Method for detecting a sediment in a magnetic-inductive flowmeter

Abstract

The invention relates to a method for detecting a sediment (21) in a magnetic-inductive flowmeter (1), wherein the magnetic-inductive flowmeter comprises: -- a magnetic-field-generating device (7) for generating a magnetic field, the magnetic field having a main axis (HA); -- a measuring tube (8) for conveying a medium, the medium comprising a liquid having solid particles distributed therein, the solid particles forming the sediment, wherein a first longitudinal plane (LE1) intersecting the measuring tube (8) intersects the main axis perpendicularly, wherein the first longitudinal plane (LE1) divides the measuring tube (8) into a first and a second partial section (TA1, TA2); and -- at least three electrodes, a first electrode (E1) of the at least three electrodes being arranged in the first partial section (TA1) such that its longitudinal axis (LA) lies in the first longitudinal plane (LE1), and a second electrode (E2) of the at least three electrodes being arranged in the second partial section (TA2), wherein the method comprises the steps of: - supplying an excitation signal to one of the at least three electrodes, in particular to the first electrode (E1); - tapping a first measurement signal at the first electrode (E1) relative to the second electrode (E2); - supplying a further excitation signal to one of the at least three electrodes, in particular to a third electrode (E3); - tapping a second measurement signal at the third electrode (E3) of the at least three electrodes relative to the second electrode (E2), wherein the third electrode (E3) is arranged in the first partial section (TA1) such that its longitudinal axis (LA) lies in the first longitudinal plane (LE1), wherein the first and second measurement signals are independent of a flow velocity of the medium; and - determining a sediment index taking into account the first and / or the second measurement signal.

Description

[0001] Method for detecting sediment in a magnetic-inductive flowmeter

[0002] The invention relates to a method for detecting sediment in a magnetic-inductive flowmeter and a magnetic-inductive flowmeter.

[0003] Magnetic-inductive flowmeters are used to determine the flow velocity and / or volumetric flow rate of a free-flowing, electrically conductive medium in a measuring tube. A magnetic-inductive flowmeter always includes a magnetic field-generating device designed to produce a magnetic field perpendicular to the horizontal axis of the measuring tube. This is typically achieved using one or more magnetic coils. To create a largely homogeneous magnetic field, pole pieces can be shaped and attached so that the magnetic field lines run essentially perpendicular to the axis across the entire cross-section of the tube. A pair of measuring electrodes attached to the outer surface of the measuring tube detects an inductively generated electrical voltage that arises when a conductive medium flows along the longitudinal axis of the measuring tube while a magnetic field is applied.Since the measured voltage depends on the velocity of the flowing fluid according to Faraday's law of induction, the flow velocity and, with the addition of a known pipe cross-sectional area, the volumetric flow rate of the medium can be determined from the measured voltage.

[0004] Level monitoring systems for flow meters are already known. In these systems, a so-called level monitoring electrode, or MME electrode (measuring fluid monitoring electrode), is positioned at the highest point of the measuring tube when it is installed horizontally, relative to the tube's cross-section. The fill level of the measuring tube is monitored by determining the conductivity of the medium between the MME electrode and the reference electrode, a metallic process connection, or one of at least two measuring electrodes. The medium always has a higher conductivity than air. If the conductivity falls below a predefined limit, partial filling is detected. This limit depends, among other things, on the specific conductivity of the medium, the nominal diameter of the measuring tube, the dimensions and position of the MME electrode, and the properties of the measuring tube lining.

[0005] One problem when conveying aqueous slurries (a mixture of liquid and solids dispersed within it, such as sand, gravel, and rock) in a pipeline is the settling of the solids at low flow rates. This leads to sedimentation.

[0006] German patent DE 10 2006 033 112 A1 discloses a method for operating a magnetic-inductive flowmeter in which a signal in the form of a current or voltage is provided at the first measuring electrode, and impedances, voltages, or currents are measured at the second measuring electrode. By comparing the measured values ​​with reference values, the presence of deposits can be determined. However, applying and tapping the signal at the measuring electrodes significantly disrupts the flow measurement.

[0007] WO 2010 / 121 908 A1 discloses a method for operating a magnetic-inductive flow meter, which includes a diagnostic mode in which a voltage is applied between the first and second measuring electrodes. This induces a current within the measuring tube, which in turn creates a potential difference at the level monitoring and reference electrodes. This potential difference can be measured and used to determine the state of the flow meter. A disadvantage of this method, however, is that the flow measurement must be interrupted for the diagnostic mode.

