The magnetic-inductive flowmeter and method for operating a magnetic-inductive flowmeter

Abstract

Magnetic-inductive flowmeter, including: - a measuring tube (1) for guiding a flowable medium, wherein the measuring tube (1) has a wall; - at least three measuring electrodes (3, 4, 11, 12) arranged in the wall, each to form a galvanic contact with the flowing medium; - at least one magnetic field-generating device (7) for generating a magnetic field passing through the measuring tube, and - a measuring circuit (16) which is designed to determine at least one first measured quantity and one second measured quantity, where measured values ​​of the first measured quantity are determined at a first pair of measuring electrodes, wherein measured values ​​of the second measured quantity are determined at a second pair of measuring electrodes or at a third measuring electrode (11) with respect to a reference potential, characterized in that that an evaluation circuit (19) is set up to determine a Reynolds number and / or a kinematic viscosity value of the medium in the measuring tube (1) using measured values ​​of the first measured quantity and the second measured quantity, which differs from the first measured quantity, wherein the measuring electrodes (3, 4, 11, 12) are arranged in the measuring tube (1) such that quotients of current measured values ​​of the first and second measured quantity correspond bijectively to the Reynolds number of the flowing medium in the measuring tube (1) at least in a Reynolds number range of 10 000 ≤ Re ≤ 100 000.

Description

[0001] Magnetic-inductive flowmeters are used to determine the flow velocity and volumetric flow rate of a fluid in a pipeline. A magnetic-inductive flowmeter has a magnetic system that generates a magnetic field perpendicular to the flow direction of the fluid. This is typically achieved using one or more coils. To create a largely homogeneous magnetic field, pole pieces are shaped and attached so that the magnetic field lines run essentially perpendicular to the transverse axis or parallel to the vertical axis of the measuring tube across the entire cross-section. A pair of measuring electrodes attached to the outer surface of the measuring tube detects an electrical voltage or potential difference perpendicular to both the flow direction and the magnetic field. This voltage or potential difference arises when a conductive fluid flows in the direction of flow with the magnetic field applied.Since the measured voltage depends on the velocity of the flowing medium according to Faraday's law of induction, the flow velocity u and, with the addition of a known pipe cross-section, the volumetric flow rate V can be determined from the induced measuring voltage U. The simplified equation applies to the measuring voltage U. U=f(Re)⋅u⋅S, where S is a nominal signal strength dependent on the sensor geometry and magnetic field, and f(Re) is a correction factor dependent on the flow profile or Reynolds number. The correction factor f(Re) is usually assumed to be constant, but this is not always the case. The nominal signal strength S is typically determined during the calibration of the measuring device and is stored within the device.

[0002] The Reynolds number of a flowing medium in a measuring tube is defined by Re=ρ⋅u⋅DNμ=u⋅DNv, with the diameter of the measuring tube DN, the density of the medium ρ, the dynamic viscosity µ and the kinematic viscosity v of the medium.

[0003] Magnetic-inductive flowmeters are sensitive to the Reynolds number of the medium in the measuring tube, as this determines the flow profile. Depending on the pipe system, measuring device, and installation scenario, the correction factor f(Re) can vary and deviate by several percent from the assumed constant value. Typically, the arrangement of the measuring electrodes and the magnetic field-generating device is optimized to ensure that the flowmeter is as linear as possible, meaning that the induced measuring voltage is independent of the Reynolds number over the widest possible range, or even for the Reynolds number range relevant to the specific application. Thus, flowmeters exhibiting deviations of a few percent in transitional flow regimes and flowmeters with deviations of approximately 0.2% for turbulent flows are already industry standard.

[0004] In conventional magnetic-inductive flowmeters, the flow velocity and the Reynolds number-dependent correction factor f(Re) cannot be determined from the measuring voltage induced in the medium and measured by the electrodes. Therefore, f(Re) is assumed to be constant. It is thus essential to adapt the electrode system, the magnet system, and the pipe geometry of the magnetic-inductive flowmeter so that f(Re) remains constant over the widest possible Reynolds number range. This adaptation always involves limitations, such as a loss of signal strength or dependence on the pole shoe geometry.

[0005] US2008127712A1 discloses a method for in-situ verification testing of electromagnetic flow meters, in which additional electrodes are used alongside the conventional measuring electrodes to inject a test current and detect the resulting "virtual voltages." These additional electrodes serve as separate injection and measurement elements, enabling the precise detection of changes in the virtual current distribution—for example, caused by deposits or alterations to the inner pipe wall. This extended electrode arrangement allows for increased sensitivity to changes in the measuring pipe's condition without requiring the removal of the flow meter or interruption of the process.

