(19)
(11)EP 2 283 325 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
02.09.2020 Bulletin 2020/36

(21)Application number: 08773365.5

(22)Date of filing:  05.06.2008
(51)International Patent Classification (IPC): 
G01F 1/60(2006.01)
G01F 1/00(2006.01)
G01F 25/00(2006.01)
G01F 23/24(2006.01)
(86)International application number:
PCT/EP2008/004504
(87)International publication number:
WO 2009/146724 (10.12.2009 Gazette  2009/50)

(54)

ELECTROMAGNETIC FLOWMETER AND METHOD WITH FULL PIPE DETECTION BY THE HELP OF A THIRD ELECTRODE

ELEKTROMAGNETISCHES DURCHFLUSS-MESSGERÄT UND -MESSVERFAHREN MIT EINER DRITTEN ELEKTRODE ZUR FESTSTELLUNG, OB DIE ROHRLEITUNG VOLLSTÄNDIG GEFÜLLT IST

DÉBITMÈTRE ET PROCÉDÉ DE MESURE DE DÉBIT DU TYPE ÉLECTROMAGNÉTIQUE COMPRENANT UNE TROISIÈME ÉLECTRODE POUR DÉTECTER SI LE CONDUIT ET ENTIÈREMENT REMPLI


(84)Designated Contracting States:
AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MT NL NO PL PT RO SE SI SK TR

(43)Date of publication of application:
16.02.2011 Bulletin 2011/07

(73)Proprietor: Siemens Aktiengesellschaft
80333 München (DE)

(72)Inventor:
  • MATZEN, Steen, Moellebjerg
    DK-6470 Sydals (DK)


(56)References cited: : 
EP-A- 1 108 989
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description


    [0001] The present invention relates to an electromagnetic flowmeter for measuring the flow rate of a fluid in a pipe. More specifically, the invention relates to full pipe detection in an electromagnetic flowmeter.

    [0002] Electromagnetic flowmeters utilize the principle of electrodynamic induction for flow rate measurement of a fluid medium. In an electromagnetic flowmeter, a magnetic field is generated across a measuring section of the flowmeter pipe through which the medium flows, which, by operation of Faraday's law, generates a voltage orthogonal to both the flow of the medium and the magnetic field. The induced voltage is measured by a pair of electrodes on opposite sides of the measuring section. This induced voltage is proportional to the flow velocity of the medium to be measured averaged over the cross section of the pipe.

    [0003] Considerable measurement errors can occur if the measuring pipe is not filled completely but only partially with the medium to be measured, because the measuring apparatus bases the measuring results that are yielded by it on a completely filled measuring pipe. In addition to incomplete of the measuring pipe, factors such as gas bubbles in the medium, corrosion of electrodes, liner damage, electrode fouling, among others, also lead to measurement errors.

    [0004] The EP 1 108 989 B1 discloses an electromagnetic flowmeter having a measuring tube through which a medium to be measured flows, first and second electrodes arranged on opposite sides of the measuring tube and coupled electrically or capacitively to the medium, and a third electrode arranged at the top of the measuring tube. A test signal is passed to the third electrode, and the response signal to this test signal is received by the first and second electrodes, wherein, based on the response signal received by the first and second electrodes, the degree of filling of the measuring tube is determined.

    [0005] The object of the present invention is to provide an improved electromagnetic flowmeter.

    [0006] The above object is achieved by an electromagnetic flowmeter for measuring the flow of a medium through a measuring pipe, comprising:
    • first and second electrodes arranged on opposite sides of said measuring pipe and coupled electrically or capacitively to said medium,
    • a differential amplifier for amplifying a potential difference in signals from the electrodes and for providing the amplified output to a flow detection circuitry,
    • means for passing a test signal as a common mode signal to said first and second electrodes, wherein said test signal comprises a sequence of symmetrical pulses whose average value over a flow detection period is zero so that the measurement of the flow is not disturbed by the test signal,
    • a third electrode arranged at the top of said measuring pipe for receiving a response signal in response to the test signal passed to said first and second electrodes, and
    • means for detecting whether said third electrode is in contact with said medium based upon said response signal.


