Underwater speed?

[Ramius]

Hi All!
I have found that some of the smartest people are here on this site. I wish to measure the speed of an R/C submarine using pressure sensor(s). I have seen the "wheel" types which seem very large. It would probably be very unsightly to have a long pitot tube sticking out the front. It just seems "logical" that if you measure the pressure of the water on the bow you could somehow calculate the speed? Yes I did look at the "eagletree" system which uses a pitot tube and I doubt since they sell sensors they would wish to provide any information. Any ideas would be very much appreciated.

Thanks in advance.

Ed

[Demon]

Look at liquid cooled PCs, some have a flow meter. You might be able to use that.

I can't see how you can avoid a pitot tube. I'd put it below the waterline when surfaced, centerlined with sub. You'd only see it when you take the sub out.

Robert

[mark_s]

Maybe you could use a flowmeter. They have a plastic impeller with a magnet and a hall effect switch to send pulses to the pic. Could be mounted inside the hall with external ports in the front and back to allow water to flow through. Just an idea.

http://www.ebay.com/itm/Water-flow-s...item2ec72fe6ba

[kellyseye]

Have you considered the effect of tides/water movement?

One method in use is to generate a magnetic field then measure the voltage induced into a pair of sensors by the water flowing through the field (it being the conductor). Obviously using the sub in fresh water will produce some problems.

[Ramius]

Thanks Guys!
All are great ideas! I already have a depth sensor accurate to 1/2" and the thought was to use a second pressure sensor mounted in a torpedo tube to look at the water pressure on the bow. Where I am stuck is how to figure out how to conver the incomming pressure to speed? I also have temperature sensors. My first guess and I could be completely wrong is that the depth sensor would be the same as the "static" pressure and that the static pressure is subtracted from the incomming pressure? I just do not have any working knowledge of how speed is done.

Best, Ed

[Demon]

I'd try some sampling. Stick the sensor/tube on the front of a piece of wood (broom handle), drag the setup in water at a certain feet/second, record the sensor values, then convert to whatever unit you want.

It all sounds very good in my head anyways.

Robert

AND TAKE PICS!

[MITechMike]

Hello Ramius
It looks like folks here were very helpful and I'm curious about what you came up with for a solution. I have a project that I'm working on and need speed through the water, using as small a sensor as I can.
Thanks
Mike

[Ramius]

Hi Mike!
Actually there are now a lot of answers to basic questions. First save yourself a lot of research by knowing that speed = PSI! As for the PIC code you can find it here at:http://www.picbasic.co.uk/forum/showthread.php?t=19572. For me I needed a "differential" pressure sensor from Seroflo ([email protected]). The reason for a differential is that as my sub dives pressure will increase so you have to subtract out "depth" pressure (Static Pressure) from the pressure on the bow (velocity Pressure). One technique I have used with my compass was to mount the sensor on top of the PIC. If you are using a surface mount PIC you will see the PIC fits comfortably under just about any sensor. You can use a CAD program to design your PC board and then convert the CAD drawing to Gerber files. It helps to create "Layers" in Cad that you can turn on and off so you can create you solder mask and "ground plane". You will probably end up with a double sided board so you can make one side say in red traces and the other side in blue traces. Do not forget to specify plated thru holes! Your board size will most likely be .625 x .625". About the only other design consideration will be to isolate the sensor from the water. Hope this helps.

Best, Ed