[0006] JP S61-75 217 A discloses a correction device for flow meters that determines the Reynolds number in real time from the measured flow signal, the set pipe diameter, and the kinematic viscosity of the medium. By continuously calculating the Reynolds number, the relevant meter constant is determined and used to correct the flow signal, particularly at low flow velocities. This enables more precise compensation of flow-dependent measurement errors and ensures accurate flow values ​​even with varying medium viscosity.

[0007] EP 0 770 855 A1 describes a magnetic-inductive flowmeter for measuring non-Newtonian fluids, which includes evaluation electronics designed to determine the flow index and apparent viscosity of a flowing medium. Two measuring electrodes are arranged in the measuring tube such that the radii intersecting the respective measuring electrodes form angles of 90° and 120°, respectively. A switch is provided to connect two coils in series, either in the same direction or in opposite directions. The potential difference between the two measuring electrodes is determined sequentially for each coil setting and used to determine the flow index and apparent viscosity of the medium.

[0008] It is therefore known in principle from the prior art to enable the determination of additional flow properties by modifying the magnetic field-generating device and the measuring electrode arrangement. However, a disadvantage of the known prior art is that determining the flow index always requires at least two measurement phases with different coil settings, in which the magnetic field must be re-stabilized before a measurement can be taken.

[0009] The invention is therefore based on the objective of providing a magnetic-inductive flow meter and a method for operating the magnetic-inductive flow meter that requires only one coil setting.

[0010] The problem is solved by the magnetic-inductive flow meter according to claim 1 and by the method for operating the magnetic-inductive flow meter according to claim 7.

[0011] The magnetic-inductive flowmeter according to the invention comprises a measuring tube for guiding a flowable medium, wherein the measuring tube has a wall, at least three measuring electrodes arranged in the wall and forming a galvanic contact with the flowing medium, at least one magnetic field-generating device for generating a magnetic field passing through the measuring tube, and a measuring circuit configured to determine at least a first measured quantity and a second measured quantity, wherein measured values ​​of the first measured quantity are determined at a first pair of measuring electrodes, and wherein measured values ​​of the second measured quantity are determined at a second pair of measuring electrodes or at a third measuring electrode with respect to a reference potential, characterized in thatthat an evaluation circuit is set up to determine a Reynolds number and / or a kinematic viscosity value of the medium in the measuring tube using measured values ​​of the first measured quantity and the second measured quantity, which differs from the first measured quantity.

[0012] It is particularly advantageous if the flow meter has at least three measuring electrodes. This allows the meter to be configured so that the Reynolds number measured at the first pair of electrodes is independent, while the Reynolds number measured at the second pair of electrodes or at the third electrode relative to the reference potential is dependent. If both measured values ​​are known, the Reynolds number can be determined. Switching the coil settings is unnecessary, and the Reynolds number can be determined in a single measurement.

[0013] The measuring circuit is preferably configured to determine a first potential difference U1 between a first pair of measuring electrodes and a second potential difference U2 between a second pair of measuring electrodes. The measured voltage is determined either by directly measuring the potential difference between the two measuring electrodes or by measuring the electrical potential prevailing at each measuring electrode with respect to a reference potential and calculating the difference. Here, U1 = f1(Re) · S1 · u and U2 = f2(Re) · S2 · u, where f1(Re) and f2(Re) each represent a Reynolds number-dependent correction factor. The measured values ​​are forwarded to an evaluation circuit that includes a storage unit containing reference values ​​and Reynolds numbers, or a mathematical function that assigns Reynolds numbers to reference values.

[0014] Alternatively, the first potential difference U1 is measured at the first pair of measuring electrodes and an electrical potential relative to a reference potential is measured at the third measuring electrode.

[0015] It is particularly advantageous if the evaluation circuit is configured to calculate quotients from the measured values, compare these with the reference values ​​in the data memory, and determine or read the Reynolds number associated with the reference value. Alternatively, a mathematical function can be stored in the memory unit that assigns the measured values ​​of the first and second measured variables, or a term that depends on the first and second measured variables, to the Reynolds number of the medium. In this case, the evaluation circuit is configured to determine the Reynolds number using the measured values ​​of the two variables and the stored mathematical function. If the Reynolds number, the flow velocity, and the pipe diameter are known, the kinematic viscosity can be calculated, for which v = u · DN / Re applies.

