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84-487: The SDV, which has been in continuous service since 1983, is used primarily for covert or clandestine missions to denied access areas (either held by hostile forces or where military activity would draw notice and objection). It is generally deployed from the Dry Deck Shelter on a specially-modified attack or ballistic missile submarines , although it can also be launched from surface ships or land. It has seen combat in

168-474: A development of the Mark 8 SDV, the Mark 9 is a very different vehicle, designed for attacking surface ships rather than deploying SEAL teams on clandestine operations. Indeed, the Mark 9 and Mark 8 share very few common parts. The Mark 9 carries two SEALs, a pilot and a navigator, and two Mark 31 or Mark 37 torpedoes for standoff attacks against ships. These torpedoes can travel up to 3 nautical miles (5.6 km) in

252-535: A diver, enabling attacks on larger and more distant enemy ships. However, the SDV is not without its weaknesses, namely its range, reliability, and mobility. The SDV's short range, which is contingent on sea state , water temperature, payload, and other factors, sometimes hinders operations. In one example, the Navy wanted to use an SDV to get a closer look at a Soviet ship anchored in a Cuban harbor 18 miles (29 km) upriver from

336-550: A fire hazard, so the more successful applications have been for space-suits, fire-fighting and mine rescue. A liquid oxygen supply can be used for oxygen or mixed gas rebreathers. If used underwater, the liquid-oxygen container must be well insulated against heat transfer from the water. Industrial sets of this type may not be suitable for diving, and diving sets of this type may not be suitable for use out of water due to conflicting heat transfer requirements. The set's liquid oxygen tank must be filled immediately before use. Examples of

420-590: A full life support and air conditioning system. The ASDS was canceled in 2009 due to cost overruns and the loss of the prototype in a fire. The Navy currently plans to replace the SDV with the Shallow Water Combat Submersible (SWCS), which will be designated the Mark 11 SDV. The SWCS was expected to enter service in 2019. The SDV program dates back to World War II . Initiated by the Office of Strategic Services Maritime Unit (OSS MU). A “submersible canoe”

504-547: A garage-shop fashion by various UDT units, and included various "Marks" such as the Mark V, VI, and VII. Intermediate numbers were assigned to some vehicles that never made it off the shop floor. All were of flooded design. The first SDV to be operationally deployed was the Mark VII, which entered service in June 1972 after being tested between 1967 and 1972. It could carry three SEALs plus a pilot sitting in compartments fore and aft. It had

588-684: A hull made from fiberglass and non-ferrous metals to hinder detection and was powered by a silver-zinc battery attached to an electric motor. The Mark VIII SDV, the model that is still in use today, began to supplant the Mark VII starting in 1983. The wet vehicle SDV program (officially named the Swimmer Delivery Vehicle, later re-designated the SEAL Delivery Vehicle after the Swimmer Delivery Vehicle Teams were renamed SEAL Delivery Vehicle Teams ) currently centers on

672-417: A large range of options are available depending on the specific application and available budget. A diving rebreather is safety-critical life-support equipment – some modes of failure can kill the diver without warning, others can require immediate appropriate response for survival. A helium reclaim system (or push-pull system) is used to recover helium based breathing gas after use by the diver when this

756-421: A larger submarine with a degree of stealth greater than that offered by small surface craft, helicopters, or other means. In exercises, the SDV has been found to excel at anti-shipping attacks, being able to attack targets in heavily-guarded fleets or docked at military bases and then slip away undetected. Additionally, it can carry larger limpet mines than those carried by a diver and has a much greater range than

840-404: A loop configured machine has two unidirectional valves so that only scrubbed gas flows to the patient while expired gas goes back to the machine. The anaesthetic machine can also provide gas to ventilated patients who cannot breathe on their own. A waste gas scavenging system removes any gasses from the operating room to avoid environmental contamination. One of the functions of a space suit

924-498: A lower profile and sonar absorbing materials. The Mark 9 SDV was intended to attack ships in shallow coastal waters that full-size submarines could not enter, and to draw attention of an enemy fleet away from the Mark 9's parent submarine. Though it proved very effective in exercises, the Mark 9 was retired starting in 1989 and was fully phased out of service by the mid-1990s due to manpower and budget constraints and because all of its capabilities save launching torpedoes were duplicated by