[0008] German patent DE 10 2018 132 058 A1 discloses a method for operating a magnetic-inductive flowmeter with which sediment in the measuring tube can be detected. For this purpose, measurement signals between at least two different electrode pairs (level monitoring electrode and measuring electrode-reference electrode) are determined, and the transfer function is calculated from these signals. Based on the determined transfer functions, the degree of sedimentation can then be determined. The basic idea is to compare measurement signals measured along a path that runs exclusively through a liquid portion of the medium with measurement signals measured along a path that passes through the sediment.

[0009] The invention is based on the objective of further developing the solution known from the prior art.

[0010] The problem is solved by the method according to claim 1 and the magnetic-inductive flow meter according to claim 11.

[0011] The inventive method for detecting sediment in a magnetic-inductive flowmeter, wherein the magnetic-inductive flowmeter comprises:

[0012] - a magnetic field-generating device for generating a magnetic field, wherein the magnetic field has a principal axis;

[0013] - a measuring tube for guiding a medium, wherein the medium comprises a liquid with solids distributed therein, which solids form the sediment, wherein a first longitudinal plane intersecting the measuring tube intersects the principal axis perpendicularly, wherein the first longitudinal plane divides the measuring tube into a first and second subsection;

[0014] - at least three electrodes, wherein a first electrode of the at least three electrodes is arranged in the first subsection or is positioned such that its own longitudinal axis lies in the first longitudinal plane, wherein a second electrode of the at least three electrodes is arranged in the second subsection, wherein the method comprises the following steps:

[0015] - Providing an excitation signal at one of the at least three electrodes, in particular at the first electrode;

[0016] - Tapping off a first measurement signal at the first electrode against the second electrode;

[0017] - Providing an additional excitation signal at one of the at least three electrodes, in particular at one electrode;

[0018] - Tapping off a second measurement signal at one of the third electrodes of the at least three electrodes against the second electrode, wherein the third electrode is arranged in the first subsection or is positioned such that its own longitudinal axis lies in the first longitudinal plane, wherein the first and second measurement signals are independent of a flow velocity of the medium;

[0019] - Determining a sediment index taking into account the first and / or second measurement signal.

[0020] In the context of this application, sediment in the pipeline primarily consists of settled rock, gravel, or sand. These materials are electrically insulating, or rather, exhibit significantly lower conductivity than the liquid portion of the medium. The sediment itself comprises not only the electrically insulating solids but also the liquid of the medium, which is located on the surface of the solids and in enclosed chambers within the sediment. Therefore, the sediment is not completely electrically insulating. This means that the electrical potential drop in the liquid is less pronounced than in the sediment. This asymmetry can be used to characterize the sediment. Furthermore, the formation of sediment from settled rock or sand is not only a very rapid but also a reversible process, unlike biofilms. Therefore, early detection of its formation is particularly advantageous.Increasing the flow velocity of the medium can stop sediment formation and dissolve the sediment.

[0021] Advantageous embodiments of the invention are the subject of the dependent claims.

[0022] One design provides that the sediment index is a quantity that depends on the electrical conductivity of the liquid and the solids.

[0023] One design includes:

[0024] - Tapping a third measurement signal at a fourth electrode against the second electrode;

[0025] - Determining a comparative value from the first and second measurement signals,

[0026] - Determining a further comparative value from the second and third measurement signals, whereby the determination of the sediment index is carried out by selecting the sediment index from the comparative values ​​depending on a selection criterion.

[0027] Determining a second sediment index allows for the consideration of asymmetry regarding the deposition or rotation of the magnetic-inductive flowmeter along the measuring tube axis.

[0028] One design provides that the sediment index and / or the comparative values ​​are determined by means of quotient calculation.

[0029] One embodiment provides that the first electrode is shaped like a circular arc and has a circular arc length l, which is projected into a cross-section through the measuring tube at a central angle a, where for the central angle a 30° < a < 60° and in particular 40° < a < 50°.

[0030] One embodiment provides that a second longitudinal plane, perpendicular to the first longitudinal plane, divides the measuring tube into a third and fourth subsection, wherein the magnetic-inductive flowmeter includes a fifth electrode, wherein the first and fifth electrodes are arranged in the third subsection, wherein the second electrode is cut through the second longitudinal plane, wherein a circular sector with a central angle α is opened in a cross-section through the measuring tube, in which the first and fifth electrodes are arranged, wherein for the central angle α, 30° < α < 60° and, in particular, 40° < α < 50°, wherein at least the first and fifth electrodes are electrically short-circuited.

