▁bottom ▁gas , ▁which ▁is ▁generally ▁carried ▁in ▁back - mount ed ▁cyl ind ers . ▁There ▁is ▁a ▁convention ▁to ▁carry ▁the ▁most ▁o xygen - rich ▁additional ▁g ases ▁on ▁the ▁right ▁side , ▁and ▁the ▁lower ▁o xygen ▁g ases ▁on ▁the ▁left ▁side . ▁This ▁practice ▁reduces ▁the ▁ch ances ▁of ▁confusion ▁at ▁depth ▁and ▁in ▁poor ▁visibility , ▁and ▁saves ▁a ▁little ▁time ▁when ▁looking ▁for ▁the ▁correct ▁gas . ▁Several ▁models ▁of ▁technical ▁d ive ▁computer ▁can ▁be ▁set ▁before ▁the ▁d ive ▁with ▁the ▁gas ▁mi xt ures ▁to ▁be ▁used , ▁and ▁will ▁indicate ▁which ▁one ▁of ▁them ▁is ▁most ▁suitable ▁for ▁the ▁current ▁depth . ▁ ▁Sur face ▁decom pression ▁▁ ▁Sur face ▁decom pression ▁is ▁a ▁procedure ▁in ▁which ▁some ▁or ▁all ▁of ▁the ▁st aged ▁decom pression ▁oblig ation ▁is ▁done ▁in ▁a ▁decom pression ▁chamber ▁instead ▁of ▁in ▁the ▁water . ▁This ▁reduces ▁the ▁time ▁that ▁the ▁diver ▁sp ends ▁in ▁the ▁water , ▁exposed ▁to ▁environmental ▁ha z ards ▁such ▁as ▁cold ▁water ▁or ▁curr ents , ▁which ▁will ▁enh ance ▁diver ▁safety . ▁The ▁decom pression ▁in ▁the ▁chamber ▁is ▁more ▁controlled , ▁in ▁a ▁more ▁comfortable ▁environment , ▁and ▁o xygen ▁can ▁be ▁used ▁at ▁greater ▁partial ▁pressure ▁as ▁there ▁is ▁no ▁risk ▁of ▁d row ning ▁and ▁a ▁lower ▁risk ▁of ▁o xygen ▁to x icity ▁conv uls ions . ▁A ▁further ▁oper ational ▁advantage ▁is ▁that ▁once ▁the ▁divers ▁are ▁in ▁the ▁chamber , ▁new ▁divers