probally by weight - you must have a lot of dirt in the air as well? Any other type of non intrusive products may get dirty or the molten metal splash would stick over time.
If the surface is smooth and quiet you may try a supersonic distance sensor.
If not, you might insert a tube into the melted material and let bubble out some gas throgh the melted material. The gas presure than provides a measure for the level.
the solution will depend on the distance, range of level changes, admissible incertainity of measurements and other additional requitements, i.e., rotation of the pot, dust, pressure around the pot, etc., etc.
Applications like these have been solved quite inexpensiviley with Microwave level transmitters such as the Micropilot M from Endress + Hauser.
The 6GHz microwave pulse can be passed though a low lead glass heat shield. The application heat does not afffect the measurement principle however siting to get a good view of the surface away from disturbing factors such as filling streams or obscuring fixed targets is important.
weber Sensors Inc. have the solution for you. They have wide range proximity sensors working at that temperatur. Contact Larfry Flanagan at 1-770-592-6630.
