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I am looking into constructing a closed and small device with arduino sensors that goes underwater. It has to be submerged into water and back several times, and that is made by someone with a rope attached to it. So it needs a way to communicate to the surface that it reached the bottom, so that the person can start pulling. It is planned for it to go no more than 30 meters under the surface.

So I though that a wireless way to comunicate from underwater to the surface just for that would be nice, and not a big trouble (Like when the pressure stops changing a LED light on in the surface). But I know nothing about eletronics, so I need help. Is there a simple way to do it? What can I use? I would like to keep it small...

And I woudn't like it to have a wire...

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    \$\begingroup\$ Wireless will probably not work because electric waves are highly attenuated by water. Some form of acoustic link will be more viable. Also, the hydrostatic pressure at 30 meters of depth is about 40 pounds per square inch so keeping your device watertight will not be easy especially since it will be battery powered so access to the batteries is necessary. Since you are going to attach it to a rope, what is the objection to using a wire which could provide power as well as data? \$\endgroup\$ – Barry Jul 20 '13 at 22:23
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    \$\begingroup\$ @Barry Just being pedantic here: Both Sonar (acoustic) and Laser (light) communications are wireless, and both are used underwater. As the OP did not specify radio in the question... \$\endgroup\$ – Anindo Ghosh Jul 20 '13 at 23:38
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RF signals are attenuated greatly by water, Very low frequency (VLF) is used for submarine communications but requires a huge antenna system. Therefore Bluetooth and other RF-based systems won't be suitable.

For a depth of 30 meters accoustic modems are a practical solution. I've done some software programming for a system that used a AquaComm: Underwater wireless modem and it has an RS232 port and simple to use commands. From memory the board and transducer combined would have taken up an area of around 100 x 100 x 200mm, so I'm not sure if that would meet your definition of "small" or not.

Optical communications would be another possibility, although the only shipping commercial products I could find had a much shorter range. I found an interesting paper Using Optical Communication for Remote Underwater Robot Operation where they reported 30 meters over the length of a pool, although that dropped to around 9 meters in a harbor. With optical it would depend a lot on the water quality and ambient light.

Because it sounds like your device is always tethered by a rope the simplest and cheapest and easiest way might be to replace the rope with a cable. Assuming there aren't any dire consequences to the sensor failing and long-term reliability isn't an issue maybe standard mains cable would do the job if properly sealed at each end. It sounds like this setup it for some form of experiment?

As mentioned in a comment at that depth keeping your device watertight won't be easy so generally keeping the underwater component of the system as small as possible and removing the need for battery access will make it easier. For the same reason it might also be worth considering leaving the Arduino above water and just place the sensor(s) underwater if possible.

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