Hey All! 

I'm super new to the group, but I'm exploring low drag options to meausure speed through the water, not based on position. I was hoping to try to convert the airspeed pitot tube setup for use in the water. 

Has anyone had any experience converting pitot tube setups for water? Any suggestions to get it working? 

Thanks all! 


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  • Really, you should try an optical flow sensor together with a light source --> https://store.3drobotics.com/products/px4flow

    There should be enough bubbles in the wake of the boat for it to work.

  • Yes, it will work.

    Years before drones, I did a project with a wakeboard fanatic, and we used a differential pressure sensor, and a simple amplifier, into A/D port of a PIC, for "Waterspeed"  :-)

    Turned out to be overkill, most older boat speedometers consists of an open tube pointing forward, with a pipe feeding a pressure gauge, properly calibrated of course.

    Oh well, at least mine had a digital display, and audible warnings if the boat went too slow, ......or too fast :-)

    And the idea was the PIC would control the throttle, but then the whole wakeboard thing died an inglorious death :-(

    The pressures in this environment are significantly higher than in air, so you can use "normal" opamps to scale/calibrate your sensors, no need for "low noise".

    I used a Motorola MPX100DP sensor.

    • Gustav thanks so much for reply! I knew it was possible, but wasn't sure what I would be up against. Did you have to go through any special design setups/ major modifications to the tubes to make it work? Was water ever an issue with the tubes/ pressure sensor? Was capillary action ever a problem?

      Do you have any pictures? I'd love to see how you did it.

    • We used a 5mm copper tube pointing forwards, thin irrigation tubing to the sensor.

      ( Water plants did rip off the sensor every now and then :-) )

      The diameter of the tubing is such that it doesn't lend itself to capillary action, but we did toy with the idea of using a small bottle in the line, to collect any water that might find its way up the tube.

      Never implemented it though......

      Calibration was a simple matter of checking the speed with a GPS, and then changing a variable in the code.

      I never took any pictures, was before digital cameras, for the masses.

      The project files were in an old 386 SX computer, I will really have to dig around to see if I can find that harddrive !

  • Optical flow?

  • I don't see why replacing the concentric pitot tubes with some type of Venturi device wouldnt work.
    It would cause a suction proportional to your speed. Just re plumb the pressure transducer to get a positive value.
  • I think you need to understand how an airspeed sensor works before you can do anything with it! the sensor uses pressure from the front of the tube to measure the airspeed the more the pressure the faster the sensor reads, if it gets clogged with water like rain while flying it will stay at that speed even if it is not moving, and the plane will think it's stalling or speeding and slow down. So while it sounds like a great idea, I'm sorry to rain on your parade but I don't think it will work. 

  • Hi

    I suggest trying the old tried and tested water wheel, you will need to work out an interface.


    airspeed to pitot water pressure  will be a real pita.. would be cool to try that but calibration is everything.

    also check here for this conversation  http://www.picbasic.co.uk/forum/showthread.php?t=17993 ;

    • Survmatic, thanks so much for the suggestion. As you can see above, I'm dealing with extremely heavy chop for the size of the boat, so I need a deep water solution as opposed to a hull bound solution. minimizing drag here is critical for the speeds I'm looking to hit. 

      I'm with you on the calibration. Probably still going to try it :) Seems like this is the best option, just have to get it to work! 

      Awesome posts on that link. Thanks a ton.

    • not sure how the pitot tube you were going to use matches with this, but this has suggestions on pitot tube design for the water:


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