Magnetic-inductive flow meter with a partially constricted measuring tube flow cross-section

A magnetic-inductive flow meter including a measuring tube, a magnetic circuit device, and two electrodes for detecting a measurement voltage. The measuring tube includes an inflow section, a measurement section that adjoins an inflow section, and an outflow section that adjoins the measurement section. A flow cross section of the measurement section is smaller than an inlet-side flow cross section of the inflow section and smaller than an outlet-side flow cross section of the outflow section. The electrodes are located on or in opposite electrode sections in the measurement section.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a magnetic-inductive flow meter with at least one measuring tube, at least one magnetic circuit device for implementing a magnetic circuit, and at least two electrodes for detecting a measurement voltage. The measuring tube has an inflow section, a measurement section, which adjoins the inflow section, and an outflow section, which adjoins the measurement section. A flow cross section of the measurement section is both smaller than an inlet-side flow cross section of the inflow section and smaller than an outlet-side flow cross section of the outflow section. The electrodes are located on or in opposite electrode sections in the measurement section of the measuring tube. Moreover, the invention also relates to a measuring tube for a magnetic-inductive flow meter.

2. Description of Related Art

The measurement engineering foundation for flow rate measurement with a conventional magnetic-inductive flow meter uses a measurement tube of a nonmagnetic material, for example, of a plastic or a nonmagnetic metal. The measuring tube is on a flow side in the region of the magnetic field generated by a magnetic circuit device. The measuring tube is not electrically conductive or is insulated electrically from the measurement fluid by an insulating lining. In operation, the magnetic field generated by the magnetic circuit device permeates the measuring tube at a measurement section in a direction that is essentially perpendicular to the flow direction. If a measurement fluid with a minimum electrical conductivity is flowing through the measuring tube, charge carriers in the conductive measurement fluid are deflected by the magnetic field. The charge carriers create an electrical potential difference on electrodes which are located perpendicular to the magnetic field and to the flow direction. The charge carriers are detected with a measurement device and are measured as a voltage. The measured voltage is proportional to the flow velocity of the charge carriers which are moved with the measurement fluid such that the flow rate in the measuring tube can be deduced from the flow velocity.

The sensitivity of the magnetic-inductive flow meter and the accuracy of the measurement which can be taken with the magnetic-inductive flow meter depend, among other things, on the magnetic field, which is generated with the magnetic circuit device in the region of the measurement section of the measuring tube, the geometry of the measurement section and the arrangement of the electrodes. The geometry of the arrangement relates to the homogeneity of the magnetic field produced in the region of the measurement section, the flow conditions of the measurement fluid in the measurement section and, thus, also the electrical field generated by the charge separation in the measurement section, which is the basis for the measurement. The tuning of these different components of the magnetic-inductive flow meter to one another is crucial to attain accurate measurements.

Varying the cross section of the measuring tube beyond its longitudinal extension and, therefore beyond its extension in the flow direction is known from the conventional art. The inlet-side flow cross section of the inflow section conventionally has the geometry of the process connection. Therefore, conventionally, a circular flow cross section having the nominal width of the pipe in the process is connected to the magnetic-inductive flow meter. The corresponding applies to the outlet-side flow cross section of the outflow section, which likewise faces the process and which can be connected to the process. When “flow cross section” is addressed here, it always means the free cross sectional area of the measuring tube which has been measured perpendicular to the flow direction and which is available to the flow, and, therefore without the wall thickness of the measuring tube at the pertinent site.

German Patent Application 10 2008 057 755 A1, which corresponds to U.S. Pat. No. 8,286,503 B2, for example, discloses that a flow cross section of an inlet-side end of an inflow section decreases toward a measurement section and an outlet-side flow cross section of the measurement section increases, in turn, to the outlet-side flow cross section of the outflow section of a measuring tube. The change of the cross section has the advantage that the flow velocity of the measurement fluid is increased in the region of the measurement section and, accordingly, a greater effect is also achieved for the charge separation as a result of the magnetic field in the measurement section.

The variable cross sectional geometry of the measuring tube beyond its longitudinal extension is achieved in the conventional art by comparatively complex production techniques, for example by casting a corresponding metal measuring tube, by internal high pressure forming or by injection molding of a plastic measuring tube. The production effort and the associated costs have, for a long time, prevented the use of magnetic-inductive flow meters for low cost, mass applications, for example as domestic water meters. This is due not only to the production costs associated with the measuring tube, but also to the altogether comparatively demanding hardware and measurement-engineering structure of a magnetic-inductive flow meter.

