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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, 2013 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\$ Jul 20, 2013 at 23:38
  • \$\begingroup\$ @AnindoGhosh -- on the other hand, "wireless" is an old term for radio. \$\endgroup\$ Feb 23, 2023 at 13:24

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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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Everyone mentioned 'high attenuation', but there's a LOT MORE to 'why radio doesn't work well underwater', than just attenuation. One of the 'tried-and-true' methods of underwater (diver applications) of course, is the 'hydrophone'. Easy to build, cheap to cost-out, and pretty durable (simply epoxy 'hermetically seal' the hydrophone). The downside to hydrophones are two primary ones: First, you are 'tethered' by wiring...even if you're wearing your hydrophone, your arduino and a battery pack...the arduino would still need a 'data line'...because 'wifi-bluetooth-etc.' doesn't work underwater (attenuation, remember?). The other method not-quite as old as hydrophones, is 'optical light'...but the downside here is 'water clarity'. Diving salt water in the Bahamas? Optimum range. Diving the Mississippi? ROFL...forget it, a trained catfish works better as a courier. Laser doesn't really change the game here...and careful with SONAR...as various wildlife groups have successfully accomplished a myriad of laws against high power units, as well as a host of ranges of use...seems water mammals like dolphins, whales, etc., go self-terminating when there's 'too much noise'...just ask the Navy. For a simple 'RC submarine', you have a couple of options...wired ends it all, right there, of course. BUT...if you want 'real wireless action', you're most-likely going to have to do some searching on 'RC Submarine multichannel radios'...and probably pick up the whole kit as a consumer unit. If you're wanting to simply dive with a friend, full face mask style, then there are SEVERAL companies out there building compact, lightweight hydrophone radios...OTS, Neptune, etc...but brace yourself, most start around $800...and if you get into 'multi-channel, data modem, professional rig' systems (like Kirby-Morgan), you can forget that next used car...systems start around $2,700 and go to over $5,000...and forget 'military surplus'...their encoding and power range is 'high controlled' and not released to surplus sales at all...and even if you get one, it's like having a H.U.D. from an F-16...plan on a visit from the DoD if someone reports you.

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  • \$\begingroup\$ This answer digresses from the question \$\endgroup\$
    – Voltage Spike
    Feb 23, 2023 at 15:59
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    \$\begingroup\$ Hi Lance, great that you're getting involved, and wanting to share your knowledge. However, I'd suggest you have a look at the site tour electronics.stackexchange.com/tour, and the guidance on how to provide good answers electronics.stackexchange.com/help/how-to-answer . You should aim to provide a clear answer to the question asked, and adding more structure to your answer would help considerably. Try editing to add some headings for the various options, and then listing the pros and cons. Also consider adding links to relevant external references. \$\endgroup\$
    – colintd
    Feb 23, 2023 at 16:55

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