A common first use of double tanks or cylinders is when penetrating a wreck or cave when deep diving (>130 fsw). Double cylinders are the backbone and foundation of a technical divers gear configuration. Double cylinders provide more air supply and reduce the amount of weight you have to carry on a weight belt. Most technical divers prefer larger volume low-pressure steel cylinders made by manufacturers like Pressed Steel and Faber. In the US these cylinders generally have a working pressure of 2,640psi (180 bar). A lower pressure cylinder does not require a high pressure fill, which can be harder to get at a recreational dive shop, to achieve a reasonable air supply. The cylinders do allow for higher volumes when necessary. The newer line of Pressed Steel cylinders are rated for high-pressure fills but are also stamped with a new volume rating, meaning you don’t have to over-fill the cylinder.
Popular sized low-pressure steel cylinders are 80, 95, 104, 112, and 120 cubic feet. The new popular Pressed Steel E8 series cylinders are 100 and 130 cubic feet.
Some divers prefer standard aluminum 80 cylinders and band them together to make a set of doubles. These cylinders are lighter than steel cylinders, but offer different buoyancy characteristics especially when low on air.
When setting up a set of doubles, it’s important to weight yourself so that you can stay submerged at 20ft if your cylinders were empty.
Cost: $600 to $1000