A method for determining the electrical conductivity of a mass flow in a three dimensional target volume including the steps of placing electrodes in a measuring connection with the target volume； supplying alternating voltage or alternating current to the tar-get volume between two of the electrodes and measuring the current or the voltage between the electrodes； determining a state space model which defines the relationships between the electrical conductivity, the voltage and the current in the target volume and the evolution of the electrical conductivity as a function of time； comparing the currents and/or the voltages according to the state space model with the supplied and the measured ones； and modifying the state space model to decrease any differences. The electrodes are placed substantially within one plane； and the state space model is determined so as to the time-dependent flow field of the mass flow within the target volume.
