Numerical computations of the extension of circular cylindrical shaped samples in a dual wind-up drum rheometer of Sentmanat extensional rheometer type M. L. Sentmanat, Rheol. Acta 43, 657 (2004); R. Garritano and H. Berting, US Patent No. 7,096,728 (08/29/2006) are presented. These time-dependent computations are fully three dimensional and based on theoretically ideal configurations. If necking or sample rupture does not occur, the elongation will resemble as ideal uni-axial if the initial sample diameter is decreased sufficiently. An initial diameter larger than 0.5 mm may result in large deviations from ideal uni-axial deformation.