EP2844960A1

EP2844960A1 - Measurement device for measuring the flow velocity of a medium

Info

Publication number: EP2844960A1
Application number: EP13719549.1A
Authority: EP
Inventors: Marcus Wolff; Henry BRUHNS
Original assignee: Zylum Beteiligungs GmbH and Co Patente II KG
Current assignee: Zylum Beteiligungs GmbH and Co Patente II KG
Priority date: 2012-04-30
Filing date: 2013-04-30
Publication date: 2015-03-11

Abstract

The invention relates to a measurement device for measuring the flow velocity of an electrically conductive medium in a volume which is permeated by a magnetic field, having - a means for generating the magnetic field, - at least two electrodes, and - an evaluation unit which evaluates a signal of the electrodes and calculates the flow velocity, the at least two electrodes being connected to a switch which is designed to short-circuit the electrodes.

Description

Measuring device for measuring the flow rate of a med

description

The present invention relates to a measuring device for measuring the

Flow rate of an electrically conductive medium in a volume penetrated by a magnetic field, comprising means for generating the magnetic field, at least two electrodes and an evaluation unit which evaluates a signal of the electrodes and calculates the flow rate.

The invention also relates to a method for measuring the

Flow rate of an electrically conductive medium in a permeated by a magnetic field volume.

Such methods and measuring devices for measuring the flow rate of a medium have been known for some time by the term "magnetically inductive flow measurement." Magnetic-inductive flowmeters typically have a measuring tube through which the electrically conductive medium flows

Permanent magnets generated. According to Faraday's law, the moving charges are separated in the magnetic field. The at least two electrodes are typically disposed on opposite sides of the measuring tube and measure a voltage which, in the ideal case, is proportional to the flow velocity of the charge carriers, i. H. to the flow rate of the electrically conductive medium. Such methods require only a very low electrical conductivity, so z. B. also the

Flow rate of tap water can be determined.

The decoupling of the signal to be measured can be done either galvanic or capacitive. In a galvanic decoupling the electrodes are in electrical contact with the medium. Here is a certain

Minimum conductivity of the medium required. In addition, the electrodes must be sufficient on their permanent contact with the medium

be corrosion resistant. In capacitive signal extraction, the electrodes are formed as large-area plates of a capacitor and are located on the outside of the tube, so are not in contact with the medium. The measuring devices of this type typically used today use electromagnetic alternating fields generated by electromagnets to generate the required magnetic field. This leads to the induction of interference voltages at the electrodes. These must be specifically suppressed by filters. For a simple and energy-efficient operation, it would thus be desirable to replace the electromagnets operated with alternating voltage by permanent magnets with a static magnetic field. When using permanent magnets, however, additional measurement errors are observed, which represent a drift of the induced voltage measured at the electrodes. This drift is a random DC voltage value that varies over time and is superimposed on the actual, induced measured value. This measurement error can be caused by electrostatic or electrochemical charges. Such measurement errors can not be excluded by statistical methods.

It is therefore an object of the present invention to provide a measuring device and a method of the type mentioned above in which the measurement of the flow velocity of the medium is not falsified by such measurement errors.

According to a first aspect of the invention, the object is achieved in that the at least two electrodes of the measuring device are connected to a switch which is designed to short-circuit the electrodes.

According to a further aspect of the invention, the object is achieved by a method of the type mentioned, in which the electrodes are short-circuited during a first period, in particular a period between 0.3 and 1 seconds, the short circuit during a second period, in particular one Period between 1 and 3 seconds, is opened, a useful signal is read from the electrodes and the flow rate is calculated based on the read-out useful signal.

