Scuba diving

Scuba diving is the sport of swimming underwater with specialized equipment that make it possible to breath underwater. The term "scuba", sometimes written as SCUBA, is an acronym for Self Contained Underwater Breathing Apparatus.

Seeing underwater

Even with proper equipment and clear conditions, humans cannot see as far in water as they can in air. Multiple factors impact overall visibility in water including refraction, desaturation, and turbidity. Visibility is typically expressed in feet or meters. Less visibility means increased danger from disorientation or from buddies becoming separated.

Refraction

Objects underwater appear about 25% closer and 33% larger when viewed through a dive mask due to refraction. This can cause objects to be just out of reach when attempting to grasp them, despite appearing within reach.

Desaturation

Water desaturates sunlight, meaning colors are absorbed and become less visible as depth increases. Longer wavelength colors, starting with red, are the first to be absorbed at shallower depths followed by shorter wavelength colors as depth increases. At a depth of about 30 meters (roughly 98 feet) very few colors other than shades of blue and grey are visible to the human eye. Flashlights can therefore be useful to divers even during the day.

Turbidity

Turbidity is the quality of being cloudy or opaque due to particles suspended in the water, such as sand, mud, and algae. Turbidity can also influence desaturation. The greater the turbidity of the water, the more quickly colors are absorbed. The content of the suspended particles can also impact the visible color spectrum; for example, algae often shades objects in the water green. Shallow turbid water can therefore sometimes appear to have less color than deep, clear water.

Hearing underwater

Sounds typically heard while scuba diving include your own breathing, boats passing overhead, and clicks from fish that are feeding. Sound travels roughly 4 to 5 times faster underwater than it does through air due to waters density, which is roughly 800 times that of air. This makes the directionality of sound difficult to discern because sound waves arrive at both ears at virtually the same time. This can lead to danger, particularly because of the difficulty in determining the location and direction of boats operating nearby. Thus, the best thing to do while underwater and hearing a boat is to stay underwater at a safe depth away from the surface until the boat is no longer audible.

Breathing underwater

The air we breath is primarily composed of about 79% nitrogen ($N_2$) and 21% oxygen ($0_2$). The air compressed in a scuba tank contains the same ratio of nitrogen and oxygen. When we breath, our lungs replace carbon dioxide in our blood with oxygen from the air we've inhaled to participate in metabolic functions. However, the nitrogen you breath is absorbed into your blood and tissues. Under pressure, your body will absorb greater amounts of nitrogen; thus the longer and deeper you dive, the more nitrogen you will absorb.

During ascension, the nitrogen that has dissolved into your tissue expands, re-enters your blood stream, and travels back to the lungs. As you breath out, the nitrogen is expelled until your body returns to a normal state. This process can continue after you've reached the surface, and takes roughly 24 hours before fully normalized. Thus back-to-back dives that occur in under a 24 hour period can slowly increase your overall nitrogen absorption.

Excessive nitrogen absorption can cause decompression sickness (DCS), also known as the bends, which is the formation of nitrogen bubbles in the blood. The faster a diver ascends, the more likely it is for these bubbles to form. Symptoms of decompression sickness include fatigue, clouded thinking, muscle and joint aches, vomiting, weakness, poor balance, vertigo, skin rash, coughing up blood, and paralysis. Severe cases can be lethal. Though symptoms tend to begin within 15 minutes to 12 hours after diving, they may take up to 48 hours to appear.

Preventing decompression sickness

DSC can be prevented by limiting the amount of time spent underwater according to dive tables. Dive computers will automatically estimate nitrogen absorption as well, and alert divers to the possibility of excessive nitrogen absorption. It's important to note, however, that dive tables and computers only estimate nitrogen absorption and other factors, such as dehydration, drug use, water temperature, dive altitude, and repetitive dives can increase the rate of absorption in ways that are unlikely to be factored into these estimates.

