Buoyancy or lift is created when water is displaced by an object that weighs less than the volume of water that is displaced by that object. If the object is heavier than the volume of water it displaces, it sinks. If the object is equal in weight to the water it displaces, it is considered to be neutrally buoyant and will in theory "suspend".
So to increase lift, you must choose a material to attach to your rig that weighs less that the water you are diving in. Salt water weighs @ 64 pounds per cubic foot, fresh water weighs @ 62 pounds per cubic foot.
You must also take into consideration pressures at various depths and their effect on the chosen material. The deeper you dive, the more potential for compression of the chosen material. At sea level, pressure is measured at 14.7 pounds of pressure per square inch (psi). For every 33' of water depth (one atmosphere, one bar, etc) an additional 14.7 pounds of pressure per square inch is experienced. For instance, at a depth of 66 feet, around 44 psi (3x14.7) is applied to objects.
As mentioned, pool noodles, which weigh much less than the water they displace at sea level, may not offer as much buoyancy benefits the deeper you dive due to the compression of the material. otoh, they may do the trick for you, especially if you dive shallow.
Closed cell foam such as Core-Cell, Divinycell, and Last-A- Foam, will perform much better than pool noodles at the lower end of recreational dive depths. Many closed cell foam products are also easily shaped to create "collars" for additional buoyancy around very negative areas of the camera rig such as wide angle lenses. One key to a good design though is to keep the buoyant material located above the "ballast". Otherwise the camera rig will have the constant, undesirable (imho) tendency to go "belly-up".
Closed cell foam is a very popular material for boat building. If there are any boatyards in your area, ask for a scrap piece.
Spec sheets are available on-line for most closed cell foam products. The data included in a material spec sheet will usually list the weight of the foam and the compression strength. The 1/2" thick, 5 pound rated Core-Cell (weighs @ 5# per cubic foot) wrapped around my strobe arms is durable and rated to withstand pressures well beyond recreational dive limits.
btw:
if a cu ft of core cell weighs 5# and a cu ft of water weighs 64#, then @ 59# of lift would be created by one cu ft of the material.
hth,
b
