HK1036651A - Apparatus for measuring the flow of a medium to be measured through a measuring tube - Google Patents

Description

Device for measuring the flow of a medium to be measured through a measuring tube

The invention relates to a device for measuring the flow rate of a medium to be measured flowing through a measuring tube in the axial direction of the measuring tube, comprising a magnetic structure which generates a magnetic field which extends through the measuring tube and essentially transversely with respect to the axis of the measuring tube, at least one measuring electrode which is arranged in the lateral region of the measuring tube and is electrically or capacitively connected to the medium to be measured, and a computing/control unit which provides information on the volume flow rate of the medium to be measured in the measuring tube using a measuring voltage induced in the measuring electrode.

The electromagnetic flowmeter measures the volume flow by utilizing the principle of electric induction: charge carriers of the medium to be measured which move perpendicularly relative to the magnetic field generate a voltage at a measuring electrode which is likewise arranged perpendicularly to the flow direction of the medium to be measured. This induced voltage is proportional to the average flow velocity of the medium to be measured in the entire cross section of the measuring tube; and thus proportional to the volume flow.

Considerable measurement errors can occur if the measuring tube is not filled to its full extent but is only partially filled with the medium to be measured, but such measuring devices are based on measurement results which occur on a completely filled measuring tube. In order to eliminate this error source, a device has already been disclosed which determines the volume flow rate taking into account the corresponding filling level information of the filling measuring tube. Thus, german utility model G9103046.3 has described an electromagnetic flow measuring device in which both electromagnets can be excited individually or jointly as desired, and if so, can be excited in the same direction or in opposite directions as desired. For determining the volume flow, at least two voltage values which are detected in different excitation states of the electromagnetic structure (for example, excitation of the two electromagnets in the same direction and in opposite directions) and which are tapped off via a corresponding pair of measuring electrodes are used. The calculation unit then processes the measurement signal with the aid of the empirically determined parameters to derive a flow rate output signal, in which case errors due to partial filling of the measurement tube are eliminated. To ensure that at least one pair of measuring electrodes is electrically connected to the medium to be measured and can be used to generate a measuring signal even in the case of very low filling of the measuring tube, the flow measuring device described in G9103046.3 has two pairs of measuring electrodes, one of which is arranged above the cross section of the measuring tube and the other at the bottom.

Furthermore, the prior art also discloses that, in addition to two measuring electrodes being arranged in the central region of the measuring tube, in each case one measuring electrode in the upper region of the measuring tube and the other measuring electrode in the bottom region of the measuring tube, the bottom measuring electrode is often grounded. The voltage measured at the measuring electrodes is used to identify the filling degree of the measuring tube, while at the same time the desired information on the volume flow through the measuring tube can be derived from the voltage values measured at the measuring electrodes.

In order to be able to use the known devices for determining the filling degree of the measuring tube in general, the supply resistance of the test voltage applied to the test electrodes must be relatively high (for example, in the order of 100k Ω). Only in this way can devices for measuring volume flow be used universally for a large range of media to be measured which have to be included, the electrical conductivity of which, as is known, differs greatly from one another.

The drawbacks of the known solutions are evident when, instead of using a compact structure, in which the actual sensor and the electronic part are housed in a casing, a measuring device is used in which the sensor is arranged in the process and connected to the remote conversion electronics via a connecting wire (usually a coaxial cable). Such a connecting lead produces a partial voltage which is dependent on the length of the coaxial cable, and no measurable voltage component is available at all at the test electrode when the connecting lead exceeds a certain length. The known measuring devices for detecting the filling degree of the measuring tube are therefore limited to a certain range of applications, namely only in compact devices or in devices in which the connecting line between the sensor and the remote electronic device does not exceed approximately 10 meters.

The invention is based on the object of providing a universally applicable and cost-effective device for measuring a volume flow and/or for detecting a filling level of a measuring tube.

This object is achieved in that at least one test electrode is provided in the upper region of the measuring tube, that a computing/control unit transmits a test signal to the test electrode, and that the computing/control unit applies a signal in response to the test signal, said response signal being received via the measuring electrode to provide information about the filling degree of the measuring tube. According to the invention, a so-called "empty pipe detection" is realized by applying a test signal to the test electrode, which "empty pipe detection" means the identification of the measuring pipe as being completely filled or only partially filled or completely empty. If the measuring tube is filled, said test signal is displayed as a response signal at the measuring electrode. On the other hand, if the measuring tube is only partially filled or empty, there is no or an interfering electrical connection between the top test electrode and the measuring electrode. Thus, no response signal or other attenuated response signal appears at the measuring electrode.

Although in principle one measuring electrode is sufficient for determining the volume flow and for detecting an "empty pipe detection", an advantageous embodiment of the invention also provides a second measuring electrode which is arranged in the region of the measuring pipe opposite the first measuring electrode. The two measuring electrodes are preferably diametrically opposed to each other in the central region of the measuring tube.

