The Interface Meter cosists of a probe attached to a polyethylene tape at one end, and at the other end a reel containing the electronic components of the instrument. Two stainless steel conducting cables run down the length of the tape from the reel to the probe. The probe incorporates an infra-red beam and a detector receiving the light signal. When the probe is submerged in any liquid the light is refracted away from the detector causing the system to go into alarm mode. If the layer is NAPL (conductivity less than 50 μS/cm) the alarm will sound with a steady tone and light. When the probe hits water (conductivity greater than 50 μS/cm) the infra-red alarm circuit is overridden and the conductivity alarm is activated, noticeable by a rapid intermittent tone and flash of light.