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1008-550: A naturally hypoxic environment. They need to be lightweight and to be reliable in severe cold including not getting choked with deposited frost. A high rate of system failures due to extreme cold has not been solved. Breathing pure oxygen results in an elevated partial pressure of oxygen in the blood: a climber breathing pure oxygen at the summit of Mount Everest has a greater oxygen partial pressure than breathing air at sea level. This results in being able to exert greater physical effort at altitude. The exothermic reaction helps keep

1092-435: A number of hoses and electrical cables twisted together and deployed as a unit. This is extended to the divers through the diver umbilicals. The accommodation life support system maintains the chamber environment within the acceptable range for health and comfort of the occupants. Temperature, humidity, breathing gas quality, sanitation systems, and equipment function are monitored and controlled. An atmospheric diving suit

1176-461: A part of the fighting team. The SDV is silver-zinc battery powered and equipped with propulsion, navigation, communication, and life-support equipment. The batteries directly power the electric motor that drives the single screw propeller. Because they are all electric, SDVs are extremely difficult to detect using passive sonar, and their small size makes them hard to detect using other means. The Mark 8 Mod 1 SDV can deliver four fully equipped SEALs to

1260-454: A pendulum rebreather. Breathing hoses can be tethered down to a diver's shoulders or ballasted for neutral buoyancy to minimise loads on the mouthpiece. A mouthpiece with bite-grip , an oro-nasal mask , a full-face mask , or a sealed helmet is provided so that the user can breathe from the unit hands-free. A store of oxygen, usually as compressed gas in a high pressure cylinder, but sometimes as liquid oxygen , that feeds gaseous oxygen into

1344-401: A portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system . Rebreather technology may be used where breathing gas supply is limited, such as underwater, in space, where the environment is toxic or hypoxic (as in firefighting), mine rescue, high-altitude operations, or where the breathing gas

1428-465: A straight line, carry a 330-pound (150 kg) warhead, and are capable of sinking ships as large as cruisers . In addition to torpedoes, the Mark 9 also carried limpet mines and satchel charges in a large cargo compartment aft of the pilot and co-pilot. The Mark 9 is designed to clandestinely approach enemy vessels while submerged, surface to fire torpedoes, and then escape unnoticed. As such, its design incorporates stealth characteristics, including

1512-620: A surface ship deploying an SDV with a crane, further limiting the SDV's mobility and usage. Modifying a surface ship to launch and recover the SDV through an underwater door, like the Italian Navy had done for its human torpedoes in WWII, would have helped alleviate this problem. The Special Boat Service of the United Kingdom Special Forces operates three Mark 8 Mod 1 vehicles. Dry Deck Shelter A dry deck shelter ( DDS )

1596-425: A wide enough bore to minimise flow resistance at the ambient pressure in the operational range for the equipment, are usually circular in cross section, and may be corrugated to let the user's head move about without the tube collapsing at kinks. Each end has an airtight connection to the adjacent component, and they may contain a one-way valve to keep the gas circulating the right way in a loop system. Depending on

1680-523: Is a removable module that can be attached to a submarine to allow divers easy exit and entrance while the boat is submerged. The host submarine must be specially modified to accommodate the DDS, with the appropriate mating hatch configuration, electrical connections, and piping for ventilation, divers' air, and draining water. The DDS can be used to deploy a SEAL Delivery Vehicle submersible , Navy divers , or Combat Rubber Raiding Craft (CRRC). In UK service it

1764-435: Is a small one-man articulated submersible of roughly anthropomorphic form, with limb joints which allow articulation under external pressure while maintaining an internal pressure of one atmosphere. Breathing gas supply may be surface supplied by umbilical, or from a rebreather carried on the suit. An emergency gas supply rebreather may also be fitted to a suit with either surface supply or rebreather for primary breathing gas. As

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1848-402: Is calcium hydroxide, which is relatively cheap and easily available. Other components may be present in the absorbent. Sodium hydroxide is added to accelerate the reaction with carbon dioxide. Other chemicals may be added to prevent unwanted decomposition products when used with standard halogenated inhalation anaesthetics. An indicator may be included to show when carbon dioxide has dissolved in

