Method for predictive maintenance and/or method for determining electrical conductivity in a magneto-inductive flow-measuring device

A method for predictive maintenance for a magneto-inductive flow-measuring device or method for determining electrical conductivity of a medium flowing through a magneto-inductive flow-measuring device, the magneto-inductive flow-measuring device includes:

• a magnet system, which produces a magnetic field passing through a measuring tube essentially transversely to a measuring tube axis;
• at least two measuring electrodes coupled with the medium, having a defined rest potential and being arranged in a region of the measuring tube lying essentially perpendicularly to the magnetic field;
• and a control or evaluation unit, which delivers information concerning volume- or mass-flow of the medium on the basis of measurement voltage induced in the measuring electrodes, the method comprising the steps of:
• applying a first exciting signal with a first frequency (f1) and at least a second exciting signal with a second frequency (f2), or an exciting signal containing at least two frequencies (f1, f2,) to the measuring electrode in such a manner that the average value of the exciting signals or exciting signal coincides, at least approximately, with the average value of electrode potential of the measuring electrode;
• detecting the conductivity (χ) of the medium or change on the surface of the measuring electrode on the basis of at least one impedance value measured between the measuring electrode and the reference potential;
• determining optimum frequencies (f1, f2, . . . ) of a pulse sequence, or sequences, at which the at least one impedance value is ascertained, as a function of the medium;
• and
• ascertaining the conductivity of the medium on the basis of ascertained impedance values, when, at least two pulse sequences following one after the other and having different frequencies, the impedance values are equal within predetermined tolerance ranges, wherein:
• frequency (f) of the pulse sequence is successively changed, until the phase value of the ascertained impedance values reaches a minimum, and, at that frequency of the pulse sequence, at which the phase value is minimum, the associated amplitude value of the impedance values is drawn upon for determining resistance (Rm), or for determining conductivity of the medium.