A system for detecting an interface between a polymer-rich phase and a solvent-rich phase comprising a liquid-liquid separator configured to receive a polymer solution as an inlet stream comprising solvent, polymer and unreacted monomer produced in a solvent-based polymerization reactor through an inlet feed, wherein a tank is configured to provide a residence time of at least 20 minutes and to permit the stream to separate into the polymer rich phase and the solvent rich phase; a first sonic transponder for sending a first sonic signal from either a top or bottom of the liquid-liquid separator and for receiving a first reflected portion of the sonic signal, the reflected portion of the sonic signal created by the passage of the sonic signal through a liquid-liquid interface between the solvent rich phase and the polymer rich phase, wherein the first sonic transponder is positioned such that it transmits the signal which travels perpendicularly to the liquid-liquid interface is provided.