Electromagnetic flowmeter in shielded lines

An electromagnetic flowmeter of high accuracy comprising:

• A. a primary including a flow tube through which the fluid being metered is conducted to intercept a magnetic field established by an electromagnet having a coil to induce a voltage therein which is transferred to a pair of electrodes mounted at diametrically-opposed positions on the tube, said coil being excited by a rectangular wave whose frequency is low relative to the frequency of a standard A-C power line;
• B. a secondary coupled to said primary, said secondary including a pair of input amplifiers, each having an inverting input, a non inverting input and an output;
• C. a transmission line to couple said primary to said secondary in a manner avoiding capacitive effects, said line being constituted by a pair of cables each having an inner conductor and an outer shield, the inner conductor of the cables connecting the electrodes of the primary to the respective non-inverting inputs of the amplifiers, the outer shields thereof being connected to the respective inverting inputs and outputs of the amplifiers, and a common shield surrounding said cables, and
• D. a differential amplifier coupled to the outputs of said pair of amplifiers to provide a differential output which is alternately applied by signal switching means to first and second sample-and-hold circuits during intervals when the magnetic flux in said tube is in its steady-state condition.