Diving Buoyancy

Issues of Concern

Effective buoyancy control is essential for proper gas management, and depth control is only a component of this solution. Divers can reduce their air consumption by making fewer adjustments to buoyancy and maintaining a streamlined hydrodynamic attitude effectively. Any reduction in a diver's effort at depth can lessen decompression stress, and effective buoyancy control significantly aids this goal. Experienced diving cinematographers understand that video and still images captured beneath the surface are more likely to be stable when the cinematographer is neutrally buoyant. During dives into caves or wrecks, maintaining minimal contact with surfaces is advised to reduce the risk of injury and preserve visibility.

Gear Basics for Buoyancy

The primary modifiers of personal buoyancy control include the buoyancy control device or jacket, thermal protection, lead weights, and the diver's lungs.

Intrinsic Factors

According to Archimedes' principle, if a swimmer remains still, they displace a volume of water equal to the body volume submerged. To float, their upward buoyant force must exceed the downward gravitational force (see Image. Archimedes' Principle). Internal factors in the human body that affect buoyancy include the percentage of body fat and the amount of gas in the lungs.

Lung volume has the most significant self-generated effect on buoyancy. When neutral, the diver can fine-tune their buoyancy with their breath. To descend, they exhale, ascend by inhaling, or take partial breaths to stay neutral.

A high body fat percentage has a small positive effect on buoyancy that can theoretically allow an adult human to maintain neutral buoyancy. However, this situation is not ideal. To be neutrally buoyant in freshwater, an adult must have 60% body fat, which correlates to class III obesity. Most snorkelers and divers are relatively healthy and require a lead weight to descend.

Although their volumes are comparatively small, the air volume in the gastrointestinal tract, sinuses, and middle ear can significantly influence the risk of barotrauma but has little effect on buoyancy.

Salinity, Thermal Protection, and Buoyancy

Water salinity has the most significant external impact on positive buoyancy. When salt is dissolved in water, its density increases, as does the buoyant force it exerts. Swimmers who remain still in freshwater and exhale typically become negatively buoyant and sink. Ocean water has a salinity of approximately 35 parts per thousand (ppt). The Dead Sea has a salinity of 337 ppt, making it nearly impossible to sink in while swimming.

After salinity, thermal protection is the second most significant external factor influencing positive buoyancy. A dive skin made of nylon, Lycra, spandex, or other material is neutrally buoyant. However, these materials are impractical in cold water. The degree of positive buoyancy is influenced by the type of exposure suit worn, whether a dry suit or a wetsuit of 3, 5, or 7 mm thickness. A dry suit typically requires approximately 3 lbs (1.36 kg) of additional lead weight for a 7-mm wetsuit to compensate for air added during a dive to avoid a suit squeeze.

Tanks and Lead

Scuba tanks vary in their buoyancy based on their material. A 12-L aluminum tank is 1.6 pounds (0.72 kg) negative when full and 2.8 pounds (1.28 kg) positive at 500 psi. A 12-L steel tank is about 9 pounds (4 kg) negative when full and 3 pounds (1.36 kg) negative when empty.

The rule of thumb for lead weight to counteract the positive buoyancy of a wetsuit is 2 to 3 lbs (1-1.36 kg) of buoyancy per mm of neoprene. Depending on the buoyancy control device, a diver may need up to 4 lbs (2 kg) of lead to counteract the positive buoyancy of a wetsuit.

A ballpark figure for the amount of lead required for a diver in a 7-mm wetsuit with full scuba gear is 10% of the diver's body weight. In addition, the average difference in lead requirement when transitioning from freshwater to saltwater diving warrants adding 4 to 7 lbs (1.8-3.2 kg) of lead to the diver.

There is no difference between the effects of solid lead and lead shot on buoyancy. The latter is typically contained in a bag and is more malleable when placed in an integrated pouch, in a buoyancy control device, or between tanks. Lead shot may also reduce the severity of injuries when dropped. All lead attachment systems require a quick-release mechanism for emergency ascents. Lead is preferred for diving due to its high density, ease of casting, corrosion resistance, and low cost.