SUMMARY OF THE INVENTION

The primary object of this invention is to provide a magnetic-inductive flow meter with a measuring tube in which a high measurement sensitivity and measurement accuracy are structurally supported, and, moreover, a measuring tube that is easily and thus economically producible.

The aforementioned object is achieved in a magnetic-inductive flow meter with a measuring tube in which a distance between the electrode sections in the measurement section of the measuring tube is greater than the largest inside diameter of the inlet-side flow cross section of the inflow section of the measuring tube. The distance between the electrode sections in the measurement section of the measuring tube means the distance from wall to the wall of the measuring tube in the sections in which there are electrodes, regardless of the insulating linings of the measuring tube in this section, and also regardless of possible recesses in the measuring tube into which the electrodes are inlet. When the inside diameter of the inlet side flow cross section of the inflow section is referred to herein, it is then assumed that the inlet-side flow cross section of the inflow section is the area of a circle. This results solely from the fact that magnetic-inductive flow meters must be connected to pipes with a circular cross section of the process system and, thus, in contrast to the flow cross section in the measurement section having circular or circular area flow cross sections and, thus, have only a single inside diameter there.

The great distance between the electrode sections in the measurement section of the measuring tube widens the distance available to charge separation beyond the amount that has been conventionally practiced, and, of added importance, the area over which a magnetic field can be introduced into the medium is increased beyond the conventional amount. This is because, conventionally, pole shoes of the magnetic circuit device are provided on the wall sections of the measurement section which are perpendicular to the electrode sections in the measurement section of the measuring tube. This aspect of the invention increases the sensitivity of the magnetic-inductive flow meter in a geometrical-structural manner and improves the measurement accuracy since, at the distances between electrode sections disclosed herein, a largely homogeneous magnetic field is produced over large parts of the volume in the measurement section.

In embodiments of the magnetic-inductive flow meter and its measuring tube in accordance with the aspects of invention, the flow cross section of the measurement section is essentially rectangular and has a length/width ratio of greater than 3:1 and, in implementations, greater than 3.5:1. In these length/width ratios, the length is defined as the distance between the electrode sections in the measurement section of the measuring tube. Implementations of this design standard result in an unusually flat flow channel which promotes flow conditions that have improved measurement accuracy. The short walls which define the “width” accommodate the electrode sections and on the “long” walls which are essentially perpendicular thereto there are, in implementations, the opposite poles of the magnetic circuit device. Especially good results are achieved with implementations having a length/width ratio of 3.74:1.

In embodiments of the magnetic-inductive flow meter and of the measuring tube for this flow meter, the ratio of the inlet-side flow cross section of the inflow section to the flow cross section of the measurement section is greater than 1.8:1, and, in implementations, greater than 2.0:1, and, in implementations, greater than 2.2:1. It has been found that, at the large distance between the sections of the electrode in the measurement section of the measuring tube, a relatively speedy tapering of the flow cross section can be implemented without adversely affecting the flow in the measurement section of the measuring tube. This applies especially in conjunction with the aforementioned length/width ratio of the flow cross section in the measurement section.

Embodiments of magnetic-inductive flow meter in accordance with aspects of invention are fundamentally suited for use for all connection-side nominal widths, but are especially suitable for connection-side nominal widths of the measuring tube which are smaller than a 10 mm, and, in implementations, smaller than 40 mm. This is due to the fact that the extension of the measurement section beyond the outside dimension of the inlet-side flow cross section between the electrode sections for magnetic-inductive flow meters of these sizes is not perceived as disruptive, since, for example, a housing can be easily produced to be so large that it also still encompasses the geometry of the measurement section which is discharging something. This may be a problem in magnetic-inductive flow meters with much greater nominal diameters.

In particular, there are now various possibilities for configuring and developing the magnetic-inductive flow meter according to aspects of the invention and the measuring tube for this flow meter according to aspects of the invention. In this respect, reference is made to the description of exemplary embodiments in conjunction with the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a magnetic-inductive flow meter1with a measuring tube2and with a magnetic circuit device3for implementing a magnetic circuit and with two electrodes, of which only one electrode4is visible in the drawings. The electrodes4are used to detect a measurement voltage, which is established when a conductive medium is flowing through the measuring tube2. Then, in the medium, a charge separation is established in the direction of the electrodes4, when the magnetic circuit device3generates a magnetic field perpendicular to the flow direction and perpendicular to the imaginary axis of the opposing electrodes4. In the exemplary embodiment shown inFIG. 1, the magnetic circuit device3consists of two opposing pole plates3awith one coil3beach, which are energized by trigger electronics that are not detailed here. Likewise the magnetic closing of the magnetic circuit device is not explicitly shown.

