SINGLE-POINT CORRECTION FOR PARALLEL DISKS RHEOMETRY

The nonhomogeneous nature of the flow in the parallel disks rheometer necessitates the differentiation of the measured quantities (torque and normal force) with respect to the shear rate at the disk rim. Commercial instruments do not calculate true material functions online, rather they report apparent Newtonian values, i.e., ones obtained assuming the material functions are constants. In this work, we present a single-point correction technique to obtain approximate values for material functions without numerical differentiation. The advantage of the single-point correction method is that it gives more accurate results than the apparent Newtonian values and it takes less time than numerical differentiation. This can, therefore, be useful in quality control laboratories and for process-line measurements where reasonably accurate data are needed in a short time. A single-point correction is applied to the parallel disks device for the shear viscosity and a new correction method for the normal stress coefficients is also developed. The accuracy of these approximate methods is tested with experimental results for a polymer melt and a polymer solution. The correction for both shear viscosity 77 and normal stress coefficient lir = ‘’Pj—avoids the numerical differentiation of the data and can be easily implemented in software that provide online material functions. © 1985, The Society of Rheology. All rights reserved.