Method and device for contactless level and interface detection

An apparatus for determining the thickness of a material layer of a substance held in a container, which substance includes a first material disposed in an upper layer and a second material disposed in a lower layer, the apparatus comprising:

• antenna placed at a predetermined distance from a container portion and aimed through the substance towards the container portion;
• a comparator unit adapted to compare a signal transmitted through the antenna and reflected signals from an upper first surface portion of the upper layer, from an upper second surface portion of the second layer, and from the container portion;
• and
• a control unit adapted to vary the frequency of the transmitted signal over a frequency band to determine phase displacement between the transmitted signal and the reflected signals, the control unit including a calculator adapted to determine optical distances from the antenna to the first and second surface portions and to the container portion, dependent on the phase displacements, determine the thickness of one of said layers dependent on the phase displacement through and index of refraction of that layer, and determine the thickness of the other of said layers dependent on said predetermined distance and the thickness of said one of said layers.