1932-400: Is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen , as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be

2016-704: Is formally named the Special Forces Payload Bay (SFPB) and was procured under ‘Project Chalfont’. Training is conducted at the purpose-built Chalfont Shore Facility (CSF) constructed by BAE Systems at HMNB Clyde . It is used by the Astute -class submarine . The United States Navy 's DDSs are 38 feet (12 m) long and 9 feet (2.7 m) high and wide, add about 30 tons to its host submarine's submerged displacement, can be transported by trucks or C-5 Galaxy airplanes, and require one to three days to install and test. They have three HY-80 steel sections within

2100-420: Is important, such as in space stations and space suits. Lithium peroxide also replenishes the oxygen during the scrubbing reaction. Another method of carbon dioxide removal occasionally used in portable rebreathers is to freeze it out, which is possible in a cryogenic rebreather which uses liquid oxygen. The liquid oxygen absorbs heat from the carbon dioxide in a heat exchanger to convert the oxygen to gas, which

2184-613: Is less of a problem. The Soviet IDA71 rebreather was also manufactured in a high altitude version, which was operated as an oxygen rebreather. Anaesthetic machines can be configured as rebreathers to provide oxygen and anaesthetic gases to a patient during surgery or other procedures that require sedation. An absorbent is present in the machine to remove the carbon dioxide from the loop. Both semi-closed and fully closed circuit systems may be used for anaesthetic machines, and both push-pull (pendulum) two directional flow and one directional loop systems are used. The breathing circuit of

2268-427: Is more economical than losing it to the environment in open circuit systems. The recovered gas is passed through a scrubber system to remove carbon dioxide, filtered to remove odours, and pressurised into storage containers, where it may be mixed with oxygen to the required composition for re-use, either immediately, or at a later date. The life support system provides breathing gas and other services to support life for

2352-695: Is specially enriched or contains expensive components, such as helium diluent or anaesthetic gases. Rebreathers are used in many environments: underwater, diving rebreathers are a type of self-contained underwater breathing apparatus which have provisions for both a primary and emergency gas supply. On land they are used in industrial applications where poisonous gases may be present or oxygen may be absent, firefighting , where firefighters may be required to operate in an atmosphere immediately dangerous to life and health for extended periods, in hospital anaesthesia breathing systems to supply controlled concentrations of anaesthetic gases to patients without contaminating

2436-399: Is sufficient to freeze the carbon dioxide. This process also chills the gas, which is sometimes, but not always, desirable. A breathing hose or sometimes breathing tube on a rebreather is a flexible tube for breathing gas to pass through at ambient pressure. They are distinguished from the low-, intermediate-, and high-pressure hoses which may also be parts of rebreather apparatus. They have

2520-427: Is the earliest type of rebreather and was commonly used by navies for submarine escape and shallow water diving work, for mine rescue, high altitude mountaineering and flight, and in industrial applications from the early twentieth century. Oxygen rebreathers can be remarkably simple and mechanically reliable, and they were invented before open-circuit scuba. They only supply oxygen, so there is no requirement to control

2604-764: Is the only SDV officially in use by the US Navy and Royal Navy. It is an upgrade of the earlier Mark 8 Mod 0 SDV. The Mod 1 is quieter, faster, more efficient, and has a longer range than the Mod 0. Its updated electronics, materials, and battery and motor systems gives it twice the range and 1.5 times the speed of the Mod 0. Another advantage of the Mark 8 Mod 1 over its predecessor is that it is built from aluminium instead of plastic reinforced fiberglass, making its hull sturdier and roomier. The sturdier hull means that it can be deployed from CH-46 Sea Knight and CH-53E Super Stallion helicopters, although SDVs often break or explode when dropped in

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2688-522: Is then available again to react with more carbonic acid. 100 grams (3.5 oz) of this absorbent can remove about 15 to 25 litres (0.53 to 0.88 cu ft) of carbon dioxide at standard atmospheric pressure. This process also heats and humidifies the air, which is desirable for diving in cold water, or climbing at high altitudes, but not for working in hot environments. Other reactions may be used in special circumstances. Lithium hydroxide and particularly lithium peroxide may be used where low mass

