Problems of Electromagnetic Blood Flowmeters

Abstract

Although practical electromagnetic flowmeters are satisfactorily used in industry, many problems still remain unsolved for measuring blood flow with implantable electromagnetic flowmeters. In this paper, four main problems of such flowmeters are discussed as follow:
1) Relation between the flow rate and the electromotive force under various conditions,
2) Drift of the baseline,
3) Inaccuracy of the phasic blood flow pattern measured by an electromagnetic flowmeter which has a non-uniform magnetic field,
a) The conductivity of the wall can not be neglected.
b) The magnetic field of the probe is not uniform.
c) The velocity profile of the fluid is not axially symmetric.
If the magnetic field is not uniform, the sensitivity of the transducer probe varies as a function of the velocity profile of the fluid, even when the average flow velocity remains constant. In this paper, it has been both theoretically and experimentally verified that, when measuring pulsatile flows, the sensitivity actually does vary greatly due to the non-uniform magnetic field. To reduce this error, the design of a suitable exciting coil of the probe is necessary.
The cause of the drift of the baseline is also discussed and a method for reducing it is proposed.The drift of the baseline is greatly influenced by electrostatic coupling between the exciting circuit and the detecting circuit and by the characteristics of the detecting electrodes. In this paper, the characteristics of various kinds of metal electrodes are discussed and it is shown that a platinized platinum electrode is the most suitable for this purpose.
It is also shown that the drift of the baseline is greatly reduced by balancing electrostatic coupling between the exciting and the detecting circuits.