A flow meter includes a measuring tube for conveying a fluid to be measured and a
magnetic field system, which has at least one
field coil, through which an
exciter current (IM) flows, at least at times, for producing a
magnetic field passing through the fluid at least partly perpendicularly to a
stream direction. In the method, the
voltage (UH) instantaneously driving the
exciter current (IM) is changed at a second point in time, t2, from the second
voltage level, Udrv, to a third
voltage level, Ushort, especially to a third voltage level which is constant or controlled to be constant, in order to achieve a sinking of the
electrical current strength of the
exciter current (IM) instantaneously flowing in the at least one
field coil from the maximum current value, Im, to an
electrical current end value, Iczu, especially a constant end value, predetermined for the exciter current (IM) In such case, the third voltage level, Ushort, is chosen smaller than the second voltage level, Urev. For determining a third point in time, t3, which corresponds to the reaching of the
electrical current end value, Ic, the exciter current IM is registered, at least at times. Based on this, a first
time constant, Tshort, for the
magnetic field system is determined, which corresponds to a time span, t3−t2,
lying between the second point in time, t2, and the third point in time, t3, and / or a second
time constant, Trev+Tshort, for the magnetic field
system, which corresponds to a time span, t3−t1,
lying between the first point in time, t1, and the third point in time, t3. For determining a diagnosis value representing an instantaneous operating state of the flow meter, the determined first
time constant, Tshort, is then compared with a predetermined first reference value, T1ref, and / or the determined second time constant, Trev+Tshort, is compared with a predetermined second reference value, T2ref.