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How SCUBA Works
by Craig C. Freudenrich, Ph.D.

Special thanks to Aaron Harmon, Susan Kaliski and Scott Powell of Down Under Surf & SCUBA for their technical assistance with this article.

In 1943, the famed oceanographer Jacques Cousteau, along with Emile Gagnan, invented the aqualung, more commonly referred to as SCUBA (self-contained underwater breathing apparatus). SCUBA made divers more mobile and revolutionized exploration of the oceans. Since then, many advances in SCUBA technology have made the equipment easier to use, safer and more affordable, allowing many people to enjoy this fascinating adventure. The Professional Association of Diving Instructors (PADI) says that each year, almost 1-million people get certified for recreational SCUBA diving. You can undertake weekend outings to explore offshore shipwrecks and coral reefs or go on longer dive vacations to exotic locations, perhaps meeting such creatures as sharks, dolphins and whales.


Photo courtesy NOAA/Dept. of Commerce
A SCUBA diver

In this edition of HowStuffWorks, we will look at the underwater world, examine the SCUBA equipment and explore how your body reacts to the underwater environment. You'll also find out what you need to do to take part in this sport.

The Equipment
The underwater environment is hostile to humans and offers unique challenges to the SCUBA diver with respect to breathing, temperature control, visibility and buoyancy. The basic equipment that you need for SCUBA diving allows you to cope with the underwater environment. In total, you carry 60 to 75 pounds (27 to 34 kg) of equipment with you into and out of the water.

Breathing Apparatus
Typical recreational SCUBA divers breathe either compressed air (78 percent nitrogen, 21 percent oxygen) or an oxygen-enriched, nitrogen-oxygen combination called Nitrox (64 to 68 percent nitrogen, 32 to 36 percent oxygen). The gas is contained in a cylinder that you carry on your back. The typical cylinder is made of aluminum, weighs about 31 pounds (14 kg) empty and holds 80 cubic feet (2,265 L) of air at 3000 pounds per square-inch (psi), or 204 atmospheres (ATM). This volume of gas would approximately fill a phone booth and weighs about 7 pounds (3.2 kg).


SCUBA gas cylinders


SCUBA regulator, showing second stage (left) and first stage (right)
You cannot breathe directly out of the tank because the high pressure would damage your lungs. Therefore, the cylinder is fitted with a regulator. The regulator does two things: It reduces the pressure from the tank to a safe level for you to inhale, and it supplies air on-demand. To accomplish these tasks, regulators have two stages:

  • First stage - The first stage attaches to the cylinder. It reduces the pressure from the tank (3000 psi or 204 ATM) to an intermediate pressure (140 psi or 9.5 ATM).
  • Second stage - The second stage is connected to the first stage by a hose. It reduces the pressure from the intermediate pressure to ambient water pressure (such as 1 to 5 ATM depending upon depth). The second stage also supplies air, either only when you inhale (typical operation) or continuously (emergency operation).
The first stage consists of high-pressure and intermediate-pressure chambers, separated from each other by either a valve-diaphragm combination or a piston, which is in contact with the ambient water pressure. The high-pressure chamber receives air directly from the cylinder, while the intermediate-pressure chamber is in contact with the ambient water pressure through the diaphragm or piston. The system operates like this:
  1. You inhale, thereby lowering the pressure in the intermediate-pressure chamber to below the ambient water pressure.
  2. The water pressure pushes inward, opening the valve or piston.
  3. The open valve connects the high-pressure chamber with the intermediate pressure chamber.
  4. Air flows from the high-pressure chamber into the intermediate-pressure chamber, thereby increasing the pressure in the intermediate-pressure chamber.
  5. When the pressure in the intermediate-pressure chamber equals the ambient water pressure, the valve or piston closes.
  6. The process repeats when you inhale again.


Operation of a regulator's first stage

The first stage usually has several ports with hoses that lead to the second stage as well as to other devices, such as an additional second stage, tank-pressure gauge and/or buoyancy control device (BCD -- more on this later).


