In a Rheotens test, the tensile force needed for elongation of an extruded filament is measured as a function of the draw ratio. In this way, the melt strength can be assessed, which is an important property in many polymer processes like fibre spinning, film blowing, and blow moulding. Several LDPE grades with different melt index and polymerised by both tubular and autoclave reactor technology were investigated. It is demonstrated that to assess melt strength, Rheotens tests have to be compared at constant extrusion pressure, while comparison at constant mass flow rate can be highly misleading. At constant extrusion pressure, Rheotens curves are invariant not only with respect to temperature but also with respect to average molar mass, and polymer grades with similar branching structure and molar mass distribution fall onto a common mastercurve. Two distinct and different Rheotens mastercurves were found for grades produced by tubular versus autoclave reactor technology, indicating distinct differences in the branching structure. Irrespective of the processing conditions, all LDPE melts investigated fail by brittle fracture, and a true rupture stress in the range of 1 to 2 MPa was found.