An ultrasonic sensor is fixed to the base of a storage tank. The tank contains water 2.4 m deep, with a layer of oil floating on top of the water.

The sensor sends a sound pulse vertically upwards. The pulse travels through the water at 1500 m/s and through the oil at 1300 m/s. It reflects from the top surface of the oil and returns to the sensor. The total time between sending the pulse and detecting its reflection is 4.2 ms.