Importance

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Abstract

In magnetic induction flow measurement in which a flow rate of a fluid is measured by a DC magnetic field having a periodically reversed polarity, an apparatus is used to compensate a DC offset voltage generated in an amplifying circuit in an electromagnetic flowmeter in which a signal voltage produced across electrodes is sampled and stored in each of a pair of sampling periods where the magnetic field takes a positive polarity and a negative polarity, respectively. A difference between the sampling values is formed as an output signal voltage. The signal voltage is sampled and stored in a compensation period following the pair of sampling periods to produce a compensation voltage to be superimposed onto the signal voltage, and compensation is made on the signal voltage by using the compensation voltage which is held until a succeeding compensation period. The apparatus includes: a pair of the electrodes provided on a tube through which an electrically conductive fluid flows; a differential amplifier to which the pair of electrodes are respectively connected through capacitors; an adder having two inputs one of which is connected to an output of the differential amplifier; an ampifier having an input connected to an output of the adder; an integration circuit having a resistor, a resistor and a capacitor and an operational amplifier; and a switch inserted between the amplifier and the integration circuit for sampling the signal voltage in the sampling period.

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First Claim