Navy SEAL Diving Gear
US Navy SEALs use 3 main types of underwater breathing gear : open circuit compressed air , closed circuit (100% oxygen) (LAR V Draeger) and closed circuit (mixed gas) (MK 15, MK 16). Other US Special Operations Forces that have an amphibious capability also use these systems.
SCUBA (open circuit)
Open circuit SCUBA (self-contained underwater breather apparatus), as used by the SEALs and other U.S. special ops units, consist of cylinders of compressed air worn on the diver's backs, typically 2 aluminum cylinders, each holding 80 cubic feet of oxygen, otherwise known as 'Twin 80s'. They are open circuit systems which means that the exhaled air is released into the water. Seal Delivery Vehicle Team divers may also utilize the open circuit breathing systems fed from air tanks inside swimmer deliver vehicles. Aside from the size and weight of the tanks, the downside of close circuit systems is the tell-tale trail of bubbles released into the water that are visible to the naked eye and to infrared.
LAR V Draeger Rebreather
The LAR V Draeger rebreather, designated as the MK 25, is a closed circuit SCUBA device. Running on 100% oxygen, all expelled breath is recycled into the closed circuit where it is filtered for carbon-dioxide. The result is a complete elimination of expelled bubbles which makes the Draeger ideal for clandestine amphibious operations. With a maximum depth of 70 feet, the LAR V Draeger rebreather cannot operate as deep as open circuit SCUBA systems. The unit's relatively small size and front-worn configuration makes them suitable for shallow water operation. Dive duration is affected by depth, water temperature and oxygen consumption rate. LAR V Draegers are used by Navy SEALs, Special Forces Dive Teams, USMC Force Recon and Division Recon, and other SOF units.
Closed Circuit Mixed Gas Rebreather
Like the LAR V Draeger, the MK 15 and MK 16 Closed Circuit Mixed Gas Rebreathers recycle the diver's expelled breath while filtering for CO2. Unlike the pure oxygen used in the Draeger, the MK 15 / MK 16 dilutes the oxygen supply with another gas (typically air but may also use Timux or Heliox as the dilutant). This mixture maintains a present partial pressure of oxygen (PPO2) level and allows for operation at much greater depth than the Draeger (depths down to 1,800 feet have been recorded).