Considering you've gone this far (I'm impressed!!!!!) you may prefer to calculate the displacement figure yourself.........if so......
First determine how much water the housing will displace. Using exterior measurements, height times width times length will give you the volume of the housing in cubic inches ( or centimeters if you choose.......).
Once you calculate the housing's volume, multiply it by the weight of water. If you plan on diving in salt water, use that figure. Salt water weighs 64 pounds per cubic foot or .037 pounds per cubic inch (fresh water weighs slightly less, @ 62 # per cu. ft).
cu foot equal 12"*12"*12" = 1728 cubic inches
64#/1728cu. in.=0.037#/cu. in.
Say your housing has measurements of:
height=8"
width=6"
depth=12"
8*6*12=576 cubic inches
576cu in times 0.037#/cu in = 21.31 pounds
Therefore a box that measures 8*6*12 will displace 576 cubic inches of salt water. The calculations above also point out that 576 cubic inches of salt water weighs about 21.3 pounds. Therefore to make the entire box neutrally buoyant, you will have to add enough additional weight to the interior of the housing so that the weight of the housing, camcorder, and the additional lead equal 21.3 pounds.
For instance, say the housing weighs 9 pounds dry and the camcorder weighs 2 pounds........then you would have to add about 10 more pounds to the interior of the box to make the rig neutral. Of course it would be most acceptable to add the additional weight to the exterior, but if you do, remember:
The mass you add to offset the buoyancy will also displace water. Therefore you will have to also take that volume into consideration (iow, a piece of lead that weighs 5 pounds on land weighs less in the water because the lead itself is displacing water.)
This is all based on Archimedes Principle.......that buoyancy is created by displacing water and that the weight of the water displaced will create an equal amount of lift.
If for instance an object displaces water that weighs 2 pounds, then 2 pounds of lift is created. And if that object weighs less than 2 pounds, it will float. And if the object weighs more than 2 pounds, it will sink. And finally, if the object weighs exactly 2 pounds, it will be neutrally buoyant.
Also take into consideration that any external lighting equipment used might be negatively buoyant and that can help offset the buoyancy created by the housing.
good luck with your project!!!
b