Transient Normal Force Transducer Response in a Modified Weissenberg Rheogoniometer

Abstract

A theoretical and experimental study for measuring meaningful transient normal force in the cone-and-plate geometry of the Weissenberg rheogoniometer type apparatus is presented. Meissner's modifications (J. Appl. Polym. Sci., 16, 2877 (1972)) are adapted to our Weissenberg rheogoniometer. The apparatus is stiffened and the closed loop feedback system is replaced by an open loop cantilever spring to extend the capability of the Weissenberg rheogoniometer to ascertain the transient normal force measurement. Due to these modifications, the transient gap variation in the instrument is limited to less than 1 μ which causes negligible effect on the normal stress measurement response. However, depending on the geometrical factors of the apparatus, the characteristic response time of the normal force measuring system in some cases, might not be small in comparison to the characteristic time of the testing fluid. The factors influencing the characteristic response time of the normal force measuring system are determined and a criterion is established which should be met to minimize the influence of the instrument response on the rheological measurements of the test fluid. Time constants as functions of geometrical and experimental factors associated with gap separation velocity for two Newtonian incompressible liquids are measured and compared with the calculated values. The theory is qualitatively in agreement with the experimental findings, yet for some cases disagreements of up to 43% between the measured and calculated time constants are observed. The dominant effect of gap angle is also confirmed.