[0016] Flow measurement circuits are well-known. Their function is to detect very small absolute values ​​and changes in the measured quantity. Numerous different configurations exist, each with its own advantages and disadvantages. For example, a circuit can be designed to measure the potential at one of the measuring electrodes relative to a reference potential. This allows the flow rate to be determined even if one of the two electrodes fails. Suitable reference potentials include the housing potential or ground potential. Alternatively, a circuit can be designed to detect and record the potential difference between two electrodes.A measuring circuit therefore typically includes an analog-to-digital converter that converts the incoming signals—in this case, the potential difference currently present at the respective pair of measuring electrodes—into digital data, which is then further processed or stored by an evaluation circuit. However, other digital measurement converters or transmitters are also known and suitable for digital measurement technology.

[0017] The evaluation circuitry is designed to process the measured values ​​acquired by the measuring circuitry and to determine the desired measured quantity from the noise. An evaluation circuitry therefore typically includes microprocessors, amplifiers, and noise filters. The measuring and evaluation circuitry can be modular and communicate via a wireless connection, or they can be part of a single measuring and evaluation electronics unit housed within the measuring instrument's enclosure.

[0018] According to the invention, the measuring electrodes are arranged in the measuring tube such that quotients of current measured values ​​of the first and second measured quantity correspond bijectively to the Reynolds number of the flowing medium in the measuring tube, at least in a Reynolds number range of 10,000 ≤ Re ≤ 100,000, in particular 5,000 ≤ Re ≤ 500,000 and preferably 1,000 ≤ Re ≤ 1,000,000.

[0019] The position of the third measuring electrode is optimized such that the ratio of the first and second potential differences U1 / U2 is bijective with respect to the Reynolds number of the flowing medium in the measuring tube. This optimization can be performed experimentally or using a simulation method, such as finite element simulations.

[0020] The quotient U1 / U2 is calculated assuming that the flow rate in the measuring tube is constant, or that the induced measuring voltage originates from a common flow rate u. U1U2=f1(Re)f2(Re)⋅S1S2=g(Re)⋅S1S2.

[0021] In the case that g(Re) is invertible, the following also holds: Re=g−1(U1⋅S2S1⋅U2), where g -1 The inverse function of g. The bijectivity of the quotient can be most easily achieved by positioning the first and second measuring electrodes in the measuring tube such that the first correction factor f1(Re) is independent of the Reynolds number over the entire Reynolds number range. In this case, the second correction factor f2 must correspond bijectively with the Reynolds number. Therefore, the position of the third measuring electrode is ideally chosen so that the change in the correction factor f2(Re), or the slope of the quotient, is as large as possible for different Reynolds numbers.

[0022] The positioning of the individual measuring electrodes in the measuring tube is crucial for determining the Reynolds number or the kinematic viscosity. Determining the Reynolds number requires measurements of two quantities. The first quantity is determined by the measuring circuit using a pair of measuring electrodes. Preferably, the same applies to the second quantity. However, the position of the individual measuring electrodes cannot be chosen arbitrarily. The ratio of the measured values ​​of the first and second quantities must be bijective with the Reynolds number of the flowing medium in the measuring tube. This means that the ratios of the measured values ​​of the first and second quantities, measured over a Reynolds number range, can be described by a function that is bijective with respect to the Reynolds number, or alternatively, that the set of ratios of the measured values ​​over a Reynolds number range corresponds bijectively to the associated set of Reynolds numbers.

[0023] This is achieved, for example, by attaching the first pair of measuring electrodes diametrically to the measuring tube, as is common with conventional flow meters, and arranging the third measuring electrode, or the third and fourth measuring electrodes, offset from the first pair of measuring electrodes.

[0024] Thus, the measured values ​​of the first measurand are essentially independent of the Reynolds number, and the measured values ​​of the second measurand are dependent on the Reynolds number. However, this is not the only way to implement a flow meter according to the invention. It is also conceivable that all measuring electrodes are arranged such that the measured values ​​taken at the measuring electrodes over the Reynolds number range or over a part of the Reynolds number range are dependent on the Reynolds number. In this case, however, the ratio of the measured values ​​must correspond bijectively to the Reynolds number so that the Reynolds number can be determined.

