Urethane potting of acoustic transducers for acoustic travel-time current meters

Piezo-ceramic acoustic transducers are electrically excited to cause dimensional changes that can generate a pressure/displacement wave. This is the transmit function. Changes in strain in the piezo-ceramic transducer generate a voltage and such changes in strain are the result of acoustic pressure/displacement stress on the transducer making it a receiver. To work in transmit and receive the electrodes of the transducer need to be electrically insulated yet pressure exposed. Immersion in an insulating fluid such as oil (castor oil is often used) can insulate yet expose the transducer to pressure but a window between the oil and the water (assuming the transducer is to work submerged in water) is still needed. Immersion of the transducer in an insulating solid solves the window problem. And there are two common materials that provide this insulating solid: epoxy and polyurethane. This paper will enumerate the experiences these authors have had with these two plastics but especially urethane.