In order to utilize the symmetrical condition and the redundant measurement as a result of this, the test electrode is arranged relative to the two measuring electrodes in such a way that the distance to each of the two measuring electrodes is substantially the same. The test signal output by the test electrode thus produces substantially the same response signal at each of the two measurement electrodes. In this case, if a large difference occurs, this indicates a malfunction in certain circumstances, for example a malfunction of one of the measuring electrodes.

An advantageous further development of the device according to the invention provides for a second test electrode to be arranged substantially diametrically opposite the first test electrode, the first test electrode preferably being arranged at the apex of the measuring tube and the second test electrode preferably being arranged at the lowest point of the measuring tube. A further development of the device according to the invention provides that one of the two test electrodes is grounded. The grounded test electrode is preferably an electrode arranged in the bottom region of the measuring tube. As indicated above, the second test electrode, which serves as a reference electrode, is not absolutely required. A ground plate (ground plate) can also be used as a reference potential, said plate being formed, for example, on a flange for fixing the flow metering device in the pipe system.

A preferred embodiment of the device according to the invention proposes that the test signal is a symmetrical pulse signal. Such an embodiment has advantages over an arbitrary asymmetric signal: that is, in the case of an asymmetrical signal, a relatively high energy density of the test signal can produce an electrochemical potential shift in the medium to be measured. If the test signal is designed as a symmetrical pulse, the potential shift is approximately zero on average.

A preferred embodiment of the device according to the invention provides a calculation/control unit which correlates the response signals on the test electrode and the measuring electrode/measuring electrodes with the test signal. This is a simple and reliable way of identifying whether a response signal to the test signal is present at said measuring electrode/electrodes.

In addition, according to an advantageous further development of the device according to the invention, a computing/control unit is provided which determines the filling level information by comparing the response signals measured at the test electrode and/or at the measuring electrode/measuring electrodes with a predetermined reference signal. These reference signals are determined beforehand in the different measurement processes under the current process and system conditions, respectively. Tolerances are thereby designed in the relevant devices and are made relatively small, as a result of which the quality of the measurement can be increased, so that a reliable detection of incompletely filled measuring tubes can be achieved.

A preferred embodiment of the device according to the invention provides a memory unit in which a preferred reference signal is stored.

The invention is explained in more detail with reference to the following drawings, in which:

figure 1 shows a schematic view of a first embodiment of the device according to the invention,

figure 2 shows a schematic view of a second embodiment of the device according to the invention,

fig. 3 is a flow chart of the associated drive calculation/control unit.

Fig. 1 shows a schematic view of a first embodiment of the device according to the invention, and fig. 2 shows a schematic view of a second embodiment of the device according to the invention. The main difference between the two embodiments is that in the first embodiment there is only one measuring electrode 4 and one test electrode 6, whereas in the second embodiment there are two measuring electrodes 4,5 and two test electrodes 6, 7.

In each of the two embodiments, the medium 11 to be measured flows through the measuring tube 2 of the flow meter (not shown in this figure) in the direction of the axis 10 of the measuring tube. The medium 11 to be measured is at least slightly electrically conductive. The measuring tube 2 itself is made of a non-conductive material or at least lined with a non-conductive material.

Since the magnetic field is perpendicular to the flow direction of the medium 11 to be measured and is usually generated by two magnets (likewise not visible in the drawing) arranged diametrically, charge carriers in the medium 11 to be measured migrate to the measuring electrode 4 or to the opposing measuring electrodes 4, 5. The voltage generated at the measuring electrode 4 or at the measuring electrodes 3,4 is proportional to the average flow velocity of the medium 11 to be measured in the cross section of the measuring tube 2, i.e. it is a measure for the volume flow of the medium 11 to be measured in the measuring tube 2. Incidentally, the measuring tube 2 is connected to the pipe system through which the medium 11 to be measured flows via a connection element, for example a flange, which is not shown in the drawing.

In both cases shown, the measuring electrodes 3,4 are in direct contact with the medium to be measured; however, as mentioned above, the connection may also be a capacitive connection.

The measuring electrodes 4,5 and the test electrodes 6,7 are connected to the computing/control unit 8 via connecting leads 12,13 or 14,15, 16. Said calculation/control unit 8 is connected to the input/output unit via a connection 17. A memory unit 10 is assigned to the calculation/control unit 8.

The device 1 according to the invention preferably operates as follows: the calculation/control unit 8 is arranged at predetermined time intervals t_MThe test signal is passed to the test electrode 6. The test signal is preferably a symmetrical pulse. As already explained above, the potential fluctuations induced in the medium 11 to be measured when a symmetrically configured test signal is applied are at least approximately zero on average. The volumetric flow measurement is therefore hardly disturbed at all by the superimposed process of detecting the filling level of the measuring tube 2.

If the medium 11 to be measured is located between the test electrode 6 and the measuring electrode 4 or the measuring electrodes 4,5, the test signal transmitted to the test electrode 6 appears as a response signal at the measuring electrode 4 or at the measuring electrodes 4, 5. The calculation/control unit 8 preferably determines the response signal by performing a correlation operation between the test signal and the response signal at the respective measuring electrode 4, 5.

