BlueTooth under water does not work

[Admin]

I'm trying to get about 4-5ft of underwater wireless transmission of data for my robot fish. Reporting my findings . . .

My class 1 bluetooth (bluesmirf + asus dongle) doesn't work more than ~10 inches deep under water. The water is deionized, but figure that won't make a difference.

In air I've tested it to ~75m, and I'm sure it can go further but haven't tried. Its rated for 100m in air.

Figured it was worth trying and reporting results for others . . .

Next up, I'm going to do some underwater tests of my 400MHz Easy Radio module and report the findings next week. If that doesn't work, I'll try a few more transceivers on the net.

[Joker94]

yuor problem may be the density of the air vs water

 the water may have acted like putting the bluetooth inside a faraday cage, stopping the transmittion

[Tsukubadaisei]

As a medical/systems engineer, I've heard a lot about this problem.

As you know BlueTooth(from now on BT) is a 2.4GHz electromagnetic wave. In other words, BT is a kind of microwave. Water molecules absorb  microwaves very easily. That is why you wont be able to read the signal once you are deep underwater.
I've seen some (old) experimental medical devices that used BT. One of them was a pill that patient had to ingest. But, once ingested, the operator loses all the communication with the pill(human body is 70% water). That is why now doctors use pills with flash memory.

Trivia: That is exactly the same reason, submarines dont use radar. Instead they use sonar. Maybe you should use sonar to communicate with your robot.

Trivia2: Wi-fi and cellphone wont work either, for the same reasons.

If you increase the output power of the BT module you may get a few extra inches. A better solution would be radio(http://www.freepatentsonline.com/6058874.html). Serial communication through sonar is the best, I believe.
That is pretty much everything I know, unfortunately it is not my area of studies.

[Admin]

Any technically competent knows that EM waves (at communication frequencies) don't penetrate water very well.

Subs can't use it for communication, for example. So we know it can't penetrate for more than a few kilometers.

The pill didn't have much battery room, so had to use a low power transmitter, and a *very* tiny antenna, so understandably has short range.

But the issue was, how far could it penetrate with a normal antenna/battery? What if you had a RC sub that only went 2 feet deep, could it penetrate just 2 feet? My robot fish will operate in shallow water, not far from the receiver. I figured with 100m in air bluetooth range, I could at least get 2 feet underwater range. I also operate in de-ionized water which allows em waves to travel about 10x further than in sea water.

This test was to quantitatively define penetration range - less than a foot with a decent transmitter/antenna at 2.4GHz in pure water.

I'm finishing up some calculations on penetration depth vs transmission frequency. According to the math, dropping the frequency to 400MHz doesn't seem to help much - it only doubles the penetration depth to about ~2 feet. I'll run some experiments this week or next. I just need to squeeze out 4-5 feet depth and I'm good to go!

Oh and my math states that EM reflection off the water surface is negligible compared to absorption by water, ruling out various antenna tricks.

[Afroman]

The lower the frequency, the better. As I recall, when submerged submarines communicate via RF, they do it at sub 100Hz frequencies with antennas hundreds of meters long, allowing them to communicate with other subs an entire ocean away. Obviously this is not practical for your application, but depending on the data rate you need, you may be able to get away with using the 27Mhz band. Cheap Nikko radio control submarines have to operate underwater after all   Hmmm... I wonder if a 1Mhz 555 based AM transmitter/receiver combo would work...

[Admin]

I managed to get 1.5 feet depth transmitting with the 400MHz Easy Radio modules and a ~5 inch antenna. Thats as deep as my smaller test tank goes. 38400 baud rate.

I'll be testing in much deeper water soon.

(all done in de-ionized water, which allows transmission to go much further than in sea/fresh water)

[Asellith]

Can you put a base unit in the water to receive sonar transmissions and rebroadcast with the BT or radio modules? Just a small float should work great unless its for open seas operation.

[Admin]

Can you put a base unit in the water to receive sonar transmissions and rebroadcast with the BT or radio modules? Just a small float should work great unless its for open seas operation.

Nope, audio modems are huge (relatively speaking), so won't fit in my fish . . .

[bill black]

RF transmission in water...be glad it's de-ionized fresh water and not nasty old salt/solids laden seawater-but you already knew that, I am sure...
here's a link that will save you some time: http://www.qsl.net/vk5br/UwaterComms.htm
and another that may be of interest:http://www.science.ca/askascientist/viewquestion.php?qID=1767

I hope this is of some use.
Bill Black
Sebastian, FL

Furthermore...Thanks for doing all the work to build and maintain this site. You do a great job.

[Admin]

RF transmission in water...be glad it's de-ionized fresh water and not nasty old salt/solids laden seawater-but you already knew that, I am sure...
here's a link that will save you some time: http://www.qsl.net/vk5br/UwaterComms.htm
and another that may be of interest:http://www.science.ca/askascientist/viewquestion.php?qID=1767

Yeap, I've been aware of that first link for about ~2 years now. Its the only good source I can find online and it has really helped. Its like 22 years old though . . . lol . . .

I've written up a program using those equations to help estimate the attenuation that I'll share when I have time. I might get to test my transmitter in deep water Thursday, and soon after I'll report results.

[bill black]

If you already have that one- you've probably seen this as well-but in case you have not:
http://www.docstoc.com/docs/2867858/RF-Communications-under-water
I remembered running across this earlier this year: http://techon.nikkeibp.co.jp/english/NEWS_EN/20090217/165752/
That product is from this company: http://www.wirelessfibre.co.uk/index.php?page=solutions
Seatooth- clever branding if it works.

My interest is in waterborne (ROV & ASV) robots as well. It is a twisted mind that (as if robotics weren't sufficiently complex) adds high pressure fluid exclusion to the equation.
Good day!