Interface detection and level sensing in mixed material tanks and oil separators can be a challenge. There are two good ways of determining the height and location of your layers. The first is the best when you have a nonconductive layer and a highly conductive/high dielectric layer. A capacitance level sensor can tell you the height of the high dielectric (water) layer and will mostly ignore the oil (low dielectric) layer. If this works for your application, it is the most maintenance-free method for determining interface location and one layer height.
Floats with varying displacements can be used to float on each layer as long as the layers have large enough density differences. These work well, but because they are mechanical, buildup and friction will cause maintenance issues in the long run.
When you come across tough applications or want other options for a critical or maintenance intensive application, please contact the good folks at Jasper Engineering, email@example.com or 952-938-6504.