A first unit rotating with the centrifugal rotor forms an electric resonance circuit having at least one variable oscillation property; and a sensing member rotating with the rotor is arranged to sense a displacement of an interface formed between two media undergoing centrifugal separation in the rotor,...http://www.google.es/patents/US3986663?utm_source=gb-gplus-sharePatente US3986663 - Centrifugal separator with sensing means