Inside a regulator's second stage
The regulator's second stage consists of:

  • Plastic chamber with an outer rubber diaphragm that is in contact with ambient water pressure
  • Purge button
  • Inner valve that is connected to a movable lever
  • Exhaust valve
  • Mouthpiece
The second stage is connected by a hose to the intermediate-pressure chamber of the first stage. This is how the second stage operates:
  1. You inhale, thereby lowering the pressure within the second stage to below the ambient water pressure.
  2. The water pressure presses in on the diaphragm membrane and moves the lever.
  3. The lever's movement opens the inlet valve. This allows air to flow into the second stage from the first stage, and into your lungs through the mouthpiece.
  4. When you exhale, the pressure in the second stage exceeds the ambient water pressure and pushes out on the membrane.
  5. The membrane moves away, allowing the lever to return to its normal position and thereby closing the inlet valve.
  6. The increased second-stage pressure opens the exhaust valve and allows the exhaled air to leave the second stage.
  7. When you inhale again, the exhaust valve closes and the process repeats.


Operation of the regulator's second stage

The regulator must be cleaned with freshwater after each dive to eliminate salt water, silt and debris that would prevent the movements of the various valves and membranes and corrode the parts. Regulators should also be serviced at least once per year. Because the regulator is one of the most important pieces of equipment, many divers choose to purchase their own regulators (instead of rent) so that they can be confident that the regulator is in good working order and has been maintained well.

The final parts of the breathing apparatus are accessories that contain emergency or alternate air supplies. They include the following:

  • Pony tanks - These are smaller cylinders that strap onto the main cylinder. Pony tanks contain air and have their own regulators. They provide enough air for many emergency situations, such as an ascent from a fairly deep depth.
  • Spare air unit - The spare air unit has the regulator built directly into the on/off valve. It is lightweight and can be carried in the pocket of a BCD. It is designed to provide only enough air to allow you to ascend from a shallow depth.
  • Snorkel - This is a small, J-shaped, lightweight breathing tube with a mouthpiece on one end. It attaches to your mask. When at the surface, the snorkel allows you to breathe outside air when you are swimming face-down, thereby conserving tank air.

Thermal Protection
To keep warm underwater, divers wear insulating suits, either wet suits or dry suits. The wet suit traps a thin layer of water between the insulating rubber of the suit and your body. Your body heat warms the water, which then keeps you warm. Wet suits should fit snugly (a loose wet suit will constantly leak in cold water). Wet suits come in short (covers only the arms and torso) or full-body lengths.

In contrast to wet suits, dry suits are made of a double-walled material with an insulating air space between the layers. They have tight fitting necks, wrists and ankles to prevent water from leaking in. They keep you warm because air is a better insulator than water and because you can wear undergarments with them.

The choice of wet versus dry suits depends on the water temperatures encountered during the dive:

  • Bare skin or nylon wet suit - 82 to 90 degrees Fahrenheit (28 to 32 Celsius)
  • Shorty wet suit - 78 to 90 F (25 to 28 C)
  • Full-body wet suit - 68 to 85 F (20 to 29 C)
  • Dry suit - below 72 F (22 C)
Wet suits and dry suits also have accessories including gloves, boots, vests and hoods.

Buoyancy Control
Underwater, it is important to control your depth at pre-determined levels set in your dive plan. To do so, you must be able to control your buoyancy, the upward force of the water on you. Buoyancy is caused by a difference in pressure between the upper part and lower part of an object. It is related to the object's weight and density, which determines the weight of water displaced by that object.

Buoyancy control device, showing front (left) and back (right)

To control buoyancy, divers use a buoyancy control device (BCD), which is also called a buoyancy compensator (BC), and lead weights. The BCD is a vest consisting of a coated rubber bladder that can be inflated or deflated with low-pressure air, either directly from the regulator's first stage or by mouth through an inflation tube. BCDs usually have a backpack type harness for holding the air cylinders and come in front-collar, vest and back-mounted styles. BCDs contain several pockets for equipment.

Because the wet suit itself is buoyant, you must add additional weights to counter this buoyancy. The weights can be attached to separate belts that the diver wears. Weights can also be inserted into the pockets of BCDs, and some newer BCDs have weight belts integrated into them.

Information Underwater
Divers have numerous gauges that provide information. Typically, they carry a gauge that tells them the air pressure in the cylinder, a gauge that tells them their depth and a compass for navigation. These gauges are often arranged on a single console that clips to the BCD. In addition, some divers may also carry a dive computer on their wrist to keep track of their depth and allowable bottom times. The dive computer consists of a battery-powered microprocessor that is programmed with the dive plan. The computer keeps track of depth and time and calculates the diver's allowable bottom time about 200 times per second.

Information gauges (left) and wrist-worn dive computer (right)

Vision and Locomotion
When you're diving, you wear a mask so you can both see and close off your nose from water. Masks can be single face plates or double face plates, and can be made with customized, prescription lenses for divers who wear eyeglasses.

