What is the formula for calculating the amount of air needed in the BCD at depth?

The formula for calculating the amount of air needed in the Buoyancy Control Device (BCD) at depth focuses on the relationship between the diver's total weight and the buoyancy they experience underwater.

At depth, a diver's total weight remains constant. However, the buoyancy of the diver will change depending on the water pressure and the displacement of the water column. To achieve neutral buoyancy, which is essential for safe and controlled diving, you need to balance the diver's weight with the buoyancy at that depth.

The correct formula indicates that the air needed in the BCD is calculated by subtracting the buoyancy at depth from the total weight. This means if the buoyancy effect is greater than the diver's weight, you need to add air to the BCD to counteract that excess buoyancy and achieve neutral buoyancy to ascend, descend, or hover effectively. Thus, this answer provides a clear and practical understanding of how divers must manage their buoyancy during a dive using the BCD.

In contrast, the other formulas either add elements that aren't necessary for this calculation or misrepresent the relationship between weight and buoyancy at depth, leading to potential confusion in achieving proper buoyancy control underwater.