Buoyancy Control Devices

Early forms of the buoyancy control device resembled a horse collar, appearing approximately 20 years after Jacques Cousteau launched the aqualung. In 1961, Maurice Fenzy designed the horse collar or Adjustable Buoyancy Life Jacket that was inflated by mouth. In 1968, Joe Schuch and Jack Schammel developed a buoyancy compensator vest that featured a smaller buoyancy ring behind the diver and a midriff section with volume to lift the diver out of the water if one or both of its carbon dioxide (CO₂) cartridges were activated for an emergency ascent. In 1972, Watergill developed the first wing-style jacket with a cummerbund, shoulder straps, an inflator, and an integrated weight system. Since then, buoyancy control devices have undergone minor changes and variations to enhance comfort, functionality, and safety.

Before the advent of the buoyancy control device, divers had to add their lead accurately to ensure they were slightly negative at the beginning of the dive and slightly positive at the end. Although this remains the goal even with a buoyancy control device, the diver can add air to a buoyancy control device as needed to compensate and lift heavy objects from the seabed (up to 60 lbs or 27 kg) by adding up to 30 L of air. Separate lift bags are recommended when lifting objects from the deep.

Today, air can be added to a buoyancy control device in 3 ways—by mouth, through an inflator hose connected to the regulator, or through a carbon dioxide cartridge (one-time use). During training, divers are taught to add or release air to the buoyancy control device in small increments and wait for their buoyancy to adjust before making further adjustments, thereby minimizing the risk of uncontrolled ascents or descents.

Neutral Buoyancy Summation

The goal of combined buoyancy systems is to obtain neutral buoyancy, which maximizes the diver's trim. Trim is the diver's attitude regarding balance with the direction of motion in the water. Effective trim control reduces the effort during swimming by reducing the sectional area of the diver as they pass through the water. A slight head-down trim is ideal for reducing down-thrust during finning, silting, and fin impact with the bottom, all while allowing the diver to look down comfortably. A stable horizontal trim requires the diver's center of gravity to be directly below the center of buoyancy (the centroid).

When a diver is on the surface with a full tank of air, they can adjust their lead so that when their buoyancy control device is fully deflated and they take a half breath of air, they float at eye level in the water. This method is the standard for the initial assessment of neutral buoyancy.

As divers descend, the neoprene in their wetsuit and the residual air in their buoyancy control device are further compressed. To achieve neutral buoyancy at depth, divers may need to either fill their lungs with air or add air to their buoyancy control device. During ascent, releasing air from the buoyancy control device and performing a long, slow exhalation are necessary to manage the expanding air (according to Boyle's law) and to control buoyancy, lung inflation, and ascent rate. 

Other advantages of neutral buoyancy maintenance include reduced energy expenditure, decreased decompression stress, improved gas management, and a sense of freedom and weightlessness without experiencing free fall. With neutral buoyancy, the diver can hover upside down and peer into crevices and holes without fear of striking the bottom. Divers can also lie on their backs in the water column without sinking to view surface marine creatures and boats. Most importantly, they can relax and focus on work or simple enjoyment while minimizing environmental impact.[3]

Clinical Significance

To understand the injuries related to buoyancy problems, healthcare practitioners must identify when the injury occurred during the dive. Accordingly, each phase of a dive will be examined, highlighting the potential injuries unique to uncontrolled buoyancy at each stage.

Uncontrolled descent can occur due to being overweight or unprepared; equipment malfunction, such as buoyancy control device leak and flooding; or poor environmental conditions, such as down currents and rip tides. Most of these can be prevented on the surface through careful preparation and equipment checks by the diver and their dive buddy.

