Capacitative electromagnetic flow meter

A capacitative electromagnetic flow meter comprising:

• a measurement tube made of insulating material through which flows a fluid to be measured;
• an exciting coil wound on a magnetic pole arranged facing a periphery of said measurement tube, that supplies flux in a direction orthogonal to a tube axis direction of said measurement tube;
• a pair of face electrodes arranged facing a periphery of an outer wall of said measurement tube in directions respectively orthogonal to a direction of said flux and said tube axis direction of said measurement tube;
• guard electrodes arranged so as to cover said face electrodes from said periphery of said outer wall, maintaining a prescribed separation with said face electrodes;
• an exciting circuit that supplies an exciting current of a frequency of at least a commercially available frequency to said exciting coil;
• a pre-amplifier that amplifies a detection signal detected through an electrostatic capacitances between said fluid to be measured and said pair of face electrodes, respectively, and between said face electrodes and said guard electrodes;
• a cable whereby said face electrodes and guard electrodes are connected with said pre-amplifier;
• a signal processing unit that outputs a flow rate of said fluid to be measured from an output signal of said pre-amplifier;
• a cylindrical yoke forming a magnetic return circuit of an exciting magnetic field arranged coaxially with said measurement tube and so as to cover a periphery of said exciting coil;
• a coil fixing plate of non-magnetic material electrically configured and fixed to said cylindrical yoke, covering said exciting coil;
• and
• an earth ring provided at both ends of said measurement tube, whereby a metal pipe that is coaxially arranged with said cylindrical yoke and said cylindrical yoke are configured and fixed symmetrically and electrically connected with respect to an axis connecting centers of said pair of face electrodes and a tube axis of said measurement tube, at a periphery of said cylindrical yoke,
• wherein said exciting circuit comprises filter means that controls a waveform of said exciting current such that an exciting flux waveform has a flat section;
• and
• a value of an electrostatic capacitance formed between said face electrodes and said guard electrodes is smaller than a value of the electrostatic capacitance between said fluid to be measured and said face electrodes.