Diver's mask and fin

To swim easily in the water, you wear fins on your feet. Fins come in a variety of styles and colors, including full-feet and half-feet designs.

Accessories

Dive knife
You can also carry the following accessories:

  • Dive knife - small knife used by divers to cut themselves free if equipment gets tangled
  • Dive slate board - small board to write on, used when divers must communicate with each other (Some dive boards are actually Magna doodles.)
  • Dive light - flashlight for illuminating objects underwater
  • Safety float - float with a line and dive flag that stays on the surface and warns passing boaters that there are divers beneath the surface
  • Signaling device - device such as a whistle or air horn, used by a diver to draw attention to himself on the surface if he gets separated from partners or dive boat
You may also choose to keep a dive kit on the boat, containing various items to repair equipment, books for planning and logging dives and first-aid kits for treating injuries.

Diving Physics, Physiology and Hazards
Dissolved Gases
The amount of any gas that can be dissolved in a liquid depends on the partial pressure of the gas over the solution and the nature and temperature of the liquid. If you increase the pressure of the gas, more gas will dissolve in solution. As for the nature of the solvent, water dissolves a different amount of gas than mineral or cooking oil does. If you increase the temperature of the liquid, less gas will dissolve. Another factor relevant to SCUBA diving is time: The longer you are at a given depth (pressure), the more nitrogen will dissolve in solution.
Underwater, your body must deal with two major issues: pressure and temperature. Pressure affects the amount of nitrogen and oxygen gases that dissolve in your blood and tissues. Pressure also affects your ears and sinuses. The ability of water to absorb your body heat can lower your body temperature and put you at risk for hypothermia.

Problems: Dissolved Gases Under Pressure
The air we breathe is a mixture of mostly nitrogen (78 percent) and some oxygen (21 percent). When you inhale air, your body consumes the oxygen, replaces some of it with carbon dioxide and does nothing with the nitrogen. At normal atmospheric pressure, some nitrogen and oxygen is dissolved in the fluid portions of your blood and tissues. As you descend under the water, the pressure on your body increases, so more nitrogen and oxygen dissolve in your blood. Most of the oxygen gets consumed by your tissues, but the nitrogen remains dissolved. Increased nitrogen pressure has two problematic effects on your body: nitrogen narcosis and residual nitrogen.

First, when the nitrogen partial pressure reaches high levels, usually those experienced when you reach depths of about 100 ft (30 m) or more, you experience a feeling of euphoria called nitrogen narcosis. The feeling of euphoria is like that experienced when a dentist or anesthesiologist gives you nitrous oxide (laughing gas). Nitrogen narcosis can impair your judgement and make you feel relaxed or even sleepy -- meaning you could start to ignore your instruments, your dive buddy and even drown. Narcosis comes on suddenly and without warning, but can be relieved by ascending to a shallower depth because the nitrogen starts to come out of solution as pressure decreases.

Dive Tables
The U.S. Navy and other diving organizations have modeled how your body absorbs nitrogen as you follow various dive profiles, and made various dive tables that you can use to calculate how much nitrogen will be absorbed by your body (see NAUI Online: Dive Tables). In all of the tables, there are corresponding times and depths at which you won't have to undergo decompression -- these are called "no decompression" limits. Generally, the deeper you dive, the shorter you can stay there -- the "no decompression" time decreases as depth increases. Recreational divers must plan their dives to stay within these limits so that they minimize the risk of decompression sickness. If you follow the tables, you have less than a 0.5-percent chance of getting "the bends". Your diving instructor will show you how to use these tables to plan a safe dive. Also, dive computers have these tables programmed into them and use the algorithms to calculate your safe bottom time.
Second, the amount of excess nitrogen in your tissues depends on how deep you dive and the amount of time you spend at those depths. The only way that you can rid your body of residual nitrogen, excess nitrogen in your tissues, is to ascend to the surface, which relieves the pressure and allows the nitrogen to come out of solution. If you ascend slowly, the nitrogen comes out of solution slowly. However, once you reach the surface, you still have residual nitrogen in your system, so you must relax before your next dive and give your body time to get rid of the residual nitrogen before you dive again. In contrast, if you ascend rapidly, the nitrogen comes out of your blood quickly, forming bubbles. It's like opening a can of soda: You hear the hiss of the high-pressure gas and you see the bubbles caused by the gas rapidly coming out of solution. This is what happens in your blood and tissues. When nitrogen bubbles form in your system, a condition known as decompression sickness or "the bends", they block tiny blood vessels. This can lead to heart attacks, strokes, ruptured blood vessels in the lungs and joint pain (one of the first symptoms of decompression sickness is a "tingling" sensation in your limbs). The best way to avoid decompression sickness is to minimize residual nitrogen by adhering to the "no decompression" depths and bottom times provided by dive tables. If you violate the "no decompression" limits, you have to stay underwater longer, for various times at pre-set depths (determined by dive tables), to allow the nitrogen to come out of your system slowly. This can present problems because you're dealing with a limited air supply; and if you ignore the decompression guidelines, you will suffer "the bends," have to be airlifted to a decompression chamber and be decompressed under emergency medical conditions. It's a life-threatening situation.

