Electromagnetic flowmeter having corrosion protection of measuring electrodes, and method incorporating the same

The present invention relates to electromagnetic flowmeters, and in particular, to cathodic protection of measuring electrodes in electromagnetic flowmeters.

Electromagnetic flowmeters, also known as magnetic inductive 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 perpendicular 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 measured by these electrodes is proportional to the flow velocity of the medium to be measured averaged over the cross section of the pipe.

The measuring electrodes used in electromagnetic flowmeters may be made from a range of different materials depending on the application that the flowmeter is intended for. The suitability of the material of the measuring electrodes for a given media is often not a trivial task, since corrosion effects have to be taken into account. Sometimes a very exotic (expensive) material like platinum has to be used in order to withstand the corrosion effects of a specific media. Furthermore in case of corrosion the noise level on the flow measurement is increased.

The object of the present invention is to provide an improved electromagnetic flowmeter having corrosion protection of the measuring electrodes. The above object is achieved by an electromagnetic flowmeter in accordance with claim 1. The above object is also achieved by a method in accordance with claim A .

The underlying idea of the present invention is to provide a technique of corrosion protection of measuring electrodes of an electromagnetic flowmeter that obviates the task of selecting suitable materials measuring electrodes for different kind of media. The proposed method uses a constant direct current to lower the electrode potential of the measuring electrodes to a level to render it immune to corrosion with respect to the medium. Thus it is possible to use cheaper material for the measuring electrodes.

In one embodiment, said threshold value is determined based upon a Pourbaix diagram of the material of that metallic measuring electrode, such that when the electrode potential of that metallic measuring electrode is lowered below said threshold value, that metallic measuring electrode achieves a state of immunity to corrosion independent of the pH of the medium. This set up can be advantageously used for many applications irrespective of the nature of the medium, without the user having to adjust the electrode potential of the measuring electrodes for each instance.

In a preferred embodiment, said flowmeter further comprises flow measurement means for measurement of flow of said medium based upon output signals from said measuring electrodes, wherein said current source has an output impedance that does not significantly lower the input impedance of said flow measurement means. The advantage of this embodiment is that the measurement system does not inflict on the electrode voltage signal which can have relative high output impedance. The output impedance depends on the media conductivity and the electrode diameter. The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1 is an electromagnetic flowmeter according to one embodiment of the present invention, and

FIG 2 is an exemplary Pourbaix diagram illustrating corrosion effects on the material of a measuring electrode with varying pH of the medium.

An electromagnetic flowmeter apparatus 10 according to one embodiment of the present invention is now illustrated referring to FIG 1. A fluid medium 14, whose flow rate is to be measured, flows through a measuring tube 12, along the direction of the axis 40 of the measuring tube 12. The medium 14 to be measured is electrically conductive, at least to a slight extent.

The flowmeter 10 includes a pair of measuring electrodes 16 and 18 arranged on opposite sides of the measuring tube 12 and in contact with the medium 14. A third electrode 20 is usually provided at the bottom of the measuring tube 12 and is grounded. A magnetic arrangement is provided including electromagnets 34 and 36 that generate a magnetic field 38, oriented perpendicularly to the flow direction of the medium 14. On account of this magnetic field 38, charge carriers in the medium 14 migrate to the measuring electrodes 16 and 18 of opposite polarity. The potential difference which builds up across the electrodes 16 and 18 is proportional to the flow velocity of the medium 14 averaged over the cross- sectional area of the measuring tube 12. A flow measurement means, such as a differential amplifier 26, amplifies this potential difference (i.e. the difference in the output signals 30 and 31 from the measuring electrodes 16 and 18 respectively) and provides the amplified output 32 to flow detection circuitry 28. The flow detection circuitry 28 calibrates the output 32 of the differential amplifier 26 to units of flow velocity or flow rate, and provides an output to output circuitry (not shown) .

The measuring electrodes 16 and 18 being made from electrically conductive metallic material are prone to corrosion effect from the medium 14. In order to determine the tendency to corrode of the metallic material of the measuring electrodes, a Pourbaix diagram may be used. An exemplary Pourbaix diagram 50 is illustrated referring to FIG 2. The illustrated Pourbaix diagram 50 is a plot of electrode potential E of a measuring electrode (in volts V), represented along the axis 52, and pH of the medium, represented along the axis 54. As known to one skilled in the art, a Pourbaix diagram maps out possible stable (eguilibrium) phases of an electrochemical system. Predominant ion boundaries are represented by lines. Referring to FIG 2, such stable phases are denoted by the regions 56, 58, 60 and 62 in the Pourbaix diagram 50. Regions 56 and 60 indicate states of corrosion, region 58 indicates a state of passivation, while the region 62 indicates a state of immunity. Based upon the above described Pourbaix diagram 50, cathodic protection of the measuring electrodes 16 and 18 may be achieved by lowering the electrode potential each of the measuring electrodes 16 and 18 to within the region 62, such that the material of the electrodes become immune to corrosion.

Referring back to FIG 1, in accordance with the present invention, the flowmeter 10 is provided with one or more current sources 22 and 24 for passing a constant direct current to each of the measuring electrodes 16 and 18 that will lower the electrode potential of these electrodes with respect to the medium 14 below a threshold value beyond which these electrodes achieve a state of immunity with respect to the medium 14. The advantage of incorporating a direct current source at the measuring electrodes is that it does not interfere with the flow signal from the measuring electrodes. This is because flow detection circuitry (28) usually comprises a high pass filter that rejects the DC voltage (caused by the current source and the inherent DC voltage at the electrodes) . Further, as seen from FIG 2, if the electrode potential of a measuring electrode is lowered beyond a specific threshold value 62, the measuring electrode would achieve a state of immunity regardless of the pH of the medium. This set up can be thus advantageously used for many applications irrespective of the nature of the medium, without the user having to adjust the electrode potential of the measuring electrodes for each instance.

Referring to FIG 1, the differential amplifier 26 has high input impedance, for example, of the order of 10 ¹² Ω. The electrode impedance of a measuring electrode is a function of the conductivity of the medium and the mechanical properties of the electrode, such as the diameter. For example, for an electrode diameter of lmm and media conductivity of 5μS/cm, the resulting electrode impedance is of 10 ⁶ Ω. The flow signal (output signals) from the measuring electrodes is thus a voltage source of relative high output impedance. Thus, in a preferred embodiment, the current sources 22 and 24 have a high output impedance that not inflict on the flow signal 32 by lowering the input impedance of the flow measurement means 26.

The advantage of the present invention is that it makes it possible to use electrodes of relative poor corrosion properties in connection with very aggressive media. The main advantage of this is lowering the sensor cost and minimizing the stock of different sensors with different electrode material .

Summarizing, the present invention provides a system and method for corrosion protection of measuring electrodes in electromagnetic flowmeters. In accordance with the present invention, an electromagnetic flowmeter for measuring the flow of a medium through a measuring tube comprises first and second metallic measuring electrodes arranged on opposite sides of said measuring tube and in contact with said medium. The rate of flow of said medium in said measuring tube is measurable by measuring output signals from said measuring electrodes representative of a potential difference between said measuring electrodes induced by the flow of said medium in said measuring tube in the presence of a magnetic field. The proposed flowmeter further comprises a current source for passing a constant direct current to each of said metallic measuring electrodes. The constant direct current passed to each metallic measuring electrode is adapted to lower the electrode potential of the respective measuring electrode with respect to said medium below a threshold value to render that metallic measuring electrode immune to corrosion with respect to said medium.

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 spirit or scope of the present invention as defined by the below-mentioned patent claims.