Magnetic-inductive flowmeter having a conductivity measuring device and method for operating a magnetic-inductive flowmeter having a conductivity measuring device

Magnetic-inductive flowmeter, comprising:

• a measuring tube for guiding a conductive medium,
• a magnetic field generator for generating a magnetic field permeating the measuring tube at least partially perpendicular to the flow direction of the medium,
• electrodes for tapping a measuring voltage induced in the medium, and
• a conductivity measuring device for determining the conductivity of the conductive medium,
• wherein the conductivity measuring device has a measuring circuit,
• wherein the electrodes are a part of the measuring circuit,
• wherein the measuring circuit has a voltage source and a measuring unit,
• wherein the voltage source generates a measuring circuit voltage in the measuring circuit in an operating state of the conductivity measuring device,
• wherein the measuring unit measures an actual electrode voltage present between the electrodes,
• wherein the conductivity measuring device has a control unit for setting the actual electrode voltage to a predetermined, constant setpoint value in the operating state of the conductivity measuring device, and an evaluation unit for determining the electrode current flowing between the electrodes and calculating a conductivity value of the medium or a variable associated with the conductivity using an electrode current value determined by the evaluation unit and the actual electrode voltage set by the control unit, and
• wherein the predetermined constant setpoint value is stored in the control unit and the control unit is adapted for determining a control difference from a comparison of the actual electrode voltage measured by the measuring unit to the setpoint value stored in the control unit and for adjusting the measuring circuit voltage until the control difference lies at or below the predetermined value if the control difference exceeds a predetermined value, and wherein the control unit is one of P-controller, PI-controller or PID-controller.