Watertight Compartment – CAD Model

One of the most integral pieces to our new sub design is a watertight compartment. In our previous design, RoboGoby Version 1.0, we decided to avoid the difficulties of making our own watertight compartment by potting all of our electronics in epoxy. While at the time this seemed like a good idea it ended up being a big problem for us. After epoxying our electronics we couldn't access anything which meant it was harder to debug our system and to add to our platform. Based on this experience we decided to make our lives easier this year (and in the long run) by designing our own watertight compartment. The two key pieces to our compartment are the machined aluminum pieces and the watertight connectors.

Machined Parts

At first we struggled with the type of seal we were going to use for the compartment. Although we knew we wanted to use an o-ring seal, we wanted something that would give us the largest opening into the watertight compartment and keep it within the 5"x5" size of our sub. Our first thought was to use an axial o-ring seal. While this seal is effective, it takes up too much room for a number of reasons. First off this type of seal needs constant pressure around its circumference which means we would have need 8+ bolts to create a good seal. Secondly, we want to use a double seal to ensure our compartment doesn't leak – with two axial o-rings the access opening would go under 3.5", which is too small to be useful. While talking with one of our mentors, Jon Amory, he mentioned the idea of using a radial o-ring seal. This design would follow the form of our sub better (it is longer not wider) and therefore would allow us to have both a larger access hole and a double seal while still fitting inside of our 5"x5" constraint.

Radial Seal (Apple Rubber)

Axial Seal (Apple Rubber)

To design the aluminum fittings we used Apple Rubber, an rubber engineering company. They have a great introduction to o-ring design (static o-ring seal descriptions found here).

On one end of the compartment, the access end, we designed a plug and a plug housing. The plug is made out of a 4.75"x4.75"x2.5" piece of aluminum. The plug itself acts like a cork to keep the air in and water out of our compartment. The inside of the plug is hollowed out and has holes for running wires through. One side is square to match the interior of our sub body. An image of the plug is below. If you look close enough you'll also see a radial hole in the pocket -- this is for a bolt we will use to pull out the plug, a necessary feature because it will take a large amount of force.

Plug

The plug housing is a also a 4.75"x4.75"x2.5" aluminum block with a pocket  diameter of 4" cut all the way through the material. It also has four mounting holes on one of the faces to accept the 6mm bolts that hold the plug in place. There are also four bolts on the exterior (two on each side) which are used to hold the sections together. An image of the plug housing is below:

Plug Housing

And  the two fitting together (with a transparent plug housing):

Assembled Seal

The third, and final, piece is the back plug. This part covers the end of the watertight compartment that will never be opened. It is made out of a 4.75"x4.75"x2.5" piece of aluminum. It has a large square pocket 2.25" deep and 3.25"x3.75". The plug housing has such a large pocket to give us more room inside of the watertight compartment for our electronics. Similar to the plug housing, the back end of the compartment has four bolt holes (two on each side) to connect sections together. Below is a picture of the part:

Back End

We will be welding the parts to our 5"x5" sub body to finish the watertight compartment. The aluminum parts slide into the compartment about .5", more than enough room to hold it in place while being welded from the outside. A picture of the finished compartment is below:

Watertight Compartment

Watertight Connectors

As you probably noticed, the pieces talked about above have holes through them. These will in fact be used for routing the wires inside of RoboGoby. The five larger holes on the back end plug and the plug itself are holes for cord grips which will keep our electronics sealed off (read more here). The 6th and final hole, which can be found on the plug, is a threaded hole for a Presta valve. We are going to use the valve to pressurize the compartment to relieve strain on the o-rings and quickly alert us of any leaks in our housing (when combined with a pressure sensor).

Plug Assembly w/ Connectors