[0025] The evaluation circuit typically includes a storage unit, wherein the storage unit contains a first data set with reference values ​​that correlate with the first and second measured quantities and are in particular proportional to the quotient of the first and second measured quantities, wherein the storage unit contains a second data set with Reynolds numbers, wherein the first and second data sets correspond bijectively, and wherein the evaluation circuit is configured to determine the corresponding Reynolds number of the medium in the measuring tube based on the first and second measured quantities.

[0026] Suitable storage units include non-volatile memory such as flash memory or EPROM. These can be integrated into the evaluation circuitry or provided separately. The storage unit preferably contains at least one first and one second data record. The first data record contains reference values. These are derived from computer simulations or calibration measurements. The reference values ​​can, for example, be quotients of the simulated or measured values ​​of the first and second measured variables. The second data record contains Reynolds numbers corresponding to these quotients. Alternatively, pairs of a Reynolds number and a reference value can be stored in the storage unit. Another alternative is to store a mathematical function in the storage unit that assigns Reynolds numbers to the two measured variables or to a term that depends on the two measured variables.

[0027] The test measurement is carried out experimentally or using a simulation program.

[0028] Advantageous embodiments of the invention are the subject of the dependent claims.

[0029] According to a first embodiment, the measuring electrodes lie essentially in a cross-sectional plane, wherein a first radius intersecting the second measuring electrode and a second radius intersecting the third measuring electrode span an angle α with α ≥ 20°, in particular α ≥ 30° and preferably 40° ≤ α ≤ 60°, wherein the measuring circuit is configured to determine measured values ​​of the first measured quantity between the first and the second measuring electrode, and wherein the measuring circuit is configured to determine measured values ​​of the second measured quantity between the first and the third measuring electrode or at the third measuring electrode with respect to the reference potential.

[0030] It is advantageous for the flow meter to have exactly three measuring electrodes. This reduces the number of potential leakage points to three. This design can be most easily achieved by extending a conventional magnetic-inductive flow meter with two measuring electrodes by adding a third. By maintaining the required angles, a magnetic-inductive flow meter can thus be realized that fulfills the requirements for determining the Reynolds number of the medium.

[0031] According to a second embodiment, a first measuring electrode axis intersecting a first and a second measuring electrode and a second measuring electrode axis intersecting a third and a fourth measuring electrode run essentially parallel, wherein the measuring circuit is configured to determine measured values ​​of the first measured quantity between the first and the second measuring electrode, and wherein the measuring circuit is configured to determine measured values ​​of the second measured quantity between the third and the fourth measuring electrode.

[0032] In conventional magnetic-inductive flowmeters, the magnetic system and the position of the measuring electrodes are optimized so that the electrical potential at each electrode correlates linearly with the flow velocity. If a third measuring electrode is added and the potential difference between the third electrode and one of the two existing electrodes is measured, the influence of the linearized system is always included in the second measured quantity. This second quantity should, however, be Reynolds number-dependent over as wide a range of Reynolds numbers as possible.

[0033] It is therefore particularly advantageous if the measured values ​​of the second quantity are taken from the third and a fourth measuring electrode, which are arranged offset from the first pair of measuring electrodes. This also decouples the potential difference at the second pair of measuring electrodes from the two potentials of the first quantity.

[0034] It is particularly advantageous if the first pair of measuring electrodes is arranged diametrically or lying on the transverse axis of the measuring tube.

[0035] According to one embodiment, the first measured quantity and the second measured quantity are a potential difference between a pair of measuring electrodes, respectively.

[0036] To avoid common-mode interference and thus minimize noise in the signal, it is advantageous if the two measured quantities are not potentials relative to a reference potential, but rather potential differences that are measured.

[0037] According to one embodiment, the Reynolds number of the medium in the measuring tube is greater than 1000, in particular greater than 5000 and preferably greater than 10000, wherein the Reynolds number of the medium in the measuring tube is less than 1000000, in particular less than 500000 and preferably less than 100000.

[0038] According to one embodiment, the first measured quantity during the test measurement in a Reynolds number range of 10,000 ≤ Re ≤ 1,000,000 is essentially proportional to the flow rate of the medium, whereby the change in the second measured quantity during the test measurement in a Reynolds number range of 10,000 ≤ Re ≤ 1,000,000 is not constant with increasing Reynolds number.