2772-427: Is to provide the wearer with breathing gas. This can be done via an umbilical from the life-support systems of the spacecraft or habitat, or from a primary life support system carried on the suit. Both of these systems involve rebreather technology as they both remove carbon dioxide from the breathing gas and add oxygen to compensate for oxygen used by the wearer. Space suits usually use oxygen rebreathers as this allows

2856-421: Is usually necessary to eliminate the metabolic product carbon dioxide (CO 2 ). The breathing reflex is triggered by CO 2 concentration in the blood, not by the oxygen concentration, so even a small buildup of CO 2 in the inhaled gas quickly becomes intolerable; if a person tries to directly rebreathe their exhaled breathing gas, they will soon feel an acute sense of suffocation , so rebreathers must remove

2940-542: Is wasted. Continued rebreathing of the same gas will deplete the oxygen to a level which will no longer support consciousness, and eventually life, so gas containing oxygen must be added to the breathing gas to maintain the required concentration of oxygen. However, if this is done without removing the carbon dioxide, it will rapidly build up in the recycled gas, resulting almost immediately in mild respiratory distress, and rapidly developing into further stages of hypercapnia , or carbon dioxide toxicity. A high ventilation rate

3024-864: The Ohio -class submarine . The Ohio -class SSGNs are capable of supporting dual dry deck shelters. Suffren -class submarine, a nuclear attack submarines designed by the French shipbuilder Naval Group , integrate a removable dry deck shelter. It can deploy a dozen combat swimmers and embark the new PSM3G swimmer delivery vehicle (SDV). Former US Navy DDS-capable submarines include: Ethan Allen -class submarine : Sturgeon -class submarine : Benjamin Franklin -class submarine : Note: The Benjamin Franklin -class special operations attack submarines were capable of supporting dual dry deck shelters. Five Permit -class submarines were also fitted to carry

3108-614: The Caribbean Sea . The SDV could not have made the round trip to the Soviet vessel from an American ship outside of Cuba's territorial waters, so the mission had to be called off. Mark 8 SDVs saw combat during the First Gulf War , where they performed mine reconnaissance and demolition missions. In the Iraq War , Mark 8 SDVs were used to secure offshore oil and gas terminals. Several days before

3192-672: The Gulf War , Iraq War , and the US intervention in Somalia . The SDV was intended to be replaced with the Advanced SEAL Delivery System (ASDS), a larger, dry submersible that is often confused with the SDV. The SDV is flooded , and the swimmers ride exposed to the water, breathing from the vehicle's compressed air supply or using their own SCUBA gear, while the ASDS is dry inside and equipped with

3276-419: The decompression status of the diver and record the dive profile . As a person breathes, the body consumes oxygen and produces carbon dioxide . Base metabolism requires about 0.25 L/min of oxygen from a breathing rate of about 6 L/min, and a fit person working hard may ventilate at a rate of 95 L/min but will only metabolise about 4 L/min of oxygen. The oxygen metabolised is generally about 4% to 5% of

3360-429: The 1990s and reported that his subs were ready more than 90 percent of the time. The main failure of the SDV is its poor mobility. The SDV can only be effectively deployed from specially modified submarines and surface ships. Although it can be transported by C-130 airplanes, the relative scarcity of vessels capable of deploying an SDV limits its usage. Submarines are the preferred means of deployment, as enemies can see

3444-529: The Atlantic and European Command and Southern Command , and is primarily focused on supporting the activities of the Sixth Fleet . The SDV suffered from reliability concerns early in its lifespan. LCDR Doug Lowe, a member of SDV Team 1 in the 1980s, reported that his team's SDVs were operational less than 50 percent of the time. However, reliability improved with usage: LCDR Lowe later commanded SDV Team 2 in

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3528-444: The CO 2 in a component known as a carbon dioxide scrubber . By adding sufficient oxygen to compensate for the metabolic usage, removing the carbon dioxide, and rebreathing the gas, most of the volume is conserved. The endurance of a rebreather, the duration for which it can be safely and comfortably used, is dependent on the oxygen supply at the oxygen consumption rate of the user, and