Some general tips to prevent decompression sickness:

• Remain within the limits of your dive computer's estimation. If your buddy's computer shows different levels than yours, always go by the more conservative of the two estimates.
• Pause during ascension. While ascending, pause the ascension for 3-5 minutes every 3-6 meters.
• Limit multiple ascents/descents. An ideal dive consists of one descent and one slow ascent (with pauses).
• Avoid fatigue, cold, and dehydration. These conditions all increase the risk of DCS.
• Don't fly within 12-18 hours of diving.

First-aid for DSC is to administer oxygen and to call emergency medical services. Further treatment involves recompression in a hyperbaric chamber.

Nitrogen narcosis

Nitrogen narcosis is a temporary condition that causes impaired judgement when a diver is exposed to excessive nitrogen. Symptoms include euphoria or anxiety, similar to a drug induced "high". As with drugs, a divers ability to think clearly and act rationally is impaired, even though the diver may not realize it while it is happening. This impairment can be extremely dangerous, especially while underwater.

Nitrogen narcosis susceptibility varies from person to person and is most likely to occur at depths below 30 meters; however, narcosis can still occur at shallower depths. Inexperienced divers are advised to limit the depth of their dives to about 20 meters to avoid narcosis until they gain more experience and training.

Narcosis wears off quickly upon ascent. Thus, if you suspect you or your buddy is experiencing narcosis, ascend until the effects are no longer felt.

Contaminated air

Though it's standard practice for professional dive operators to have the air quality of their compressors regularly tested by an independent lab, there is always a chance that something will go wrong when filling a scuba tank with compressed air. Prior to a dive, always smell the air in the tank and do not use air that has a color, odor, or taste.

Despite this precaution, $CO_2$ contamination may occur unnoticed. For example, exhaust from a gas-powered compressor may be accidentally sucked in to the intake and cause dangerous levels of carbon monoxide ($CO$) to enter the tank. Carbon monoxide is hard for a diver to detect because it is colorless, odorless, and tasteless. In cases of contamination, it might not be until after the dive has begun that issues start to show. Signs of $CO_2$ poisoning include headache, nausea, confusion, and unconsciousness. In extreme cases, the lips and fingernail beds will turn bright red.

$CO_2$ poisoning can ultimately prove fatal if not addressed right away. Therefore, it's important to abort the dive and ascend right away if any warning signs occur. Once at the surface, breath air and seek oxygen treatment if available. If symptoms do not clear up quickly, it is important to get medical attention.

Body temperature underwater

Water conducts heat away from the body approximately 25 times faster than air, which is why a comfortable air temperature can feel uncomfortably cold in water at the same temperature. A wetsuit is therefore recommended for thermal protection in water temperatures under 30° celsius or 86° fahrenheit.

Thermoclines

Thermoclines are abrupt changes in temperature between layers of water. They are formed because cold water is denser and tends to sink under warmer water when not mixed by currents and other forces. Thicker wetsuits are needed for colder temperatures, so knowing water temperatures at different depths can be an important factor in planning a scuba diving trip.

Tides and scuba diving

Tides are the periodic rise and fall in water level caused by the gravitational pull of the moon on large bodies of water. Depending on the tide, a dive site may be significantly shallower or deeper than it was on a previous visit. Tides also cause changes in currents that impact visibility and can make dive sites difficult or dangerous. The safest time to visit a dive site is generally during a slack tide which is the period between high and low tide and therefore usually the least amount of current. Tide tables, which detail the times for high and low tides and related water conditions throughout the day, are typically available in daily marine reports online from local news sources

Currents and scuba diving

Currents are the directional movements of water through a surrounding body of water in which there is less movement. A current can dangerously overpower a diver and carry them away, but can also be useful with appropriate planning. Drift diving is the practice of scuba diving while intentionally allowing the current to carry the diver from their entry point to a planned exit point some distance away.