[0039] At Reynolds numbers below 1000, the flow is in a transitional region between turbulent and laminar flow, and f(Re) can no longer be described by a defined function. f(Re) exhibits hysteresis or varies over time. At high Reynolds numbers, the flow profile is independent of the Reynolds number, and therefore f(Re) is constant for both measured quantities. In this case, the Reynolds number cannot be determined.

[0040] According to one embodiment, the medium in the test measurement is a Newtonian fluid, in particular water, wherein in the test measurement the flow meter is inserted in a pipeline with a straight inlet section of at least DN 20 and preferably at least DN 50 such that a substantially symmetrical flow profile is present in a measuring range, wherein the measuring pipe has a diameter of DN 80.

[0041] According to one embodiment, the magnetic field-generating device comprises two coils, in particular those mounted opposite each other, which are connected in series in the same direction.

[0042] Two opposing coils connected in series with the same polarity each generate a magnetic field that points predominantly in the same direction as the magnetic field generated by the other coil. The magnetic field lines forming between the two coils run essentially parallel to an axis of symmetry connecting them. Furthermore, current flows in the same direction through coils connected in series with the same polarity. EP 0 770 855 A1 describes the magnetic field distribution of coils connected in series with the same and opposite polarities.

[0043] The inventive method for operating the inventive magnetic-inductive flow meter comprises the following process steps: - Recording a measured value of a first measured quantity and a measured value of a second measured quantity, whereby the respective measured values ​​of the two measured quantities are determined at different pairs of measuring electrodes; - Determining a Reynolds number that depends on the measured values ​​of the first and second measured quantities.

[0044] The first and second measured values ​​are acquired using a measuring circuit. The acquired measurements are then evaluated in the evaluation circuit, and the Reynolds number of the medium is determined.

[0045] It is particularly advantageous to calculate a quotient of the first and second measured values ​​and compare this with stored reference values ​​in the memory unit. In this case, the reference values ​​can be quotients of measurement data previously acquired during the calibration process at the pairs of measuring electrodes. Alternatively, a mathematical function can be stored in the memory unit that describes the relationship between the Reynolds number and the two measured quantities, or the term dependent on both measured quantities. This mathematical function can, for example, be a polynomial, particularly a higher-order polynomial, derived from the measurement data obtained during calibration. If the measured quantities satisfy the aforementioned condition of bijectivity, the Reynolds number can be determined.

[0046] According to the invention, a mapping which assigns Reynolds numbers to quotients of the first and the second measured quantity is bijective at least in a Reynolds number range of 10,000 ≤ Re ≤ 100,000, in particular 5,000 ≤ Re ≤ 500,000 and preferably 1,000 ≤ Re ≤ 1,000,000.

[0047] Advantageous embodiments of the invention are the subject of the dependent claims.

[0048] According to one embodiment, the inventive method comprises the following process step: - Forming a corrected flow velocity and / or a corrected volumetric flow rate using a correction factor, where the correction factor depends on the determined Reynolds number.

[0049] It is particularly advantageous if correction factors, especially one of the two correction factors f1 and f2, are stored in the storage unit, which serve to determine the flow velocity and / or the volumetric flow rate more accurately. The correction factors can be determined in a simulation procedure or ascertained or measured during the adjustment procedure.

[0050] According to one embodiment, the inventive method comprises the following process step: - Determining a kinematic viscosity value, wherein the kinematic viscosity value is determined using measured values ​​of the first or the second measured quantity and the determined Reynolds number.

[0051] The measured values ​​are displayed, for example, on a screen attached to or connected to the flow meter. Alternatively, the display can be part of a smartphone or laptop and receive the measured values ​​from the evaluation circuitry via a wireless connection. Other output devices commonly used in process automation include data transmission systems such as fieldbuses or real-time Ethernet.

[0052] The invention is explained in more detail with reference to the following figures. They show: Fig. 1: A cross-sectional view of a state-of-the-art magnetic-inductive flowmeter; Fig. 2: a cross-sectional view of a first embodiment of the magnetic-inductive flowmeter according to the invention; Fig. 3: a cross-sectional view of a second embodiment of the magnetic-inductive flowmeter according to the invention; Fig. 4: two diagrams, the first diagram showing the functions f1 and f2 as a function of the Reynolds number and the second diagram showing the quotient g of the two functions f1 and f2 as a function of the Reynolds number; and Fig. 5: a flowchart of an exemplary embodiment of the method for operating the magnetic-inductive flowmeter.

