In the evening of the open water tests, we reviewed video of the housings while talking (perhaps evangelizing is a better word…) to several friends at the Centro Ecológico Akumal where we were staying.
After sleeping on it, I realized that we had actually observed one other problem with the units the day before: Although the water flow was moving the housings, the videos showed that they did not quite lean at the same angle as the supporting pole. The second pivot point under the float was allowing the unit to try to right itself, reducing the tilt angle that the accelerometer would read. So the next day, while Trish was off diving with one of her students, I attached some foam floats to the support rods, which now had plenty of holes to vent any trapped bubbles, and I refashioned the float end of the support rods into a fixed ‘T’ junction.
After Trish returned, I put on my kit and we took the units in for their second real world test, again with a dummy payload. With extra buoyancy on the lines, the o-ring design performed beautifully, and even with the really fast flow of this system, we were not seeing much wobble. So I proceeded to setup the rubber bottom design, and this time I got to watch that unit slowly sink to the ground. Closer inspection revealed that the rubber end cap was slowly becoming convex. The water pressure at this deeper cave was much higher than it had been at the coastal site we used for the first test; so it was pressing the end cap inwards.
So day 2 of the field testing was nearly the mirror image of day 1, but it showed that I needed to re-enforce the soft rubber end cap somehow before that unit would behave predictably. This also explained why we had heard the noise during the pressure test in the light maker’s dive shop: the rubber must have been extremely concave during that test, so it sank to the bottom of the chamber with a “clunk”.
So another unsuccessful trial, but much was learned. So it goes…
Underwater Arduino Data Loggers 