So whenever I decided to make a case I knew I had a few restrictions
With these restrictions in mind, I first attempted to find a standard oxygen sensor and didn't have a ton of success, so I made my own.
I think looked into the best was to get air to the cell I was using and found you technically need an EDM to drill a small enough hole to sustain the pressure and reduce the flow rate. So I ended up just buying one and it had a 1/4" rubber hose that I found would nicely attach to NPT threaded nozzle. With this I had from scuba to sensor and just needed a housing
To attempt to reach the waterproof item I decided to build a cylinder with lots of tiny holes that should be small enough to resist most splashes. The cylinder would be built in 3 parts. A raw cyliner with a cavity on the side to house the electronics. One end would have a spot for a connector board to connect the sensor to the main electronics and could then be glued in with a cap to prevent water intrusion. The other end I made a screw head with a NPT threaded hole in the center. While This worked in theory, it turns out its incredibly difficult to print good screws with a 3d printer, much less ones that are water proof.
Onto method #2, Designing an enclosure for the electronics and sensor that I can waterproof togethor.
Has enough room on each side of the sensor so it can easily slide in and out, has a taper at the bottom that allows a pressure chamber to form for the oxygen portion. 4 screws allow for even tension with a upper housing. With this I needed to design the upper portion.
Initial version had sections for buttons and screen, I figured I would silicon in around the buttons. Also not pictured is a slot for micro-usb to recharge the battery. Problem was while it was easily serviceable, it was not particular waterproof and had the battery,sensor just sort of flopping around. also the connection between the main circuit board and the sensor was a cable which made assembly hard and uncertain. So I decided to design around the issue, first was a tension plate that both arranged the sensor correctly, put pressure on the senor to maintain the pressure chamber.
It took a few iterations but I realized I need to retain the tension board to the top so added screw mounts. Next I need something to hold the battery
I realized after a few versions I too needs to hold this and plugging and unplugging the battery every pass was annoying. So I moved the battery connectors to a mid board and screwed the battery holder to the tension plate. This allowed the entire assembly to come out for service to change the battyer but also allowed for just 4 screws to change the sensor that expires every year. Final change was screw holes in the top plate to also retain the main board as well as a cutout for the humidity sensor. With that I had a fully functioning and easily printable case.