[0053] The Fig. Figure 1 shows a magnetic-inductive flowmeter known from the prior art. The construction and measuring principle of a magnetic-inductive flowmeter are fundamentally known. A medium with electrical conductivity is passed through a measuring tube (1). A magnetic field-generating device (7) is positioned such that the magnetic field lines are oriented essentially perpendicular to a longitudinal direction defined by the axis of the measuring tube. A saddle coil or a pole shoe (10) with an attached coil and coil core is preferably suitable as the magnetic field-generating device (7). When a magnetic field is applied, a flow-dependent potential distribution is generated in the measuring tube (1), which is detected by two measuring electrodes (3, 4) mounted opposite each other on the inner wall of the measuring tube (1). These are generally arranged diametrically and form an electrode axis.The lines are intersected by a transverse axis perpendicular to the magnetic field lines and the longitudinal axis of the pipe. Based on the measured measuring voltage U, the flow velocity can be determined, taking into account the magnetic flux density, and, additionally considering the pipe's cross-sectional area, the volumetric flow rate of the medium can be determined. To prevent the measuring voltage applied to the first and second measuring electrodes (3, 4) from being conducted away through the pipe (8), the inner wall is lined with an insulating material, for example, a plastic liner (2). The magnetic field generated by a magnetic field-generating device (7), for example, an electromagnet, is produced by a direct current of alternating polarity, pulsed by an operating circuit. This ensures a stable zero point and makes the measurement insensitive to influences from electrochemical disturbances.A measuring circuit is configured to read the measuring voltage applied to the first and second measuring electrodes (3, 4), and an evaluation circuit is designed to determine the flow velocity and / or volumetric flow rate of the medium. Commercially available magnetic-inductive flowmeters have two additional electrodes (5, 6) besides the measuring electrodes (3, 4). Firstly, a level monitoring electrode (5), ideally located at the highest point in the pipe (8), serves to detect partial filling of the measuring tube (1) and is configured to transmit this information to the user and / or to take the fill level into account when determining the volumetric flow rate. Secondly, a reference electrode (6), which is usually located diametrically opposite the level monitoring electrode (5) or at the lowest point of the pipe cross-section, serves to ensure adequate grounding of the medium.

[0054] Prior art magnetic-inductive flowmeters are optimized, according to their measuring electrode positioning and the design of the magnetic field-generating device, such that the flowmeter is linear, i.e., that the correction factor f(Re) is essentially constant for a given measuring range. Thus, as a first approximation, the following simplification applies: U=f⋅S⋅u, where f is assumed to be constant for a Reynolds number range, and S is determined via an adjustment, i.e. measured in a known measurement environment and then stored in the measuring instrument for determining the flow velocity and / or volumetric flow rate.

[0055] The Fig. Figure 2 shows a schematic cross-section of a first embodiment of the flow meter according to the invention. The first and second measuring electrodes (3, 4) are arranged diametrically and adapted to the magnetic field-generating device such 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 of the measuring tube (15) define a central angle α. The measuring circuit (16) is designed such that it detects a first potential difference U1 between the first and the second measuring electrode (3, 4) and a second potential difference U2 between the first and the third measuring electrode (3, 11), with U1 = f1(Re) · S1 · u and U2 = f2(Re) · S2 · u, where f1(Re) and f2(Re) each describe a Reynolds number-dependent correction factor.The position of the third measuring electrode (11) and its central angle α are optimized such that the quotient of the first and second potential differences U1 / U2 is bijective with respect to the Reynolds number of the flowing medium in the measuring tube, or that a mathematical function mapping the Reynolds number to the quotient is bijective. The optimization of the arrangement can be carried out experimentally or by means of a simulation method, for example, using finite element simulations.

[0056] The quotient U1 / U2 is calculated as follows: U1U2=f1(Re)f2(Re)⋅S1S2=g(Re)⋅S1S2.

[0057] In the case that g(Re) is invertible, the following also holds: Re=g−1(U1⋅S2S1⋅U2), where g -1The inverse function of g. The bijectivity of the quotient can be most easily achieved by positioning the first and second measuring electrodes in the measuring tube such that the first correction factor f1(Re) is independent of the Reynolds number over the Reynolds number range. In this case, the second correction factor f2 must correspond bijectively with the Reynolds number.