    [0007] The above object is also achieved by a method for measuring flow of a medium through a measuring pipe, comprising:
    • amplifying, by means of a differential amplifier, a potential difference in signals from first and second electrodes arranged on opposite sides of said measuring pipe and coupled electrically or capacitively to said medium and providing the amplified output to a flow detection circuitry,
    • passing a test signal as a common mode signal to said electrodes, wherein said test signal comprises a sequence of symmetrical pulses whose average value over a flow detection period is zero so that the measurement of the flow is not disturbed by the test signal,
    • measuring a response signal received at a third electrode arranged at the top of said measuring pipe in response to said test signal passed to said first and second electrodes, and
    • detecting whether said third electrode is in contact with said medium based upon said response signal.


    [0008] The underlying idea of the present invention is to perform full pipe detection on an electromagnetic flowmeter without major changes in the existing circuitry for flow detection and electrode impedance measurement. The proposed full pipe detection works further for remote installation and with very high/ low media conductivity.

    [0009] The test signal comprises a sequence of symmetrical pulses whose average value over a flow detection period is zero. This ensures that the flow measurements are not disturbed by the test signal.

    [0010] In a preferred embodiment, the means for detecting whether said third electrode is in contact with said medium is based upon a comparison of the electrode-to-ground impedance determined for said third electrode with electrode-to-ground-impedance of at least one of said first and second electrodes. This provides a higher reliability in detecting full pipe with different media conductivities.

    [0011] In an exemplary embodiment, the flowmeter further comprises means for measuring electrode-to-ground impedance for said first electrode, said means for measuring electrode-to-ground impedance for said first electrode further comprising:
    • means passing said test signal to said first electrode,
    • means for terminating the response signal from said third electrode to ground, and
    • means for measuring a potential difference between the first and the second electrode in response to said test signal passed to said first electrode.


    [0012] Electrode impedance measurement is useful in gauging various parameters such as conductivity of the medium, gas bubbles in the medium, corrosion of electrodes, fouling of electrodes, liner damage, and so on. The above embodiment facilitates electrode impedance measurement and full pipe detection using the same detection circuitry.

    [0013] To effectively determine the electrode-to-ground impedance of the third electrode, in an exemplary embodiment, the third electrode is terminated to ground via a resistor, wherein the electrode-to-ground impedance of the third electrode is determined by measuring a voltage across said resistor. The resistor also serves the purpose of terminating the input when the third electrode is not in contact with the medium, thus minimizing the noise due to stray currents.

    [0014] The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

    FIG 1 is a schematic diagram of an electromagnetic flowmeter in accordance with one embodiment of the present invention, and

    FIG 2 is an exemplary graphical plot of impedance measurement of the top electrode when the medium is potable water and the measuring pipe is full,

    FIG 3 is an exemplary graphical plot of impedance measurement of the top electrode when the medium is potable water and the measuring pipe is not full,

    FIG 4 is an exemplary graphical plot of impedance measurement of the top electrode when the medium is demineralized water and the measuring pipe is full, and

    FIG 5 is an exemplary graphical plot of impedance measurement of the top electrode when the medium is demineralized water and the measuring pipe is not full.



    [0015] An electromagnetic flowmeter apparatus 10 according to one embodiment of the present invention is now illustrated referring to FIG 1. A fluid medium 12, whose flow rate is to be measured, flows through a measuring pipe 14, along the direction of the axis 22 of the measuring pipe 14. The medium 12 to be measured is electrically conductive, at least to a slight extent.

