Modern day Scuba can be accredited to basically two mechanisms developed in the early twentieth centuries. First is the open-circuit scuba where a diver’s exhales are vented into the water and then the closed circuit system where the carbon dioxide is removed and oxygen is recirculated by the pressure developed from the new gas.
The very first rebreathers were discovered for the purpose of rescue and escape. These were closed-circuit and they were eventually modified for military applications thanks to lower bubbles and better stealth. Commercially, the first known closed-circuit rebreather was built by an Englishman, Henry Fleuss back in 1878. His entire apparatus consisted of a rubber mask, a breathing bag, about 50 to 60% O2 in a copper tank and CO2 scrubbed by rope yarn in caustic potash. This gave him dive duration of about three hours!
It was not until 1910 that the oxygen rebreather was perfected by Sir Robert Davis who was able to reproduce it commercially in larger quantities.
Open Circuit Scuba
Commercially, this kind of scuba set was first developed in 1943. It consisted of mounted cylinders on the back with compressed air pumped in using a demand regulator and then directly exhaled into the water near the tank.
Ted Eldred (leader and founder of KRD) is given the onus of developing the single hose two stage regulators also called the Porpoise Scuba Gear.