tube, at least within a Reynolds-number range of 10,000≤Re≤100,000.
15. The magnetic-inductive flow meter of claim 12, wherein the at least three measuring electrodes are arranged substantially in a cross-sectional plane, wherein a first radius intersecting a second measuring electrode of the first measuring electrode pair and a second radius intersecting the third measuring electrode span an angle of at least 20°, wherein the measuring circuit is configured to determine measured values of the first measurement variable between the second measuring electrode and a first second measuring electrode of the first second measuring electrode pair and to determine measured values of the second measurement variable between the first and third measuring electrodes or at the third measuring electrode relative to a reference potential.
16. The magnetic-inductive flow meter of claim 15, wherein the angle spanning the first radius and the second radius is at least 40° but no more than 60°.
17. The magnetic-inductive flow meter of claim 12, wherein a first measuring electrode axis intersecting the first measuring electrode pair and a second measuring electrode axis intersecting the third measuring electrode and a fourth measuring electrode are substantially parallel, wherein the measuring circuit is configured to determine measured values of the first measurement variable between the first measuring electrode pair and to determine measured values of the second measurement variable between the third and fourth measuring electrodes.
18. The magnetic-inductive flow meter of claim 12, wherein, in a test measurement, the first measurement variable behaves substantially proportionally to a flow rate of the medium within a Reynolds-number range of 10,000≤Re≤1,000,000, and wherein, in the test measurement, a change in the second measurement variable within a Reynolds-number range of 10,000≤Re≤1,000,000 with increasing Reynolds number is not constant.
19. The magnetic-inductive flow meter of claim 12, wherein, in a test measurement, the medium is a Newtonian fluid, wherein, in the test measurement, the flow meter is introduced into a pipeline with a straight inlet section of at least DN 20 such that a substantially symmetrical flow profile is present in a measurement region, and wherein the measuring tube (1) has a diameter of DN 80.
20. The magnetic-inductive flow meter of claim 12, wherein the magnetic-field-generating device includes two oppositely attached coils that are connected in series in the same direction.
21. A method for operating a magnetic-inductive flow meter, the method comprising: providing a magnetic-inductive flow meter according to claim 12; detecting a measured value of the first measurement variable and a measured value of the second measurement variable, wherein the respective measured values of the first and second measurement variables are determined at different measuring electrode pairs; and determining a Reynolds number dependent upon the measured values of the first and second measurement variables.
22. The method of claim 21, further comprising calculating a corrected flow rate and/or a corrected volumetric flow using a correction factor, which depends upon the determined Reynolds number.
23. The method of claim 21, further comprising determining a kinematic viscosity value of the medium using measured values of the first or the second measurement variable and the determined Reynolds number.
24. The method of claim 21, wherein a map that assigns Reynolds numbers to quotients of the first and second measurement variables is bijective, at least within a Reynolds-number range of 1,000≤Re≤1,000,000.
25. The method of claim 24, wherein the map that assigns Reynolds numbers to quotients of the first and second measurement variables is bijective, at least within a Reynolds-number (Re) range of 10,000≤Re≤100,000.
Description
[0055] The invention is explained in greater detail with reference to the following figures. The following are shown:
[0056] FIG. 1: a cross-sectional diagram of a magnetic-inductive flow meter according to the prior art;
[0057] FIG. 2: a cross-sectional diagram of a first exemplary embodiment of the magnetic-inductive flow meter according to the invention;
[0058] FIG. 3: a cross-sectional diagram of a second exemplary embodiment of the magnetic-inductive flow meter according to the invention;
[0059] FIG. 4: two graphs, wherein the first graph shows the functions ƒ.sub.t and ƒ.sub.2 as a function of the Reynolds number, and the second graph shows the quotient g of the two functions ƒ.sub.1 and ƒ.sub.2 as a function of the Reynolds number; and
[0060] FIG. 5: a flow diagram of an exemplary embodiment of the method for operating the magnetic-inductive flow meter.