    [0016] The flowmeter 10 includes a pair of electrodes 24 and 26 arranged on opposite sides of the measuring pipe 14 and coupled electrically or capacitively to the medium 12. A magnetic arrangement is provided including electromagnets 16 and 18 that generate a pulsed magnetic field 20, oriented perpendicularly to the flow direction of the medium 12. In an exemplary embodiment, the magnetic field 20 is a quasi-stationary DC magnetic field having a frequency of 6.25Hz for a mains supply of 50Hz, or a frequency of 7.5Hz for a mains supply of 60Hz. On account of this magnetic field 20, charge carriers in the medium 12 migrate to the electrodes 24 and 26 of opposite polarity. The potential difference which builds up across the electrodes 24 and 26 is proportional to the flow velocity of the medium 12 averaged over the cross-sectional area of the measuring pipe 14. A differential amplifier 36 amplifies this potential difference (i.e. the difference in the signals 54 and 52 from the electrodes 24 and 26 respectively) and provides the amplified output 60 to flow detection circuitry 64. The flow detection circuitry 64 calibrates the output 60 of the differential amplifier 36 to units of flow velocity or flow rate, and provides an output to output circuitry (not shown). In the illustrated example, the electrodes are in direct contact with the medium 12 to be measured. However, the coupling may also be of a capacitive nature.

    [0017] As described earlier, since the flowmeter 10 bases the measuring results that are yielded by it on a completely filled measuring pipe 14, considerable measurement errors can occur if the measuring pipe 14 is not filled completely but only partially with the medium 12 to be measured as shown in FIG 1. For the purpose of detecting whether the measuring pipe 14 is completely filled, a third electrode 28 is provided at the top of the measuring pipe 14. The impedance of the top electrode 28 to ground is indicative of whether the top electrode 28 is wetted by (i.e., in contact with) the medium 12. A fourth electrode 30 is provided at the bottom of the measuring pipe 14 and is grounded. A test signal 32 is passed to the electrodes 24 and 26 as a common mode signal, for example, via two capacitors (constant current source). In an exemplary embodiment, the test signal 32 is a pulsed current signal, comprising a sequence of symmetrical pulses such that the average value the test signal 32 is zero over a flow detection period. This ensures that the flow measurements are not disturbed by the test signal 32. In a preferred embodiment, the pulses have a repetition rate (i.e., pulse frequency) above a corner frequency (for example, above 200 Hz) to ensure that electrode impedance measurements at that frequency are asymptotically equal to the theoretical value of media impedance. For example, in case of a magnetic field frequency of 6.25Hz for a mains supply of 50Hz, the flow detection period is taken to be 20 milliseconds. In case of a magnetic field frequency of 7.5Hz for a mains supply of 60Hz, the flow detection period is taken to be 16.67 milliseconds. In the above examples, the test signal 32 may have a pulse frequency 400Hz for a magnetic field frequency of 6.25Hz, or a pulse frequency 480Hz for a magnetic field of frequency 7.5Hz. The test signal 32 passed to the electrodes 24 and 26 is transformed into voltage in the medium 12, which appears as a response signal 56 at the top electrode 28. Since the test signal 32 appears at the electrodes 24 and 26 as a common mode signal that is rejected by the differential amplifier 36, the output 60 of the differential amplifier 36 comprises essentially a flow velocity measurement signal.

    [0018] The top electrode is terminated to ground by a resistor 40. The voltage induced response signal 56 at the top electrode 28 forces a current into the resistor 40 via the electrode-to-ground impedance of the top electrode 28. Electrode-to-ground impedance of the top electrode 28, different to zero, may be measured only when the top electrode is in contact with the medium 12. With no media contact at the top electrode 28, no signal is transferred to the resistor 40 that appears as very low impedance (in theory, zero impedance) to the detection circuitry, which thus detects an incompletely filled pipe. The resistor 40 further serves the purpose of terminating the input when the top electrode 28 is not in contact with the medium 12, thus minimizing noise due to stray currents. The electrode-to-ground impedance of the top electrode 28 is determined from a ratio of difference of the positive and negative values of the detected voltage across the resistor 40 over a flow detection period (i.e. 20 milliseconds for a 50Hz system, and 16.67 milliseconds for a 60 Hz system) to the current from the test signal 32.

    [0019] As shown in FIG 1, the detected voltage across the resistor 40 is buffered by a buffer amplifier 38. Summation circuitry 48 is provided for summing the output 58 of the buffer amplifier 38, with the output 60 of the differential amplifier 36. The output 62 of the summation circuitry 48 thus comprises a top electrode impedance signal superimposed on the flow velocity measurement signal. This output 62 is transmitted to the flow detection circuitry 64 for simultaneous measurement of flow velocity as well as full pipe detection based top electrode impedance measurement as described above.

