Blood viscosity measurements in both extensional and shear flow by a falling ball viscometer.

A falling ball viscometer is used to measure whole blood and plasma viscosities. The principle of the device is to determine the fall time of a ball contained in a disposable syringe; the periodic run of the sphere prevents blood sedimentation during the measuring cycle but does not prevent the aggregation process which takes place with a short relaxation time. The analysis of the flow field is discussed relative to previous papers. The study of the dynamics of the falling ball provides two characteristic rates of deformation for the mixed extensional and shear flow field. The Reynolds numbers are calculated using different velocities and radii. Results on blood and plasma viscosity of patients suffering from rheumatoid arthritis are given in relation with clinical and biological indexes.