Equalization and Barotrauma

The pressure difference between the head-up and head-down positions on the surface is sufficient to cause barotrauma if a diver cannot equalize. A diver can equalize the pressure in their middle ear and sinuses using the Valsalva maneuver or other exercises described elsewhere. A positively or neutrally buoyant diver on the surface may need to use the anchor line to slowly pull themselves down to depth to allow equalization maneuvers to be effective as they descend.

Equalization is more easily performed in a head-up rather than a head-down position. Once the pressure in the sinuses and middle ear is equal to the pressure in the surrounding water, the diver may continue to descend while equalizing or swim comfortably at that depth. An uncontrolled descent does not allow time for equalization, leading to the collapse of the eustachian tube and sinus ostia, trapping low-pressure air (causing a squeeze) in the middle ear or sinuses.[4] A squeeze can also occur in a mask or dry suit if pressure is not equalized in these spaces.

The tympanic membrane can rupture inward when the pressure is not equalized in the middle ear. The rupture often occurs unilaterally at first, causing severe nystagmus and vertigo, which can lead to disorientation and drowning. Similarly, negative sinus pressure can also cause an extreme headache and vomiting with similar results.[5][6]

Uncontrolled Descent Considerations

Uncontrolled descent can also damage the benthic environment or cause physical trauma to the diver by striking objects, including other divers. Marine hazards such as sea urchin spines, coral, and stonefish may be encountered inadvertently, with severe consequences. These incidents offer valuable teachable moments, particularly for divers who have been injured by or who have injured coral reefs. These delicate reefs can take hundreds of years to grow, but may be damaged or destroyed in a moment by fins or hands.

Once a diver descends below 100 ft (33 m), they must also be mindful of nitrogen narcosis. Also referred to as the rapture of the deep, this condition was first described in 1826. However, it was not until pressure chamber experiments in 1935 that nitrogen metabolism was identified as the causative agent.[7] Symptoms include euphoria, time disorientation, giddiness, decreased reaction time, and a false sense of security. A diver can continue to descend unaware while consuming their limited air supply.

Maintaining neutral or slightly positive buoyancy at depth keeps the diver off the reef, wreck, or cave floor, reducing environmental damage and allowing for streamlined movement at a stable depth.

Common Buoyancy Mistakes During Ascent

Novice or experienced but unpracticed divers often make common mistakes while preparing to ascend, including failing to release air added to the buoyancy control device at depth and holding their breath on the way up. As the air expands due to reduced pressure, the lift in the buoyancy control device and lungs can cause the diver to lose control of their ascent, allowing pulmonary hyperinflation to develop.

As the lungs become hyperinflated, the diver may experience a fullness in the chest, which can lead to pulmonary barotrauma. Small alveoli capillaries can burst, forcing gas into the bloodstream. In the small gas exchange vessels, this can cause arterial blockages and small pulmonary infarcts. Once the air enters the venous system and larger vessels, it forms bubbles, which can coalesce, resulting in a pulmonary air embolism. An arterial air embolism can migrate into the central circulation and the brain, causing a cerebrovascular infarct. Slow and constant exhalation can avoid this cascade on the ascent.[8]

Many problems that arise during descent may also occur during ascent, but can be worse. Elevated pressures in the sinuses and middle ear can cause the eustachian tubes and sinus ostia to collapse. As the pressure increases, the diver experiences a reverse squeeze, accompanied by a severe headache and ear pain. The pressure can significantly damage the ostia and eustachian tubes, causing epistaxis, tympanic membrane rupture with resultant vertigo and hearing loss, and ultimately, stenosis and scarring of both the tubes and the ostia.

Treatment of Barotrauma and Decompression Sickness

The diver can manage a squeeze or reverse squeeze during the dive by ascending or descending in small increments to relieve the pressure on the ostia and eustachian tubes, respectively. Once the pain and excess pressure are relieved, attempts to equalize using Valsalva or other maneuvers may be re-attempted. This method is typically successful, and descent or ascent may be resumed with more frequent efforts to equalize along the way.

