Abstract: A method for measuring interface levels between fluids is described, characterized in that, with the aid of a magnetic field forming means, a variable magnetic field is formed in one of the fluids whereby a counterflowing magnetic field is established, being a function of the fluid properties with respect to the portion of conducting fraction in the fluid, and conductivity of the fraction; and the properties of the mentioned fluid are registered by registering the dominating impedance, alternatively resonance frequency, of the system; and by corresponding registrations of different fluid layers at the respective height levels, and in the existing interphase layers, and thereafter by mutual comparison of the mentioned properties, one or more exsisting multiphase levels are determined. A device to carry out the method is also described.