We have talked about nitrogen under pressure, but what about oxygen? High-pressure oxygen can cause convulsions, seizures and drowning. Oxygen toxicity comes on quickly and without warning. For most divers breathing compressed air, this won't occur until they've reach about 212 ft (65 m) below the surface -- usually deeper than "no decompression" limits. However, for divers breathing Nitrox, oxygen toxicity will occur at a shallower depth because the oxygen partial pressure in the gas mixture is higher. The best advice for avoiding oxygen toxicity is to be aware of your depth limit and stick to it.

One final note about gases under pressure: They must flow freely in and out of your lungs at all times during your dive. If you hold your breath while ascending, the gases inside will expand and could block the circulation in your lungs (embolism) or even rupture your lungs (pneumothorax). Therefore, never hold your breath while breathing from SCUBA gear!


Effects of SCUBA diving on the body

Ears and Sinuses
Within your head and skull bone are air spaces, sinuses within the bone itself, and air pockets in the ear canal. As you descend in the water, water pressure squeezes the air in these spaces, causing a feeling of pressure and pain in your head and ears. You must equalize the pressure in these spaces by various methods, such as closing your nostrils and gently blowing your nose. If properly equalized, your sinuses can withstand the increased pressure with no problems. However, sinus congestion caused by cold, flu or allergies will impair your ability to equalize the pressure and may result in damage to your eardrum.

Hypothermia
A water temperature below body temperature draws heat from the body. It is important to have proper thermal protection (wet or dry suits) to avoid hypothermia. Shivering is your body's response to lower body temperature and one of the beginning symptoms of hypothermia; you should end your dive if you begin to shiver.

Other Risks
Increased physical exercise underwater can lead to fatigue, dehydration, and intestinal or skeletal muscle cramps. Divers should be aware of their physical limits and not push their boundaries.

While there are many risks involved in SCUBA diving, new divers can minimize the dangers through proper education and training. Open-water certification programs emphasize diving physiology, diving hazards and safe diving practices. A trained diver can enjoy the sport safely with minimal health risks.

SCUBA Training
Recreational Diving Spots
Popular activities in SCUBA diving include wreck diving and reef diving. There are numerous dive spots throughout the world, including the U.S. Atlantic coast, California, Mexico, the Caribbean Islands, Hawaii, Australia and the Mediterranean Sea. See the dive directories in the Links section for incredible dive locations.
To train for SCUBA diving, you should be in reasonably good physical condition. It would not hurt to have a medical check up and discussion with your physician prior to training. The first step is to take an open water certification course (PADI or NAUI). For this course, you must be at least 10 years old. The course addresses:

  • Orientation - receive a basic introduction to the sport
  • Academic training - learn about diving physiology and hazards, SCUBA equipment, safety, use of dive tables, planning and emergency procedures
  • Skill training in confined environment - practice diving skills in a pool or other confined body of water
    • clear a mask that's filled with water
    • recover a regulator after it has come out of your mouth
    • put on and take off equipment in the water
    • perform neutral-buoyancy techniques
    • establish proper weighting
    • do a controlled emergency ascent
    • breathe from a buddy's air supply
  • Open-water skills - demonstrate the same skills in an open-water environment (river, quarry, lake, ocean)

    You will make at least four open-water dives as part of your open-water training.

You need your open-water certification card to rent dive equipment. Although you do not need to renew your certification, refresher courses are advised for certified divers who have not gone diving in a long time.

After open-water certification, you may decide to pursue further dive training at several levels:

  • Amateur levels
    • advanced training
    • rescue diving
  • Professional ratings
    • master training
    • dive master
    • instructor
    • master SCUBA trainer
See PADI Recreational Programs for details. Regardless of your level of training, SCUBA diving is an exciting sport that lets you explore unknown environments and interact with your surroundings in a whole new way.

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