The Basics

Air Has Weight

Yes, air actually has weight. The weight of air exerts pressure on your body—about 14.7 psi (pounds per square inch). This amount of pressure is called one atmosphere of pressure because it is the amount of pressure the earth’s atmosphere exerts. Most pressure measurements in scuba diving are given in units of atmospheres or ATA.

Pressure Increases With Depth

The weight of the water above a diver exerts pressure on their body. The deeper a diver descends, the more water they have above them, and the more pressure it exerts on their body. The pressure a diver experiences at a certain depth is the sum of all the pressures above them, both from the water and the air.

Every 33 Feet of Salt Water = 1 ATA of Pressure

Pressure a diver experiences = Water Pressure + 1 ATA (from the atmosphere)

Total Pressure at Standard Depths*

Depth / Atmospheric Pressure + Water Pressure = Total Pressure

• 0 feet / 1 ATA + 0 ATA = 1 ATA
• 15 feet / 1 ATA + 0.45 ATA = 1.45 ATA
• 33 feet / 1 ATA + 1 ATA = 2 ATA
• 40 feet / 1 ATA + 1.21 ATA = 2.2 ATA
• 66 feet / 1 ATA + 2 ATA = 3 ATA
• 99 feet / 1 ATA + 3 ATA = 4 ATA

_{*This is only for saltwater at sea level}

Water Pressure Compresses Air

Air in a diver’s body air spaces and dive gear will compress as pressure increases (and expand as pressure decreases). Air compresses according to Boyle’s Law.

Boyle’s Law: Air Volume = 1/Pressure

Not a math person? This means that the deeper you go, the more air compresses. To find out how much, make a fraction of 1 over the pressure. If the pressure is 2 ATA, then the volume of the compressed air is ½ of its original size at the surface.

Pressure Affects Many Aspects of Diving

Now that you understand the basics, let’s look at how pressure affects several key aspects of diving.

Equalization

As a diver descends, the pressure increase causes the air in their body’s air spaces to compress. The air spaces in their ears, mask, and lungs become like vacuums as the compressing air creates a negative pressure. Delicate membranes, like the eardrum, can get sucked into these air spaces, causing pain and injury. This is one of the reasons that a diver must equalize their ears for scuba diving.

On ascent, the reverse happens. Decreasing pressure causes the air in a diver’s air spaces to expand. The air spaces in their ears and lungs experience a positive pressure as they become overfull of air, leading to pulmonary barotrauma or a reverse block. In a worst-case scenario, this could burst a diver’s lungs or eardrums. To avoid a pressure-related injury (such as an ear barotrauma) a diver must equalize the pressure in their body’s air spaces with the pressure around them.

To equalize their air spaces on descent, a diver adds air to their body air spaces to counteract the “vacuum” effect by:

• Breathing normally, which adds air to their lungs every time they inhale.
• Adding air to their mask by breathing out their nose.
• Adding air to their ears and sinuses by using one of several equalization techniques.

To equalize their air spaces on ascent, a diver releases air from their body air spaces so that they do not become overfull by:

• Breathing normally, which releases extra air from their lungs every time they exhale.
• Ascending slowly and allowing the extra air in their ears, sinuses, and mask to bubble out on its own.

Buoyancy

Divers control their buoyancy (whether they sink, float up, or remain “neutrally buoyant” without floating or sinking) by adjusting their lung volume and buoyancy compensator (BCD).

As a diver descends, the increased pressure causes the air in their BCD and wetsuit (there are small bubbles trapped in neoprene) to compress. They become negatively buoyant (sinks). As they sink, the air in their dive gear compresses more and they sink more quickly. If they do not add air to their BCD to compensate for their increasingly negative buoyancy, a diver can quickly find themselves fighting an uncontrolled descent.

Conversely, as a diver ascends, the air in their BCD and wetsuit expands. The expanding air makes the diver positively buoyant, and they begin to float up. As they float towards the surface, the ambient pressure decreases and the air in their dive gear continues to expand. A diver must continuously vent air from their BCD during ascent or they risk an uncontrolled, rapid ascent, which can be quite dangerous.

A diver must add air to their BCD as they descend and release air from their BCD as they ascend. This may seem counterintuitive until a diver understands how pressure changes affect buoyancy.

Bottom Times

Bottom time refers to the amount of time a diver can stay underwater before beginning their ascent. Ambient pressure affects bottom time in two important ways.

Increased Air Consumption Reduces Bottom Times

The air that a diver breathes is compressed by the surrounding pressure. If a diver descends to 33 feet, or 2 ATA of pressure, the air they breathe is compressed to half of its original volume. Each time the diver inhales, it takes twice as much air to fill their lungs than it does at the surface. Consequently, this diver will use their air up twice as quickly, hence reducing their bottom time.

Increased Nitrogen Absorption Reduces Bottom Times

The greater the ambient pressure, the more rapidly a diver’s body tissues will absorb nitrogen. Without getting into specifics, a diver can only allow their tissues a certain amount of nitrogen absorption before they begin their ascent. Otherwise, they run an unacceptable risk of decompression illness without mandatory decompression stops. The deeper a diver goes, the less time they have before their tissues absorb the maximum allowable amount of nitrogen.

Because pressure becomes greater with depth, both air consumption rates and nitrogen absorption increase the deeper a diver goes. Therefore, one of these two factors will limit a diver’s bottom time.

Rapid Pressure Changes Can Cause Decompression Sickness (the Bends)

Increased pressure underwater causes a diver’s body tissues to absorb more nitrogen gas than they would normally contain at the surface. If a diver ascends slowly, this nitrogen gas expands gradually, and the excess nitrogen is safely eliminated from their tissues. However, the body can only eliminate nitrogen so quickly. If a diver goes through too great of pressure change too quickly, their body cannot eliminate all of the expanding nitrogen, leading to potentially harmful nitrogen bubbles forming in their tissues and blood.

These nitrogen bubbles can cause decompression sickness (DCS) by blocking blood flow to various parts of the body, potentially causing serious conditions such as strokes, paralysis, and other life-threatening issues. Rapid pressure changes are one of the most common causes of DCS.

The Greatest Pressure Changes Are Closest to the Surface

The closer a diver is to the surface, the more rapidly the pressure changes occur.

Depth Change / Pressure Change / Pressure Increase

• 66 to 99 feet / 3 ATA to 4 ATA / x 1.33
• 33 to 66 feet / 2 ATA to 3 ATA / x 1.5
• 0 to 33 feet / 1 ATA to 2 ATA / x 2.0

Observe what happens really close to the surface:

• 10 to 15 feet / 1.30 ATA to 1.45 ATA / x 1.12
• 5 to 10 feet / 1.15 ATA to 1.30 ATA / x 1.13
• 0 to 5 feet / 1.00 ATA to 1.15 ATA / x 1.15

A diver must compensate for the changing pressure more frequently the closer they are to the surface. The shallower their depth:

• the more frequently a diver must manually equalize their ears and mask.

• the more frequently a diver must adjust their buoyancy to avoid uncontrolled ascents and descents.

Divers must take special care during the last portion of the ascent. It is crucial to never shoot straight to the surface after a safety stop. The last 15 feet represent the greatest pressure change and need to be ascended slowly.

Most beginner dives are conducted within the first 40 feet of water for safety purposes and to minimize nitrogen absorption and the risk of DCS. This practice is essential. Nevertheless, it is important to keep in mind that it is more challenging for a diver to control their buoyancy and equalize in shallow water compared to deeper water because the pressure changes are more extreme!