Types of currents

There are several types of currents that divers encounter. Permanent offshore currents are currents that flow continuously in one direction, such as the North American Gulf Stream. However, divers are most often affected by longshore and rip currents.

Longshore currents are currents that run parallel to the shoreline and are generally caused by waves that approach the shore at an angle and influenced by tides and proximity to other currents. These influences can cause longshore currents to sometimes reverse direction, so awareness and careful planning are important when dealing with longshore currents. As indicated by the name, longshore currents can span long distances; thus when planning, the entry point of a dive in a long current should be well upstream of the planned exit point.

Rip currents are narrow flows of fast-moving water that usually run perpendicular to the shore. These currents are known for quickly pulling beach-goers, swimmers, and divers out to sea. Rip currents are formed by water flowing through the narrowing of passageways the water flows through, such as between sandbars or breaks in a coral reef. The best way to get out of a rip current is to swim parallel to the shore until free of the current.

Waves, surf, and scuba diving

Waves form as wind pushes on the surface of the water. As waves travel and encounter shallower water, the wave is lifted while the bottom of the wave slows down. As the wave steepens and the bottom slows, surf is formed when the wave topples over and "breaks" near the shoreline. Fetch is the measure of how long and hard the wind blows across the water unobstructed and is the main factor in the size of the wave and resulting intensity of the surf. Surf entry should not be attempted by scuba divers without special training.

Surge

Waves mostly affect divers near the surface, such as during shore dives or when getting on or off a boat, or in shallow water when they cause surge. Surge is the back-and-forth motion of water caused by waves overhead. If a surge is strong, divers are involuntarily moved by the motion of the water and have little control over the direction in which they move. To avoid the danger of being thrown against rock or coral formations during intense surge, it's best to move to deeper water where surge is less severe.

Diving with aquatic life

As long as they are out of arms reach, underwater animals tend to ignore humans; unlike on land where most animals will flee or avoid human presence. This ability to get close when diving means divers must take special care not to disturb or disrupt wildlife. A diver should therefore be aware of where their body and equipment are in proximity to their surroundings at all time to avoid disturbing the underwater environment. This means not stirring up sediment and taking care not to let equipment make contact with the environment; even a slight touch can harm some of the delicate creatures that live underwater.

Injuries from aquatic life

Staying mindful and avoiding unintentional contact with surroundings also helps avoid injury to the diver from cuts, snags, and dangerous wildlife. Some aquatic animals, such as jellyfish or fire coral, have nematocysts, which are cells that produce toxins that, when touched, can cause symptoms ranging from a mild sting to intense pain, anaphylactic shock, or paralysis. In addition to avoiding direct contact with surroundings, divers can wear full length exposure protection.

Some animals, like scorpion-fish and lion-fish, have poisonous spines that can cause serious injury or even death. These animals do not attack divers; however, due to their camouflage, they are easy to make contact with accidentally.

Despite being one of the greatest fears while diving, attacks from sharks, eels, and barracudas are one of the least likely causes of injury. Even then, "attacks" are not territorial or aggressive; they are usually either defensive or due to mistaken identity. To avoid attacks, divers should not feed, corner, grab, or act aggressively toward any animal. This includes not putting hands blindly into holes or around corners. Jewelry and loose clothing should also be avoided because they can easily be mistaken for a flailing fish.

Physical forces at play when scuba diving

Buoyancy

In scuba diving, buoyancy is the upward force that water exerts on a diver. Objects that displace a greater weight of water than the object weighs will float, such as a boat, and are said to have positive buoyancy. Objects that displace a lesser weight of water than the object weighs will sink, such as a rock, and are said to have negative buoyancy. An object that displaces water equal to the weight of the object itself will not float or sink, and thus has neutral buoyancy.

Divers adjust their buoyancy using a buoyancy control device (BCD) to control the depth of their dive while in the water. For example, a diver will maintain positive buoyancy to rest on the surface, while a negative buoyancy is used descend. The majority of time during a successful dive is spent at a neutral buoyancy, which allows the diver to "hover" and swim easily through the water.

