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Ping1D mount inside PVC pipe hull - thoughts please

Hi, i’ve tested the Ping1d on my jetski - impressive.
Next step is designing the robot boat for mapping with ardurover, but looking for pointers please

Q1. Mounting inside PVC pipe hulls - any issues with sonar propogating through the curved PVC surface. Probably 80mm pipe hulls with 2mm pvc thickness - but well curved and no bubbles.

Q2. Do you recommend wet mounting inside the hull -mineral oil or similar or external mount?

We’ll be testing - but any indication of upper boat speed limit for ping at around 20m assuming no bubbles etc would be helpful, - thanks in advance
Paul.

Hi @KiwiPaul,

Would definitely recommend an external mount if possible. Sound reflects at changes of acoustic impedance, and the PVC/water boundary has a sound transmittance of roughly 63%. Assuming you managed to get a wet-mounting fluid with the same density as PVC (meaning no loss going from ping1d to pipe, and pipe back to ping1d on the return) you’d still only get at most 0.63\times 0.63 \approx 40\% return compared to the usual amount (because of the pipe-water then water-pipe transitions). A water-filled pipe would have twice the transitions, so would have a return transmittance of 0.63^4 \approx 16\% of the normal amount. To make matters worse the pipe is curved, so the returning sound isn’t hitting normal to the surface, which means more of it is reflected away.

Is there a particular reason for the pipe mount? If it’s for physical protection of the sonar you could mount it so that there’s a hole for the sound to go through, without needing the whole device exposed. You could also put some mesh (plastic, or marine-grade stainless) in front of it. That would still reduce the return sensitivity a little bit, but it should be marginal if you don’t have too much of the flat face covered/blocked :slight_smile:

For a rough estimate, consider:

  1. you’ve got 40m of distance for the sound to travel (to get from and back to the sonar)
  2. the ping sonar echosounder/altimeter has a beamwidth of 30 degrees
  3. sound travels in fresh water at ~1480m/s

Putting that into an image, the maximum distance you can travel and still detect a ping is ~10.7m:

Considering the time period that’s over, your t_{echo}=\frac{40m}{1480m/s}\approx0.027s.
Converting that into the boat speed, you get v_{boat}=\frac{10.7m}{0.027s}\approx 397m/s, which from a quick google is apparently roughly as fast as a handgun bullet through air (sound in air travels at ~343m/s).

It’s worth noting that the receiver won’t be quite as sensitive at an angle as it is to sound coming straight on (like it would with a stationary sensor pointed directly at a flat surface), but at such a short distance the receiver angle will depend entirely on the angle of the boat on the water surface relative to the target surface below - speed is only a concern if it’s causing your boat/sonar to tilt significantly :slight_smile:

Wow! - External mount it is - reason for internal was streamlining and protection - both those are secondary to performance and reliability. The environment is muddy harbour so not a tough one.

re speed - we’ll keep it below handgun velocities for safety reasons :slight_smile: - but it suggests that bubbles/turbulence/tilt aside we should get good readings beyond our ability to push the boatlet through the water.

Great answers - many thanks - lots of testing ahead.

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