[0031] One embodiment provides that the magnetic-inductive flowmeter includes a sixth electrode, wherein the sixth electrode is arranged in the third subsection, and wherein the sixth electrode is short-circuited with the first and fifth electrodes.

[0032] One embodiment provides that the magnetic-inductive flow meter is arranged in a pipeline, with an earthing disc arranged between the pipeline and the magnetic-inductive flow meter.

[0033] One embodiment provides that an electrode axis intersecting the first and third electrodes intersects the first longitudinal plane. Another embodiment provides that the sediment index comprises a sediment height, a sediment fraction relative to an inner diameter, or a sediment fraction relative to an inner cross-section of the measuring tube.

[0034] The magnetic-inductive flowmeter according to the invention comprises:

[0035] - a magnetic field-generating device for generating a magnetic field, wherein the magnetic field has a principal axis;

[0036] - a measuring tube for guiding a medium, wherein a first longitudinal plane intersecting the measuring tube intersects the principal axis perpendicularly, the first longitudinal plane dividing the measuring tube into a first and second subsection;

[0037] - at least three electrodes, wherein a first electrode of the at least three electrodes is arranged in the first subsection or is positioned such that its own longitudinal axis lies in the first longitudinal plane, wherein a second electrode of the at least three electrodes is arranged in the second subsection,

[0038] - a measuring and / or evaluation circuit which is electrically connected to the at least three electrodes, wherein the measuring and / or evaluation circuit is configured to carry out the method according to the invention.

[0039] The invention is explained in more detail with reference to the following figures. They show:

[0040] Fig. 1: a cross-section through a magnetic-inductive flowmeter according to the state of the art;

[0041] Fig. 2: a cross-section through an embodiment of the magnetic-inductive flowmeter according to the invention;

[0042] Fig. 3: a cross-section through a design of a measuring electrode;

[0043] Fig. 4: a cross-section through a further embodiment of the magnetic-inductive flowmeter according to the invention;

[0044] Fig. 5: a graph with three examples of sediment index changes as a function of increasing sediment height; and

[0045] Fig. 6: a process diagram of an embodiment of the method according to the invention.

[0046] Fig. 1 shows a cross-section through a magnetic-inductive flowmeter 1 known from the prior art for determining a flow velocity-dependent measured quantity. The flow velocity-dependent measured quantity is typically the current flow velocity of the flowable and electrically conductive medium to be monitored. Alternatively, the flow velocity-dependent measured quantity can be a volumetric flow rate or – if the medium density is known – a mass flow rate.

[0047] The magnetic-inductive flowmeter 1 generally comprises a magnetic field-generating device 7 for generating a magnetic field. The generated magnetic field itself has an imaginary principal axis HA, which lies within an imaginary plane of symmetry of the generated magnetic field distribution and / or an imaginary plane of symmetry of the measuring tube 8. The magnetic field-generating device 7 can comprise at least one magnetic coil. Magnetic-inductive flowmeters 1 with two opposing magnetic coils are known. Alternatively or additionally, the magnetic field-generating device 7 can comprise a permanent magnet for generating a magnetic field. The at least one magnetic coil itself can be a cylindrical coil or a saddle coil. Furthermore, magnetic field-guiding components can be part of the magnetic field-generating device 7.The magnetic field-generating device 7 can thus have at least one coil core, at least one pole shoe and / or at least one field guidance component (e.g. a field guide plate) for guiding the magnetic field outside the medium.

[0048] The magnetic-inductive flowmeter 1 further comprises a measuring tube 8 for guiding the medium to be monitored. The measuring tube 8 typically comprises a metallic support tube whose inner surface is provided with an electrically insulating liner 2. The liner 2 is in contact with the medium. Alternatively, the measuring tube 8 can also directly have a support tube made of an electrically insulating material (e.g., plastic or ceramic), so that a liner 2 can be omitted. The measuring tube 8 can have a round cross-section, as shown, or—at least in the measuring section—a rectangular cross-section. To clearly locate the individual electrodes on the measuring tube 8, the measuring tube 8 is subsequently divided into imaginary subsections. This division is also applied to the embodiments according to the invention shown in Figures 2 to 4.Thus, there exists an imaginary horizontal first longitudinal plane LE1, which divides the measuring tube 8 into a first and second subsection TA1 and TA2. The first subsection TA1 is the upper half of the measuring tube 8, and the second subsection TA2 is the lower half. The two subsections TA1 and TA2 are essentially the same size. The first longitudinal plane LE1 is intersected perpendicularly by the principal axis HA.

