Interface level switches are used to monitor the level of two liquids with different specific gravity values. When two different liquids are contained within the same tank, but are unable to mix with each other, an emulsion layer forms and creates a barrier between the two liquids. When two insoluble materials mix, the liquid with the heavier molecules will sink to the bottom while the lighter ones will stay on top, oil and water is perfect example of this.

Underground cement basins, wash pits, or any area that collects runoff of different liquids will often need to be pumped out. In the case of oil and water, the float on an interface liquid level sensor will sink through the oil but float on the water level allowing clear indication of the interface level. This can allow pumps to siphon off the oil to be reclaimed or recycled. Other applications use water or steam to help extract and clean the oil that comes from a well, and can even be used to help transport it. However the water must then be separated through a process appropriately called separation. Separation is used to remove the additives, solvents, catalysts, impurities, etc. Interface float level switches are critical to the operation of these difficult applications.