Diving buoyancy

By Marty McCafferty, EMT-P, DMT, and Patty Seery, MHS, DMT

From the first diving lessons, students learn that three elements intervene in the control of buoyancy: the stabilization vest, called "stab", the weights and the lung volume. While one can easily understand the need for adequate ballasting, it is less obvious to understand all that this implies. Two errors are commonly made when it comes to ballasting, both by novices and experienced divers: on the one hand the tendency to use too much weight and on the other hand the wearing of a weight that is not adapted to changing circumstances, Equipment and environment may vary from dive to dive.

Avoid getting tired unnecessarily

Improper ballasting hinders the achievement of neutral buoyancy. However, many "overweighted" divers do not realize that they are overweight. This excess weight forces them to further inflate their stab in order to achieve neutral buoyancy, which can prevent them from maintaining a horizontal position. Vertical, the increased resistance force of water increases the effort required to evolve underwater and the amount of air breathed. An "underweighted" diver can also become exhausted by trying not to go back up. Greater physical exertion not only increases the consumption of respiratory gas, but can also increase decompression stress.

Adequate ballasting

You may have heard a diver say, "This is the weight I always use." If an underwater buoyancy test and previous experience may prove useful, this type of statement should not be the last word in a ballast conversation. Indeed, a correct ballasting requires reflection and practice and varies according to the circumstances. A diver can gain or lose weight (in terms of muscle mass or body fat) and his physical shape can fluctuate. In addition, the equipment (especially the wet suit) wears out and must be replaced from time to time. Finally, the diving environments also vary. All of these factors have an impact on buoyancy and require ballast adjustment.

In order to determine the amount of weight required, body weight, the type of wetsuit worn, the weight of the equipment and the type of environment in which the dive will take place must be taken into account. When wearing a full wet suit of 6 mm thickness, it is recommended to start with a weight equivalent to 10% of the body weight. With a combination of 3 mm, this starting weight can be reduced by half. Keep in mind, however, that these percentages are only a starting point.

Dry suits and thicker neoprene require more weight to counter their increased buoyancy. Body composition (muscle density, for example) also influences the amount of weight needed. An aluminum diving bottle will also require more ballasting than a steel bottle.

Salt water is denser than fresh water and increases the buoyancy of submerged objects, which therefore require more weight to sink. Diving training generally begins in freshwater environments such as a swimming pool, a quarry or a lake. Beginners divers should therefore consider increasing their ballast if they dive at sea wearing the same suit. The amount of extra weight needed depends on one person to another. Performing a buoyancy test in each situation allows you to determine exactly how much weight to add.

Ballast fixing

There are different methods of ballasting. The most common is the lead belt, on which lead blocks are threaded, or to which are attached pockets that can contain lead pellets or flexible shot bags. Belts are easy to drop in an emergency as long as they are not trapped under other items of equipment. A shoulder harness or ballast harness is sometimes used when wearing a thicker suit requiring an amount of weight that it would be uncomfortable to wear around the waist.

The lead pockets integrated into the stab and the harness systems offer two advantages compared to belts: their comfort is often much higher and the weight is easier to balance. But unlike belts that have a single release loop, harnesses and integrated systems can have multiple release loops or handles. This is crucial information to share with your partner before a dive, and to keep in mind in an emergency. One disadvantage of some lead bags is the difficulty of adding or removing weight if an adjustment is necessary.

The importance of a balanced ballast

In addition to wearing an adequate amount of ballast, the weight should be well distributed in order to optimize the position and balance of the diver underwater. A horizontal position will provide the diver with better hydrodynamics. To do well, you should wear the same amount of ballast on each side of the body to avoid tipping over to one side or the other during the dive. The weight and position of the bottle must also be taken into account when placing the ballast. It is sometimes useful to raise or lower the bottle in the strap of the stab in order to contribute to an optimal positioning of the diver underwater. Back inflation stabs tend to push the diver forward (head down) into the water. It may therefore be wise to move the weights backwards to counterbalance this tendency. Attaching lead bags to the back of the stab can help balance the diver, but could be dangerous in an emergency because the help of the buddy will be necessary to drop them, if necessary.

With regard to the fins, some have positive buoyancy, others neutral and still others negative. Each type of fins may require compensation. Ankle weights can help counterbalance greater buoyancy in the legs, but are also more difficult to drop. If the fins have a negative buoyancy and push the legs down, one solution is to place the pellets higher on the body or to reassemble the bottle in the strap in order to raise the center of gravity and to maintain itself more easily at the horizontal.

It is essential to check before each dive where and how the pellets of your partner are placed and know how to drop them in an emergency.

By learning how to determine the amount of weight needed, the diver not only increases the pleasure of diving, but also its safety. A good knowledge of the necessary basic ballast and the factors influencing buoyancy will help it adapt to a variety of environments and conditions.

How to perform a buoyancy test

Immerse yourself in a place where you have no feet and empty all the air from your stab. When you take a normal breath, the water should reach your eye level. When you exhale, you need to run just enough so that the water covers your head. Then your next normal inspiration should bring you up so that the water is again at eye level.

Adjust your weights little by little (about 1 kg at a time). It may be wise to add a little more weight than necessary to counter the increased buoyancy of a partially empty aluminum bottle at the end of the dive.

If you are properly weighted, you should be able to move effortlessly to a depth of 4,5 m at the end of the dive with an aluminum bottle containing 500 psi (approximately 34 bar) and a deflated stab.