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The underwater environment is hostile to humans. It offers unique challenges with respect to breathing, temperature control, buoyancy and pressure. SCUBA -- which stands for self-contained underwater breathing apparatus -- equipment allows you to cope with the harsh underwater environment. Let’s find out how.

Breathing Underwater
Typical recreational SCUBA divers breathe either compressed air – air that is 78 percent nitrogen and 21 percent oxygen -- or an oxygen-enriched, nitrogen-oxygen combination called nitrox. Nitrox is 64 to 68 percent nitrogen and 32 to 36 percent oxygen. The gas is contained in a cylinder, or tank, that you carry on your back.

You cannot breathe directly out of the tank because the high pressure would damage your lungs. So, it 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. The first stage attaches to the cylinder. It reduces the pressure from the tank (3000 psi) to an intermediate pressure (140 psi). The second stage connects to the first stage by a hose. It reduces the pressure from the intermediate pressure to ambient water pressure. The second stage also supplies air, either only when you inhale (typical operation) or continuously (emergency operation). 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. Divers clean their regulators with freshwater after each dive. This gets rid of salt water, silt and debris that could damage the regulator, causing it to malfunction.

Image courtesy OAR/National Undersea Research Program (NURP) A dry suit

Besides the big, main tank, divers are also equipped with accessories that contain emergency or alternate air supplies. For deep diving, smaller cylinders called pony tanks strap onto the main cylinder. Pony tanks provide enough air for many emergencies, like ascent from a fairly deep depth. Next is the spare air unit. It’s designed to provide just enough air for ascent from a shallow depth. Finally, there’s the 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 facedown. It helps conserve tank air.

Staying Warm Underwater

Heavy Air! Basic SCUBA equipment weighs 60 to 75 pounds! A typical cylinder is made of aluminum, weighs about 31 pounds empty and holds 80 cubic feet of air at 3000 pounds per square-inch (psi). This volume of gas would approximately fill a phone booth and weighs about seven pounds.

To keep warm underwater, divers wear insulating suits -- wet suits or dry suits –- and gloves, boots, vests and hoods. 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 come in short (covers only the arms and torso) or full-body lengths. 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.

Which suit you wear depends on the water temperature. Bare skin or a nylon wet suit is appropriate for water that is 82 to 90 degrees Fahrenheit. For water at 78 to 90 degrees F, a shorty wetsuit will work. A full-body suit is needed for temperatures in the 68 to 85 degrees F range and for anything consistently below 72 degrees F, you’ll need a dry suit.

Controlling Buoyancy Underwater
Underwater, it is important to control your depth. 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. To control buoyancy, divers use a buoyancy control device (BCD) and lead weights. The BCD is a vest consisting of a coated rubber bladder that can be inflated or deflated with low-pressure air. Because the wet suit itself is buoyant, you must add additional weights to counter this buoyancy.

Handling Pressure Underwater
The air we breathe is a mixture of mostly nitrogen and some oxygen. 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 is consumed by your tissues, but the nitrogen remains dissolved. Increased nitrogen pressure has two problematic effects on your body: nitrogen narcosis and residual nitrogen.

Nitrogen narcosis can impair your judgment 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.

First Breath 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).

Residual nitrogen – excess nitrogen in your tissues -- depends on how deep you dive and the amount of time you spend at those depths. The longer you are at a given depth (pressure), the more nitrogen will dissolve in solution. The only way that you can rid your body of residual nitrogen is to ascend to the surface. This relieves the pressure and allows the nitrogen to come out of solution. If you ascend slowly, the nitrogen comes out of solution slowly. 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, they block tiny blood vessels. This condition, known as decompression sickness or the bends, is serious and can lead to heart attacks and strokes.

High-pressure oxygen can also be a problem. It can cause convulsions, seizures and drowning. Oxygen toxicity comes on quickly and without warning. The best advice for avoiding oxygen toxicity is to be aware of your depth limit and stick to it.

SCUBA + STUFFY NOSE = TROUBLE! Within your head and skull bone are air spaces, sinuses within the ebone itself, and air pockets in the ear canal. As you SCUBA dive, water pressure squeezes the air in these spaces, causing a feeling of pressure and pain in your head and ears. A diver must equalizd the pressure in these spaces by closing his nostrils and gently blowing his nose. If properly equalized, your sinuses can handle the increased pressure. However, a stuffy nose caused by cold, flu or allergies can keep you from equalizing the pressure. And this can damage your eardrum.