    [0020] The electrode-to-ground impedance of the top electrode 28 depends on the conductivity of the medium 12. To ensure a higher reliability in detecting a full pipe with media having different conductivities, the full pipe detection is carried out based on a comparison of the electrode-to-ground impedance determined for the top electrode 28 with the electrode-to-ground-impedances of at least one of the electrodes 24 and 26. If the electrode-to-ground impedance of the top electrode 28 is comparable to, or lies within predetermined tolerances around electrode-to-ground impedance of the electrodes 24 and 26 , it is an indication that the top electrode 28 is in contact with the medium (i.e., the measuring pipe 14 is completely filled). If the electrode-to-ground impedance of the top electrode 28 is significantly lesser than electrode-to-ground impedance of the first and second electrodes 24 and 26, it is an indication that the top electrode 28 is not in contact with the medium 12 (i.e., the measuring pipe 14 is not completely filled).

    [0021] FIGs 2-5 illustrate the variation of the top electrode impedance with varying media conductivities. In FIG 2, an exemplary plot 76 of the electrode-to-ground impedance (in ohms) of the top electrode (represented along an axis 72) with time in seconds (represented along an axis 74) is shown, wherein the medium is potable water and measuring pipe is completely full. The mean impedance as calculated from this plot is 549.96 ohms. In FIG 3, a plot 86 of the electrode-to-ground impedance (in ohms) of the top electrode (represented along an axis 82) with time in seconds (represented along an axis 84) is shown in a scenario when measuring pipe is not completely full, the medium being the same, i.e. potable water. The mean impedance as calculated in this case is 101.86 ohms. As can be seen the electrode-to-ground impedance of the top electrode is greatly reduced when the top electrode is not in contact with the medium. However, as illustrated in FIG 4 and 5, the measured electrode-to-ground impedance of the top electrode is significantly higher when a medium of lower conductivity, such as demineralized water, is used. In FIG 4, an exemplary plot 96 of the electrode-to-ground impedance (in ohms) of the top electrode (represented along an axis 92) with time in seconds (represented along an axis 94) is shown, wherein the medium is demineralized water and measuring pipe is completely full. The mean impedance as calculated from this plot is 4615.96 ohms. In FIG 5, a plot 106 of the electrode-to-ground impedance (in ohms) of the top electrode (represented along an axis 102) with time in seconds (represented along an axis 104) is shown in a scenario when measuring pipe is not completely full, the medium being the same, i.e. demineralized water. The mean impedance as calculated in this case is 108.83 ohms. In order to detect full pipe even with very high/ low conductivities, the information on the electrode impedance can be used for adapting the full pipe detection level to the actual media conductivity. The information on electrode impedance of the top electrode can be further used for detecting small air bubbles gathered at the top of the flowmeter. This will show as an increased noise level in the top electrode impedance measurement.

    [0022] Referring back to FIG 1, to determine the electrode-to-ground impedance of the electrode 24, a test signal (typically, a current signal) is injected at only the electrode 24 and the differential voltage between the electrodes 24 and 26 by the differential amplifier 36 is measured. The electrode-to-ground impedance of the electrode 24 is determined by calculating a ratio of the differential voltage between the electrodes 24 and 26 and the current at the test signal. Likewise, to determine the electrode-to-ground impedance of the electrode 26, a test signal (typically, a current signal) is injected at only the electrode 26 and the differential voltage between the electrodes 26 and 24 by the differential amplifier 36 is measured. The electrode-to-ground impedance of the electrode 26 is determined by calculating a ratio of the differential voltage between the electrodes 26 and 24 and the current at the test signal. Switching means 42 is adapted to terminate the output signal 58 of the buffer amplifier 38 to ground (position 44) during impedance measurements of electrodes 24 and 26, and to couple the output 58 to the summation circuitry 48 (position 46) during full pipe detection. Advantageously, test signal used in electrode impedance measurement may be the same test signal 32 generated from the same signal source that is used for full pipe detection. Further advantageously, the detection circuitry and the associated filtering circuitry for electrode impedance may be the same as used for flow detection. In the illustrated embodiment, a flow transmitter analog-to-digital converter (not shown) samples the differential electrode signal with a relative high sampling rate (e.g. 9.6KHz) and the same data is used for electrode impedance and flow detection by using a correlation detection method. Electrode impedance measurement for the electrodes 24 and 26 is useful in gauging various parameters such as conductivity of the medium, gas bubbles in the medium, corrosion of electrodes, fouling of electrodes, liner damage, and so on.

