Methods for operating a liquid pump having a rotor are provided for conveying liquids. The viscosity of the conveyed liquid, such as blood, may be determined while taking into account measured values of operating parameters of the pump. The pump may be operated in a predetermined region of a family of characteristic curves. The family of characteristic curves may link at least two operating parameters of the pump to each other, and, in the predetermined region, at least three parameters of the pump may be sensed and taken into account in order to determine the viscosity. Because of the operation in a predetermined region of a family of characteristic curves, the viscosity of the conveyed liquid can be determined in addition to the volumetric flow rate through the pump and the pressure difference across the pump when an appropriate selection is made from a sufficient number of sensed operating parameters.
