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Diving Technology

The content and activities in this topic will work towards building an understanding of the challenges that are faced when diving in high pressure and how modern technology has advanced the exploration of the world ocean.

Historical Diving Technology

<p><strong>Fig. 9.21.</strong> An air-filled wooden container called a diving bell permitted divers to work under water.</p><br />

People have known for centuries that they could carry air under water. Diving bells were first used in Europe in the 1600s for salvaging cannons, metals, and other precious materials from shallow bays and lakes. Diving bells were made from wooden barrels, metal kettles, and other containers, that were inverted to maintain an air space and weighted so that they would sink to the bottom. In 1690 Edmund Halley—the English scientist who discovered Halley’s Comet—designed a diving bell with a window that could be used for undersea exploration. His design was used for nearly 100 years (Fig. 9.21). The Halley diving bell had two lead-weighted barrels—a large working barrel and a smaller air-refill barrel. Both barrels had openings on the bottom that allowed air and water to enter. Leather hoses attached to the barrels allowed air to flow to a person working inside the bell and to a diver working outside the bell in the ocean. By the late 1700s inventors had devised ways to pump air continuously into diving bells, eliminating the need for a refill barrel. More advanced diving bells are still used today.


Compressed Air

<p><strong>Fig. 9.22.</strong> A hard-hat dive suit used from the late1800s to the 1950s.</p><br />

Because pressure increases quickly with depth, air cannot flow down from the surface without additional force. But, air can be forced under water if it is compressed. Compression pushes the air particles closer together, increasing the pressure. This provides the force to move air to divers under water.


The first underwater divers wore waterproof canvas suits and heavy copper and brass helmets called hard-hats (Fig. 9.22). From the late1800s until the 1950s, hard-hat divers breathed compressed air that was continuously supplied from pumps on the surface. Hard-hat divers could remain under water for long periods of time, but their movement and vision were highly restricted.


Despite its awkwardness, this same technology is still used today in underwater construction and submarine rescue operations (Fig. 9.23 A). Modern diving gear uses watertight rubber “drysuits” and lighter, smaller helmets (Fig. 9.23 B) or facemasks and insulating rubber “wetsuits” (Fig. 9.23 C).

<p><strong>Fig. 9.23.</strong> Common diving systems include (<strong>A</strong>) atmospheric diving suits, (<strong>B</strong>) modern diving helmet, and (<strong>C</strong>) regulator and mask.</p><br />


<p><strong>Fig. 9.24.</strong> A scuba regulator adjusts the pressure of air inhaled by a diver so that it is about equal to the pressure of the surrounding water.</p><br />

The dream of being able to move about freely under water became a reality in 1943 when Jacques Cousteau and Emil Gagnan perfected the demand regulator, better known as scuba, an acronym that stands for self-contained underwater breathing apparatus. This device allows a diver to breathe safely and easily from a tank of compressed air (Fig. 9.24). Compression forces a given mass of gas into a smaller volume, making it more dense than atmospheric air. As the diver descends, the regulator adjusts airflow so that the air is at the same pressure as the water and the diver’s lungs remain at their normal size.


Problems with Compressed Air

Scuba gear allows divers to descend more comfortably, stay under longer, and go deeper than they can as breath-holding free divers. However, problems related to pressure differences are more serious for divers using compressed air than for free divers. Scuba diving is an exciting sport, but no one should attempt it without instruction by a professional. Scuba divers must learn how pressure affects gases, how to use specialized equipment, and how to plan for safe dives. Certification requires careful training, passing a written test, and demonstrating practical skills. For safety, dive shops require that customers be certified before they can rent equipment or have their tanks refilled with compressed air.


Scuba divers must breathe in and out continuously. Holding their breath is dangerous because trapped air expands when a scuba diver ascends. For example, the breath held in ascending from 30 m expands the lungs to four times their size at depth—enough to rupture them. Bubbles of air can escape from the ruptured lungs into the circulatory system, blocking the flow of blood to vital parts of the body.


<p><strong>Fig. 9.25.</strong> Gas dissolved in a bottle of soda forms bubbles when the pressure is decreased.</p><br />

Another risk from scuba diving comes from the effect of pressure on gases that are dissolved in the blood. A familiar example of a gas dissolved under pressure greater than 1 atm is a bottle of carbonated soda. As long as the bottle remains closed and the pressure inside stays high, the gas (carbon dioxide) remains in solution. When the cap is removed, the pressure is released and the gas that was dissolved in the water begins to come out of solution, forming bubbles (Fig. 9.25).


