Method and apparatus for determining the thickness of material layers of a container-held substance comprising a first material disposed in an upper layer and a second material disposed in a lower layer, by transmitting a radio signal through the substance towards a container portion; receiving reflected signals from a surface of the upper layer, a surface of the second layer, and the container portion; varying the frequency of the transmitted signal to determine phase displacement between transmitted and reflected signals; determining optical distances to the surfaces and the container portion, dependent on the phase displacements; determining the thickness of one of said layers dependent on phase displacement through and index of refraction of that layer; and determining the thickness of the other layer dependent on the thickness of said one of said layers.