[0049] In principle, a magnetic-inductive flowmeter 1 according to the prior art has at least two opposing measuring electrodes 3, 4. The at least two measuring electrodes 3, 4 are intersected by the imaginary longitudinal plane LE1. The illustrated prior art further shows a level monitoring electrode 5, which is arranged in the first subsection TA1, and a reference electrode 6, which is arranged in the second subsection. The reference electrode 6 is connected to an electrical reference potential. The function of a level monitoring electrode 5 includes the detection of the fill level in a measuring tube 8. Thus, it has also been possible, among other things, to measure the electrical resistance between the level monitoring electrode 5 and a reference electrode 6 or the process connection, which may be equipped, for example, with grounding discs or ground electrodes.The measured electrical resistance increases or the electrical conductivity decreases abruptly when the measuring tube 8 changes from a fully filled to a partially filled state. In this case, air would be present instead of the medium, at least in some areas, between the level monitoring electrode 5 and the reference electrode 6, so that the conductivity of the air would be included in the measured electrical resistance or conductivity. The corresponding change at the level monitoring electrode is detected by the measuring and / or evaluation circuit, and an output signal regarding the fill level of the measuring tube 8 is sent by an output unit (e.g., a display unit, a signal lamp, or a connector). The level monitoring electrode 5 is also referred to in the literature as an empty-pipe detection electrode, or EPD electrode for short.The function of a reference electrode 6 is to ensure potential equalization between the medium and the sensor (measuring tube, electrodes, etc.). The reference electrode 6 provides an alternative grounding option to the conventional grounding disc. Furthermore, the potential difference between measuring electrodes 3, 4, and the reference electrode 6 can be used to evaluate the flow rate, for example, for analyzing the flow profile. The reference electrode 6 is typically electrically connected to the housing (not shown) of the transmitter unit (not shown) and / or the pipeline (not shown). The housing is typically connected to the protective earth. Magnetic-inductive flowmeters are already available on the market whose reference electrode 6 is not connected to earth. This means that the reference electrode is not directly connected to an external reference potential.

[0050] All four electrodes 3-6 shown are electrically connected to a measuring and / or evaluation circuit 9. The measuring and / or evaluation circuit 9 is configured to measure a measuring voltage across the two measuring electrodes 3, 4 and to determine a flow velocity-dependent measured quantity from this. Furthermore, the measuring and / or evaluation circuit 9 is configured to perform a conductivity measurement between the level monitoring electrode 5 and one of the other electrodes (e.g., the reference electrode 6) in order to detect partial filling. The measuring and / or evaluation circuit 9 is also electrically connected to the magnetic field-generating device 7 and configured to provide an operating signal to the magnetic field-generating device 7. The operating signal can be a time-varying voltage signal.The measuring and / or evaluation circuit 9 can include at least one printed circuit board, a microprocessor, electronic components (e.g. electrical resistors, capacitors, logic electronic components, power supply, filters, etc.), conductor tracks and / or connectors.

[0051] Commercially available magnetic-inductive flowmeters 1 typically have a transmitter unit (not shown) with a display. The transmitter unit can be directly connected to the housing (not shown) that encloses the measuring tube 8. In this case, with the magnetic-inductive flowmeter in its usual mounting orientation, the transmitter unit would be located in the upper part of the flowmeter.

[0052] Alternatively, the transmitter unit can be arranged separately from the measuring tube 8 and housing, allowing the operator to position it almost anywhere from the pipeline. The measuring and / or evaluation circuit 9 or the magnetic field-generating device 7 and / or the electrodes 3-6 are then connected via cables to a sensor output (e.g., a connector) in the housing. The sensor output itself is then also connected, or can be connected, to the remote transmitter unit via a cable.

[0053] Fig. 2 shows a cross-section QS through an embodiment of the magnetic-inductive flowmeter 1 according to the invention, with four electrodes E1, E2, E3, E4. According to the invention, at least three electrodes are provided. A first electrode E1 of the at least three electrodes is arranged in the first section TA1 or positioned such that its own longitudinal axis LA lies in the first longitudinal plane LE1. As shown in the embodiment, the first electrode E1 can be a measuring electrode from which the flow-velocity-dependent measuring voltage is tapped or can be tapped during operation of the magnetic-inductive flowmeter. A second electrode E2 of the at least three electrodes is arranged in the second section TA2. In the embodiment shown in Fig. 2, the second electrode E2 is a reference electrode that is electrically connected to a reference potential 23.A third electrode E3 of the at least three electrodes is arranged in the first subsection TA1 or positioned such that its own longitudinal axis LA lies in the first longitudinal plane LE1. In the embodiment shown in Fig. 2, the third electrode E3 is a level monitoring electrode which – in conjunction with the measuring and / or evaluation circuit 9 – is configured to detect partial filling.