The body of a scuba diver under water is much like soda bottled under pressure. The diver’s lungs are filled with air at an ambient pressure, pressure equal to the surrounding water. Air from the lungs dissolves in the blood and circulates throughout the body. High pressures at depth force greater amounts of gas to dissolve in the blood and tissues than the body is accustomed to at sea level (1 atm).


Air is a mixture of gases, mostly nitrogen and oxygen. Our bodies use oxygen rapidly in respiration. However, we do not use the nitrogen gas that comprises over 78 percent of air. Under normal 1 atm conditions people exhale nitrogen, but during scuba diving, excess nitrogen gas is forced under pressure into the tissues, where it accumulates in increasing amounts with depth and time under water until the tissues are fully saturated.


At a depth of 30 m or deeper, dissolved nitrogen gas can cause nitrogen narcosis. Divers affected by nitrogen narcosis feel intoxication similar to that from alcohol. Their reaction times are slowed, and their judgment and reasoning are impaired. No diver is immune to the effects of nitrogen narcosis, but susceptibility and the depth at which the symptoms occur vary. The symptoms may be relieved, with no lasting ill effects, by ascending to a shallower depth.


<p><strong>Fig. 9.26.</strong> Bubbles of nitrogen gas may form around the joints when pressure decreases. Note that the bubbles are not drawn to scale.</p><br />

Decompression sickness is a much more serious nitrogen-related problem. Decompression is the reduction in pressure during ascent from deeper water. In decompression, excess nitrogen in the blood comes out of solution and forms bubbles the way an open bottle of carbonated soda does (Fig 9.25). If the nitrogen bubbles are tiny enough to travel through the circulatory system, they are exhaled normally from the lungs. However, if the bubbles are larger, they may block tiny blood vessels, causing mild symptoms such as a tingling “pins and needles” sensation. If the bubbles are too large to get through the circulatory system, they can kill a diver by blocking the flow of blood to vital organs such as the brain, spinal cord, or heart. They can lodge in the joints, causing a painful condition called the “bends,” or they can seriously damage nerves (Fig. 9.26).


Divers can get decompression sickness if they

  • stay down too long,
  • dive too deep,
  • come up too fast,
  • make too many dives in too short a time,
  • exercise heavily during the dive,
  • travel to altitudes above 2,500 m too soon after diving, or
  • are in poor physical condition.


<p><strong>Fig. 9.27.</strong> Dive table used by scuba divers to plan safe diving depths and times. Click the link below to see the table in PDF.</p><br />

Standard dive tables, based on those developed for U.S. Navy divers, show the maximum times scuba divers may remain safely at specific depths (Fig. 9.27). Divers use these tables to plan dives that avoid decompression problems. They must be very cautious not to exceed the safe time and depth limits. Dive computers assist divers in these tasks. Divers also must ascend at slow, safe rates between 0.3 and 1 m per second, about the rate that a small bubble of air floats to the surface, to avoid decompression problems.


If severe symptoms such as numbness, pain, or unconsciousness occur, a stricken diver must be rushed to a recompression chamber and attended by a physician. A diver with decompression sickness is placed inside the recompression chamber, which is then tightly sealed. Pressure inside the chamber is increased so that the nitrogen bubbles lodged in the victim’s body re-dissolve. Then the pressure inside the chamber is gradually decreased to 1 atm as the diver breathes out excess nitrogen gas.


Recreational scuba divers rarely stay at a fixed depth. Dive computers take into account the variable depths of a dive when calculating bottom time, or how long divers have until they need to start ascending to the surface. Dive computers even account for the rates at which different kinds of body tissues absorb and release nitrogen. Dive computers are a valuable piece of diving equipment, but they can also fail, so divers should have backup systems to prevent nitrogen accumulation. Other factors that affect the rate of nitrogen accumulation and release include water temperature, amount of exertion, and the diver’s age, quantity of body fat, and state of health.


Technical scuba divers often need to work at depths that exceed the safety limits set for recreational divers. They use special gas mixtures that replace nitrogen with helium. Helium is expensive, but it is absorbed more slowly than nitrogen. For example, divers using compressed air may work to depths just under 65 m; with helium-oxygen mixtures they can work at depths down to 130 m. Ironically, oxygen, the gas vital for life, is also a limiting factor in scuba diving with compressed gases. At about 7 atm of pressure a condition called oxygen toxicity can cause convulsions or unconsciousness. Special gas mixtures with less oxygen are prepared for technical diving that exceeds the safe limit.


