How to transmit data underwater over 40m

Question

Dear All,

I would need to transmit data in salted water in both ways on a distance of 40 to 50 meters maximum with no cable.

The quantity of data is not important only constarint is to use to much energy and not to expensive. Apparently the best option would be ultrasound:

I found some interesting device on internet:

http://www.benthowave.com/products/BII-7520omnidirectionaltransducer.html

As I'm a mechanical engineer, I have no clue on which material I should buy to test underwater data transmission...

So I have 3 Questions:

1/ Is the ultra sound option the only one for my application, if not, what do you recommand ?

2/ In the case of ultrasound communication, which reference of equipment should I buy (maybe in the link I gave before or you have better options?)

3/ If you think of other options than ultrasound, do you have any reference of electronic system I should buy for testing?

Thanks a lot,

Alexandre

michaelkellett · Answer

There are a couple of points in your responses to Doug's questions that need some more detail. 7 - Data rate - I'm not sure what units o/s are but let's assume it to be 10 x 64 bit words per second - you can tell us if that won't do. 13 - Cost target - will you make just one or lots - if lots then how many - is the price target for both ends ? 30$ is close to impossible - certainly only achievable for quantities in 100k+ I haven't done business with Benthowave, they look like a proper competent company - but not one that sells $5 sensors. You might, just about, be able to make a cheapo echo sounder/fish finder kind of transducer for $5 - maybe you could find a Chinese source. But then you need to package it up with the electronics, battery and however the data gets in an out - and all to a standard that will work under water. The data rate is low so you can use a simple modulation scheme - where you just drive the transmitter at full power for short bursts. With a 40kHz transducer 640 bits per second is quite feasible. You'll need a robust error detection/correction system so both transmitter and receiver will need a little micro processor. Look at this: https://www.amazon.co.uk/Lucky-Finders-Portable-Fishing-Sounder/dp/B06XFG59QW/ref=sr_1_1_sspa?keywords=echo+sounder&qid=… The sensor would probably do but it wont be very omnidirectional. You could buy a couple of these fish finders, check what frequency they drive the transducer with and go from there. Some of the things you've said suggest you want to buy a complete working system, these people do such stuff: https://waterlinked.com/underwater-communication/ MK

jw0752 · Answer

Hi Alexandre, If a low data rate is permissible you might find that you can get more distance with less power by using a lower frequency carrier. Infra-sound would carry greater distance as it is less easily absorbed in the medium. John

dougw · Answer

How large can the communication system be? How much power is available? Is the water turbulent? Is the water clear or murky? Are there any obstacles in the water? (Is it a straight line-of-sight)? Is there marine life that cannot be disturbed? What data rate is adequate? How deep is it? Can an antenna stick out of the water? Is there movement of one or both ends of the link? Does the link need to be omni-directional? Does the communication need to be real-time? (could a robot go back and forth?) What is the cost target? How much time is available for development? There are a large number of answers to your question, including the technologies mentioned in other responses above and commercial communication systems and a couple of others I can think of, but if you want to narrow it down to a solution that works well for you, it would help to try to answer these questions.

colporteur · Answer

Good luck in your endeavour. Engineers and developers have been trying to do it for GPS for a number of years. Surface floating relay stations that receive signal from the underwater device is one option. As the US military has discovered, transmitting from underwater is possible with very low frequency signals but the power, transmission rate and the technology, 2Km long antennas make it rather difficult.

michaelkellett · Answer

In a past life I designed piezo electric microphones and accelerometers, so I know a bit about piezo transducers in general. We did look at making the cheapo echo sounder type transducers but the plan never got far. I've designed some electronics for reversing sensor systems - a lot of which is similar to echo sounding in water. In underwater comms I suspect that the modulation scheme will be the key to getting a good reliable link. MK

michaelkellett · Answer

Here is a circuit to drive a 40kHz air transducer for a reversing system. It's about as cheap and simple as it gets. The RX amplifier is shown too. A depth sounder (cheap) works the same way. Q3 is driven by a signal from a uP timer to give burst of pulses at the operating frequency of the transducer. The transformer is a 1:10 and designed so that its inductance tunes the piezo element to the operating frequency. The piezo transducer is connected between SE1 and GND. D1 protects the transistor from the huge signal on the transformer secondary. VSD was 5V and we got about 50V pk-pk at SE1. There was a 40kHz bandpass filter connected to PR1 and the output of that went to an ADC port on a micro. For communications you could start by trying the modulation and encoding schemes used by IR remote controllers. One of these uses 0.6ms on/0.6ms off for 0 and 1.2ms on/0.6ms off for 1, this will give bursts of at least 24 cycles (at 40kHz) which should work quite well. MK