[0054] To improve clarity, the illustration of the magnetic field-generating device has been omitted. Fig. 2 further shows a measuring tube 8 filled with a medium. The medium consists of a liquid 20 containing dispersed solids. A typical example of such a medium is rock and / or sand in water. Due to their higher density compared to the liquid, the solids are deposited in the lower section (second section TA2) of the measuring tube 8 and form a sediment 21. The sediment 21 covers the second electrode E2. The method according to the invention takes advantage of the fact that the electrical conductivity of the sediment 21 (liquid-solid mixture) is many times lower than the electrical conductivity of the liquid 20.

[0055] The embodiment shown in Fig. 2 further includes a measuring and / or evaluation circuit 9, which is configured to provide an excitation signal to one of the at least three electrodes. The excitation signal can be an AC voltage signal with a frequency selected from a frequency range of 1 Hz to 10 kHz. Alternatively, the excitation signal can be a multi-frequency voltage signal in which the voltage changes periodically at at least two frequencies. In the illustrated embodiment, the excitation signal is supplied via a series resistor 10 electrically connected to the third electrode. The excitation signal can be generated with a voltage source relative to a reference potential, preferably ground potential.

[0056] Furthermore, the measuring and / or evaluation circuit 9 is configured to tap a first measurement signal at the first electrode E1 against the second electrode E2 after the excitation signal has been provided via the first electrode E1 or the second electrode E2. The first measurement signal can be one or more measured values ​​of a local electrical conductivity of the medium along a measuring path 15. The measuring path 15 runs partly through the liquid 20 of the medium and partly through the sediment 21 that has already formed in the lower region of the measuring tube 8.

[0057] Furthermore, the measuring and / or evaluation circuit 9 is configured to tap a second measurement signal at a third electrode E3 of the at least three electrodes against the second electrode E2. This also occurs after the excitation signal has been provided via the third electrode E3 or the second electrode E2. The second measurement signal is only determined after the first measurement signal has been determined. The second measurement signal can be a single measured value or multiple measured values ​​of the local electrical conductivity of the medium along a measurement path 16. The measurement path 16 also runs partly through the liquid of the medium and partly through the sediment 21 in the lower region of the measuring tube 8.

[0058] According to the invention, the first and second measurement signals are independent of the (current) flow rate of the medium. Instead, the first and second measurement signals can depend on a property of the medium. Thus, the first and second measurement signals can be a single measured value or multiple measured values ​​that depend on the local electrical conductivity of the medium.

[0059] According to the invention, the measuring and / or evaluation circuit 9 is configured to determine a sediment index taking into account the first and / or second measurement signal. The sediment index represents the degree of sedimentation in the measuring tube 8. Thus, the sediment index can include a sediment height, a sediment fraction at an inner diameter, or at an inner cross-section of the measuring tube 8. A measured value of the first and / or second measurement signal can be linked to the sediment index via a mathematical function. The mathematical function can be stored in the measuring and / or evaluation circuit 9, in particular in a memory. Alternatively, a lookup table can be stored that links the sediment index to the values ​​of the first and / or second measurement signal.

[0060] The sediment index informs the operator of the magnetic-inductive flowmeter 1 that sediment 21 is forming in the measuring tube 8. The operator can then temporarily increase the flow velocity of the medium, thus slowing down or even interrupting the sediment formation process in the measuring tube 8. By monitoring the sediment index, an ideal time for reducing the flow velocity of the medium can be determined. This has the advantage that the process does not need to be operated continuously at high flow velocities. This reduces operating costs.

[0061] The sediment index can be a quantity that depends on the electrical conductivity of the liquid and the solids.

[0062] The illustrated embodiment of the magnetic-inductive flowmeter according to the invention has a fourth electrode E4, which is a measuring electrode. The fourth electrode E4 is positioned opposite the first electrode, such that a measuring electrode axis intersecting the first and fourth electrodes E1, E4 runs parallel to or within the first longitudinal plane LE1.

