you want to determine the required volume of the dive tank for your next project (important for subs, which shall dive statically).
The following steps can already be done on a theoretical basis on the drawing board with a few assumptions, before buying the wrong diving tank.
Therefore a look from the opposite side is maybe helpful:
Consider a well balanced sub, which is floating statically beneath the surface (neutral buoyancy). How much water must be removed from the tank so that the sub is floating at its foreseen waterline?
Step 1: Put all future equipment (RC, motors, servos, batteries etc.) into the WTC; except the tank and its connected components.
Step 2: Insert a corresponding mass (e.g. lead) at the tank position; corresponding to the mass of the tank and its components.
Step 3: Connect the sub to a scale (maybe your friend is a fisher man and has a scale that can be borrowed) and put the sub into the bath tub, a small pond or similar environment (sub totally submerged). The sub should remain in horizontal orientation.
Step 4: Measure the weight of the submerged model.
Step 5: Now carefully: Lift the sub – still attached to the scale – until it has reached the desired waterline and repeat the measurement (sub in horizontal or at desired orientation). It will have gained weight.
Step 6: Difference between both measurements is the volume of the dive tank you need to achieve neutral buoyancy for your sub.
One should add some 15 % to 20% to be sure that the sub will dive statically; in case a small air bubble is hiding under the skin of the boat.
Reason for this “reverse” approach is the following:
A trimmed submerged sub is balanced within its environment (neutral buoyancy) and it will need lift to surface until it has reached its nominal waterline position. It is exactly this “negative” lift which is necessary to submerge and achieve neutral buoyancy (statically well balanced under water).
The rest, how it will behave during cruise under water, is a matter of the individual sub shape and depth rudder efficiencies and has been explained in other contributions within this forum.
I just did it for my Revell GATO (300 ml dive tank necessary).