    [0023] The present invention is advantageous in a number of ways. First, the proposed full pipe detection test does not interfere with the flow detection measurement and uses the same circuitry as used in flow detection and electrode impedance measurement. Furthermore, in the illustrated embodiments, 50Hz/ 60Hz rejection on the full pipe detection is inherent. Also, the proposed full pipe detection works also for remote installation and with very high/ low media conductivity.

    [0024] Summarizing, the present invention deals with an electromagnetic flowmeter for measuring the flow of a medium through a measuring pipe. The proposed flowmeter includes first and second electrodes arranged on opposite sides of the measuring pipe and coupled electrically or capacitively to the medium. A common mode test signal is passed to the first and second electrodes. A third electrode is provided at the top of the measuring pipe for receiving a response signal in response to the test signal passed to the first and second electrodes. The flowmeter comprises means for detecting whether said third electrode is in contact with said medium based upon said response signal.

    [0025] Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the scope of the present invention as defined by the below-mentioned patent claims.


    Claims

    1. An electromagnetic flowmeter (10) for measuring the flow of a medium (12) through a measuring pipe (14), comprising:

    - first and second electrodes (24, 26) arranged on opposite sides of said measuring pipe (14) and coupled electrically or capacitively to said medium (12),

    - a differential amplifier (36) for amplifying the potential difference in signals (54, 52) from the electrodes (24, 26) and for providing the amplified output (60) to a flow detection circuitry (64),

    - means for passing a test signal (32) as a common mode signal to said first and second electrodes (24, 26), wherein said test signal (32) comprises a sequence of symmetrical pulses whose average value over a flow detection period is zero so that the measurement of the flow is not disturbed by the test signal,

    - a third electrode (28) arranged at the top of said measuring pipe (14) for receiving a response signal (56) in response to the test signal (32) passed to said first and second electrodes (24, 26),

    - means for detecting whether said third electrode (28) is in contact with said medium (12) based upon said response signal (56).


     
    2. The flowmeter (10) according to claim 1, wherein said means for detecting whether said third electrode (28) is in contact with said medium (12) comprises means for determining an electrode-to-ground impedance of said third electrode (28), wherein said detection is based upon a comparison of the electrode-to-ground impedance determined for said third electrode (28) with electrode-to-ground-impedance of at least one of said first and second electrodes (24, 26).
     
    3. The flowmeter (10) according to any of the preceding claims, further comprising means for measuring electrode-to-ground impedance for said first electrode (24), said means for measuring electrode-to-ground impedance for said first electrode (24) further comprising:

    - means passing said test signal (32) to said first electrode (24),

    - means for terminating the response signal from said third electrode (28) to ground, and

    - means for measuring a potential difference between the first and the second electrode (24, 26) in response to said test signal (32) passed to said first electrode (24).


     
    4. The flowmeter (10) according to any of the preceding claims, wherein said third electrode (28) is terminated to ground via a resistor (40), the electrode-to-ground impedance of said third electrode (28) being determined by measuring a voltage across said resistor (40).
     
    5. A method for measuring flow of a medium (12) through a measuring pipe (14), comprising:

    - amplifying, by means of a differential amplifier, a potential difference in signals from first and second electrodes arranged on opposite sides of said measuring pipe and coupled electrically or capacitively to said medium and providing the amplified output to a flow detection circuitry,

    - passing a test signal (32) as a common mode signal to said electrodes (24, 26), wherein said test signal comprises a sequence of symmetrical pulses whose average value over a flow detection period is zero so that the measurement of the flow is not disturbed by the test signal,

    - measuring a response signal (56) received at a third electrode (28) arranged at the top of said measuring pipe (14) in response to said test signal (32) passed to said first and second electrodes (24, 26), and

    - detecting whether said third electrode (28) is in contact with said medium (12) based upon said response signal (56).


