The numerical complexity of Discrete Element Method (DEM) simulations generally forces an idealisation of DEM models, making the calibration process the key to realistic simulation results. When calibrating cohesionless, free-flowing bulk materials, individual simple experiments are commonly used as reference for the calibration, such as the angle of repose in various test methods. Regardless of the experiment, the calibration is regularly performed by trial and error, systematic variation of the parameters, or using optimisation algorithms until a suitable combination of parameters is found. A problem of the calibration, which is often ignored, is the ambiguity of these parameter combinations. Thus, there are usually a variety of contact parameters that can map the same macroscopic reference value. This paper deals in detail with the ambiguity of parameter combinations during the calibration process. It shows which standard tests can be used to generate different experimental reference values for the calibration. The results of simulations with systematic parameter variation highlight the problem of ambiguity. Subsequently it will be shown how the combination of different tests can significantly reduce the acceptable parameter combinations. A modified draw down test will be presented as a calibration test which can deliver different reference values in a single test. Hence, this calibration test allows to obtain an almost unique set of DEM parameter. (C) 2018 The Authors. Published by Elsevier B.V.