I'm building sensors for an underwater robot and was wondering if it's possible to waterproof ultrasonic range finders like this one to work underwater. I have made waterproof enclosures before. However, I don't know what material will allow the ultrasonic signals to transmit. Will the actual piezo transduce have to touch the water, or could there be a plastic barrier in between the water and the sensor?
The thing is cheap enough to just try it and see.
You can't just point it at a window of some sort into the water, the pulse would start in air, and almost all of it would reflect off of the impedance mismatch into the water.
You will have to take the grill off the transducer, and have the transducer contact your 'window medium', that is, the plastic between the transducer and the water. Use thermal grease (because you have it on hand) to prevent air voids between the transducer and the plastic film. The Sonoluminescence FAQ, which address' issues about coupling transducers to a flask of water, says 'There's nothing special about the glue. Obviously you need something rigid/brittle rather than rubbery.' so a hard epoxy might be better than grease, but harder to get off again.
It is possible the front surface of the transducer is 'live', so watch out for that.