3612-412: The DDS. Rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas

3696-592: The MABOT oil terminal and KAAOT Oil Terminals, in part using SDVs. The terminals were quickly seized with no casualties, and explosives which were found on the terminals were made safe by Polish GROM operators. In 2003, SEALs using SEAL Delivery Vehicles swam ashore along the Somali coastline and emplaced covert surveillance cameras. Known as cardinals, the cameras were designed to watch likely target locations for wanted terrorists as al-Qaeda and its affiliates began to regroup in

3780-400: The Mark 8. The pilot and navigator operate the vehicle from a prone position and lay side by side. The prone position gave the Mark 9 a low profile and enabled it to operate in very shallow water, although SEALs reported that staying prone for the entire duration of an operation was uncomfortable. The Mark 9's sleek profile and independent diving planes enabled it to be especially agile. It

3864-475: The Mark VIII Mod 1. The SDV was first developed in 1975 for use among UDT/SEAL teams. The early Mark 8 Mod 0 SDVs had a PRC104 UHF radio for use underwater. The newer model Mark 8 Mod 1 has a dual sliding canopy and quick release hatch. SDVs carry a pilot, co-pilot, and four person combat swimmer team and their equipment to and from maritime mission objectives on land or at sea. The pilot and co-pilot are often

3948-537: The SDV. SDVs are launched and recovered by surface ships using a crane. They can also be airdropped (uncrewed) into an operational area from a C-130 Hercules airplane. Finally, the SDV can be launched from the shore. An SDV can be launched from one platform and recovered by another. USS  John Marshall demonstrated this capability during a multilateral exercise in the Mediterranean when it recovered and then launched another country's SDV. The Mark 8 Mod 1 SDV

4032-450: The USN, the first one built by Electric Boat . The first, designated DDS-01S ("S" for starboard opening outer door), was completed in 1982. The remaining five, DDS-02P ("P" for port opening), -03P, -04S, -05S, and -06P, were built between 1987 and 1991 and were built by Newport News Shipbuilding . The shelters are maintained by a combined effort of Navy divers stationed on the teams and workers of

4116-599: The air that the staff breathe, and at high altitude, where the partial pressure of oxygen is low, for high altitude mountaineering. In aerospace there are applications in unpressurised aircraft and for high altitude parachute drops, and above the Earth's atmosphere, in space suits for extra-vehicular activity . Similar technology is used in life-support systems in submarines, submersibles, atmospheric diving suits , underwater and surface saturation habitats, spacecraft, and space stations, and in gas reclaim systems used to recover

4200-419: The ambient pressure breathing volume components, usually called the breathing loop in a circulating flow rebreather, and the make-up gas supply and control system. The counterlung is an airtight bag of strong flexible material that holds the volume of the exhaled gas until it is inhaled again. There may be a single counterlung, or one on each side of the scrubber, which allows a more even flow rate of gas through

4284-441: The ambient pressure breathing volume, either continuously, or when the user operates the oxygen addition valve, or via a demand valve in an oxygen rebreather, when the volume of gas in the breathing circuit becomes low and the pressure drops, or in an electronically controlled mixed gas rebreather, after a sensor has detected insufficient oxygen partial pressure, and activates a solenoid valve. Valves are needed to control gas flow in

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4368-766: The beginning of the 2003 invasion of Iraq , two SDV teams were launched from Mark V Special Operations Craft in the Persian Gulf. Their objectives were the hydrographic reconnaissance of the Al Basrah (MABOT) and Khawr Al Amaya (KAAOT) Oil Terminals . After swimming under the terminals and securing their Mark 8 Mod 1s, the SDV SEALs spent several hours taking pictures and surveying Iraqi activity on both platforms before returning to their boats. On March 20, 2003, SEALs from SEAL Team 8 and 10 (31 SEALs, 2 Navy EOD , 1 USAF combat controller, and several Iraqi interpreters) moved to seize

4452-470: The breathing gas is a mixture of oxygen and metabolically inactive diluent gas. These can be divided into semi-closed circuit, where the supply gas is a breathable mixture containing oxygen and inert diluents, usually nitrogen and helium, and which is replenished by adding more of the mixture as the oxygen is used up, sufficient to maintain a breathable partial pressure of oxygen in the loop, and closed circuit rebreathers, where two parallel gas supplies are used:

