an ultrasonic transducer for sensing liquid within an article , such as a vassel or pipe , without contacting the liquid has a body , a sealed capsule of a flexible material containing a liquid capable of transmitting ultrasonic energy carried by the body from which a head portion of the capsule extends to contact a wall of the article . within the body is a piezo - electric element in contact with a portion of the sealed capsule to transmit ultrasonic energy to and receive energy from the liquid in said capsule as it passes through the capsule head without the need for a coupling compound .