Patent ID: 8044843

Claim:
A method 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 method comprising: transmitting a radio signal, from antenna means placed at a predetermined distance from a container portion, through the substance towards the container portion; receiving, in said antenna means, 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; varying the frequency of the transmitted signal over a frequency band to determine phase displacement between the transmitted signal and the reflected signals; determining optical distances from the antenna means to the first and second surface portions and to the container portion, dependent on the phase displacements; determining the thickness of one of said layers dependent on the phase displacement through and index of refraction of that layer; and determining the thickness of the other of said layers dependent on said predetermined distance and the thickness of said one of said layers.