The medium 11 to be measured and other external system and measurement process conditions greatly influence the reliability of the final measurement of the volume flow by the device 1 according to the invention. A specific test signal is therefore transmitted to the test electrode 6 in the case of a specific system and measurement process, for example, with a completely filled measuring tube 2. The corresponding response signal is determined at the measuring electrode 4 or the measuring electrodes 4,5, respectively. The measured response signal is stored in the memory unit 10 as a reference response signal or as an ideal response signal. All the actual response signals determined subsequently are compared with this reference response signal. If the actual response signal is within a predetermined tolerance Δ around the ideal response signal, this indicates that the measurement of the mass flow rate is performed in a completely filled measuring tube 2. On the other hand, if the actual response signal is outside the predetermined tolerance a around the ideal response signal, the calculation/control unit 8 informs the operator, for example via an input/output unit, that the measuring tube 2 is not completely filled.

Fig. 3 shows a flow chart relating to the drive calculation/control unit 8. As already mentioned above, under predetermined system and measurement process conditions, at the beginning of the actual acquisition of the measured values, a test signal is transmitted to the measuring electrode 6 and the corresponding response signal at the measuring electrode 4 is stored as an ideal response signal or both the corresponding response signals at the measuring electrodes 4,5 are stored as ideal response signals. Furthermore, a tolerance Δ of the response signal is specified. It is advisable to use a measuring tube 2 which is completely filled with the medium 11 to be measured for determining the reference response signal.

At program point 18, the measurement, calculation step is started. By means of a timer (program point 19) and a time inquiry (program point 20), at a predetermined time t_MThe determined test signal is then passed to the test electrode 6 at program point 21. The response signal at the at least one measuring electrode 4,5 is measured at a program point 22. If the actual response signal is within a predetermined tolerance delta around the ideal response signal, the calculation is performed by the calculation/control unit 8 according to the ideal state. At a predetermined time t_MAfter which the subsequent corresponding program point is repeatedly executed. If the measurements and calculations at the program points 22,23 show that the actual response signal is outside the predetermined tolerance Δ around the ideal response signal, the operator is indicated by the input/output unit 10 that the measuring tube 2 does not have a defined degree of filling of the measuring tube 2.

List of reference numerals

1 apparatus according to the invention

2 measuring tube

3 measuring the tube axis

4 first measuring electrode

5 second measuring electrode

6 first test electrode

7 second test electrode

8 calculation/control unit

9 output/input unit

10 memory cell

11 medium to be measured

12 connecting wire

13 connecting wire

14 connecting wire

15 connecting wire

16 connecting wire

17 connecting wire

Claims (11)

1. A device for measuring the flow of a medium to be measured flowing through a measuring tube in the direction of the axis of the measuring tube, which device has a magnetic structure which generates a magnetic field which passes through the measuring tube and extends substantially transversely with respect to the axis of the measuring tube, at least one measuring electrode which is arranged in the lateral region of the measuring tube and is electrically or capacitively connected to the medium to be measured, and a computing/control unit which provides information about the volume flow of the medium to be measured in the measuring tube using a measuring voltage induced in the measuring electrode, wherein at least one test electrode (6; 7) is provided in the upper region of the measuring tube (2), the computing/control unit (8) transmits a test signal to the test electrode (6; 7), and the computing/control unit (8) uses the response signal of the test signal to provide information about the degree of filling of the measuring tube (2), the response signal is received via the measuring electrodes (4; 5).

2. An arrangement as claimed in claim 1, wherein a second measuring electrode (5; 4) is provided in the region of the measuring tube (2) opposite the first measuring electrode (4; 5).

3. An arrangement as claimed in claim 2, wherein the two measuring electrodes (4,5) are diametrically opposed to one another in the central region of the measuring tube (2).

4. A device as claimed in claim 1,2 or 3, wherein the test electrode is arranged relative to the two measurement electrodes such that: the distance to each of the two measuring electrodes is substantially the same.

5. The device as claimed in claim 1,2,3 or 4, wherein a second test electrode (7; 6) is provided, which is substantially diametrically opposite the first test electrode (6; 7), the first test electrode (6; 7) preferably being arranged at the apex of the measuring tube (2) and the second test electrode (7; 6) preferably being arranged at the lowest point of the measuring tube (2).

6. The device of claim 1,2,3,4 or 5, wherein one of the two test electrodes (6,7) is connected to ground.

7. The apparatus of claim 1 or one or more of claims 2 to 6, wherein the test signal is a symmetrical pulse.

8. The device according to claim 1 or 7, wherein the calculation/control unit (8) correlates the response signals at the test electrodes (7; 6) and/or the measuring electrodes (6; 7)/measuring electrodes (6,7) with the test signals.

9. The device as claimed in claim 1,7 or 8, wherein the computing/control unit (8) determines the filling degree information of the measuring tube (2) by comparing the response signals measured at the test electrodes (6; 7) and/or at the measuring electrodes (4; 5)/measuring electrodes (4,5) with predetermined reference signals.

10. The device as claimed in claim 9, wherein the predetermined reference signal is determined by the filling degree determined with the measuring tube (2) and is a function of the respective specific process parameter.

11. A device as claimed in claim 9 or 10, wherein a storage unit (10) is provided for storing a predetermined reference signal.