Cave and technical divers live by the rule of thirds: Always reserve a third of your available breathing gas for self- or buddy rescue. While side-mounted tanks do not provide a single, common air supply, the rule of thirds still applies. Typically, a diver will breathe no more than a third of one tank, then switch to the alternate tank. The dive is turned when both tanks are at least two-thirds full, allowing enough reserve gas for delays or for buddy rescue. Rather than sharing air by way of a long hose, side-mount divers can simply pass an entire tank to their diving partner.Proponents of this system point out that this not only simplifies air sharing, but the use of separate tanks may also lessen the impact of certain types of equipment failures and will preserve adequate reserve gas even in the event of a total regulator or valve failure.
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