4536-692: The breathing volume, and gas feed from the storage container. They include: Oxygen sensors may be used to monitor partial pressure of oxygen in mixed gas rebreathers to ensure that it does not fall outside the safe limits, but are generally not used on oxygen rebreathers, as the oxygen content is fixed at 100%, and its partial pressure varies only with the ambient pressure. Re breathers can be primarily categorised as diving rebreathers, intended for hyperbaric use, and other rebreathers used at pressures from slightly more than normal atmospheric pressure at sea level to significantly lower ambient pressure at high altitudes and in space. Diving rebreathers must often deal with

4620-406: The capacity of the scrubber to remove carbon dioxide at the rate it is produced by the user. These variables are closely linked, as the carbon dioxide is a product of metabolic oxygen consumption , though not the only product. This is independent of depth, except for work of breathing increase due to gas density increase. There are two basic arrangements controlling the flow of breathing gas inside

4704-401: The carbon dioxide absorbent: 4KO 2 + 2CO 2 = 2K 2 CO 3 + 3O 2 . A small volume oxygen cylinder is needed to fill and purge the loop at the start of use. This technology may be applied to both oxygen and mixed gas rebreathers, and can be used for diving and other applications. Potassium superoxide reacts vigorously with liquid water, releasing considerable heat and oxygen, and causing

4788-416: The complications of avoiding hyperbaric oxygen toxicity, while normobaric and hypobaric applications can use the relatively trivially simple oxygen rebreather technology, where there is no requirement to monitor oxygen partial pressure during use providing the ambient pressure is sufficient. Rebreathers can also be subdivided by functional principle as closed circuit and semi-closed circuit rebreathers. This

4872-578: The country, however the cameras only took one image a day and captured very little. In American service, the SDV is deployed with SEAL Delivery Vehicle Team 1 (SDVT-1), based in Pearl Harbor , and SEAL Delivery Vehicle Team 2 (SDVT-2), based in Little Creek, Virginia . SDVT-1 operates on behalf of Central Command and Pacific Command in the Middle East and Indian and Pacific Oceans. SDVT-2 operates in

4956-468: The diluent, to provide the bulk of the gas, and which is recycled, and oxygen, which is metabolically expended. Carbon dioxide is considered a waste product, and in a correctly functioning rebreather, is effectively removed when the gas passes through the scrubber. There have been a few rebreather designs (e.g. the Oxylite) which use potassium superoxide , which gives off oxygen as it absorbs carbon dioxide, as

5040-485: The dim lights of the instrument panel; SEALs describe riding in an SDV as like "being locked in a little black coffin deep under the water." A 2011 article reported that out of 2,600 SEALS roughly 230 are trained to operate the SDVs. SDVs are generally launched from a Dry Deck Shelter on the back of a submarine , although they can also be deployed from amphibious carriers and other surface ships equipped to launch and recover

5124-439: The diver continues to inhale. Oxygen can also be added manually by a button which activates the demand valve. Some simple oxygen rebreathers had no automatic supply system, but only the manual feed valve, and the diver had to operate the valve at intervals to refill the breathing bag as the volume of oxygen decreased below a comfortable level. All rebreathers other than oxygen rebreathers may be considered mixed gas rebreathers, as

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5208-410: The gas composition other than removing the carbon dioxide. In some rebreathers the oxygen cylinder has oxygen supply mechanisms in parallel. One is constant flow ; the other is a manual on-off valve called a bypass valve; both feed into the same hose which feeds the counterlung. Others are supplied via a demand valve on the counterlung. This will add gas at any time that the counterlung is emptied and

5292-435: The granules by size, or by moulding granules at a consistent size and shape. Gas flow through the scrubber may be in one direction in a loop rebreather, or both ways in a pendulum rebreather. The scrubber canister generally has an inlet on one side and an outlet on the other side. A typical absorbent is soda lime , which is made up of calcium hydroxide Ca(OH) 2 , and sodium hydroxide NaOH. The main component of soda lime