[0063] The measuring and / or evaluation circuit 9 can advantageously also be configured to tap a third measurement signal at a fourth electrode E4 against the second electrode E2. For this purpose, an excitation signal is provided at the fourth electrode E4 or the second electrode E2. The third measurement signal can also be a measured value dependent on the local conductivity of the medium along a measuring path 19. The measuring path 19 runs partly through the liquid of the medium and partly through the sediment 21 in the lower region of the measuring tube 8. To ensure more stable monitoring of sediment formation, the measuring and / or evaluation circuit 9 can be configured to determine a comparison value from the first and second measurement signals and a further comparison value from the second and third measurement signals.Based on the two determined comparison values ​​and taking a selection criterion into account, the sediment index is determined. The selection criterion can, for example, specify that the higher comparison value should be used to determine the sediment index or that the higher comparison value should be selected as the sediment index.

[0064] Further measurements can be taken between the first and third electrodes E1, E3 along a measurement path 17 and / or between the fourth and third electrodes E4, E3 along a

[0065] Measurement path 18 is provided. In the example shown, measurement paths 17 and 18 run exclusively through the liquid of the medium. If no sediment 21 is present, the measurement signals along measurement paths 15, 17, 18, and 19 must be similar or even identical.

[0066] The sediment index and / or the comparative values ​​can be determined, for example, by calculating quotients.

[0067] Fig. 3 shows a cross-section through a configuration of a measuring electrode. The measuring electrode shown could be either the first electrode E1 or the fourth electrode E4. The first electrode E1 in Fig. 3 is circular arc-shaped and has an arc length l, which is projected through the measuring tube 8 at a central angle α in a cross-section QS. For the central angle α, 30° < α < 60°, and in particular, 40° < α < 50°. It is advantageous if the circular arc-shaped electrode has a radius of curvature that corresponds to a radius of the measuring tube 8.

[0068] Fig. 4 shows a cross-section through a further embodiment of the magnetic-inductive flowmeter 1 according to the invention. The magnetic-inductive flowmeter 1 has a measuring tube 8 which can be divided as follows. An imaginary second longitudinal plane LE2, extending perpendicular to the first horizontal longitudinal plane LE1, divides the measuring tube 8 into a third and fourth subsection TA3, TA4. In a top view, the third subsection TA3 is the entire left part of the measuring tube 8 and the fourth subsection TA4 is the entire right part of the measuring tube 8. In the third subsection TA3, a fifth and sixth electrode E5, E6 are arranged in addition to the first electrode E1. The fifth electrode is arranged in the second subsection TA2, whereas the sixth electrode E6 is arranged in the first subsection TA1.A fourth electrode E4 – as shown – can also be provided, arranged in the fourth subsection TA4 and oriented opposite the first electrode E1. Further electrodes can also be arranged in the fourth subsection TA4, resulting in a mirror-symmetrical arrangement of the electrodes. A circular sector with a central angle α is drawn in a cross-section QS through the measuring tube 8, in which the first and fifth electrodes E1, E5 – in this case, the first, fifth, and sixth electrodes E1, E5, E6 – are arranged. For the central angle α, 30° < α < 60°, and in particular, 40° < α < 50°.

[0069] At least the first and fifth electrodes, E1 and E5, are electrically short-circuited. This means that these electrodes are electrically connected to each other via a cable or an electrically conductive connector. This has the advantage of significantly reducing the flow profile dependency of the magnetic-inductive flowmeter.

[0070] It may also be required that an electrode axis EA intersecting the first and third electrodes E1 and E3 intersects the first longitudinal plane LE1. This presupposes that the first and third electrodes E1 and E3 are not arranged at a common electrode height in the measuring tube 8. Fig. 5 shows a graph with three examples of sediment index changes with increasing sediment height. The three examples differ in the nominal diameter of the measuring tube used. The sediment index [in %] is plotted as a function of the sediment height normalized to the nominal diameter of the measuring tube. The setup used for the measurements / simulations comprises a pipeline in which an inline magnetic-inductive flowmeter according to the configuration of Fig. 4 is arranged. A grounding disk, connected to a reference potential, is located between the pipeline and the magnetic-inductive flowmeter.

[0071] On the x-axis, circular markers indicate the height at which the first, fifth, and sixth electrodes are positioned. Since the height of the fifth and sixth electrodes depends on the nominal diameter, their positions are represented by oval markers covering a sediment depth range. The solid line shows the results for a setup where the measuring tube has a nominal diameter of DN100 (i.e., 100 mm). The dashed line shows the results for a setup where the measuring tube has a nominal diameter of DN200 (i.e., 200 mm). The dotted line shows the results for a setup where the measuring tube has a nominal diameter of DN600 (i.e., 600 mm). What is already suggested by the solid line is clearly evident in the other two lines: the respective sediment index of the DN200 and DN600 magnetic-inductive flowmeters increases continuously and exhibits three distinct steps.The respective levels are located relative to the standardized sediment height at the positions of the individual electrodes in the measuring tube. When the sediment reaches the height of one of the three electrodes E1, E5, E6 – which are electrically short-circuited – the sediment index increases sharply. For example, the sediment index jumps to approximately 30% when the lowest electrode of the measuring electrodes (i.e., the fifth electrode, E5) is covered. If the electrode located on the bisector of the measuring tube's cross-section (i.e., the first electrode, E1) is covered, the sediment index jumps to approximately 60%. If the uppermost electrode (i.e., the sixth electrode) is also covered, the sediment index jumps to approximately 90%.

