All interface meters operate with a two sensor (switch) system. The first sensor, either optical or ultrasonic, is responsible for detecting liquids, regardless of the nature of the liquid. These sensors cannot distinguish between hydrocarbon and water, so a second sensor is necessary in order to make this determination. All interface meters use a simple conductivity sensor to determine if the liquid detected by the first sensor is water (conductive) or hydrocarbon (non-conductive).
Optical based sensors usually use blunt steel posts as their conductivity electrode and are mounted adjacent to the optical prism. This shape of electrode (located immediately adjacent to the optical crystal) gets coated with viscous product and stays coated for a long time, until the probe is lowered far enough into the water where the hydro-static pressure is strong enough to squeeze the coating product up the probe's body. Additionally, having a probe body that extends below the location of the electrodes increases the amount of product that must move before the electrode can "see" the water.
the HS-2 difference
The biggest difference between our conductivity sensor and the competition's is the shape of our electrodes. Our sensor uses a thin stainless steel wire which is protected within the tip of the probe. The thin wire is a shape that viscous hydrocarbon liquids have a hard time holding onto and so this electrode clears very quickly as the probe passes from the LNAPL layer into the water below. You can see the configuration of our probe tip in the image above.