5376-475: The inspired volume at normal atmospheric pressure , or about 20% of the available oxygen in the air at sea level . Exhaled air at sea level contains roughly 13.5% to 16% oxygen. The situation is even more wasteful of oxygen when the oxygen fraction of the breathing gas is higher, and in underwater diving, the compression of breathing gas due to depth makes the recirculation of exhaled gas even more desirable, as an even larger proportion of open circuit gas

5460-401: The intermediate reaction, the carbonic acid reacts exothermically with sodium hydroxide to form sodium carbonate and water: H 2 CO 3 + 2NaOH –> Na 2 CO 3 + 2H 2 O + heat. In the final reaction, the sodium carbonate reacts with the slaked lime (calcium hydroxide) to form calcium carbonate and sodium hydroxide: Na 2 CO 3 + Ca(OH) 2 –> CaCO 3 + 2NaOH. The sodium hydroxide

5544-435: The internal pressure is maintained at one atmosphere, there is no risk of acute oxygen toxicity. This is an underwater diving application, but has more in common with industrial applications than with ambient pressure scuba rebreathers. Different design criteria apply to SCBA rebreathers for use only out of the water: Mountaineering rebreathers provide oxygen at a higher concentration than available from atmospheric air in

5628-534: The large volumes of helium used in saturation diving . The recycling of breathing gas comes at the cost of technological complexity and specific hazards, some of which depend on the application and type of rebreather used. Mass and bulk may be greater or less than open circuit depending on circumstances. Electronically controlled diving rebreathers may automatically maintain a partial pressure of oxygen between programmable upper and lower limits, or set points, and be integrated with decompression computers to monitor

5712-613: The maintenance company Oceaneering International . They have expected useful lives of about 40 years each. The first submarine to have an operational dry deck shelter was the USS ; Cavalla , which was fitted with the DDS in 1982 and first deployed with it in 1983. It is deployed on the Virginia -class submarine , the Los Angeles -class submarine , the Seawolf -class submarine :, and

5796-488: The mission area, be "parked" or loiter in the area, retrieve the SEALs, and then return to the launch site. The SEALs sit upright in the Mark 8, with the pilot and co-pilot/navigator facing forward and the other four facing aft. For air, the SEALs rely on their own air tanks or rebreathers , supplemented by compressed air tanks on the SDV. The crew and passenger compartment in the Mark 8 is small, cramped, and pitch black except for

5880-671: The outer glass-reinforced plastic (GRP) fairing: a spherical hyperbaric chamber at the forward end to treat injured divers; a smaller spherical transfer trunk; and a cylindrical hangar with elliptical ends. The hangar can support a SEAL Delivery Vehicle (SDV) submersible, six Navy SEALs to man the SDV, and a crew of Navy Divers to operate the DDS and launch the SDV; or 20 SEALs with four Combat Rubber Raiding Craft (CRRC). The SDV release team consists of 2 officers, 2 enlisted technicians, and 18 divers. The two SEAL delivery vehicle teams report to Naval Special Warfare Group 3 . There are currently six portable dry deck shelters in use by

5964-400: The personnel under pressure in the accommodation chambers and closed diving bell. It includes the following components: The life support system for the bell provides and monitors the main supply of breathing gas, and the control station monitors the deployment and communications with the divers. Primary gas supply, power and communications to the bell are through a bell umbilical, made up from

6048-402: The photo, benefit from easier field repair if a tear or hole while helical corrugations allow efficient drainage after cleaning. Breathing hoses are usually long enough to connect the apparatus to the user's head in all attitudes of their head, but should not be unnecessarily long, which will cause additional weight, hydrodynamic drag , risk snagging on things, or contain excess dead space in

6132-418: The rebreather, known as the pendulum and loop systems. In the pendulum configuration, the user inhales gas from the counterlung through a breathing hose, and exhaled gas returns to the counterlung by flowing back through the same hose. The scrubber is usually between the breathing hose and the counterlung bag, and gas flow is bi-directional. All of the flow passages between the user and the active absorbent in

6216-467: The scrubber are dead space – volume containing gas which is rebreathed without modification by the rebreather. The dead space increases as the absorbent is depleted. Breathing hose volume must be minimised to limit dead space. In the loop configuration, the user inhales gas through one hose, and exhales through a second hose. Exhaled gas flows into the scrubber from one side, and exits at the other side. There may be one large counterlung, on either side of

