Electromagnetic flow meter

Abstract

An electromagnetic flow meter for measuring the flow of blood through a conduit. Flow rate is measured by applying an electromagnetic field around the conduit, with the moving electrolytic blood inducing a current that is sensed by electrodes. The flow meter includes a substantially 'E'-shaped magnetic core structure as well as an 'E'-shaped hinged cover piece for defining a complete flux loop. In the circuitry, a floating ground arrangement is provided for enhancing the stability and sensitivity of the meter and to ensure appropriate patient isolation. A strong magnetic field, covering the sensing region and with little flux leakage, is alternately switched in polarity. The magnetic coil driver circuit employs an arrangement of switches for facilitating very fast polarity reversal. A two-level current source in the coil current driver greatly enhances the meter's efficiency and power dissipation characteristics. Extensive provisions, both electrical and mechanical, are made to reduce the effects of capacitive coupling, spurious induced currents, electromagnetic interference, and susceptibility to noise and external magnetic fields. A dual gated sample hold circuit samples the sensor signal during steady intervals. A second dual sample-and-hold averaging circuit minimizes the effects of base-line drift in the sense signal and low-frequency noise in the amplifier stage. The flow meter includes a two section error detection circuit which generates an error signal either when the flow signal exceeds power supply limits or when the rate of change of the flow signal exceeds a clinically reasonable rate.