[0072] Fig. 6 shows a process diagram of an embodiment of the method according to the invention.

[0073] In process step I, an excitation signal is provided at one of at least three electrodes, e.g., the first electrode E1. The excitation signal can be an AC voltage signal with a frequency selected from a frequency range of 1 Hz to 10 kHz. Alternatively, the excitation signal can be a multi-frequency voltage signal in which the voltage changes periodically at at least two frequencies. Furthermore, the excitation signal can have a variable voltage amplitude.

[0074] In process step II, a first measurement signal is tapped at the first electrode against the second electrode. The first and second electrodes can be located at different electrode heights. The electrode height is determined by the distance from a horizontal axis intersecting the respective electrode to the lowest point on the inner contour of the measuring tube. The first electrode can be electrically connected (i.e., short-circuited) to other electrodes. The second electrode is preferably positioned on the measuring tube such that, when in use (i.e., when the magnetic-inductive flowmeter is used in a pipeline for measurement), it is located lower than or at the same height as the first or third electrode.According to the invention, the positioning of the second electrode is chosen such that, in the event of sediment formation, the second electrode is covered by the sediment first or simultaneously with the first or third electrode.

[0075] In process step III, a second measurement signal is tapped at a third electrode of the at least three electrodes against the second electrode. For this purpose, an excitation signal is first provided at, for example, the third electrode. Here, too, the third electrode and the second electrode can be located at different electrode heights. According to the invention, the third electrode and the first electrode are located at different electrode heights.

[0076] Process steps II and III are preferably carried out one after the other.

[0077] Further process steps may be included. For example, in process step IV, a third measurement signal can be taken from a fourth electrode relative to the second electrode. For this purpose, an excitation signal is applied, for example, to the fourth electrode. The fourth electrode can, for instance, be located at the same electrode height as the first electrode.

[0078] In a subsequent process step V, a comparative value can be determined from the first and second measurement signals. Furthermore, another comparative value can be determined from the second and third measurement signals. The comparative values ​​can be determined, for example, by calculating ratios. Thus, one of the two comparative values ​​can be determined from the ratio of the first and second measurement signals, and the other comparative value from the ratio of the second and third measurement signals.

[0079] In process step VI, a sediment index can be determined taking into account the first and / or second measurement signal. The sediment index can be determined by selecting it from the reference values ​​based on a selection criterion. The selection criterion may require that only the higher reference value be used in determining the sediment index. REFERENCE SYMBOL LIST

[0080] 1 magnetic-inductive flow meter

[0081] 2 liners

[0082] 3, 4 Measuring electrode

[0083] 5 Level monitoring electrode

[0084] 6 Reference electrode

[0085] 7 magnetic field generating device

[0086] 8 measuring tube

[0087] 9 Measuring and / or evaluation circuit

[0088] 10 Series resistor

[0089] 15-19 Measurement path

[0090] 20 Liquid

[0091] 21 Sediment

[0092] 23 Reference potential

[0093] E1 first electrode

[0094] E2 second electrode

[0095] E3 third electrode

[0096] E4 fourth electrode

[0097] E5 fifth electrode

[0098] E6 sixth electrode

[0099] LE1 first longitudinal plane (horizontal dividing plane)

[0100] LE2 second longitudinal plane (vertical dividing plane)

[0101] TA1 first section (above)

[0102] TA2 second subsection (below)

[0103] TA3 third section (right)

[0104] TA4 fourth section (left)

