<p>Measurements of elongational properties of polymer melts with counter-rotating drums according to Sentmanat have become very popular over recent years. However, some results contrary to those obtained from classical elongational rheometers are published, particularly at higher elongations at which the uniformity of sample deformation plays a decisive role. In this paper, it is reviewed how the uniformity of sample geometry was determined in pioneering devices, and experimental results were verified by applying various deformation modes and different types of rheometers. Concerning the commercial counter-rotating drums insets, uniform sample deformation is often assumed but rarely verified. To overcome this deficiency, a video equipment synchronized with the rheometer functions was installed. On commercial products of a high density polyethylene and a linear polypropylene, it was shown that maxima of tensile stress or elongational viscosity, respectively, are accompanied by non-uniform elongations. An algorithm is presented for the numerical description of the sample edges. From a comparison of the width averages as a function of experimental time with the corresponding data for the uniform deformation, conclusions with respect to the uniformity of a sample can be drawn and the range assessed, in which an evaluation is based on reliable measurements.</p> Graphical Abstract <p></p>