Methods and systems are provided for monitoring a solid-liquid interface, including providing a vessel configured to contain an at least partially melted material; detecting radiation reflected from a surface of a liquid portion of the at least partially melted material; providing sound energy to the surface; measuring a disturbance on the surface; calculating at least one frequency associated with the disturbance; and determining a thickness of the liquid portion based on the at least one frequency, wherein the thickness is calculated based on L=(2m-1)v.sub.s/4f, where f is the frequency where the disturbance has an amplitude maximum, v.sub.s is the speed of sound in the material, and m is a positive integer (1, 2, 3, . . . ).
This invention was made with U.S. Government support under National Renewable Energy Laboratory (NREL) Subcontract No. ZDO-2-30628-03 under Department of Energy (DOE) Contract No. DE-AC36-98GO10337, awarded by DOE. The U.S. Government has certain rights in this invention.