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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?

Benjamin Chasnov
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  • You do realize that ultrasonic rangefinders work by measuring the time it take a pulse to travel through air, right? And there is no air underwater, right? –  Jun 13 '13 at 20:52
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    @DavidKessner - no, they work by measuring the time it takes for the signal to return through whatever medium it has been coupled into. Water will cary it just fine, the issue is more coupling to the water so that the signal returned therefrom is the first strong enough reflection to register, and any reflections off the coupling medium are too soon / too weak to falsely trigger. Generally a different sort of transducer is used, and coupled with some kind of semi-solid material, or immersed in oil in contact with the hull. – Chris Stratton Jun 13 '13 at 20:59
  • @ChrisStratton You are correct for sonar in general, but not for the specific unit that the OP linked to (I wasn't clear in my prev comment). The sonar unit needs to be optimized for the medium, as the frequency and software need to be tweaked. The emitter and sensor are also different for the medium. –  Jun 13 '13 at 21:12
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    If I were making an underwater robot, I would get a cheap fish finder that has a NEMA output (a serial port that tells you the depth). The sensor unit is already waterproof and made for water. The main unit might be on the big size, however, but can be put inside a normal waterproof box. –  Jun 13 '13 at 21:14
  • The speed of sound in water is significantly different...looks like about four times what it is in air. I'm sure this will significantly change things. – Phil Frost Jun 13 '13 at 23:54
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    No, sonar can't be made waterproof. If it could, the navy would be using it in submarines. Oh, wait... – Olin Lathrop Jul 14 '13 at 12:07
  • Given the caveats above about sensor matching etc. etc. it might be entirely doable to bond the transducer to the inside of the hull of your boat / enclosure and just calibrate out the error. – John U Aug 13 '13 at 11:35

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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.

Bobbi Bennett
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  • How about submerging the transducer it in an oil filled container (PC overclockers have been using [mineral oil](http://en.wikipedia.org/wiki/Computer_cooling#Liquid_submersion_cooling) to submerge computers in for cooling so it must not be too corrosive for circuit boards, but I'm not sure about the transducer diaphram), and then submerging that oil filled container in water? Some commercial Sonar Transducers use oil filled housings. – Johnny Jun 14 '13 at 00:36
  • That sounds like an easy (a bit messy) test. But, there will be impedance transitions, oil to container wall, and container wall to water, that might give unwanted reflections. And for all I know, the transducer -is- something like a bimetallic affair that is optimum for air, not for water, and not a piezo slab that would couple to water pretty well. Like I said, it is cheap enough to experiment with and find out. – Bobbi Bennett Jun 14 '13 at 02:08