[0061] FIG. 1 shows a magnetic-inductive flow meter known from the prior art. The structure and measuring principle of a magnetic-inductive flow meter are known, in principle. A medium having an electrical conductivity is conducted through a measuring tube (1). A magnetic-field-generating device (7) is attached in such a way that the magnetic-field lines are oriented to be substantially perpendicular to a longitudinal direction defined by the measuring tube axis. A saddle coil or a pole shoe (10) with mounted coil and coil core is preferably suitable as a magnetic-field-generating device (7). When the magnetic field is applied, a flow-dependent potential distribution arises in the measuring tube (1), which potential distribution is tapped with two, opposing measuring electrodes (3, 4) attached to the inner wall of the measuring tube (1). As a rule, these are arranged diametrically and form an electrode axis, or are intersected by a transverse axis which runs perpendicular to the magnetic-field lines and the longitudinal axis of the tube. On the basis of the measured measuring voltage U, taking into account the magnetic flux density, the flow rate can be determined, and, additionally taking into account the tube cross-sectional area, the volumetric flow of the medium can be determined. In order to prevent the discharge of the measuring voltage present at the first and second measuring electrodes (3, 4) via the pipe (8), the inner wall is lined with an insulating material—for example, a plastic liner (2). The magnetic field produced by a magnetic-field-generating device (7), e.g., an electromagnet, is generated by a direct current of alternating polarity clocked by means of an operating circuit. This ensures a stable zero-point and makes the measurement insensitive to influences caused by electrochemical interference. A measuring circuit is designed to read out the measuring voltage present at the first and second measuring electrodes (3, 4), and an analysis circuit is designed to ascertain the flow rate and/or the volumetric flow of the medium. Commercially available magnetic-inductive flow meters have two further electrodes (5, 6) in addition to the measuring electrodes (3, 4). For one, a fill-level monitoring electrode (5), optimally attached to the highest point in the pipe (8), is used to detect a partial filling of the measuring tube (1) and is designed to forward this information to the user and/or to take into account the fill-level in the determination of the volumetric flow. Furthermore, a reference electrode (6), which is usually attached diametrically to the fill-level monitoring electrode (5) or to the lowest point of the tube cross-section, is used to ensure sufficient grounding of the medium.
[0062] Magnetic-inductive flow meters according to the prior art are optimized according to their measuring electrode positioning and configuration of the magnetic-field-generating device in such a way that the flow meter is linear, i.e., that the correction factor ƒ (Re) is substantially constant for a specified measurement range. Thus, the following simplification applies in a first approximation:
U=ƒ.Math.S.Math.u,
where ƒ is assumed to be constant for a Reynolds-number range, and S is determined via a calibration, i.e., measured in a known measuring environment and then stored in the meter for determining the flow rate and/or the volumetric flow.
[0063] FIG. 2 shows a schematic cross-section of a first exemplary embodiment of the flow meter according to the invention. The first and second measuring electrodes (3, 4) are arranged diametrically and are adapted to the magnetic-field-generating device in such a way that the flow meter is linear over the specified Reynolds-number range. In addition to the first and second measuring electrodes (3, 4), a third measuring electrode (11) is arranged in the measuring tube (1). A second radius (14), intersecting the third measuring electrode (11), and a transverse axis (15) of the measuring tube span a central angle α. The measuring circuit (16) is designed such that it taps a first potential difference U.sub.1 between the first and second measuring electrodes (3, 4) and a second potential difference U.sub.2 between the first and third measuring electrodes (3, 11), where U.sub.1=ƒ.sub.1(Re).Math.S.sub.1.Math.u and U.sub.2=ƒ.sub.2(Re).Math.S.sub.2.Math.u, where ƒ.sub.1(Re) and ƒ.sub.2(Re) each describe a correction factor dependent upon the Reynolds number. The position of the third measuring electrode (11) or the central angle α is optimized in such a way that the quotient of the first and second potential differences U.sub.1/U.sub.2 behaves bijectively to the Reynolds number of the flowing medium in the measuring tube, or that a mathematical function which maps the Reynolds number to the quotient is bijective. The arrangement can be optimized experimentally or by means of a simulation method—for example, by means of finite element simulations.
[0064] For the quotient U.sub.1/U.sub.2, there results:
[00004]
[0065] If g(Re) is invertible, the following also applies:
[00005]
where g.sup.−1 is the inverse function of g. The bijectivity of the quotient can most easily be realized by attaching the first and second measuring electrodes in the measuring tube in such a way that the first correction factor ƒ.sub.1(Re) is independent of the Reynolds number over the Reynolds-number range. In this case, the second correction factor ƒ.sub.2 must correspond bijectively to the Reynolds number.
[0066] The measuring circuit (16) is designed to tap a potential difference between the first and second measuring electrodes (3, 4) and a potential difference at the first and third measuring electrodes (3, 11) or to measure an electrical potential at the third measuring electrode in relation to a reference potential. The measurement data are forwarded to an analysis unit, which comprises a memory unit in which reference values and Reynolds numbers are stored. The analysis circuit is designed to determine the Reynolds number of the medium in the measuring tube from the measured measurement data and the stored reference data. If the Reynolds number is known, the kinematic viscosity can be calculated with the aid of the measured values of the first or the second measurement variable, or of the already-determined flow rate or of the volumetric flow. The measuring circuit, analysis circuit, and memory unit can be arranged on an electronics unit in a different way from that depicted in the schematic diagram.
