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I know that Bluetooth operating at the standard 2.4GHz has a hard time penetrating water (I guess the water "absorbs" the signal). However, at what depth does this really start? In my case I am thinking about a device that operates within 1 meter of the surface and would transmit to someone nearby on land. Is a reliable pairing possible to attain and maintain? If so, is it doable with standard transmitters? That is, would there need to be any special signal amplifiers or the like?

Furthermore, if this is possible, is it only possible with Bluetooth classic or would BLE fare similarly?

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    \$\begingroup\$ I doubt there is much info on this specific use case. Why not put a bluetooth device in a waterproof case and a bathtub? \$\endgroup\$
    – Passerby
    Commented Oct 18, 2016 at 4:07
  • \$\begingroup\$ What kind of water do you mean? Seawater? Fresh Ground water? \$\endgroup\$ Commented Oct 18, 2016 at 4:18
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    \$\begingroup\$ The added salts and minerals make the em absorption worse. \$\endgroup\$
    – Passerby
    Commented Oct 18, 2016 at 4:40
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    \$\begingroup\$ @marcman: considering the facts that Bluetooth and microwaveovens operate at very similar frequencies (ca. 2.4GHz) and microwave heating works mainly by absorption of RF by water contained in the food you can imagine that also the Bluetooth signal will be absorbed (i.e. attenuated) quite effectively by water. \$\endgroup\$
    – Curd
    Commented Jan 11, 2018 at 11:23
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    \$\begingroup\$ I paired my Series 2 Apple watch with waterproof headphones. As long as the watch was not submerged, it worked great. As soon as I submerged my wrist, the music stopped. I tried swimming a few laps, and had intermittent music (every time my left wrist was out of the water for a stroke). Didn't realize the limits of bluetooth underwater...I do now! \$\endgroup\$ Commented Mar 25, 2018 at 19:53

3 Answers 3

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The question has been studied extensively. According to this lecture, at 2.4GHz (microwave frequency as well) the 1/e (63% loss) occurs in 1.4cm. You probably can do better with 9600 baud UART. :-(

enter image description here

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Water absorbs high frequencies like 2.4GHz very well. That is how your microwave oven works so fast. Your chances of using ANY protocol at 2.4GHz (not just BlueTooth) are essentially zero. That is also why submarines use sonar and not radar underwater. Extremely low frequencies (10KHz and lower) are used to communicate to submarines when submerged.

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  • \$\begingroup\$ It is also the reason cops don't use radar when it is raining. \$\endgroup\$
    – Old_Fossil
    Commented Oct 18, 2016 at 5:14
  • \$\begingroup\$ Considering that most of those radars are X-band(~10Ghz) or higher, signal loss during precipitation will be high..LOL On the other hand, unless you've got walls of water moving around(that is what they looked like during a hurricane in Texas back in the 80's), 2.4Ghz will work with degraded ability. Higher power and directional antennas help. \$\endgroup\$
    – GB - AE7OO
    Commented Mar 24, 2020 at 13:41
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If you absolutely have to use BT/BLE (or other 2.4 GHz tech) and your depth is limited (say 1-2 m max) then I'd suggest using a floating antenna.

This is the only way a submerged device may be able to maintain a stable BT connection with another system sitting out of water.

Even if you're not using 2.4 GHz, absorption would be too highfor any practical communication if transmission has to go through water.

I'd not bet on any communication with a submerged antenna except for a military budget project.

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  • \$\begingroup\$ As far as I know, even they don't try with anything beyond 220. At 2.4Ghz, you would need a dedicated power plant just for the amplifiers, stacked arrays of antennas and someone who was WAY TOO CLOSE to listen to you... For lowish bandwidth, 27Mhz will work(I've seen remote controlled subs using CB crystals so...) \$\endgroup\$
    – GB - AE7OO
    Commented Mar 24, 2020 at 13:51

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