Oxygen rebreathers are severely depth-limited due to oxygen toxicity risk, which increases with depth, and the available systems for mixed gas rebreathers were fairly bulky and designed for use with diving helmets.
The first versions were inflated from a small disposable carbon dioxide cylinder, later with a small direct coupled air cylinder.
The British adapted the Davis Submerged Escape Apparatus and the Germans adapted the Dräger submarine escape rebreathers, for their frogmen during the war.
After World War II, military frogmen continued to use rebreathers since they do not make bubbles which would give away the presence of the divers.
Their system combined an improved demand regulator with high-pressure air tanks. The patent was circumvented by Ted Eldred of Melbourne, Australia, who developed the single-hose open-circuit scuba system, which separates the first stage and demand valve of the pressure regulator by a low-pressure hose, puts the demand valve at the diver's mouth, and releases exhaled gas through the demand valve casing.
Siebe Gorman was allowed to sell in Commonwealth countries, but had difficulty in meeting the demand and the U. Eldred sold the first Porpoise Model CA single hose scuba early in 1952.