[Physics] How to calculate how much weight is needed to sink a specific amount of air and for how deep will it sink? Understanding buoyancy

buoyancyfluid dynamicsfluid-staticspressure

Understanding Buoyancy

I'm trying to understand buoyancy so I created a question that should help me greatly.
To keep this simple my above is a photo is of a 20 ft long tank. Each line represents 1 cubic ft which each holds 62 lb of water. To get specific this full tank would be:
1' x 1' x 20 ' and would hold a total of 1,240 lb of water as each line is 1' x 1' x 1'.

The question is how do I calculate how much weight is needed to sink a specific amount of air? Also how do I calculate how far it would sink considering water pressure increases by depth. I'd like to find the MAXIUMUM amount of air I could sink with the LEAST amount of weight to be at least beyond 11 ft deep.

We can start with this example photo where I've made the section of air 30 % of 1 cubic foot and the other 70% is just of weight.
THIS IS A CLOSED CHAMBER which would be:

3.6" x 3.6" x 3.6" = .03 cubic ft = AIR (PINK)

8.4" x 8.4" x 8.4" = .07 cubic ft = WEIGHT = 43.4 lb (GREEN)

In theory this wouldn't be enough air to even lift that amount of weight so I'd assume that It would sink until it remains still due to water pressure at a specific depth.

Best Answer

Buoyancy does not really change with depth (water temperature differences can slightly change it's density). Once a sealed container weighed more than the water it displaced (negative buoyancy), it would begin to sink and go to the bottom. If it weighed less than the amount of water it displaced (positive buoyancy) it will float. So to decide if your container will sink or float, find it's outer volume (that is the amount of water it will displace), find the weight of that volume of water, if the sealed container's total weight is more than the weight of the same volume of water, it will sink to the bottom. If it is lighter than the water it will float. Neutral buoyancy, where it weighs the same as the water, and will neither sink or float, is an unstable condition.