[0105] QS cross-section

[0106] HA Main Axle

[0107] LA Longitudinal axis

[0108] EA electrode axis

Claims

PATENT CLAIMS 1. Method for detecting a sediment (21) in a magnetic-inductive flowmeter (1), wherein the magnetic-inductive flowmeter comprises: - a magnetic field-generating device (7) for generating a magnetic field, wherein the magnetic field has a principal axis (HA); - a measuring tube (8) for guiding a medium, wherein the medium comprises a liquid with solids distributed therein, which solids form the sediment, wherein a first longitudinal plane (LE1) intersecting the measuring tube (8) intersects the principal axis perpendicularly, wherein the first longitudinal plane (LE1) divides the measuring tube (8) into a first and second subsection (TA1 , TA2); - at least three electrodes, wherein a first electrode (E1) of the at least three electrodes is arranged in the first subsection (TA1) such that its own longitudinal axis (LA) lies in the first longitudinal plane (LE1), wherein a second electrode (E2) of the at least three electrodes is arranged in the second subsection (TA2), wherein the method comprises: - Providing an excitation signal at one of at least three electrodes; - Tapping off a first measurement signal at the first electrode (E1) against the second electrode (E2); - Tapping off a second measurement signal at a third electrode (E3) of the at least three electrodes against the second electrode (E2), wherein the third electrode (E3) is arranged in the first subsection (TA1) such that its own longitudinal axis (LA) lies in the first longitudinal plane (LE1), wherein the first and second measurement signals are independent of a flow velocity of the medium; - Determining a sediment index taking into account the first and / or second measurement signal.

2. The method according to claim 1, wherein the sediment index is a quantity dependent on the electrical conductivity of the liquid and the solids.

3. The method of claim 1 or 2, further comprising: - Tapping a third measurement signal at a fourth electrode (E4) against the second electrode (E2); - Determining a comparative value from the first and second measurement signals, - Determining a further comparative value from the second and third measurement signals, where the sediment index is determined by selecting the sediment index from the comparison values ​​depending on a selection criterion.

4. Method according to one of the preceding claims, wherein the sediment index and / or the reference values ​​are determined by means of quotient formation.

5. Method according to one of the preceding claims, wherein the first electrode is formed in a circular arc shape and has a circular arc length l which is projected into a cross-section (QS) through the measuring tube (8) at a central angle a, wherein for the central angle a 30° < a < 60° and in particular 40° < a < 50°.

6. A method according to any one of claims 1 to 4, wherein a second longitudinal plane (LE2) extending perpendicular to the first longitudinal plane (LE1) divides the measuring tube (8) into a third and fourth subsection (TA3, TA4), wherein the magnetic-inductive flowmeter (1) comprises a fifth electrode (E5), wherein the first and fifth electrodes (E1, E5) are arranged in the third subsection (TA3), wherein the second electrode (E2) is cut by the second longitudinal plane (LE2), wherein a circular sector with a central angle α is opened in a cross-section (QS) through the measuring tube (8), in which the first and fifth electrodes (E1, E5) are arranged, wherein the central angle α is such that 30° < α < 60° and, in particular, that 40° < α < 50°, wherein at least the first and fifth electrodes (E1, E5) are electrically short-circuited.

7. Method according to claim 6, wherein the magnetic-inductive flowmeter (1) comprises a sixth electrode (E6), wherein the sixth electrode (E6) is arranged in the third subsection (TA3), wherein the sixth electrode (E6) is short-circuited with the first and fifth (E1 , E5).

8. Method according to one of the preceding claims, wherein the magnetic-inductive flow meter (1) is arranged in a pipeline, wherein an earthing disc is arranged between the pipeline and the magnetic-inductive flow meter (1).

9. Method according to one of the preceding claims, wherein an electrode axis (EA) intersecting the first and third electrodes (E1, E3) intersects the first longitudinal plane (LE1).

10. Method according to any of the preceding claims, wherein the sediment index comprises a sediment height, a sediment fraction at an inner diameter or at an inner cross-section of the measuring tube (8).

11. Magnetic-inductive flowmeter, comprising: - a magnetic field-generating device (7) for generating a magnetic field, wherein the magnetic field has a principal axis (HA); - a measuring tube (8) for guiding a medium, wherein a first longitudinal plane (LE1) intersecting the measuring tube (8) intersects the principal axis perpendicularly, wherein the first longitudinal plane (LE1) divides the measuring tube (8) into a first and second subsection (TA1 , TA2); - at least three electrodes, wherein a first electrode (E1) of the at least three electrodes is arranged in the first subsection (TA1) such that its own longitudinal axis (LA) lies in the first longitudinal plane (LE1), wherein a second electrode (E2) of the at least three electrodes is arranged in the second subsection (TA2), wherein the third electrode (E3) is arranged in the first subsection (TA1) such that its own longitudinal axis (LA) lies in the first longitudinal plane (LE1), - a measuring and / or evaluation circuit (9) which is electrically connected to the at least three electrodes, wherein the measuring and / or evaluation circuit (9) is configured to perform the method according to one of the preceding claims.

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