Final Design

Our final design is a pressure vessel consisting of a PVC hollow cylinder of 0.1524 meters (6 inches) in nominal diameter, featuring delrin bulkheads with dual redundant fluorocarbon O-ring seals at either end of the cylinder.

The pressure vessel is placed vertically into the water such that the cylinder’s axis is perpendicular to the sea floor. PVC was chosen because it is low-cost and readily available. Delrin was chosen because of its rigidity; it deflects less than PVC and acrylic. Since the bulkhead needs to be fabricated with high precision, limited flex is a desirable trait. Both materials are resistant to seawater corrosion. After considering all options, Delrin was chosen as the material, and axial bolt pattern was chosen as the attaching method.

               The tube diameter was determined by finding the minimum amount of space our components required. For our design, a 0.1524 m (6 inches) diameter PVC tube was chosen. This corresponds to a vessel length of 0.621 m (24.5 inches). During the fabrication process, some material had to be shaved off either end. The final length of the buoy was 0.508m (20 inches)

An inflatable bladder is secured on the bottom bulkhead with a collar. Tubes run from the external bladder, through the bottom bulkhead, and into a pump, valve, and internal reservoir that comprise the variable ballast system. The variable ballast system is mounted on an internal frame which also houses an array of AA-cell Ni-MH (Nickel Metal-Hydride )  batteries.

A CTD (Conductivity, Temperature, Depth) sensor is mounted to the side wall of the top bulkhead. The sensor, which resembles a narrow tube, is oriented parallel to the cylindrical axis, allowing seawater to run through it as the vessel sinks. Its wires run through a port in the bulkhead side and into the top portion of the vessel, where the electronics tray is located. The electronics tray holds the Android phone, motor driver, IOIO board and CTD board. It is bolted to the bottom surface of the top bulkhead.

            The pressure vessel is able to rise, sink, or float by varying its volume – specifically the volume of the inflatable bladder – while keeping the total mass of the system constant. To rise, the Android sends a command via the IOIO board to the preprogrammed microprocessor. The processor simultaneously opens the solenoid valve and activates the pump for thirty seconds. Fluorinert, an inert liquid, is then pumped from the internal bladder to the external bladder, causing it to inflate. The total volume of the system increases, and the increased buoyancy force causes the vessel to rise. After thirty seconds, the valve closes to prevent fluid from reentering the internal reservoir. To sink, the exact opposite operation takes place. Fluorinert flows back into the internal bladder, decreasing the volume of the system, and allowing the gravity force of the vessel to overcome the buoyancy force.