This instrument rapidly and accurately measures depth to hydrocarbons and water in monitoring wells and tanks. It is lightweight, battery-operated, and exceptionally rugged. The ORS is built with a NEMA-3, weather-proof aluminum housing and chemically resistant probe, and 100ft tape. Measurements of depth to fluid are accurate to within 1/100 of a foot. The probe fits into openings smaller than two inches in diameter, which makes it ideal for use in narrow-gauge monitoring wells. The system's dual-function probe uses an optical prism to distinguish air from liquid and an electrical conductivity sensor to differentiate water from hydrocarbons.