The Heron Oil/Water Interface Meter has a 3/4" diameter probe and a 300' tape. The tape is marked in 1/100' increments. The meter can be used to determine the thickness and depth of both LNAPL and DNAPL. A constant tone indicates product and an intermittent tone indicates the probe tip is in water.The meter will automatically turn "off" after 10 minutes of use, but is easily reactivated with a push of the "on" button. The tough coating on the tape is easily cleaned.