Under Pressure

The Basic Consequences of Depth and Pressure in Scuba Diving

Scuba diver near the surface.
Scuba divers must understand the how depth and pressure are related to dive safely. © Getty Images

How does pressure change underwater and how do pressure changes effect aspects of scuba diving such as equalization, buoyancy, bottom time, and the risk of decompression sickness? Review the fundamentals of pressure and scuba diving, and discover a concept no one told me during my open water course: that pressure changes more rapidly the closer a diver is to the surface.

The Basics

• Air Has Weight
Yes, air actually has weight.

The weight of air experts pressure on your body - about 14.7 psi (pounds per a 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 his body. The deeper a diver descends, the more water he has above him, and the more pressure it exerts on his body. The pressure a diver experiences at a certain depth is the sum of all the pressures above him, 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 salt water 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 Effects Many Aspects of Diving

Now that you understand the basics, let's look at how pressure effects four basic aspects of diving.

1. Equalization

As a diver descends, the pressure increase causes the air in his body's air spaces to compress. The air spaces in his ears, mask, and lungs become like vacuums as the compressing air creates a negative pressure. Delicate membranes, like the ear drum, to be sucked into theses air spaces causing pain and injury. This is one of the reasons that a diver must equalize his 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 his ears and lungs experience a positive pressure as they become overfull of air, leading to a 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 his body's air spaces with the pressure around him.

• To equalize his air spaces on descent a diver adds air to his body airspaces to counteract the "vacuum" effect by

- breathing normally, this adds air to his lungs every time he inhales
- adding air to his mask by breathing out his nose
- adding air to his ears and sinuses by using one of several ear equalization techniques

• To equalize his air spaces on ascent a diver releases air from his body air spaces so that they do not become overfull by

- breathing normally, this releases extra air from his lungs every time he exhales
- ascending slowly and allowing the extra air in his ears, sinuses and mask to bubble out on its own

2. 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 his BCD and wetsuit (there are small bubble trapped in neoprene) to compress.

He becomes negatively buoyant (sinks). As he sinks, the air in his dive gear compresses more and he sinks more quickly. If he does not add air to his BCD to compensate for his increasingly negative buoyancy, a diver can quickly find himself fighting an uncontrolled descent.

In the opposite scenario, as a diver ascends, the air in his BCD and wetsuit expands. The expanding air makes the diver positively buoyant, and he begins to float up. As he floats towards the surface, the ambient pressure decreases and the air in his dive gear continues to expand. A diver must continuously vent air from his BCD during ascent or he risks an uncontrolled, rapid ascent (one of the most dangerous things a diver can do).

A diver must add air to his BCD as he descends and release air from his BCD as he ascends. This may seem counterintuitive until a diver understands how pressure changes effect buoyancy.

3. Bottom Times

Bottom time refers to the amount of time a diver can stay underwater before beginning his 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 he breathes is compressed to half of its original volume. Each time the diver inhales, it takes twice as much air to fill his lungs than it does at the surface. This diver will breath use his air up twice as quickly (or in half the time in half the time) as he would at the surface. A diver will use up his available air more quickly the deeper he goes.
• 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 his tissues a certain amount of nitrogen absorption before he begins his ascent, or he runs an unacceptable risk of decompression illness without mandatory decompression stops. The deeper a diver goes, the less time he has before his 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. One of these two factors will limit a diver's bottom time.

4. 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 bit by bit and the excess nitrogen is safely eliminated from the divers tissues and blood and released from his body when he exhales.

However, the body can only eliminate nitrogen so quickly. The faster a diver ascends, the faster nitrogen expands and must be removed from his tissues. If a diver goes through too great of pressure change too quickly, his body can not eliminate all of the expanding nitrogen and the excess nitrogen forms bubbles in his tissues and blood.

These nitrogen bubbles can cause decompression sickness (DCS) by blocking blood flow to various parts of the body, causing strokes, paralysis, and other life threatening problems. 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.

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

Look at 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 he is to the surface. The more shallow his depth:

• the more frequently a diver must manually equalize his ears and mask.
• the more frequently a diver must adjust his buoyancy to avoid uncontrolled ascents and descents

Divers must take special care during the last portion of the ascent. Never, never, shoot straight to the surface after a safety stop. The last 15 feet are the greatest pressure change and need to be taken more slowly than the rest of the ascent.

Most beginner dives are conducted in the first 40 feet of water for safety purposes and to minimize nitrogen absorption and the risk of DCS. This is as it should be. However, keep in mind that it is more difficult for a diver to control his buoyancy and equalize and in shallow water than in deeper water because the pressure changes are more extreme!