<p><strong>Fig. 9.28.</strong> A US Navy diver wearing a rebreather system practices defusing an underwater mine.</p><br />

Conventional scuba diving systems rely on a fixed supply of air to be consumed by the diver. Exhaled air leaves a conventional scuba system as bubbles. In contrast, rebreather diving systems allow divers to recycle exhaled air for repeated use (Fig. 9.28). Waste carbon dioxide gas (CO2) is removed from the exhaled air and new oxygen gas (O2) is added.


Rebreather systems allow divers to remain underwater for long periods of time, often multiple hours. Rebreathers are ideal for marine biologists and photographers because they do not release streams of air bubbles that might scare away fish like conventional scuba systems do. Military divers also use rebreathers in situations requiring long dive times such as repairing ship hulls and defusing naval mines.



Activity: Dive Planning

Use your knowledge of ocean depths and diving technology to plan a safe scuba trip to a nearby dive site. Diving occurs worldwide in locations such as coral reefs, kelp forests, rock quarries, and caves.


Submersible Vehicles

Submersibles are small vehicles that operate underwater and are designed to help discover the ocean depths. Submersibles are usually supported by a surface vessel and allow explorers to travel to depths that have not previously been possible. There are two types of submersibles: manned submersibles are operated by an on-board pilot, and unmanned ROVs (remotely operated vehicles) are operated remotely. Manned submersibles were the first type to give ocean scientists a glimpse of the deep ocean and the unique ecosystems that it supports. In 1964, the submersible Alvin (Fig. 9.30 A), which is able to carry one pilot and two observers, was the first manned submersible to be launched. Alvin’s first dive was only 35 feet, but subsequent dives have taken it to a maximum depth of 14,764 feet. Throughout its lifespan Alvin has discovered more than 300 new species of animal, and has explored the RMS Titanic a dozen times (Fig. 9.30 B). The vessel can typically dive for approximately six hours in the deep ocean. Two hours are needed for descent and two hours are needed to ascend, leaving approximately two hours for exploration on the sea floor. Alvin is still in operation and continues to make scientific discoveries in the deep ocean with assistance from its companion vessel R/V Atlantis.

<p><strong>Fig. 9.30.</strong> (<strong>A</strong>) <em>Alvin</em> was the first manned submersible and remains in operation today.</p><br />
<p><strong>Fig. 9.30.</strong>&nbsp;(<strong>B</strong>) The <em>RMS Titanic</em> has been explored using submersible vehicles. This photo of the <em>RMS Titanic</em> was taken by the ROV <em>Hercules</em> at a depth of approximately 12,500 feet.</p><br />


Remotely operated vehicles (ROVs) are a type of submersible that does not utilize an onboard pilot, and is instead operated remotely from a companion vessel or platform. ROVs can be equipped with advanced scientific equipment that allows scientists to explore and document never before seen ecosystems. The NOAA ship Okeanos Explorer has an ROV called Deep Discoverer that was launched in 2013 (Fig. 9.31 A). Deep Discoverer has a robotic arm for exploring as well as high definition cameras that are capable of capturing stunning images from great distances at high resolution (Fig. 9.31 B). ROVs have the ability to stay in deep water for days, or even weeks, at a time. Without the need to surface for oxygen, as with the pilots of manned submersibles must, ROV pilots can simply work in shifts around the clock from the Okeanos Explorer. Deep Discoverer and her research vessel are both equipped with the latest in satellite technology, which allows live remote viewing of ROV dives around the world. Live feeds of current Okeanos Explorer dives can be viewed online at Okeanos Explorer LIVE Video (if the live feeds is temporarily unavailable, come back and visit it again after a few hours).

<p><strong>Fig. 9.31.</strong> Remotely operated vehicles (ROVs) can withstand depths and pressures that humans cannot. (<strong>A</strong>) ROV <em>Deep Discoverer</em> is outfitted with a high definition camera. (<strong>B</strong>) This camera is able to produce high definition photos such as these bobtail squid eggs.</p><br />


<p><strong>Fig. 9.32.</strong> The ROV <em>Hercules</em> is designed to withstand high pressure and low temperatures found in the ocean depths.</p><br />

The ROV Hercules (Fig. 9.32) is designed with deep water exploration in mind, and working in high pressures for long periods of time requires the ROV construction to be highly durable. At its maximum depth of more than 4000 meters (2.5 miles), the air is compressed to 1/400th of its original volume at the surface. When Hercules was constructed, most of its electrical components were built into cylindrical titanium pressure housings to withstand the immense pressures associated with diving to such great depths. This allows Hercules’ scientific equipment, such as cameras, to be able to withstand high pressures and to deliver stunning underwater photographs (Fig. 9.30 B). ROVs are deployed beyond scientific exploration and are also used to maintain oil platforms, prospect shipwreck sites, and work in areas that are too dangerous for people.


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.