Effect of hematocrit value upon electromagnetic flowmeter sensitivity.

An electromagnetic flowmeter system that has an accuracy of 0.1% and is insensitive to fluid conductivity was used to study the sensitivity to steady flow of a 6-mm cannular transducer. All measurements were carried out at 37°C. Sensitivity decreased with increasing hematocrit value whether flow was laminar or otherwise. With laminar flow and a given hematocrit value, the sensitivity also decreased with increasing flow rate. Sensitivity started to rise when increasing flow rate caused departure from the laminar regime. At the lowest flow used in the laminar regime (250 ml/min), the sensitivity fell 4% when the hematocrit value was changed from 0 to 66%; at 39.5% the reduction in sensitivity was 2%. With laminar flow at hematocrit 39.5% the sensitivity fell 1% as flow increased from 250 to 1500 ml/min. At hematocrit 29.5% the sensitivity rose 1.2% when the flow regime changed from laminar to turbulent. The changes may be explained in terms of a radial distribution of cells, of which a cell-free boundary layer is a particular example. Theoretical calculations show that the thickness of an apparent cell-free boundary layer must be tens of microns to explain the observations with laminar flow and that the apparent thickness is greatly reduced with onset of turbulence. A possible mechanism is described by which the results could be explained in terms of cell orientation. ADDITIONAL