Magnetic Meters: Effects of Electrical Resistance in Tissues on Flow Measurements, and an Improved Calibration for Square-Wave Circuits

Abstract

Apparent sensitivity of magnetic meters applied to intact vessels can be altered by changes in conductivity of vessel walls or of the contained fluid, and by variable contact of the vessel with surrounding tissue or fluid. An experimental study of these problems was begun by designing a built-in magnetometer for complete electrical calibration of the square-wave meter. It was then found that flow of mercury, saline or blood of various hematocrits through nonconductive tubes gave signals equal to those calculated on the basis of Faraday's principle, providing that the magnetic field was practically uniform across the electrodes. Adequate uniformity was obtained with parallel pole faces at least 1.3 times as wide as the gap. Variable loss of signal (up to 50%), owing to contact of conductive vessels with surrounding tissue or fluid, could be practically eliminated by an electrical shield extending 2.6 times the diameter along the vessel at the electrodes. Specific conductances of canine aortas were from 0.52 to 0.73 times the conductances of blood from the same animals. Alterations in this ratio, readily produced by changes in composition of vessel or fluid, can result in significant changes of flow signal. Indirect flow calibrations require matching of the conductance ratio to that occurring at the time of measurement.