FIGS. 2 to 4, in contrast toFIG. 1, show only the measuring tube in order to emphasize its structural particulars.

InFIGS. 1 to 4, the measurement2has an inflow section2a, a measurement section2badjoining the inflow section2aand an outflow section2c, which adjoins the measurement section2b. As can be easily recognized, the flow cross section A of the measuring tube2changes greatly over the longitudinal extension of the measuring tube2and, therefore, in the throughflow direction. The flow cross section Amof the measurement section2bis both smaller than the inlet-side flow cross section Aeof the inflow section2aand also smaller than the outlet-side flow cross section Acof the outflow section2c.

The electrodes4are located on or in opposite electrode sections5a,5bin the measurement section2bof the measuring tube2, where they contact the electrical potentials arising due to charge separation and make them available as measurement voltage.

As is especially apparent fromFIG. 3, the measuring tube2shown in the figures is characterized in that the distance smbetween the electrode sections5a,5bin the measurement section2bof the measuring tube2is larger than the inside diameter soof the inlet-side flow cross section Acof the inflow section2aof the measuring tube2. By the distance smbetween the electrode sections5a,5bin the measurement section2bbeing increased relative to the inside diameter soof the inlet-side flow cross section Ac, the distance of effective charge separation and, thus, the effective measurement sensitivity of the magnetic-inductive flow meter1is increased. At the same time, with the widening of the flow cross section, the possible supporting and action surface for the pole shoes3aof the magnetic circuit device3is increased.

As can be clearly discerned fromFIG. 4, but as also follows from examiningFIGS. 2 & 3together, the flow cross section Amof the measurement section2bis essentially rectangular and, in this case, has a length/width ratio of roughly 3.7. The distance smbetween the electrode sections5a,5bin this exemplary embodiment is, therefore, approximately four times greater than the height of the inside cross section. For a flow cross section Amof the measurement section2bconfigured in this way, a favorable flow profile is achieved. This advantageously affects the attainable measurement accuracy. “Essentially rectangular” in this connection means than the flow cross section Amof the measurement section2bis bordered for the most part by wall surfaces which run in pairs parallel to one another. The wall surfaces, however, pass into one another at the junction points only at a certain radius of curvature. The flow cross section Amof the measurement section2bis unchanged in the exemplary embodiment shown here over the longitudinal extension of the measurement section2bso that a smooth flow without unnecessary perturbations can be established in the measurement section2b.

In the exemplary embodiment shown in the figures, the ratio of the inlet-side flow cross section Acof the inflow section2ato the flow cross section Amof the measurement section2bis roughly 2.2. Therefore, a considerable reduction of the flow cross section results. The illustrated structural layout is characterized in that the flow profile is, nevertheless, particularly suited for a high-quality flow rate measurement.

The inflow section2ais shaped such that it has a continuously decreasing flow cross section in a single coherent reducing region without sudden changes in cross section and without phases remaining in a constant flow cross section. The same applies analogously to the outflow section2c, which in a single coherent expansion region has a continuously increasing flow cross section which finally ends in the outlet side flow cross section Aewhich is then kept constant over a short distance. However, this region of the constant outflow cross section Acis no longer included in the expansion region.

The configuration of the measurement section2bof the measuring tube2allows an especially short construction of the entire measuring tube2. In the exemplary embodiment shown in the figures the ratio of the longitudinal extension of the measurement section2bto the longitudinal extension of the reducing region and the ratio of the longitudinal extension of the measurement section2bto the longitudinal extension of the expansion region is settled at roughly 0.9 mm. The connection-side nominal width of the measurement tube2is 15 mm. The distance smbetween the electrode sections5a,5bin the illustrated exemplary embodiment is 17.2 mm. The flow meter with the described dimensions is particularly useful, e.g., for registering water consumption in amounts conventional for households and, therefore, as a domestic water meter.

In embodiments, the measurement tube2is made of a metal pipe of nonmagnetic material. The reducing region of the inlet section2a, the expansion region of the outlet section2c, and the measurement region2bare produced without cutting by forces acting from outside on the pipe. The pipe geometry can, therefore, be very easily produced without expensive production methods, such as casting or internal high pressure forming. As such, the production costs compared to conventional measuring tubes for the illustrated magnetic-inductive flow meters are very low and thus also a use of these magnetic-inductive flow meters with these measuring tubes for mass applications in the low cost domain is possible.

FIGS. 1 to 3show that the measuring tube2in the measurement section2bhas a nonconductive lining6which can be omitted in other exemplary embodiments in which the measuring tube itself is not electrically conductive.