Method for operating a magnetoinductive flowmeter

A method for operating a magnetoinductive flowmeter comprising a measuring tube consisting of a nonconductive material, in which the medium to be measured flows,

• comprising a magnetic field generating device for generating an alternating magnetic field that permeates the measuring tube substantially perpendicularly to the longitudinal axis of the measuring tube by virtue of the magnetic field generating device being driven with a periodic control signal,
• and comprising two measuring electrodes galvanically coupled to the medium for tapping off a voltage, which is amplified by an amplifier circuit having a high-impedance input to form a measurement signal that is dependent on the flow velocity when medium is flowing,
• wherein the control signal is embodied as an alternating rectangular pulse with pause times (Mess1, Mess2) and during a period there is a first pause time (Mess1) after a positive rectangular pulse (MB+) and a second pause time (Mess2) after a negative rectangular pulse (MB−) and no magnetic field is present during the pause times (Mess1, Mess2),
• wherein that the measurement signal is evaluated during the duration of the rectangular pulses (MB+, MB−) for the purpose of determining the flow velocity of the medium and during the duration of the pause times (Mess1, Mess2) for the purpose of function monitoring,
• wherein the impedance between the two measuring electrodes is determined by way of the noise level of the measurement signal during the pause times (Mess1, Mess2), and the noise level of the measurement signal during the pause times (Mess1, Mess2) is a measure of malfunctions, wherein the impedance between the two measuring electrodes is influenced by air bubbles or foreign particles in the measurement medium,
• wherein the measurement signal is sampled repeatedly during the pause times (Mess1, Mess2) and from the detected samples a respective average value per pause time is formed,
• wherein the difference between the two average values is formed per period.