Opinions wanted for data through water

[ptribbey]

Would like your opinions on the best way to transmit data through water.
This is the scenario: An LM34 driving a pic driving some sort of transducer. The range will be 200 feet to 0 feet. the receiver will be some sort of transducer driving a pic driving an lcd. It will measure water temp. My first thought was to use a piezo device for the transmitter, and an old vexlar transducer for the receiver. If so, at what frequency? It will be used in freshwater only. Can it be done with radio? I know there are commercial units out there, but am trying to tackle it from scratch for the education.
Thank You
Paul

[Robert Reed]

Having built several sonar devices in the past, I can tell you the piezo transducers are the best way to go. The cheapest price on these would be marine depth sounder replacement transducers. These operate at a 200 KHz frequency and are generally pulsed for a couple of hundred micro seconds at 200 volts give or take. The echo signal is surprisingly large, covering the range of a few to a few hundred millivolts, depending on target distance. A very simple step up transformer wound on a small form with 15:1 ratio and driven from a 12 volt supplied transistor will drive the transducer nicely.This can easily be wound by hand on a 1/4" X 1" form. However. you will never get your readings of anything less than 2 feet due to inherant transducer ringing ( similar to Radar clutter).In the units I have designed, I had to add a short "gate out" one shot to the reciever input to kill the reciever for that short ringing time. Did I under stand you correctly, that you want to measure water temperature? The waters temperature will affect wave speed but you are cutting hairs in differentiating the measurements. Sounds like a lot of research is in order here.

[Chris Smith]

Sound ....+ Data

The advantages of sound over light are mentioned in many articles.

Sound goes thousands of miles while light is always limited to feet or yards.

RF is a waste of time.

[stevech]

first order challenge is battery power/life, assuming your device is untethered.

blue laser light is sometimes used. Acoustics more often, like pseudo-random codes for high correlation gain and operating with a poor SNR

[ptribbey]

I was hoping to mount the pic and the battery in a small container around the size of a pack of smokes. I would mount the piezo and the temp probe outside the case. The device would be clamped to a down rigger for lake superior trout fishing. Finding the thermalcline is key for a good catch.
Robert, you mentioned a 200 volt pulse to the piezo, I was hoping to use the 1" piezo offered by all electronics. I dont think they are hardy enough.
You are right when you say alot of research is needed.

[jimandy]

The device would be clamped to a down rigger for lake superior trout fishing

And the trout won't be disturbed by the pulses?

[ptribbey]

No, the fish are not disturbed. There are commercial units available, and using one always means more fish. When you fish the big lake, you use what is known as a downrigger. It is a winch with a large weight attached. You cast out your spoon and then let the water take out more line, then you clip your line to the downrigger, and lower it down anywhere up to about 200 feet.

[rshayes]

Your application would nedd a lot less power output than an echo sounder. Thes have to transmit a pulse to the bottom and detect the reflected pulse at an equal range. You only need to detect the signal after the first pass.

If an echo sounder could detect the bottom at 200 feet with a 200 volt transmitted pulse, I would expect that a one-way path would only require a 14 volt pulse, assuming that the bottom is 100 percent reflective. Using fixed pulses and varying the pulse repetition rate would be a simple way of transmitting the information. A pulse repetition rate in the hundreds of hertz with 100 microsecond (about 20 cycle) pulses would give a duty cycle of about 1 percent. A 9 volt alkaline battery might be able to supply enough energy for several hours of operation, depending on the power required to drive the transducer.

[Robert Reed]

The last sonar device I built used two 9v batteries, because I needed a regulated 15 volt supply for auxiliary circuits. I used a 1" diam. X 2" long transducer. The 180 volt pulsing( actually the `pulse' is supplied by a burst oscillator at 200 KHz for 150 usec.) was easily obtained and definately needed even though I was only shooting for 50 ft. maximum depths.The drain on the 9v batteries was insignificant due to two things-The duty cycle is quite low and I used a timed start switch which kept the unit on for 30 seconds and then shut off. This allowed periodic depth checks without worry of remembering the next day that I had forgot to turn the unit off. In our area (Lake Erie) the thermocline varies at and around 30 feet during the decent weather months. It wouldn't be hard to shoot this depth, but since the waters temperature only varies the speed somewhat, what do you get the return echo from? Obviously, some reflection occurs as a layer of water of different temp occurs,( a different medium) but it seems like it would be quite small. Two items of note - there are small units of about 2 1/2" cubed available for under $90 dollars ( these work extremely well) and there are common transducers that operate at 50 KHz. I have no experience with these beyond the fact that the lower frequency is supposed to penetrate more deeply.

[Robert Reed]

Oops! After rereading your last post, it looks like you are transmitting a pulse from downrigger ball up to the surface where it will be recieved by another device. This paints a totally different picture from how I first understood it. I assume now that you are only interested in the depth at a given temperature. And as RS Hayes mentioned could be done with less power for the reasons he gave. Most piezos for these applications can handle quite a bit of voltage in pulsed form at thier resonant frequency, but you would have to check with the manufacturer on this. I would imagine you could use any piezo down into the audio range ( possibly a hacked speaker) at lower power. I think most of these will handle over 50 V across them. The 200 KHz standard frequency was derived from the fact that this is way above any natural resonant frequencies of hull, machinery,etc., so you might have an interference problem if you get too low in freq. Also electrical components will get larger with decreasing frequency.

[ptribbey]

Thank you Robert, rshayes and others who took time to post. Yes the pulses will transmit one way temp data through the water up to the receiver at the boat. I was thinking about transmitting one 16 bit word every 30 seconds or so. That should give me resolution enough for tenths of a degree, and keep the power draw down. If I am thinking about this correctly, my immersed microphone at the boat will first need a high pass filter to cancel out the ambient noise. It should only let the 200k and up pass through to the pulse shaper.

[Robert Reed]

In my limited experience with these devices, I needed a tuned receiever at 200 KHz. this required 3 stages - each one with an LC resonant load. This produced a gain of about 10,000, much more than you would need for your application. The best receiving transducer would be another piezo for listening only.
One other problem is that your normal depth sounder will be running at 200 KHz also and cause interference when turned on. It seems like life in the electronic world is never easy.