     
    6. The method according to claim 5, wherein detecting whether said third electrode (28) is in contact with said medium (12) comprises determining an electrode-to-ground impedance of said third electrode (28), wherein said detection is based upon a comparison of the electrode-to-ground impedance determined for said third electrode (28) with electrode-to-ground-impedance of at least one of said first and second electrodes (24, 26).
     
    7. The method according to any of claims 5 and 6, further comprising measuring electrode-to-ground impedance for said first electrode (24), wherein measuring the electrode-to-ground impedance for said first electrode (24) further comprises:

    - passing said test signal (32) to said first electrode (24),

    - terminating the response signal (56) from said third electrode (28) to ground, and

    - measuring a potential difference between the first and the second electrode (24, 26) in response to said test signal (32) passed to said first electrode (24) .


     
    8. The method according to any of claims 5-7, wherein determining the electrode-to-ground impedance of the third electrode (28) comprises terminating said third electrode (28) to ground via a resistor (40) and measuring a voltage across said resistor (40).
     


    Ansprüche

    1. Elektromagnetisches Durchflussmessgerät (10) zum Messen des Durchflusses eines Mediums (12) durch ein Messrohr (14), das Folgendes umfasst:

    - eine erste und zweite Elektrode (24, 26), die auf gegenüberliegenden Seiten des Messrohres (14) angeordnet und elektrisch oder kapazitiv mit dem Medium (12) gekoppelt sind,

    - einen Differenzverstärker (36) zum Verstärken der Potentialdifferenz von Signalen (54, 52) von den Elektroden (24, 26) und zum Liefern der verstärkten Ausgabe (60) an eine Durchflussdetektionsschaltungsanordnung (64),

    - Mittel zum Weitergeben eines Testsignals (32) als ein Gleichtaktsignal an die erste und zweite Elektrode (24, 26), wobei das Testsignal (32) eine Sequenz symmetrischer Pulse umfasst, deren Durchschnittswert über eine Durchflussdetektionsperiode null ist, so dass die Messung des Durchflusses nicht durch das Testsignal verzerrt wird,

    - eine dritte Elektrode (28), die auf dem Messrohr (14) angeordnet ist, zum Empfangen eines Antwortsignals (56) als Reaktion auf das Testsignal (32), das an die erste und zweite Elektrode (24, 26) weitergegeben wird,

    - Mittel zum Detektieren, ob sich die dritte Elektrode (28) in Kontakt mit dem Medium (12) befindet, basierend auf dem Antwortsignal (56).


     
    2. Durchflussmessgerät (10) nach Anspruch 1, wobei das Mittel zum Detektieren, ob sich die dritte Elektrode (28) in Kontakt mit dem Medium (12) befindet, Mittel zum Bestimmen einer Elektrode-Masse-Impedanz der dritten Elektrode (28) umfasst, wobei die Detektion auf einem Vergleich der Elektrode-Masse-Impedanz, die für die dritte Elektrode (28) bestimmt wird, mit einer Elektrode-Masse-Impedanz der ersten und/oder zweiten Elektrode (24, 26) basiert.
     
    3. Durchflussmessgerät (10) nach einem der vorhergehenden Ansprüche, das ferner Mittel zum Messen einer Elektrode-Masse-Impedanz der ersten Elektrode (24) umfasst, wobei das Mittel zum Messen der Elektrode-Masse-Impedanz für die erste Elektrode (24) ferner Folgendes umfasst:

    - Mittel zum Weitergeben des Testsignals (32) an die erste Elektrode (24),

    - Mittel zum Abschließen des Antwortsignals von der dritten Elektrode (28) gegenüber Masse, und

    - Mittel zum Messen einer Potentialdifferenz zwischen der ersten und zweiten Elektrode (24, 26) als Reaktion darauf, dass das Testsignal (32) an die erste Elektrode (24) weitergegeben wird.


