update: link to CAD files
Finished printing the latest hull design This had to be printed in three sections due to issues with excessive overhang at the bottom portion of the hull, and is held together using M3 stainless steel cap screws. There is a hole in the center of the bottom section so an analog temperature sensor can be installed and potted.
Having replaced the large Arduino Mega with the new Sodaq Ndogo, there was now plenty of room inside the buoy to include all the extra sensors (yet another excuse to spend hours with CAD). Also, after researching static o-ring seals I have decided to test the spherical buoy design once more and integrate a 3/16″ thick gland into the lower portion of the buoy hull. The project will also use RTV as a backup to the o-ring in case the PLA is not stiff enough to properly compress the o-ring. Below is an exploded view of the new assembly.
Somewhere along the way, I spent a several weeks designing and redesigning alternative buoy shapes in CAD. I started at version V03 and ended up working my way to version V10, but I always came across the same issues…limited space inside the buoy meant I had to limit the number of onboard sensors. I had initially planned to use an Arduino Due as the brains of the project since it had a faster clock speed and larger memory compared to the standard Uno board. I also liked the idea that the Due ran at 3.3V for low power consumption which would translate to longer battery life for greater deployment times. However I could not get the Due to work with I2C when connected to my IMU so I gave up and decided to use an older Mega 2560 I had lying around which seemed to work.
Having printed a red and black version of the more recent buoy design where an o-ring is built into the upper lid of the buoy, I was concerned that the single o-ring in-between the two halves would not be adequate for prolonged ocean use. If the seal were to fail the inside would fill with water and ruin the electronics. Ideally there should be redundancy built into the design for more reliant waterproofing to protect the sensitive electronics.
However, now that I was able to hold the printed parts in my hand and watch it float in the bath tub I was very happy with the size and general shape of the buoy. The small 150mm diameter hull would make for very easy deployment, just throw it like a baseball, while still having a low environmental impact (hopefully!) when monitoring coastal areas or rivers and lakes (as mentioned by a comment by Paul).
After printing the first version of the latest design (V.04), I realized that it would be more convenient to mount all of the electronics to the underside of the lid rather than to the bottom half of the hull. This would make assembly and disassembly for maintenance much easier and keep the tangle of wires to a minimum. Below is the more recent concept idea, where the microcontroller and sensors all mount to the underside using posts and screws. This will also create a structural connection point to mount the antennas and xbee radio.