Divers with colds, congestion, or allergy symptoms are cautioned against diving as mucus, swelling of the tubes and ostia, and fluids can make equalization difficult and sometimes impossible. A reverse squeeze occurs more often in these divers. When ascending and attempts to equalize fail, an additional maneuver may be attempted to clear a reverse squeeze. By forcefully blowing out through the nose, the diver can generate negative pressure in the nasopharynx, which can open the Eustachian tubes and ostia on the opposite side of the squeeze, thereby equalizing pressure.

Finally, rapid or uncontrolled ascent can lead to decompression sickness, a condition caused by the rapid expansion and destabilization of nitrogen bubbles in the blood following nitrogen saturation at depth. Treatment may include in-water recompression with slower ascent, transport to a hyperbaric chamber for therapy, or breathing 100% oxygen to help clear the nitrogen. These interventions are discussed in detail elsewhere.[9]

Safe Ascent and Emergency Ascents

Current recommendations include a diver ascending no faster than 30 ft (9 m) per minute with a minimum safety stop at 15 ft (5 m) for 3 minutes. Additional safety stops at deeper depths are recommended based on a deeper dive and duration profiles. In addition to the normal ascent, there are 3 other emergency ascent types. These ascents are typically precipitated by low-air or out-of-air situations.

The 3 emergency ascent types, ranked from safest to least safe, are as follows:

• Alternate air source ascent: The ascending diver uses a dive buddy's octopus regulator or other independent air supply to breathe as they ascend at no greater than 30 ft (9 m) per minute.
• Emergency swim ascent: If alone (or unable to contact a buddy), the diver keeps the regulator in their mouth, attempting inhalation during ascent as the tank air expands, while slowly exhaling and dumping all buoyancy control device air, and swimming upward. The diver may spread their arms and legs to slow their ascent rate if necessary.
• Buoyancy emergency ascent: If alone, the diver ditches their weight belt or integrated weights and rockets to the surface using the residual air in the buoyancy control device for added lift while exhaling during the ascent. This ascent method should be avoided when possible.

Clinical Conclusions

Diving buoyancy is not trivial. According to data collected by the Diver Alert Network, most divers who drown have air in their tanks and their weight belts on. The majority of diving fatalities are due to cardiac events during the dive or on the surface, typically due to preexisting conditions. However, poor buoyancy control and rapid ascent were the second and third most common risk factors contributing to diver fatalities. During out-of-air emergencies, the overweight diver is 6 times more likely to die compared to their normal-weight buddies.

Although effective buoyancy control makes scuba diving a more pleasant experience, it is also critical to underwater safety and survival. Therefore, buoyancy control relies on more than simple physics; it requires a diver's total situational control over mind, body, equipment, and environment. Divers demonstrating effective buoyancy control can control their panic, recognize nitrogen narcosis at depth, and ascend to clear their sensorium when needed. These divers are also aware of their remaining air and ascend slowly with good control, so they have surplus air on the surface. In emergencies, these divers pause to assess the situation, make a plan for correction, and execute it, rather than sacrificing their buoyancy and, perhaps, their life.

Nursing, Allied Health, and Interprofessional Team Interventions

Scuba diving is inherently dangerous, with safety limits that can take years to define. Additional challenges arise when a diver cannot control their buoyancy. Understanding the conditions under which a diver is injured helps predict diagnosis and treatment strategies for clinical providers. Determining whether barotrauma occurred during ascent or descent is crucial for healthcare personnel to anticipate patient needs effectively. The mechanics and history of buoyancy control provide additional insight for healthcare staff.

The treatment strategies described in this activity are most useful for divers and boat operators, but they also apply to remote facilities with limited resources. Specialist consultation with the Diver Alert Network is recommended and is available 24/7. Advanced treatment, such as hyperbaric therapy, may be necessary and is discussed in detail elsewhere.

Nursing, Allied Health, and Interprofessional Team Monitoring

Staff who conduct regular physical screening examinations for prospective and ongoing scuba divers should consult additional information from the Underwater and Hyperbaric Medical Society and the Diver Alert Network.