Scanning Capillary Viscometry
The scanning capillary viscometer is the most advanced method for assessing the viscosity of whole blood. Despite the fact that it shares the concept of capillary flow and a U-shaped geometry with the Ostwald tube, the scanning capillary viscometer is radically different from an Ostwald tube which measures viscosity only at a single, high-shear rate level--a shear rate level which is not known to the technician. The Ostwald tube essentially assumes that blood behaves as a Newtonian fluid.
Unlike rotating viscometers or vibrational systems wherein the instrument itself must move, capillary viscometry uses calibrated tubes having inside diameter of less than 1 mm to enable the fluid to move, providing a more accurate and realistic quantitative measurement. Viscometers using blood samples less than a critical threshold volume are highly inaccurate because of the Fåhræus-Lindqvist effect, which causes measured viscosity levels to be dramatically lower than actual values. Rheovector only uses scanning capillary viscometers whose geometry ensures accurate viscosity measurements.
The blood sample is introduced into column #1 through a valve. A computer-controlled three-way valve allows blood in column #1 to fall and column #2 to rise, equilibrating by gravity. The instrument detects fluid height changes and determines viscosity by the pressure drop and flow rate automatically compensating for the effect of surface tension inside the tubes.
The flow rate is controlled by the U-shaped geometry of the disposable tube and the diameter of the horizontal capillary tube. Over the course of a viscosity test, the flow rate inside the capillary varies from an initial maximum speed down to zero when the two fluid levels in the columns reach the same height. From a single automated scan of this sweeping, decelerating motion, the capillary viscometer captures viscosity data across the complete range of shear rates experienced in the human body during a cardiac cycle. It provides consistently accurate viscosity measurements at all shear rates. Moreover, disposal of the consumable U-shaped tube after the test allows the operator to avoid cleaning up blood specimens between tests. In all its aspects, the scanning capillary technology has made viscosity testing vastly more practical than it has ever been in the past.