Patent ID: 8571816

Claim:
An electromagnetic flow meter comprising: a magnetic excitation coil arranged so that the direction in which the magnetic field thereof is produced is perpendicular to the direction of flow of a fluid flowing within a measuring tube; a magnetic excitation device providing a magnetic excitation electric current to the magnetic excitation coil with the polarity thereof switching alternatingly; a pair of electrodes disposed within the measuring tube perpendicular to the direction of flow of the fluid flowing within the measuring tube and to the direction of the magnetic field produced by the magnetic excitation coil; a differential amplifier performing differential amplification of a signal EMF produced between the electrodes, to produce an AC flow rate signal; an AC amplifier amplifying the AC flow rate signal from the differential amplifier; a sampler sampling the AC flow rate signal that has been amplified by the AC amplifier, to produce a DC flow rate signal; a DC amplifier amplifying the DC flow rate signal from the sampler; an A/D converter converting into a digital signal the DC flow rate signal amplified by the DC amplifier; and a processor calculating a flow rate of the fluid flowing within the measuring tube from the digital signal converted by the A/D converter; wherein the DC amplifier comprises: first through Nth (where N≧2) individual gain generators inputting individually the DC flow rate signal from the sampler and for applying, to the DC flow rate signal, the gain produced thereby, established so that the gains of the first through Nth individual gain generators applied to the DC flow rate signal are as sequentially larger values; a gain selector selecting, as the DC flow rate signal to the A/D converter, one of the outputs from the first through Nth individual gain generators; and a saturation preventer preventing the occurrence of saturation in a later stage individual gain generator, connected to all of the earlier-stage individual gain generators of the first through Nth individual gain generators except for the first individual gain generator; and wherein the processor comprises: a controller controlling enabling/disabling of the saturation preventing operation of the saturation preventer in the DC amplifier and for controlling a selecting operation of the gain selector, based on the calculated flow rate of the fluid.