     
    4. Durchflussmessgerät (10) nach einem der vorhergehenden Ansprüche, wobei die dritte Elektrode (28) über einen Widerstand (40) gegenüber Masse abgeschlossen wird, wobei die Elektrode-Masse-Impedanz der dritten Elektrode (28) durch Messen einer Spannung über den Widerstand (40) bestimmt wird.
     
    5. Verfahren zum Messen eines Durchflusses eines Mediums (12) durch ein Messrohr (14), das Folgendes umfasst:

    - Verstärken, mittels eines Differenzverstärkers, einer Potentialdifferenz von Signalen von einer ersten und zweiten Elektrode, die auf gegenüberliegenden Seiten des Messrohres angeordnet und elektrisch oder kapazitiv mit dem Medium gekoppelt sind, und Liefern der verstärkten Ausgabe an eine Durchflussdetektionsschaltungsanordnung,

    - Weitergeben eines Testsignals (32) als ein Gleichtaktsignal an die Elektroden (24, 26), wobei das Testsignal eine Sequenz symmetrischer Pulse umfasst, deren Durchschnittswert über eine Durchflussdetektionsperiode null ist, so dass die Messung des Durchflusses nicht durch das Testsignal verzerrt wird,

    - Messen eines Antwortsignals (56), das an einer dritten Elektrode (28), die auf dem Messrohr (14) angeordnet ist, empfangen wird, als Reaktion darauf, dass das Testsignal (32) an die erste und zweite Elektrode (24, 26) weitergegeben wird, und

    - Detektieren, ob sich die dritte Elektrode (28) in Kontakt mit dem Medium (12) befindet, basierend auf dem Antwortsignal (56).


     
    6. Verfahren nach Anspruch 5, wobei das Detektieren, ob sich die dritte Elektrode (28) in Kontakt mit dem Medium (12) befindet, Bestimmen einer Elektrode-Masse-Impedanz der dritten Elektrode (28) umfasst, wobei die Detektion auf einem Vergleich der Elektrode-Masse-Impedanz, die für die dritte Elektrode (28) bestimmt wird, mit einer Elektrode-Masse-Impedanz der ersten und/oder zweiten Elektrode (24, 26) basiert.
     
    7. Verfahren nach einem der Ansprüche 5 und 6, das ferner Messen einer Elektrode-Masse-Impedanz für die erste Elektrode (24) umfasst, wobei das Messen der Elektrode-Masse-Impedanz für die erste Elektrode (24) ferner Folgendes umfasst:

    - Weitergeben des Testsignals (32) an die erste Elektrode (24),

    - Abschließen des Antwortsignals (56) von der dritten Elektrode (28) gegenüber Masse, und

    - Messen einer Potentialdifferenz zwischen der ersten und zweiten Elektrode (24, 26) als Reaktion darauf, dass das Testsignal (32) an die erste Elektrode (24) weitergegeben wird.


     
    8. Verfahren nach einem der Ansprüche 5-7, wobei das Bestimmen der Elektrode-Masse-Impedanz der dritten Elektrode (28) Abschließen der dritten Elektrode (28) über einen Widerstand (40) gegenüber Masse und Messen einer Spannung über den Widerstand (40) umfasst.
     


    Revendications

    1. Débitmètre (10) électromagnétique pour mesurer le débit d'un fluide (12) dans un tuyau (14) de mesure, comprenant :

    - des première et deuxième électrodes (24, 26), disposées sur des côtés opposés du tuyau (14) de mesure et couplées électriquement ou capacitivement au milieu (12),

    - un amplificateur (36) différentiel pour amplifier la différence de potentiel en des signaux (54, 52) provenant des électrodes (24, 26) et pour fournir la sortie (60) amplifiée à un circuit (64) de détection de débit,

    - des moyens pour envoyer un signal (32) de test, en tant que signal en mode commun, aux première et deuxième électrodes (24, 26), le signal (32) de test comprenant une séquence d'impulsions symétriques, dont la valeur moyenne sur la durée de détection du débit est de zéro, de sorte que la mesure du débit n'est pas perturbée par le signal de test,

    - une troisième électrode (28), disposée au sommet du tuyau (14) de mesure, pour recevoir un signal (56) de réaction en réaction au signal (32) de test envoyé aux première et deuxième électrodes (24, 26),

    - un moyen pour détecter si la troisième électrode (29) est en contact avec le milieu (12) sur la base du signal (56) de réaction.


