The Siebe Gorman CDBA from three angles

The Clearance Divers Breathing Apparatus (CDBA) is a type of rebreather made by Siebe Gorman in England.

The Royal Navy used it for many years. It was for underwater work rather than for combat diving. The main oxygen 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 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 techniques, and in the 1960's and afterwards civilian divers had to retread the same ground and develop nitrox diving independently.

Instead of a weight belt there is a weight pouch at the back, full of lead shot a little over one inch/2.5 cm in diameter. In an emergency, the diver could pull a line which opened the weight pouch to jettison this weight. As the rebreather has a single pendulum breathing tube, the diver must breathe deeply to avoid carbon dioxide build-up.

For a short dive the equipment could be used without the back cylinders, using only the front cylinders; this created a very light and compact set 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 equipment relied entirely on training.

CGI image of three frogmen riding a Subskimmer underwater. Their breathing sets are the Siebe Gorman CDBA.

British "frogman's" sets used the same shape of "counter lung" 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, and no cylinders on the front, leaving the diver's front uncluttered for climbing in and out of small boats. The SCBA gave 90 minutes dive duration with no reserve. In mixture mode it was called SCMBA. Other British frogmen's sets had no back cylinders and one big oxygen cylinder across the belly: one of these modes was the UBA (Underwater Breathing Apparatus).

Note: The letters "SCBA" had two other meanings connected to breathings sets:

• Submerged Chamber Breathing Apparatus (formerly used to aerate submerged air-chambers).
• Self Contained Breathing Apparatus (its modern meaning, it means industrial breathing sets for use on land).
• The letters "SCMBA" nowadays mean the South Carolina Mortgage Brokers' Association.

External links

• CDBA Teardown (http://www.nobubblediving.com/cdba.htm): detailed description and many images.