Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
2002-05-31
2003-10-21
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
C073S861120
Reexamination Certificate
active
06634238
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electromagnetic flowmeters comprising a flow sensor, control electronics, and evaluation electronics. In the following, only flowmeters or flow sensors will be spoken of if necessary for simplicity, and to a method of operating the electromagnetic flowmeter.
BACKGROUND OF THE INVENTION
As is well known, electromagnetic flowmeters measure the volumetric flow rate of an electrically conductive liquid flowing through a pipe; thus, per definitionem, the liquid volume flowing through a pipe cross section per unit time is measured.
The flowmeter has a, usually nonferromagnetic, flow tube which is connected into the pipe in a liquid-tight manner, e.g., by means of flanges or threaded joints. The portion of the flow tube which contacts the liquid is generally electrically nonconductive, so that a voltage induced in the liquid according to Faraday's law of electromagnetic induction by a magnetic field cutting across the flow tube will not be short-circuited.
Therefore, metal flow tubes are commonly provided with a nonconductive lining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and are generally nonferromagnetic; in the case of flow tubes made completely of plastic or ceramic, particularly of alumina ceramic, the nonconductive lining is not necessary.
The magnetic field is produced by means of a coil assembly comprising at least two field coils, each of which is positioned on the flow tube along a diameter of the latter. The field coils may be air-core coils or coils with a core of soft magnetic material.
To ensure that the magnetic field produced by the field coils is as homogeneous as possible, the coils are, in the most frequent and simplest case, identical and electrically connected in series, so that in operation they can be traversed by the same excitation current. It is also known to cause the same excitation current to flow through the field coils alternately in the same and the opposite direction in order to be able to determine a flow profile and/or a liquid level in the pipe, see U.S. Pat. No. 5,493,914, or in order to be able to measure the viscosity of non-Newtonian fluids, see U.S. Pat. No. 5,646,353.
The excitation current just mentioned is produced by control electronics; it is regulated at a constant value of, e.g., 85 mA, and its direction is periodically reversed; this serves in particular to largely compensate electrochemical interference voltages developed at the electrodes. The current reversal is achieved by incorporating the field coils in a so-called T network or a so-called H network; for the current regulation and current reversal, see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.
The aforementioned induced voltage is picked off by means of at least two galvanic, i.e., liquid-wetted, electrodes, or by means of at least two capacitive electrodes, i.e., two electrodes disposed in the wall of the flow tube, for example, which in the most frequent case are arranged at diametrically opposed positions such that their common diameter is perpendicular to the direction of the magnetic field and, thus, perpendicular to the diameter on which the field coils are located. The induced voltage is conditioned by means of evaluation electronics to obtain a volumetric flow rate signal, which is recorded, displayed, or further processed.
Electromagnetic flowmeters measure volumetric flow rate with optimum accuracy if the flow in the flow tube is uniformly turbulent. Under this condition of uniform turbulence, each flowmeter is calibrated by the manufacturer, and the values of the so-called calibration factor and the zero drift, which are determined during this calibration, are electronically stored in the flowmeter.
To ensure that after its sale, the flowmeter can be operated with this accuracy in the field, the manufacturer generally specifies an undisturbed inlet section, which is a straight tube length and must be present or be inserted between the flowmeter and a spot of the pipe which disturbs or may disturb the uniform turbulence. Such pipe spots are, for instance, elbows, valves, etc.
During operation of the flowmeter, however, the uniformly turbulent flow profile thus generated may become nonuniform despite the inlet section as a result of unforeseeable events or changes in the liquid. This makes the measurement results inherently more inaccurate and in the worst case even may invalidate the measurement result without this being noticeable.
It is therefore desirable to detect such accuracy-reducing events during measurements, i.e., to derive a corresponding error signal, which then is displayed, triggers an alarm, or serves to correct the measurement result, etc.
To determine the flow profile, but particularly to compensate disturbances in the flow profile, U.S. Pat. No. 5,325,724 proposes to cause excitation currents to flow through both field coils, which produce equidirectional, but temporarily differently strong partial magnetic fields. Investigations have shown, however, that these solutions proposed in U.S. Pat. No. 5,325,724 do not produce the desired effect, namely a significantly asymmetric magnetic field, particularly in the area of the measuring electrodes.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method and a flowmeter for carrying out this method whereby in operation a significantly asymmetric magnetic field can be produced, so that the flow profile can be monitored with high reliability.
To attain this object, the invention provides a method of operating an electromagnetic flowmeter having a flow tube connected into a fluid-conveying line, said method comprising the steps of:
causing the fluid to flow through the flow tube;
causing a first excitation current of predeterminable strength, generated by means of a measuring and control circuit of the flowmeter, to flow through a first field coil mounted on the flow tube for producing a first partial magnetic field of predeterminable average strength which cuts through the fluid;
causing a second excitation current of predeterminable strength, generated by means of the measuring and control circuit, to flow through a second field coil mounted on the flow tube for producing a second partial magnetic field of predeterminable average strength which also cuts through the fluid;
varying the strength of at least one of the excitation currents in such a manner that the average strengths of the partial magnetic fields are at least temporarily different from each other;
reversing the polarity of one of the two excitation currents in such a manner that the two partial magnetic fields are at least temporarily directed opposite to each other while having different average strengths;
inducing a voltage in the moving fluid traversed by the partial magnetic fields for changing potentials applied to measuring electrodes positioned at the flow tube; and
picking off potentials applied to the measuring electrodes for producing a measurement signal derived from the voltage induced in the moving fluid.
Furthermore, the invention provides a method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and moving fluid, said flowmeter having a flow sensor comprising:
a flow tube for the moving fluid, of which an inner portion, which contacts the fluid, is electrically nonconductive, and which has a tube wall;
a first electrode positioned at or in the flow tube and a second electrode positioned at or in the flow tube, which electrodes are located on a first diameter of the flow tube;
a coil assembly, mounted on the flow tube and comprising a first field coil and a second field coil,
said coil assembly being located on a second diameter of the flow tube, which is perpendicular to the first diameter, and being operable to produce a magnetic field cutting across the tube wall and the fluid when a first excitation current flows in the first field coil and a second excitation current flows in the second field coil,
said excitation currents changing their amplitude and dir
Endress + Hauser Flowtec AG
Jones Tullar & Cooper P.C.
Patel Harshad
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