Because of the difficulties with single orifice doubles and backmounted independents, doubles were introduced to backmounted diving. Independents are two cylinders that are strapped to one's chest with separate regulators. A 1st stage failure would result in only one cylinder being used by the diver. A single orifice double was two cylinders connected together by a valve and one regulator. All gas would be lost if the first stage of a single orifice twice set failed. Today, both problems are eliminated by the isolated manifold doubles. Each cylinder can be operated independently. However, the diver can still inhale gas from both cylinders using one regulator. The doubles are usually held together by metal bands. The valves are connected with an isolation manifold so they can be separated if necessary. Doubles diving with backmounted doubles is similar to a regular single-cylinder backmount. It offers a vertical profile that is the same.
‘Caverns Measureless to Men’ by Sheck Exley, one of the pioneers of cave diving in Florida, USA. He also wore ‘Basic Cave Diving: A Blueprint for Survival’ which studies many cave diving accidents.
