To measure the thickness of a LNAPL in a well, you typically use either a NAPL/water interface probe that distinguishes between water and NAPLs or a weighted tape coated with a water and non-water indicator substance. Interface probes are also available that can measure the thickness of DNAPLs.
All interface meters indicate the presence and thickness of a hydrocarbon layer with the use of two different switches. One switch is designed to detect the presence of liquids (liquid level switch), the second switch is designed to detect the presence of conductive liquids (conductivity switch). Since hydrocarbons are nonconductive, they will only trigger the first switch, whereas water, being conductive, will activate both switches. When both switches are activated, the meters are designed only to display the conductivity switch result. In this way the units can distinguish between water and hydrocarbons.
Most meters use an optical sensor to detect hydrocarbons.