The Clearance Divers Breathing Apparatus (CDBA) is a type of

rebreather A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's breathing, exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. ...

made by Siebe Gorman in

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. The British

Royal Navy The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against ...

used it for many years. It was for underwater work rather than for combat diving. The main

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

cylinders are on the diver's back. The oxygen cylinders at the front of the diver are for bailout. In its basic mode it was an oxygen rebreather; but some of the cylinders could be replaced by diluent cylinders for

nitrox Nitrox refers to any breathing gas, gas mixture composed (excepting trace gases) of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual applicatio ...

mode (which the Navy called "mixture"), and then the set was sometimes called CDMBA. The Royal Navy was using nitrox from 1944, but did not reveal its nitrox techniques, and in the 1960s and afterwards civilian divers had to retread the same ground and develop nitrox diving independently. In later years it was called DSSCCD from "Diving Set, Self-Contained, Clearance Diver".

Design

The CDBA was very popular with the clearance divers. It is comfortable since there is no cylinder on the middle of the back, no bulky buoyancy compensator, and it requires very little weight. As the rebreather has a single "pendulum" breathing tube, the diver must breathe deeply to avoid

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

build-up. The

counterlung 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 ...

is eight litres. As with all rebreathers, the diver should breathe continuously to keep the gas flowing over the absorbent. Dives on the unit are limited to 90 minutes. Instead of a weight belt there is a weight pouch at the back, full of lead ball weights in diameter. In an emergency, the diver could pull a line which opened the weight pouch to jettison these weights. It is intended to be used with a fullface mask with one breathing tube. At first (during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

and after) a mask with an oval or circular flat window (as seen in images a
this link
was used; later the mask with the newer type of rectangular window mostly flat but folded back at the sides was used.

Modes of setting up the CDBA

The front cylinders were intended as a bailout; the main cylinders are on the back. For a short dive the set could be used without the back cylinders, using only the front cylinders; this made the set very light and compact and suitable for getting through small holes. Even with the back cylinders the diver is much more compact and streamlined and agile than most recreational open circuit scuba. The front and the back of the harness can unclip from each other at the shoulders. There is no automatic gas control: so safe use of the set relied entirely on training.

Other similar rebreathers

2 frogmen with SCBA 1950s-type rebreathers

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"frogman's" sets used the same shape of

as the CDBA but different cylinders. One type was the "Swimmer Canoeists Breathing Apparatus" (SCBA), which had oxygen cylinders on the back, vertically for better streamlining in swimming, the oxygen connections thinner, and no cylinders on the front, leaving the diver's lower front uncluttered for climbing in and out of small boats. The SCBA gave 90 minutes dive duration with no reserve. In mixture (

heliox Heliox is a breathing gas mixture of helium (He) and oxygen (O2). It is used as a medical treatment for patients with difficulty breathing because mixture generates less resistance than atmospheric air when passing through the airways of the lung ...

) mode it was called SCMBA. An old photograph seems to show the cylinders (and perhaps also the weights) in a tied or laced canvas casing rather than held on with metal clamps, and a rectangular fullface mask as with the CDBA. There is a British armed forces manual about the SCBA, dated 1984 as if the SCBA was still in use then, showing separate eyes-and-nose mask and strapped-in mouthpiece and no canvas pouch, and separate sport-type weight belt with buckle-shaped weights all at the back. The design of the Swimmer Canoeists Breathing Apparatus likely changed down the years. The LOSE (Lightweight Oxygen Swimmers Equipment), a diving rebreather formerly made by Siebe Gorman, was similar to a Swimmer Canoeist's Breathing Apparatus including the cylinder backpack. Other British frogmen's sets had no back cylinders and one or more big cylinders across the belly: one of these modes was the UBA (Underwater Breathing Apparatus).

Other meanings of those initials

*The letters "SCBA" had two other meanings connected to breathing sets: **Submerged Chamber Breathing Apparatus (formerly used to aerate submerged air-chambers). **Self Contained Breathing Apparatus (its modern meaning, it means industrial breathing sets (usually open-circuit) for use on land): see

SCBA A self-contained breathing apparatus (SCBA), sometimes referred to as a compressed air breathing apparatus (CABA) or simply breathing apparatus (BA), is a device worn to provide breathable air in an atmosphere that is immediately dangerous to ...

. For other uses of the letters "UBA" see UBA.

References

External links