[0067] FIG. 3 shows a schematic cross-section of a second embodiment of the flow meter according to the invention. The first and second measuring electrodes (3, 4), which form a first electrode pair, are intersected by a first measuring electrode axis (17) which runs parallel to a second measuring electrode axis (18), intersecting the third measuring electrode (11) and a further, fourth measuring electrode (12), and to the transverse axis (15). The third and fourth measuring electrodes (11, 12) form a second measuring electrode pair. A first radius (13), intersecting the second measuring electrode (4), and the transverse axis (15) span a central angle β. The second radius (14), intersecting the third measuring electrode (11), and the transverse axis (15) span a central angle γ. The measuring circuit (16) is designed to tap the first potential difference at the first measuring electrode pair and the second potential difference at the second measuring electrode pair. Both central angles β and γ are selected or optimized in such a way that, in a test measurement, the quotient of the first and second potential differences U.sub.1/U.sub.2 or of the measured values and the Reynolds number of the flowing medium correspond bijectively. In the simplest case, the central angle β is set to zero, and the central angle γ is adjusted until the aforementioned condition is fulfilled, especially fulfills the condition for the Reynolds-number range 10,000≤Re≤1,000,000.
[0068] FIG. 4 shows two diagrams, wherein the first diagram shows a relationship between the individual correction factors ƒ.sub.1, ƒ.sub.2 and the Reynolds number of the flowing medium in the measuring tube, and the second diagram shows a relationship between the quotients of the correction factors g and the Reynolds number of the flowing medium in the measuring tube. The two diagrams are limited to a Reynolds-number range of approximately 10.sup.3 to 10.sup.7. The correction factors ƒ.sub.1 and ƒ.sub.2 are each linked to one of the two potential differences tapped by different measuring electrode pairs. The curve of the functions ƒ.sub.1 and ƒ.sub.2 has three ranges (I, II, III). In the first and third ranges (I, III), the curve of ƒ.sub.1 is not constant. In this example, the curve has a negative slope in the first range (I) and a positive slope in the third range (III). In contrast, the curve of ƒ.sub.1 is constant in the second range (II). The flow meter is linear for this Reynolds-number range. The second function ƒ.sub.2 is bijective, at least in the second range. In the example shown, the curve of ƒ.sub.2 is also bijective in the first and third ranges (I, III). For the quotient g, this means that it is bijective in ranges one through three (I, II, III). A Reynolds number can thus be unambiguously assigned to each quotient of the measurement data of the two measurement variables. Measurement deviations can be corrected, taking into account the correction function, for the ranges in which the flow rate is sensitive to Reynolds number changes (see ranges I and III).
[0069] FIG. 5 shows a flow diagram of an embodiment of the method for operating a magnetic-inductive flow meter. In a first step, the first potential difference U.sub.1 is measured at the first measuring electrode pair. In a second step, the second potential difference U.sub.2 is measured at the second measuring electrode pair. As an alternative to measuring the potential difference, in both aforementioned steps, the potentials at the respective measuring electrodes can also be measured in relation to the reference potential, and the difference, e.g., in the analysis circuit, can be calculated. The two first steps do not have to be carried out subsequently, but can also take place simultaneously. It is also possible for the second potential difference U.sub.2 to be measured first, and then the first potential difference U.sub.1. However, measuring voltages of two measuring phases, in which different, especially opposite, direct voltages are respectively applied to the coils and in which the magnetic field has been incorporated, are usually taken into account for determining the flow rate or the volumetric flow. An offset in the measuring voltage can be compensated for thereby. The measurement of the potential difference or the potentials takes place via a measuring circuit. The analysis circuit calculates a quotient of the two measured values, especially potential differences, and compares this quotient with a Reynolds number assigned to this ascertained quotient. This Reynolds number is stored in a memory. Alternatively, a mathematical equation or a mathematical function which assigns a Reynolds number or a Reynolds-number range to a quotient can also be stored in the memory. Alternatively, data that have been ascertained in a calibration process can also be stored in the memory. The data can be the reference values measured in the calibration process, but also extrapolated values or values of a smoothed characteristic curve or fit function of the measurement data, for example. The reference values can, in a calibration process, be determined experimentally or by means of a simulation program.
LIST OF REFERENCE SIGNS
[0070] 1 Measuring tube [0071] 2 Liner [0072] 3 First measuring electrode [0073] 4 Second measuring electrode [0074] 5 Fill-level monitoring electrode [0075] 6 Reference electrode [0076] 7 Magnetic-field-generating device [0077] 8 Pipe [0078] 9 Measuring, operating, and/or analysis circuit [0079] 10 Pole shoe [0080] 11 Third measuring electrode [0081] 12 Fourth measuring electrode [0082] 13 First radius [0083] 14 Second radius [0084] 15 Transverse axis [0085] 16 Measuring circuit [0086] 17 First straight line [0087] 18 Second straight line [0088] 19 Analysis circuit [0089] 20 Memory unit [0090] 21 Coil [0091] I First range [0092] II Second range [0093] Ill Third range