Notably, salt water weighs more than fresh water and thus divers will find that they are more buoyant in salt water. Divers therefore require more weight when diving in salt water than in fresh water.

Pressure

Pressure refers to the physical force an object exerts on something it is in contact with. For a scuba diver, pressure usually refers to the weight of the water they are submerged in. Ambient pressure is the pressure of the surrounding medium or, again for divers, the surrounding water.

A $1cm^2$ vertical column of air that is the height of the atmosphere at sea level will weight roughly $1gk$; this is referred to as 1 bar or 1 atmosphere (asm). A similar column of air at $1in^2$ weighs just under $15lbs$.

Scientifically speaking, $1bar = 0.0987atm$; however, in scuba diving they tend to be used interchangeably.

The same volume of water is approximately 800 times heavier than air. As a diver descends, the weight of the water above them increases rapidly. A $10m$ ($33ft$) column of water weighs as much as a column of air (with the same diameter) that extends all the way to the top of the atmosphere. Thus every $10m$ deeper a diver swims is adds about $1atm$ of pressure.

Boyle's law: Volume and density

Atmospheric pressure refers to the ratio of weight to volume of air at a given pressure, and is most commonly measured in pounds per square inch (PSI). The PSI of air at $1bar$ (sea level) is $15psi$ because the weight of a $1in^2$ column of air weighs about $15lbs$.

As depth increases, so does the atmospheric pressure. According to Boyle's law, the same mass of air will take up less volume and become more dense as depth increases. The relationship between volume and density is inverse; thus if the density doubles the volume is halved for the same quantity of air; if the density is halved then the volume is doubled.

This is important for divers to remember because the volume of our lungs does not change when diving, but the density of the air we breath does. The implication is that the deeper you dive, more air you require. A dive at $30m$ is at $4bar$ of pressure; the air you breath is thus twice as dense as it is at a depth of $10m$ / $2bar$ and thus requires twice as much air for the same amount of time. Put another way, a scuba tank that lasts 1 hour at the surface will only last 30 minutes at $10m$, 20 minutes at $20m$, and 15 minutes at $30m$.

Equalization

Equalization is the processes of maintaining equilibrium between the body and the ambient pressure. Despite the enormous pressure exerted by water while diving, the body will naturally maintain equilibrium with the surrounding water as long as it has time to adjust gradually. This is because the body primarily consists of solids and liquids that are relatively incompressible, distribute pressure evenly, and "push back" with a force equal to the ambient pressure.

The parts of the body that contain air, such as the lungs, sinuses, and ears, remain in balance as long as the air they are filled with air at ambient pressure. As long as the pressure of air in the body is equal to the ambient pressure, it "pushes back" on the ambient pressure with equal force, thus maintaining equilibrium. Scuba equipment works by providing air that is always at the ambient pressure, regardless of depth.

Ear equalization

Divers may experience pain in the middle ear when outside pressure is greater than the pressure inside the ear. If not equalized, this pressure can lead to a series of injuries, collectively called ear squeeze. Ear squeeze can range from excessive stretching of the eardrums to tears and ruptures that allow water into the middle ear which affects balance and can increase the risk of infection. While middle ear damage is usually able to heal, the inner ear can be affected in extreme cases and lead to permanent hearing loss and trouble balancing.

The valsalva maneuver is the most common method to equalize the ears and is accomplished by pinching the nose to block the flow of air while gently blowing out through the nose. This forces equalized air through the eustachian tubes and into the middle ear. Swallowing, yawning, and wiggling your jaw may also work.

It's best to equalize before any discomfort is felt, usually about twice for every two to three meters of decent, even if you don't feel you need to. If you are unable to equalize comfortably, it's important to stop and wait until you can before continuing to descend.