6300-621: The scrubber contents from freezing, and helps reduce heat loss from the user. Both chemical and compressed gas oxygen have been used in experimental closed-circuit oxygen systems – the first on Mount Everest in 1938 . The 1953 expedition used closed-circuit oxygen equipment developed by Tom Bourdillon and his father for the first assault team of Bourdillon and Evans ; with one "dural" 800l compressed oxygen cylinder and soda lime canister (the second (successful) assault team of Hillary and Tenzing used open-circuit equipment). Similar requirement and working environment to mountaineering, but weight

6384-463: The scrubber, or two smaller counterlungs, one on each side of the scrubber. Flow is in one direction, enforced by non-return valves, which are usually in the breathing hoses where they join the mouthpiece. Only the flow passage in the mouthpiece before the split between inhalation and exhalation hoses is dead space, and this is not affected by hose volume. There are some components that are common to almost all personal portable rebreathers. These include

6468-436: The scrubber, which can reduce work of breathing and improve scrubber efficiency by a more consistent dwell time . The scrubber is a container filled with carbon dioxide absorbent material, mostly strong bases , through which the exhaled gas passes to remove the carbon dioxide. The absorbent may be granular or in the form of a moulded cartridge. Granular absorbent may be manufactured by breaking up lumps of lime and sorting

6552-442: The service, they may be made of a flexible polymer, an elastomer , a fibre or cloth reinforced elastomer, or elastomer covered with a woven fabric for reinforcement or abrasion resistance. If the woven layer is bonded to the outside surface it protects the rubber from damage from scrapes but makes it more difficult to wash off contaminants. Breathing hoses typically come in two types of corrugation. Annular corrugations, as depicted in

6636-416: The type include: A cryogenic rebreather removes the carbon dioxide by freezing it out in a "snow box" by the low temperature produced as liquid oxygen evaporates to replace the oxygen used. This may be compared with some applications of open-circuit breathing apparatus: The widest variety of rebreather types is used in diving, as the consequences of breathing under pressure complicate the requirements, and

6720-580: The water from a helicopter, making aerial deployments impractical and undesirable. The Mark 8 Mod 1 SDV has an endurance of about eight to 12 hours, giving it a range of 15 to 18 nmi (28 to 33 km) with a diving team or 36 nmi (67 km) without. The main limiting factor on endurance is not batteries or breathing gas for the SEALs, but water temperature: humans can only spend so much time in cold water, even with wetsuits , before their blood pressure drops and they become dehydrated from losing blood volume and body fluids, respectively. Despite being

6804-425: The water of the soda lime and formed carbonic acid, changing the pH from basic to acid, as the change of colour shows that the absorbent has reached saturation with carbon dioxide and must be changed. The carbon dioxide combines with water or water vapor to produce a weak carbonic acid: CO 2 + H 2 O –> H 2 CO 3 . This reacts with the hydroxides to produce carbonates and water in an exothermic reaction. In

6888-634: Was also faster than the Mark 8, reaching speeds of 7–9 knots (13–17 km/h), owing to its twin screw propellers and high-performance silver-zinc batteries . Its speed and agility led operators to compare it to flying a fighter jet or driving a sports car. The SDV is used primarily for inserting SEALs for covert operations or for placing mines on ships. It is also used for underwater mapping and terrain exploration, location and recovery of lost or downed objects, and reconnaissance missions. It has been invaluable at deploying SEAL teams in clandestine missions, as it has enabled them to land on shores inaccessible to

6972-502: Was employed by OSS MU during extensive training and exercises, but was never actually deployed for combat operations. The same capability was adopted by the American Underwater Demolition Teams (UDTs) in 1947. The one-man submersible displayed little functional military potential. However, it substantiated and characterized the need for improved and expanded UDT capabilities. After the war, development continued in

7056-619: Was invented by the Italians during World War I. The idea was successfully applied by the Italian Navy ( Regia Marina ) also early in World War II. The official Italian name for their craft was Siluro a Lenta Corsa (SLC or " Slow-running torpedo "). The vehicle was then copied by the British when they discovered the Italian operations and called it the " Sleeping Beauty " or Motorised Submersible Canoe. It

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