An apparatus for measuring oil in water for larger parts per million (ppm) of light-to-medium weight crude oils shown. The excitation signal and the fluorescent light being detected are provided and received through a single channel within the ultrasonic transducer. The target area for measuring the ppm's of oil in water is located just inside of the measurement window to prevent interference by turbidity or other oil droplets within the fluid stream. The angle between the excitation signal and the fluorescent light as transmitted and received from the single channel is very small.