Buoyancy Types

The ancient Greek scientist Archimedes was the first to discover the principle of buoyancy. He observed that an object immersed in a fluid (water in the case of divers) would be pushed up by a force equal to the weight of the fluid it displaced. In other words, buoyancy is of significant importance to you as a diver because you will witness its effects whenever you dive.

Buoyance is broken down into three types.

Positive Buoyancy

When an object weighs less than the weight of the water it displaces, this generally means the object will float on the surface. If the object were held underwater and if it were to be released, it would head straight for the surface.

Divers achieve positive buoyancy by inflating their buoyancy control devices (BCDs) while on the surface to keep them floating, and they save energy because they do not need to swim to keep afloat.

Neutral Buoyancy

The diver’s goal while underwater is to neither sink nor float. So, according to the rules of buoyancy, the amount of water displaced is equal to the combined weight of the diver and the equipment used. Neutral buoyancy—holding position in the water column without moving your arms or feet—is important when you swim over a reef because you do not want to touch or damage marine organisms, such as corals. Neutral buoyancy also prevents you from crashing into the bottom, especially important if the bottom is silty. Imagine the mess that can make. And being neutral will allow you to complete your safety stop at a fixed depth.

This feeling of weightlessness and the sensation of flying through the water in complete control is a fun part of the dive experience. This skill takes some time to master but is well worth the effort.

Negative Buoyancy

This is when the object sinks. The weight of the object is more than the weight of water it displaces. A diver achieves this by deflating the BCD and sinking.

Controlling Buoyancy

The sensation of floating in a three-dimensional underwater world is one of the most exciting aspects of diving. Being able to move up and down in any direction in between, or sideways, or just being able to float weightless and effortlessly increases the fun of the dive and is extremely relaxing. As you gain more experience, you will find it much easier to redistribute your weight to achieve optimum buoyancy and trim. The following guidelines will help.

Lead weight is used to compensate for the (positive) buoyancy effects of your body and the exposure protection worn in water. The thicker the wetsuit, the greater the positive buoyancy. While swimming underwater, you can displace more or less water by adding or dumping gas in your BCD. Your lung volume also affects buoyancy. When you breathe in (full lungs), your body size increases slightly but enough that you will displace more water than when you exhale (empty lungs). This allows you to achieve positive buoyancy at the surface and neutral buoyancy while underwater and to fine-tune buoyancy when diving.

Proper Weighting

To correctly weight yourself, you need to adjust your weights before you go for a dive or any time you change part of the equipment you dive with. This is particularly important when changing thermal protection and when you move between fresh and saltwater dive sites.

• Adjust weights so that while holding a full breath (with no gas in your BCD), you float with just the top of your head breaking the surface.
• Make sure your cylinders have only around 30 bar/500 psi of pressure when adjusting your weight. This simulates a near-empty cylinder much like at the end of a dive. This is important because your cylinder will be more buoyant at the end of your dive because air has been used and air has mass. (For example, 100 liters of air has a mass of around 130 grams. Using imperial units, 13 cubic feet of air tips the scales at approximately 1 pound.) Because of the buoyancy shift between the beginning and the end of your dive, you need to adjust your weighting. Without this additional weighting, you could float to the surface or be unable to complete a safety stop at the end of the dive. Both are potentially harmful situations.
• When you have established the correct weighting, simply dump the gas from the BCD, and breathe out through your regulator to descend.

Spend some time finding your correct weighting, and the rewards will be noticeable. Over weighting yourself can be dangerous and make it hard to control your buoyancy correctly. Experience and the guidance of more skilled dive buddies will help you build on this skill and fine-tune your buoyancy.

Moving From One Water Environment to Another

Buoyancy is affected by diving in salt or fresh water because they have different densities (unit weights) and, thus, different buoyancy characteristics. The Dead Sea (salt water) is so dense that people can float easily without needing to swim, while fresh water is less dense and to stay afloat you probably need to swim. Sea water is heavier than fresh water by 2.5% to 3% because of the greater amount of salts dissolved in it.

• When moving from fresh water to salt water, a diver has to ADD weight. There is not a precise calculation because it depends on factors other than just the buoyancy effects of salt water.
• REDUCE weight when moving from salt to fresh water. Fresh water is less dense and, therefore, has less upward force and provides less buoyancy, resulting in the necessity for less weight to be carried by the diver.
• It is recommended you complete a buoyancy check before diving in a new environment. Don’t waste a dive because your weighting is incorrect.
• Also, as you learn to relax more in the water, your control of lung volume will improve.

One interesting situation you’ll sometimes encounter when diving that is directly related to the different densities of salt and fresh water is called a halocline. Halocline describes the phenomenon when lighter fresh water floats on top of heavier salt water like layers of a cake, except these layers are usually transparent. You may not notice the layers until you see someone swim through one layer into the one above or below it. This sets up a minor turbulence when the layers of water mix, creating a very strange and noticeable optical effect as light traveling through the changing water densities is refracted at slightly different angles.