The following is the new method and the new measuring device

Based on knowledge: the said electrostatic and electrochemical charges occur at the electrodes of the measuring device, in particular near their surfaces. By the switch according to the invention, the charges but degraded (neutralized) by the switch shorts the electrodes and enforces a potential equalization in this way. If you then cancel this short circuit again, it can be assumed that a resulting immediately after this switching operation

Potential difference between the electrodes on magnetic induction

can be traced back and thus represents a measure of the flow. The

electrochemical and electrostatic processes cause no measurement error, since they act more slowly and slowly rebuild after opening the short circuit. It does not matter whether the signal extraction works according to the galvanic or capacitive principle. The evaluation of the signal measured after opening the short circuit can be carried out using the methods known from the prior art.

It is thus possible in a cost effective manner to provide a measuring device with increased accuracy. The above task is thus completely solved.

In a preferred embodiment of the invention, the switch is a

electronic switch. In contrast to mechanical switches (eg relays), electronic switches are characterized by lower energy consumption, longer service life, short switching times and low leakage currents.

Typically, the switches to be used according to the invention are always either in the open or in the closed train position, with a very short change-over phase due to technical reasons only. In other embodiments of the invention, however, provision can also be made for a targeted transition to take place between the locked closed state.

In a further embodiment, the means for generating the magnetic field is a permanent magnet. As already stated above, can thus (by the

Avoidance of electromagnets) the induction of interference voltages at the electrodes can be prevented or at least reduced.

In a preferred embodiment, the switch comprises at least one

Transistor, preferably a MOSFET, in particular a MOSFET in a CMOS IC. MOSFETs are characterized by short switching times and low

Switching resistors off. In addition, they can be produced particularly inexpensively. In one embodiment of the invention it is provided that the switch in the open state has a resistance of at least 10 GOhm, in particular at least 100 GOhm. Since the measurement of the signal in the open state of the switch, but the switch is still connected to the electrodes, the signal of the electrodes should not be falsified or weakened by too low a resistance of the open switch.

Otherwise, there would be a higher amplification of the signal of the electrodes

required, which, however, also entails an increased amplification of interference signals.

In one embodiment of the invention it is provided that the measuring device has a drive unit for the switch, wherein the drive unit comprises at least one timer. By the drive unit, the switch automatically, z. B. at regular intervals, closed and opened. The timer can itself specify predetermined time intervals between closing and opening the switch. In other embodiments, the time given by the switch could also depend on variable values, e.g. B.

The timer could be shorter at high measured flow rates

Specify time intervals as at low measured flow rates. The control unit can also specify when the evaluation of signals from the electrodes takes place. Finally, the timer can also specify a periodic sequence of switch activation and signal evaluation.

In a preferred embodiment it is provided that the control unit is designed such that the switch for the duration of a first period, in particular a period between 0.3 and 1 seconds, is closed and for the duration of a second period, in particular a period between 1 and 3 seconds, is opened, further wherein the evaluation unit is designed such that the signal of the electrodes is evaluated immediately after the second period.

In a variant of the invention, the electrodes are short-circuited for each 50 to 100 ms, in particular approximately 80 ms, and then for 10 to 50 ms,

in particular approx. 20 ms, opened for a measurement. These parameters allow a particularly good compromise between a high resolution and a short reaction time / measurement time. This configuration is based on the following considerations: the short-circuiting of the electrodes for the duration of a first time period forces a potential equalization of the electrodes and the electrostatic and electrochemical charges are reduced. To determine the induced voltage, this short circuit is then canceled. The evaluation of the signal of the electrodes takes place according to this embodiment, however, not immediately after cancellation of the short circuit, but only after a settling time, which is essentially determined by the capacitance of the measuring circuit. After this second period, during which the switch is open, the voltage applied to the electrodes, z. B. with an analog / digital converter, converted and evaluated, in particular also stored for further processing. Immediately after the storage process, the electrodes can be short-circuited again and the described process can be repeated periodically. In this way, the induced voltage can be measured time-discretely. The continuous storage of many discrete measured values enables a mathematical post-treatment to suppress statistical measurement errors (eg digital filtering or mean value calculation).

