What is critical when both the BCD and dry suit are used together to control buoyancy?

Simultaneous venting is critical when both the buoyancy control device (BCD) and the dry suit are used together to manage buoyancy effectively. This process involves releasing air from both systems at the same time to maintain neutral buoyancy while ascending or descending in the water. When divers use a dry suit, it provides an additional layer of buoyancy control. If a diver only vents air from one system without accounting for changes in the other, it can lead to an imbalance, resulting in uncontrollable ascents or descents.

For instance, if a diver begins to ascend and only vents the dry suit while leaving air in the BCD, the diver may become overly buoyant and experience a rapid ascent, which is dangerous. Likewise, if only the BCD is vented, the dry suit can cause the diver to descend too quickly. Thus, simultaneous venting is vital to manage the combined buoyancy provided by both systems and ensure a safe diving experience.