[0058] The measuring circuit (16) is configured to detect a potential difference between the first and second measuring electrodes (3, 4) and a potential difference between the first and third measuring electrodes (3, 11), or to measure an electrical potential at the third measuring electrode relative to a reference potential. The measurement data are forwarded to an evaluation unit, which includes a storage unit containing reference values ​​and Reynolds numbers. The evaluation circuit is configured to determine the Reynolds number of the medium in the measuring tube from the measured data and the stored reference data. If the Reynolds number is known, the kinematic viscosity can be calculated by adding the measured values ​​of the first or second measured quantity, or the previously determined flow rate or volumetric flow rate.The measuring circuit, evaluation circuit and storage unit can be arranged on an electronic unit, unlike in the schematic representation.

[0059] The Fig. Figure 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) that runs parallel to a second measuring electrode axis (18) and the transverse axis (15), which intersects the third measuring electrode (11) and a further, fourth measuring electrode (12). The third and fourth measuring electrodes (11, 12) form a second measuring electrode pair. A first radius (13), which intersects the second measuring electrode (4), and the transverse axis (15) form a central angle β. The second radius (14), which intersects the third measuring electrode (11), and the transverse axis (15) form a central angle γ. The measuring circuit (16) is configured to detect 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 chosen or optimized such that, in a test measurement, the quotient of the first and second potential differences U1 / U2, or 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 met, in particular the condition for the Reynolds number range 10,000 ≤ Re ≤ 1,000,000.

[0060] The Fig. Figure 4 shows two diagrams. The first diagram illustrates the relationship between the individual correction factors f1 and f2 and the Reynolds number of the flowing medium in the measuring tube, while the second diagram illustrates the relationship between the quotients of the correction factors g and the Reynolds number of the flowing medium in the measuring tube. Both diagrams are limited to a Reynolds number range of approximately 10 3 up to 107The correction factors f1 and f2 are each linked to one of the two potential differences measured by different pairs of electrodes. The function f1 and f2 exhibits three regions (I, II, III). In the first and third regions (I, III), the function f1 is not constant. In this example, the function has a negative slope in the first region (I) and a positive slope in the third region (III). In the second region (II), however, the function f1 is constant. The flow meter is linear for this Reynolds number range. The second function f2 is bijective at least in the second region. In the illustrated example, the function f2 is also bijective in the first and third regions (I, III). Therefore, the quotient g is bijective in regions one through three (I, II, III). Thus, each quotient of the measured data of the two quantities can be uniquely assigned a Reynolds number.For the areas where the flow rate is sensitive to changes in the Reynolds number (see areas I and III), measurement deviations can be corrected by taking the correction function into account.

[0061] The Fig.Figure 5 shows a flowchart of one embodiment of the method for operating a magnetic-inductive flowmeter. In a first step, the first potential difference U1 is measured at the first pair of measuring electrodes. In a second step, the second potential difference U2 is measured at the second pair of measuring electrodes. Alternatively, instead of measuring the potential difference, the potentials at the respective measuring electrodes can also be measured relative to the reference potential in both of the aforementioned steps, and the difference can be calculated, for example, in the evaluation circuit. The first two steps do not have to be performed sequentially but can also be carried out simultaneously. The second potential difference U2 can also be measured first, followed by the first potential difference U1.Typically, however, the measurement voltages of two measurement phases, in which different, especially opposing, DC voltages are applied to the coils and in which the magnetic field has stabilized, are used to determine the flow velocity or volumetric flow rate. This allows for compensation of any offset in the measurement voltage. The potential difference or potentials are measured using a measuring circuit. The evaluation circuit calculates a quotient of the two measured values, especially potential differences, and compares this quotient with a Reynolds number assigned to it. This value is stored in memory. Alternatively, the memory can also contain a mathematical equation or function that assigns a Reynolds number or a range of Reynolds numbers to a quotient.