The choice of the duration of the first period depends on the time required for the electrostatic and electrochemical charges to be sufficiently degraded. This can depend on several factors, eg. Example of the geometry of the measuring device, the nature of the flowing medium, the flow rate of the medium and the material and the surface of the electrodes. Accordingly, in other embodiments of the invention also significantly shorter or longer periods may be provided. In particular, first time periods between 100 and 300 milliseconds or first time periods between 1 and 10 seconds would be conceivable.

The duration of the second period should be chosen so that the

Settling time of the measuring circuit is essentially comprises. Since very different capacitances of the measuring circuit and thus very different settling times are also conceivable here, depending on the configuration of the measuring circuit, different values are also possible for the duration of the second period in different embodiments of the invention, in particular periods between 100 milliseconds and 1 second or periods between 3 and 10 seconds. According to one embodiment of the invention, it is provided that a zero-point signal is read out immediately at the beginning of the second time period and the flow rate is calculated based on the zero-point signal and the useful signal, in particular based on the difference between the zero-point signal and the useful signal. By difference between zero and

The useful signal can also be deducted from any drift voltage remaining despite the short-circuiting of the electrodes, which is not due to the flow of the medium. In further embodiments, it would also be conceivable for more than one signal to be used to determine the zero point or for another calibration to be recorded before the measurement of the useful signal and used to calculate the flow rate.

In a further embodiment of the invention it is provided that the

Evaluation unit has a filter circuit for suppressing high-frequency noise. Such a filter circuit can, for. B. be designed as a simple low-pass filter, since in most applications thereof

It can be assumed that the flow velocity of the medium varies only slowly *. But there are also more expensive filters, eg. B. active filters or statistical

Filter methods, conceivable.

According to a further variant of the invention, it is provided that with a suitable measuring circuit (triggered sample and hold circuit) only the proportion of the induced voltage is measured and voltages dependent on the electrode kinetics are not taken into account. Advantageously, electrochemically caused offset voltages could thus be eliminated from the measurement signal.

In a further embodiment of the invention it is provided that the

Evaluation unit is designed to calculate the flow rate by means of a calibration table. As stated above, in the measuring apparatus and method of the present invention, the flow rate calculation formulas known in the art may be used. These formulas take z. B. on that a linear

There is a relationship between the voltage measured at the electrodes and the flow rate of the medium. In practice, however, it has been shown that non-linearities often occur, which are different from the respective ones Design and possibly even of conditions when installing the measuring device, eg. B. in a pipe. Advantageously, therefore, a calibration is performed, d. H. the measuring device is flowed through with a medium of known flow rate and the resulting

Electrode voltage tapped and stored in a table. The

Calibration table can be stored in the evaluation unit and the

Flow rate can be calculated from the table. Alternatively, it is also conceivable that the evaluation unit first stores only the signals of the electrodes and a subsequent calibration takes place, d. H. the flow rates for recorded voltage values of the electrodes are only calculated later.

In order to enable the above-described functionalities on the evaluation unit, it can be provided, in particular, that the evaluation unit has a microcontroller. Likewise, it can be provided that the

MicroController controls a mounted on the measuring device display so that the current flow rate or the total volume of the flowed through medium can be read. Advantageously, the measuring device also has an interface with which the detected values can be transmitted to a computer or via the calibration data from the

Measuring device can be stored.

The electrodes of the measuring device advantageously have surfaces with high quality, so that electrical deposits are minimized and a detachment of the deposits is simplified.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. Show it :

Fig. la is a schematic representation of an inventive

Measuring device in which the signal extraction takes place according to the galvanic principle; Fig. Lb is a schematic representation of an inventive

Measuring device, in which the signal extraction takes place according to the capacitive principle;

Fig. 2 is a greatly simplified diagram of the voltage waveform to the

Electrodes of a measuring device according to the invention;

Fig. 3 is a perspective view of an inventive

Measuring device;

FIG. 4a is a side view of the measuring device of FIG. 3; FIG.