     
    2. Débitmètre (10) suivant la revendication 1, dans lequel le moyen de détection, si la troisième électrode (28) est en contact avec le milieu (32), comprend un moyen de détermination d'une impédance électrode à terre de la troisième électrode (28), la détection reposant sur une comparaison de l'impédance électrode à terre déterminée pour la troisième électrode (28) à une impédance électrode à terre d'au moins l'une de la première et de la deuxième électrodes (24, 26).
     
    3. Débitmètre (10) suivant l'une quelconque des revendications précédentes, comprenant, en outre, un moyen de mesure de l'impédance électrode à terre pour la première électrode (24), le moyen de mesure de l'impédance électrode à terre pour la première électrode (24) comprenant, en outre :

    - un moyen pour envoyer le signal (32) de test à la première électrode (24),

    - un moyen pour mettre fin au signal de réaction de la troisième électrode (28) à la terre, et

    - un moyen de mesure d'une différence de potentiel entre la première et la deuxième électrodes (24, 26) en réaction au signal (32) de test envoyé à la première électrode (24).


     
    4. Débitmètre (10) suivant l'une quelconque des revendications précédentes, dans lequel la troisième électrode (28) est mise à la terre par l'intermédiaire d'une résistance (40), l'impédance électrode à terre de la troisième électrode (28) étant déterminée en mesurant une tension aux bornes de la résistance (40).
     
    5. Procédé de mesure du débit d'un milieu (12) dans un tuyau (14) de mesure, dans lequel :

    - on amplifie, au moyen d'un amplificateur différentiel, une différence de potentiel en des signaux provenant de la première et deuxième électrodes disposées sur des côtés opposés du tuyau de mesure et couplées électriquement ou capacitivement au milieu et on fournit la sortie amplifiée à un circuit de détection de débit,

    - on envoie un signal (32) de test, en tant que signal en mode commun, aux électrodes (24, 26), le signal de test comprenant une séquence d'impulsions symétriques, dont la valeur moyenne sur la durée de détection du débit est de zéro, de sorte que la mesure du débit n'est pas perturbée par le signal de test,

    - on mesure un signal (56) de réaction reçu à une troisième électrode (28) disposée au sommet du tuyau (14) de mesure en réaction au signal (32) de test envoyé aux première et deuxième électrodes (24,26), et

    - on détecte si la troisième électrode (28) est en contact avec le milieu (12) sur la base du signal (56) de réaction.


     
    6. Procédé suivant la revendication 5, dans lequel détecter si la troisième électrode (28) de mesure est en contact avec le milieu (12) comprend déterminer une impédance électrode à terre de la troisième électrode (28), la détection reposant sur une comparaison de l'impédance électrode à terre déterminée pour la troisième électrode (28) à une impédance électrode à terre d'au moins l'une de la première et de la deuxième électrodes (24, 26).
     
    7. Procédé suivant l'une quelconque des revendications 5 et 6, comprenant, en outre, mesurer une impédance d'électrode à terre pour la première électrode (24), mesurer l'impédance électrode à terre pour la première électrode (24) comprenant, en outre :

    - envoyer le signal (32) de test à la première électrode (24),

    - acheminer le signal (56) de réaction de la première électrode (28) à la terre, et

    - mesurer une différence de potentiel entre la première et la deuxième électrode (24, 26) en réaction au signal (32) de test envoyé à la première électrode (24).


     
    8. Procédé suivant l'une quelconque des revendications 5 à 7, dans lequel déterminer l'impédance électrode à terre de la troisième électrode (28) comprend mettre la troisième électrode (28) à la terre par l'intermédiaire d'une résistance (40) et mesurer une tension aux bornes de la résistance (40).
     




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    Cited references

    REFERENCES CITED IN THE DESCRIPTION



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    Patent documents cited in the description