Free AirAir at Atmospheric Pressure

Free air refers to air at atmospheric pressure, which is the pressure exerted by the weight of air in the Earth’s atmosphere. This entry will discuss the importance of understanding free air in the context of scuba diving, its relationship with compressed air, and the role it plays in dive planning and safety.

Atmospheric Pressure

Atmospheric pressure, also known as air pressure or barometric pressure, is a measure of the force exerted by the weight of air molecules on a given area. At sea level, the average atmospheric pressure is approximately 14.7 pounds per square inch (psi) or 1 atmosphere (atm). This pressure decreases with increasing altitude, as there is less air above to exert a force.

Free Air vs. Compressed Air

Scuba diving relies on the use of compressed air, which is air that has been pressurized and stored in a scuba tank. This allows divers to carry a sufficient supply of breathable gas for extended periods underwater. Compressed air is typically pressurized to around 200-300 bars (roughly 2,900-4,350 psi), depending on the capacity of the scuba tank.

Free air, on the other hand, is the air we breathe at the surface, at atmospheric pressure. The difference between free air and compressed air is critical in understanding various aspects of scuba diving, including buoyancy, gas consumption, and decompression sickness.


Buoyancy is the upward force exerted by a fluid (in this case, water) that opposes the weight of an object (the diver and their equipment) submerged in it. The buoyancy force is equal to the weight of the fluid displaced by the object. In scuba diving, understanding the relationship between free air and compressed air is essential for managing buoyancy.

A scuba diver’s buoyancy is affected by the volume and weight of the air in their lungs, as well as the air in their buoyancy control device (BCD). As a diver descends and the surrounding water pressure increases, the volume of the air in their lungs and BCD decreases, following Boyle’s law. This causes a decrease in buoyancy, and the diver must add air from their compressed air supply to maintain neutral buoyancy.

Gas Consumption

A diver’s rate of gas consumption is influenced by factors such as depth, exertion, and individual breathing patterns. The deeper a diver goes, the higher the ambient pressure, which affects how the body uses air. The relationship between free air and compressed air becomes important in understanding this aspect of diving.

At depth, the increased ambient pressure causes the volume of compressed air to decrease as it is inhaled from the scuba tank. However, the air in the diver’s lungs remains at the same pressure as the surrounding water. This means that, at greater depths, divers consume more air per breath, as the inhaled air is more compressed than at the surface.

Decompression Sickness

Decompression sickness (DCS), also known as “the bends,” is a potentially dangerous condition caused by the formation of gas bubbles in a diver’s tissues as they ascend to the surface. Nitrogen, which makes up 78% of the air we breathe, is absorbed into the body’s tissues while diving. As a diver ascends and the surrounding pressure decreases, the absorbed nitrogen comes out of solution and forms gas bubbles, which can cause pain, tissue damage, and even death.

Understanding the relationship between free air and compressed air is crucial in avoiding DCS. Divers must ascend slowly, allowing the dissolved nitrogen to be released from the tissues gradually, and follow appropriate decompression procedures, such as safety stops and decompression stops, depending on the dive profile

Dive Planning and Safety

A thorough understanding of free air and compressed air is essential for proper dive planning and safety. Divers must consider factors such as gas consumption, decompression limits, and surface intervals to ensure a safe and enjoyable dive experience.

Gas Management

Effective gas management begins with calculating the required air supply for the planned dive. Divers should account for their breathing rate, depth, bottom time, and the need for a reserve supply in case of emergencies. Understanding the differences between free air and compressed air is crucial in determining the appropriate volume of air required for a dive.

Dive Tables and Computers

Dive tables and dive computers are essential tools for planning dives and managing decompression. These tools take into account the differences between free air and compressed air to calculate safe ascent rates, decompression stops, and no-decompression limits.

Dive tables, developed by the U.S. Navy and other organizations, provide guidelines for divers based on depth, time, and repetitive dive profiles. Dive computers, on the other hand, offer real-time information and calculations during the dive, allowing for greater flexibility and more accurate decompression management.

Breathing Techniques

Proper breathing techniques are essential for conserving air and managing buoyancy during a dive. Divers should practice slow, deep, and controlled breathing to maximize the efficiency of air consumption. Additionally, awareness of the differences between free air and compressed air can help divers adapt their breathing patterns to the changing pressures and gas densities at depth.

Surface Intervals

Surface intervals are the time spent at the surface between dives, allowing the body to off-gas excess nitrogen absorbed during the dive. A sufficient surface interval is critical in reducing the risk of decompression sickness on subsequent dives. Dive tables and computers take into account the differences between free air and compressed air to calculate appropriate surface intervals based on the dive profile.


Understanding the concept of free air, or air at atmospheric pressure, is vital for scuba divers. This knowledge helps divers manage buoyancy, gas consumption, and decompression risks effectively. By considering the differences between free air and compressed air, divers can plan and execute safe dives, ensuring they have an enjoyable and secure underwater experience.