Figures 4b, 4c are cross-sectional views of those shown in Figures 3 and 4a

Measuring device.

In Fig. La a simplified representation of a measuring device according to the present invention with galvanic signal extraction is shown. The

Measuring device 10a in this case has two electrodes 12a, 12b which are in contact with the medium flowing through a pipe 11. Not shown is a magnet which generates a magnetic field (directed into the plane of the drawing). When the medium is flowing, this results in the charge separation and the attachment of positive particles 13a and negative particles 13b on the surfaces of the electrodes 12a, 12b. The electrodes 12a, 12b are connected via leads 14a, 14b to a switch 16 and a differential amplifier 20. The switch 16 is connected in parallel with the inputs of the differential amplifier 20. Of the

Differential amplifier advantageously has a very low

Temperature dependence and a low offset drift.

The switch 16 is switched by a drive unit 18, wherein the

Drive unit 18 has a timer (not shown) and also a

Evaluation unit 19 controls. The evaluation unit 19 has an analog / digital converter (not shown) and a memory unit.

FIG. 1b shows an exemplary embodiment of a measuring device 10b with capacitive signal extraction. The electrodes 12a, 12b are thus arranged outside the tube 11. The interconnection of the remaining elements corresponds to the interconnection of the elements in the measuring device 10a with galvanic signal extraction.

In Fig. 2 is the diagram (here only very simplified sketched) of a

Voltage profile at the electrodes of a measuring device according to the invention shown. During a first period 21, the switch is closed so that the electrodes and the inputs of the differential amplifier are short-circuited. After the first period, the switch is open for a second period of time 22. Immediately after opening the switch, a first voltage is applied to the electrodes. This first voltage can be measured as a zero point signal and used to calculate a corrected signal. During the second period 22, a transient takes place whose course depends, inter alia, on the capacitance of the measuring circuit. Typically, this leads to an increase in the voltage, with the voltage increase gradually leveling off. After expiration of the second time period, a measurement of the useful signal takes place during a third, very short time period 23, thus represented in the diagram as the time. For example, the measurement could be for a period of 1 ps. After the measurement of the useful signal, the switch is closed again for a first period of time and a new cycle begins.

Depending on the configuration of the measuring device, in particular of the electrodes and of the measuring circuit, completely different voltage profiles may result.

Fig. 3 shows a perspective view of a device according to the invention

Measuring device 10. The measuring device 10 has openings 30a, 30b for the inlet or outlet of the medium. The measuring device 10 also has a metallic, cylindrical casing 31. The casing 31 has as few recesses and openings as possible in order to suppress interference signals as much as possible. Placed on the casing 31 is a housing 32 for the evaluation unit and the drive unit. The evaluation unit and the drive unit can, for. B. be realized on the microcontroller. The housing 32 has an opening for a USB socket 34, so that a microcontroller of the measuring device 10 can be connected to a computer. The

Measuring device also has a display 33, on which the current

Flow rate and the total volume of flow through the medium can be read. The measuring device shown in Fig. 3 has no connection for an external power supply, but is powered by an integrated battery. Thus, interference can be prevented by fluctuations of an external power supply.

FIG. 4 a shows a side view of the measuring device shown in FIG. 3.

4b shows a cross section through the measuring device from FIG. 4a and FIG. 3. The cross section shows magnets 40a, 40b which are arranged on opposite sides of a bushing 45. The magnets 40a, 40b are thereby brought into position by screws 42 fastened to the housing holders 41a, 41b. When the cylindrical sheath 31 is removed, the screws 42 are accessible and the brackets can be loosened and removed. Thus, the magnets 41a and 41b become accessible and can be exchanged.

As can be seen from the cross-sectional view in FIG. 4B, the openings 30a, 30b have larger diameters than the passage 45. Thus, an increased flow rate is achieved in the passage 45. However, it is also conceivable in other embodiments that the implementation has the same or a larger diameter than the openings.

