Method of operating an electromagnetic flowmeter

An electromagnetic flowmeter for a fluid flowing in a line, comprising:

• a flow tube connectable into the line conducting the fluid;
• a measuring and control circuit;
• a coil assembly fed by said measuring and control circuit, said coil assembly producing a magnetic field cutting across said flow tube by means of a first field coil mounted on said flow tube and by means of a second field coil mounted on said flow tube;
• at least two measuring electrodes for picking off potentials which are induced in the fluid flowing through said flow tube and traversed by the magnetic field;
• and
• means connected at least intermittently to said measuring electrodes for producing at least one measurement signal derived from the potentials induced in the fluid, wherein:
• said first field coil is traversed at least intermittently by a first excitation current, and said second field coil is traversed at least intermittently by a second excitation current;
• and
• the two excitation currents are adjusted by means of said measuring and control circuit in such a manner that at least intermittently, a first partial magnetic field, produced by means of said first field coil, has an average strength which is different from an average strength of a second partial magnetic field, produced simultaneously by means of said second field coil.

A method of operating an electromagnetic flowmeter for measuring the volumetric flow rate of an electrically conductive and moving fluid, the 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 with respect to the flow tube, a second electrode positioned with respect to the flow tube, which electrodes are located on a first diameter of the flow tube;
• and a coil assembly, mounted on the flow tube and comprising a first field coil and a second field coil, the 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, the method comprising the step of:
• changing the amplitude and direction of the excitation currents periodically during each cycle of the excitation currents such that:
• during a first quarter cycle, the excitation currents are equal, have a constant value, and flow through the field coils in the same direction, a first direction;
• during a second quarter cycle, the first excitation current has the constant value and flows through the first field coil in an opposite direction to the first direction, the second excitation current is less than the constant value by a constant amount and flows through the second coil in the first direction;
• during a third quarter cycle, the excitation currents have the constant value and flow through the field coils in the opposite direction;
• and during a fourth quarter cycle, the first excitation current has the constant value and flows through the first field coil in the first direction, and the second excitation current is less than the constant value by the constant amount and flows through the second field coil in the opposite direction;
• forming first, second, third and fourth voltages from the two potentials during the first, second, and fourth quarter cycles, respectively;
• forming a first voltage difference from the first and third voltages, which serves to compute a volumetric flow rate signal,
• forming a second voltage difference from the second and fourth voltages,
• forming a quotient from the second and first voltage differences, said quotient is determined during a calibration step of the electromagnetic flowmeter under uniformly turbulent flow conditions and stored as a device constant in the flowmeter;
• continuously forming instantaneous values of the quotient in operation, which are compared with the device constant;
• and
• triggering an alarm when a predeterminable threshold is exceeded and/or the volumetric flow rate signal is corrected.