Pressure Housings

Pressure Housings are used to enclose electronics underwater. There are two primary types of housings.
Pressure compensated housings are fluid filled and are maintained at ambient pressure. A flexible diaphragm is typically used to transmit the external pressure to inside the housing. Benign substances such as mineral oil or silicone oil are used for the compensating fluid which must completely fill the inside of the housing leaving no air pockets. Components that are air or gas filled cannot be used, because they will implode under pressure.
One-atmosphere housings maintain constant pressure inside. Typically they are air filled, but sometimes dry nitrogen or other gases are used to prevent condensation. Cylindrical- and spherical-shaped housing designs are strongest. Rectangular and other shapes are usually only used in shallow water applications. Common materials selected for immersion in seawater are anodized aluminum, stainless steel, titanium and plastic.
Cylinders are the most common housing geometry. Flat or hemispherical end caps are typically retained with bolts or are threaded into the main tube. Glass spheres are also sometimes used for pressure housings. The clarity of the glass can be taken advantage of by optical sensors and controllers.
Things to consider when designing or selecting an underwater pressure housing include depth rating, safety margin and material selection.