Sinus equalization

There should be no need to explicitly equalize the sinuses because the passageways to sinus cavities allow the free transfer of air and pressure from surrounding areas of the body. However, colds and sinus congestion can block these passageways and block equalization, initially causing pain, then causing the sinuses to fill with blood and mucus, a condition called sinus squeeze. There is no way to equalize sinuses while underwater, thus anyone experiencing the initial symptoms should abort the dive before further damage is caused.

To prevent sinus squeeze, some divers take decongestants prior to diving. However, this is generally cautioned against because the medication may wear off while the diver is still underwater, causing the sinus passageways to swell shut and prevent equalization on ascent; this is referred to as a reverse block.

Mask equalization

The artificial air pocket between your mask and face must also be equalized during descent. The mask can be equalized by gently breathing out through your nose every few feet during descent.

Reverse blocks

When ascending, expanding air typically escapes from the ears, sinuses, and mask without the need for intervention by the diver. A reverse block occurs when the expanding air cannot escape. Though uncomfortable and sometimes painful, reverse blocks are usually less severe than other equilibrium issues. In the event of a reverse block, descend until the pain subsides, yawn and swallow to stretch the eustachian tubes, then slowly attempt the ascent again.

The danger of rapid ascension

Recall that Boyle's law states that volume and density are inversely proportional. When descending, equalization is necessary to prevent the reduction of volume from crushing the air pockets inside our bodies, particularly in the middle ear, and causing injury. Conversely, when ascending, equalization is required to prevent over-expansion of these air pockets as air volume increases.

The worst case scenario of rapid ascent is fatal over-expansion of the lungs. Over expansion of the lungs can occur in just $4ft$ of water. This is why "the number one rule of diving" is to breath continuously and never hold your breath.

Another danger associated with rapid ascension is arterial gas embolism (AGE), or air embolism, which is caused by air bubbles that block blood flow to the brain. This can result in unconsciousness, paralysis, or even death. Victims of an air embolism should be given oxygen as first-aid and taken to a hospital immediately.

Chest pain, difficulty breathing, excessive fatigue, nausea/vomiting, or unconsciousness in someone that has just ascended should result in immediate medical assistance. Most symptoms show within a few minutes of surfacing.

Scuba equipment

Casual divers often rent their equipment, however it's generally advised that each diver purchase their own fins, mask, snorkel, and wet suit boots to ensure proper fit and comfort on every dive. Additional gear can be easily rented and includes; a buoyancy control device (BCD), weight belt, tank, exposure suit, and regulator.

Equipment care

After each dive, all equipment should be rinsed free of saltwater, mud, sediment, and other contaminants that can cause damage or corrosion. Equipment should be completely dry before being put away, and stored in a cool dry place away from direct sunlight.

Masks should be put in a protective case (usually with the snorkel detached) to prevent damage, especially while carrying or traveling. Fins should be stored without any bends, which could become permanent, and often include inserts to keep the foot pockets from sagging or otherwise deforming.

Exposure suites

• Dive skins are lightweight, stretchy exposure suites. They offer basic protection from sunlight and the stings and abrasions that can result from contact with objects and wildlife underwater. They also offer minimal thermal protection when used alone, but this can be enough in particularly warm/tropical waters. Dive skins are often worn under wet suits to make putting them on easier and reducing the flow of cold water through the suit.
• Wetsuits are somewhat thicker than dive skins and generally made of neoprene foam, which is a poor conductor and thus a good insulator. Depending on thickness and other accessories, wetsuits - along with wetsuit hoods, boots, and gloves - can provide thermal protection for water temperatures down to about 10° C or 50° F.
• Drysuits are used for colder water and seal at the neck and wrists to keep water out and the diver surrounded by a layer of air, which is an effective insulator against cold. Dry suits are generally used in water temperatures below 10° C (50° F). Drysuits require specialized training to use effectively.

Exposure suit comfort ranges

Though these ranges may vary by individual tolerance, they should be accurate for most people: