A lack of buoyancy control is one of the most common root causes of diving injuries, and even fatality. Divers without adequate buoyancy control face issues such as barotrauma, marine life injuries, and uncontrolled ascents. Without appropriate buoyancy control, divers can also dive deeper than intended, increasing air consumption and unintentionally altering their dive profiles. The key to managing all of these risks is to get your buoyancy just right, and keep it that way.
What is Buoyancy Control?
We were all introduced to the physics of buoyancy control in our open water courses, but it can be easy to forget the basics: When your weight equals your displacement, you are neutrally buoyant. During a dive, your body will displace a specific volume of water, and that volume of water weights a certain amount. If you and your equipment are lighter than the displaced volume of water, you will float upward. If you outweigh the water you have displaced, you will sink. During your dive, your breathing, the compression of your exposure protection with depth, and the changing pressure of your tank can change your buoyancy, and you can offset that change in buoyancy by adding or venting air from your buoyancy control device (BCD).
Many divers rely on the dive shop, their instructors, or simply what they used for their last dive to determine their weight needs. However, it’s critical to not underestimate the importance of proper weighting. Perform a weight check regularly to make sure that you have enough lead to dive safely, but especially that you are not overweighted. If you are diving in a new location, a new environment, or have changed your equipment configuration, you should always perform a weight check. Wetsuit thickness, the use of drysuits, switching from steel to aluminum tanks, or even using a new wetsuit of the same thickness can all change your buoyancy significantly. With a BCD containing enough air to account for the air in your tank that you’ll breathe during the course of the dive, and a full tank, you should float at eye-level in the water with a normal breath and descend as you exhale. At the end of the dive, with a close to empty tank and an empty BCD, you should be neutral with a normal breath.
Buoyancy Changes during the Dive
You should start every dive with some air in your BCD. As you descend and the increasing water pressure compresses your exposure protection and the air in your BCD, add small amounts of air as needed to maintain neutral buoyancy. At the end of your dive, you’ll need to vent air as you ascend, and both the air in your BCD and your exposure protection expand once again and increase your total displacement. At your safety stop, your BCD should contain the least amount of air of the whole dive. If you can feel a hump of air in your BCD, you are overweighted and should consider removing some weight for the next dive. In addition to keeping yourself neutral in the water, make sure that you can achieve good horizontal trim – reducing your cross section in the water will reduce the effort it takes to move forward through the water, and make it easier to avoid damaging coral or other benthic reef life.
For more information on safe diving practices, visit DAN.org