Fig. 4C shows a cross-sectional view through the plane in which the electrodes 12a, 12b are arranged. The electrodes are attached to base bodies 15a, 15b. The base body 15a, 15b are, when the sheath 31 is removed, accessible from the outside, so that the electrodes 12a, 12b can optionally be replaced with the base bodies 15a, 15b.

Not shown in FIG. 4C is the microcontroller on which the drive unit and the evaluation unit are located. The microcontroller should be arranged in the housing 32. Likewise, the switch 16 and an amplifier 20 are in the housing 32. In other embodiments, however, it can also be provided that the switch 16 is arranged directly on the electrodes 12a, 12b in order to achieve the lowest possible short circuit resistance. The leads 14a, 14b from the electrodes 12a, 12b to the switch 16 and amplifier 20 are thus different according to this embodiment long. In other embodiments, it may be provided that the

Supply lines are formed symmetrically.

The supply lines 14a, 14b are shown only shortened in Fig. 4C.

Advantageously, reinforced wires or strands are used for the leads, so that a low short-circuit resistance is achieved.

Claims

claims

1. Measuring device for measuring the flow rate of an electrically conductive medium in a penetrated by a magnetic field

Volume comprising

a means for generating the magnetic field,

at least two electrodes and

an evaluation unit which evaluates a signal of the electrodes and calculates the flow rate,

characterized in that the at least two electrodes are connected to a switch which is adapted to short the electrodes.

2. Measuring device according to claim 1, characterized in that the

Switch is an electronic switch.

3. Measuring device according to claim 1 or 2, characterized in that the means for generating the magnetic field is a permanent magnet.

4. Measuring device according to one of the preceding claims, characterized

characterized in that the switch comprises at least one transistor,

Preferably, a MOSFET, in particular a MOSFET in a CMOS IC comprises.

5. Measuring device according to one of the preceding claims, characterized

characterized in that the switch in the open state has a resistance of at least 10 GOhm, in particular at least 100 GOhm.

6. Measuring device according to one of the preceding claims, characterized

characterized in that the switch in the on state has a resistance of at most 1 ohm, preferably at most 0, 1 ohm and more preferably at most 10 mOhm.

7. Measuring device according to one of the preceding claims, characterized

characterized in that the measuring device comprises a drive unit for the switch, wherein the drive unit comprises at least one timer.

8. Measuring device according to claim 7, characterized in that the

Drive unit is designed such that the switch for the duration of a first period, in particular a period between 0.3 and 1 seconds, is closed and is open for the duration of a second period, in particular a period between 1 and 3 seconds, further the evaluation unit is designed such that the signal of the electrodes is evaluated immediately after the second Zeitrau m.

9. Measuring device according to one of the preceding claims, characterized

characterized in that the evaluation unit has a filter circuit for suppressing high-frequency interference signals.

10. Measuring device according to one of the preceding claims, characterized

characterized in that the evaluation unit is adapted to calculate the flow rate by means of a calibration table.

11. Method for measuring the flow rate of an electric

conductive medium in a volume penetrated by a magnetic field with at least two electrodes, with the steps:

Short-circuiting the electrodes during a first period, in particular a period between 0.3 and 1 second, opening the short circuit during a second period, in particular a period between 1 and 3 seconds,

Reading a useful signal from the electrodes and Calculate the flow rate based on the

read-out useful signal.

12. The method according to claim 11, characterized in that the readout of the useful signal takes place immediately after the second period.

13. The method according to claim 11 or 12, characterized in that

a zero point signal is read out immediately at the beginning of the second period and the flow rate is calculated on the basis of the zero point signal and the useful signal, in particular based on the difference between the zero point signal and the useful signal.

14. The method according to any one of claims 11 to 13, characterized in that the method steps are carried out periodically.