Amessiah · Answer

Check this link... Engineer making low budget rx tx underwater communication device. Manual both in English and Russian. Working range up to 300m (and some updated uWave devices - up to 1-3 km). Maybe somebody still need it. 
 github.com/.../diy.github.io

neuromodulator · Answer

Alternatives are, radiofrequency, Light and sound. RF has a pretty hard time penetrating salty water, and you require very low frequencies. Laser can get longer distances, but it has to be aimed and still won't go much farther than RF. Then ultrasound can get to very long distances, many kms if needed, but there is always the tradeoff of range and rate. And still ultrasound will always have limited latency, because of speed of sound in water.

michaelkellett · Answer

You could buy a Fishfinder like this - all you want is the sensor (well two of them) so you might find that more cheaply. Then you need to build a load of electronics to drive the sensor at one end and receive signals at the other . This isn't mechanical engineering ! You MUST specify the data rate to get anywhere with this - it affects everything ! MK

michaelkellett · Answer

If you don't mind waiting you can get a sensor for about £10 https://www.aliexpress.com/item/32792494513.html?spm=a2g0o.detail.1000060.2.4fec2ea1KciMwo&gps-id=pcDetailBottomMoreThis… or a classic Seafarer type (called NASA here) for more dosh but quick: https://www.allgadgets.co.uk/marine/pc/NASA-Depth-Transducer-p31.htm?gclid=EAIaIQobChMIw7uQncur5gIVh-FRCh1cDgepEAQYByABE… The Seafarer thing is just a PZT disc about 3cm diameter and 1cm or so thick potted into the plastic holder with resin. It's not that waterproof from the top and is meant to be installed through a boat hull. There is an article here that will give you some ideas about how to drive the transducer - it's not free https://www.elektormagazine.com/magazine/elektor-198406/45247/ MK

ahenon · Answer

Hello Doug,

Thanks a lot for your questions, please find my answers below:

How large can the communication system be? 10x10x20 cm each
How much power is available? enough to transmit data during 48hrs -> see data rate
Is the water turbulent? There can be 3 knots/hr stream
Is the water clear or murky? Murky
Are there any obstacles in the water? (Is it a straight line-of-sight)? No obstacle
Is there marine life that cannot be disturbed? No
What data rate is adequate? 10 o/s
How deep is it? 25 meters max
Can an antenna stick out of the water? No
Is there movement of one or both ends of the link? yes on one end
Does the link need to be omni-directional? Yes
Does the communication need to be real-time? (could a robot go back and forth?) Real time
What is the cost target? from 30$ or 100$ max (minimum is the best)
How much time is available for development? 6 months to 12 months

So do you see any options?

Thanks a lot for your help,

Alexandre

clem57 · Answer

Here is another discussion which shows the problems: RF communication through salt water . Basically without the resources of the US Navy, you would be hard pressed within the budget you have. Clem

vimarsh_ · Answer

What I think is that based on your budget you can try to create your own device (a transponder) because I didn't find a device that can fulfill all that.

Considering speed of sound in water is 1.4km/s sound is the best choice. It is also faster in see water. 50m is not much.

Probably using normal powerful transmitting devices will be enough and transmission will be real-time.

50/1400=0.03571428571s. That will be the time taken, almost real-time.

RF and light are bad ideas. For salt water, considering refractive index and density that can harm the earlier mentioned communication means. So sound will be best.

You can also look into how submarines communicate. They remain submerged for a long time and receive signal from air. That will help to decide how to transmit the data.

How to transmit data underwater over 40m

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