[0062] Alternatively, the memory can also contain data determined through a calibration procedure. This data can be the reference values ​​measured during the calibration process, but also extrapolated values ​​or values ​​from, for example, a smoothed characteristic curve or a fit function of the measurement data. The reference values ​​can be determined experimentally in a calibration procedure or using a simulation program. Reference symbol list 1 measuring tube 2 liners 3 first measuring electrode 4 second measuring electrode 5 Level monitoring electrode 6 Reference electrode 7 magnetic field generating device 8 pipe 9 Measuring, operating and / or evaluation circuit 10 pole shoe 11 third measuring electrode 12 fourth measuring electrode 13 first radius 14 second radius 15 Transverse axis 16 Measuring circuit 17 first straight 18 second straight 19 Evaluation circuit 20 storage units 21 coil I first area II second area III third area

Claims

[1] Magnetic-inductive flowmeter comprising: - a measuring tube (1) for guiding a flowable medium, wherein the measuring tube (1) has a wall; - at least three measuring electrodes (3, 4, 11, 12) arranged in the wall, each to form a galvanic contact with the flowing medium; - at least one magnetic field-generating device (7) for generating a magnetic field passing through the measuring tube, and - a measuring circuit (16) which is designed to determine at least one first measured quantity and one second measured quantity, where measured values ​​of the first measured quantity are determined at a first pair of measuring electrodes, wherein measured values ​​of the second measured quantity are determined at a second pair of measuring electrodes or at a third measuring electrode (11) with respect to a reference potential, characterized by , that an evaluation circuit (19) is set up to determine a Reynolds number and / or a kinematic viscosity value of the medium in the measuring tube (1) using measured values ​​of the first measured quantity and the second measured quantity, which differs from the first measured quantity, wherein the measuring electrodes (3, 4, 11, 12) are arranged in the measuring tube (1) such that quotients of current measured values ​​of the first and second measured quantity correspond bijectively to the Reynolds number of the flowing medium in the measuring tube (1) at least in a Reynolds number range of 10 000 ≤ Re ≤ 100 000. [2] Flow meter according to claim 1, wherein the measuring electrodes (3, 4, 11, 12) lie essentially in a cross-sectional plane, wherein a first radius (13) intersecting the second measuring electrode (4) and a second radius (14) intersecting the third measuring electrode (11) span an angle α with α ≥ 20°, in particular α ≥ 30° and preferably 40°≤α≤60°, wherein the measuring circuit (16) is configured to determine measured values ​​of the first measured quantity between the first and the second measuring electrode (3, 4), wherein the measuring circuit (16) is set up to determine measured values ​​of the second measured quantity between the first and the third measuring electrode (3, 11) or at the third measuring electrode (11) with respect to a reference potential. [3] Flow meter according to claim 1, wherein a first measuring electrode axis (17) intersecting a first and a second measuring electrode (3, 4) and a second measuring electrode axis (18) intersecting a third and a fourth measuring electrode (11, 12) are essentially parallel, wherein the measuring circuit (16) is configured to determine measured values ​​of the first measured quantity between the first and the second measuring electrode (3, 4), wherein the measuring circuit (16) is set up to determine measured values ​​of the second measured quantity between the third and the fourth measuring electrode (11, 12). [4] Flow meter according to any one of the preceding claims, where the first measured quantity in a test measurement within a Reynolds number range of 10,000 ≤ Re ≤ 1,000,000 is essentially proportional to the flow rate of the medium, where the change in the second measured quantity during the test measurement is not constant within a Reynolds number range of 10,000 ≤ Re ≤ 1,000,000 as the Reynolds number increases. [5] Flow meter according to claim 4, where in the test measurement the medium is a Newtonian fluid, in particular water, wherein in the test measurement the flow meter is inserted in a pipeline with a straight inlet section of at least 20 DN and preferably at least 50 DN in such a way that a substantially symmetrical flow profile is present in a measuring range, wherein the measuring tube (1) has a diameter of DN 80. [6] Flow meter according to one of the preceding claims, wherein the magnetic field generating device (7) comprises two coils (21) which are arranged in series in the same direction, in particular opposite each other. [7] Method for operating a magnetic-inductive flowmeter according to one of the preceding claims, comprising the method steps: - Recording a measured value of a first measured quantity and a measured value of a second measured quantity, where the respective measured values ​​of the two quantities are determined at different pairs of measuring electrodes, where a mapping which assigns Reynolds numbers to quotients of the first and second measured quantities is bijective at least in a Reynolds number range of 10,000 ≤ Re ≤ 100,000; - Determining a Reynolds number that depends on the measured values ​​of the first and second measured quantities. [8] Method according to claim 7, comprising the method steps: - Calculating a corrected flow velocity and / or a corrected volumetric flow rate using a correction factor, the correction factor depends on the determined Reynolds number. [9] Method according to claim 7 and / or 8, comprising the method steps: - Determining a kinematic viscosity value, where the kinematic viscosity value is determined using measured values ​​of the first or the second measured quantity and the determined Reynolds number.

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