If you are trying to measure the interface between two liquids, first you must determine if the liquids will separate when settled. The liquids separate by either differing molecular properties such as a polar solvent and a nonpolar compound being immiscible, or because they simply have a differing specific gravity.
It is important to keep in mind that if there is separation between the two liquids, it does not guarantee the ability to measure the interface. Float sensors depend on the specific gravity between two liquids and will not distinguish between differing molecular properties.
For liquids, water is the reference with a specific gravity of 1. Less dense liquids such as ethanol and propane have specific gravity measures of .787 and .500 respectively, while more dense liquids such as mercury has a specific gravity of 13.663.
A common application for interface level measurement involves oil and water. Since oil is less dense with a specific gravity of .7, it will float on top of the water which has a specific gravity of 1. An effective level measurement will include two floats with differing specific gravities. The first float must sit on top of the oil, so it will need to have a specific gravity less than .7.
The second float must be dense enough to sink below the oil, and sit on top of the water. Here at APG we use a float with a specific gravity of .92 in order to achieve this.
The specific gravity between the two liquids must be different enough to allow a single float to sink below the less dense liquid (oil) and rest on the denser (water). Determining the minimum specific gravity gap required between two liquids is difficult since a number of factors are at play. However, we suggest a difference of at least .05 between two liquids.
If you have questions about specific gravity and interface level measurement please contact us and we will be glad to help.