How Does the Buoyancy of a Scuba Tank Change as It Empties

When exploring the underwater world, divers rely on scuba tanks to breathe, but the weight and buoyancy of these tanks significantly impact a diver's experience. A 70 cubic foot scuba tank typically holds about 5 pounds of air, which can be adjusted with weight belts to maintain neutral buoyancy. This article explores how the buoyancy of scuba tanks changes as they become empty and discusses the preferences and experiences of various divers with different tank types.

The Basics of Scuba Tank Buoyancy

A well-designed scuba tank should be filled with air in a way that allows the diver to maintain neutral buoyancy. This is achieved by adding or removing weights to counteract the buoyancy changes as the tank empties. A full tank is usually negatively buoyant, meaning it wants to sink, while an empty tank is positively buoyant, meaning it wants to float.

Experience and Work with Various Diving Tanks

The author has spent considerable time diving with and evaluating different types of scuba tanks. Most divers opt for a single 72 to 80-cubic-foot aluminum tank, which is suitable but has its drawbacks. This type of tank has more positive buoyancy compared to steel tanks when full, and it carries an additional 3 pounds of buoyancy when nearly empty. This extra buoyancy can make it challenging to stay underwater, as more weight must be added to the weight belt.

Aluminum vs. Steel Tanks

Steel tanks are another popular choice, as seen from the author's experience with a 100 cubic foot steel tank. These tanks are significantly heavier than a 72-cubic-foot steel tank, both in and out of water. However, the negative buoyancy of a steel tank means it is less likely to float when empty, which is a significant safety advantage. The issue arises when the 100 cubic foot steel tank begins to float with only 1000 psi of air left. This is inconvenient for diving and counterproductive to safety. As a result, the author sold the 100 cubic foot steel tank after just one use.

Twin Steel Tanks

The ease of using a pair of twin steel 50-cubic-foot tanks is noteworthy. These tanks have a consistent weight profile, making them easier to manage when fully filled and completely empty, similar to the 72-cubic-foot tanks. The author found these tanks particularly useful for collecting abalone specimens during thesis work. The ability to dive into shallow reefs without wave interference was a significant advantage. The only drawback was the limited bottom time due to the tank’s weight.

Steel 60 Cubic Foot Tanks

A preferred choice for boat diving, the 60 cubic foot steel tank offered several benefits. Typically, dive boats refill tanks to 1800 psi, but the author found that the 60 cubic foot tanks provided a full fill. Additionally, on boats that fill to 2500 psi, a 60 cubic foot steel tank held more air than a single 80 cubic foot aluminum tank. The downside was the need to have the tank hydrostatically tested, as the author's has been erroneously failed by a technician with incorrect tools.

Conclusion

Properly understanding and managing the buoyancy changes of scuba tanks as they empty is critical for a comfortable and safe diving experience. Each type of tank has its pros and cons, and divers should choose the one that best suits their needs and preferences. The experiences shared here highlight the importance of considering tank type, weight management, and safety when planning a dive.