▁risk ▁of ▁vest ib ular ▁D CS ▁is ▁to ▁ensure ▁ade qu ate ▁initial ▁decom pression , ▁and ▁to ▁make ▁the ▁switch ▁to ▁nit ro x ▁at ▁a ▁relatively ▁shall ow ▁depth ▁( less ▁than ▁ 3 0   m ), ▁while ▁using ▁the ▁highest ▁accept ably ▁safe ▁o xygen ▁fraction ▁during ▁decom pression ▁at ▁the ▁switch . ▁ ▁Deep ▁technical ▁div ing ▁usually ▁involves ▁the ▁use ▁of ▁several ▁gas ▁mi xt ures ▁during ▁the ▁course ▁of ▁the ▁d ive . ▁There ▁will ▁be ▁a ▁mixture ▁known ▁as ▁the ▁bottom ▁gas , ▁which ▁is ▁optim ised ▁for ▁limit ing ▁in ert ▁gas ▁nar cos is ▁and ▁o xygen ▁to x icity ▁during ▁the ▁deep ▁sector ▁of ▁the ▁d ive . ▁This ▁is ▁generally ▁the ▁mixture ▁which ▁is ▁needed ▁in ▁the ▁largest ▁amount ▁for ▁open ▁circuit ▁div ing , ▁as ▁the ▁consumption ▁rate ▁will ▁be ▁greatest ▁at ▁maximum ▁depth . ▁The ▁o xygen ▁fraction ▁of ▁the ▁bottom ▁gas ▁suitable ▁for ▁a ▁d ive ▁deeper ▁than ▁about ▁ ▁will ▁not ▁have ▁sufficient ▁o xygen ▁to ▁reli ably ▁support ▁conscious ness ▁at ▁the ▁surface , ▁so ▁a ▁travel ▁gas ▁must ▁be ▁carried ▁to ▁start ▁the ▁d ive ▁and ▁get ▁down ▁to ▁the ▁depth ▁at ▁which ▁the ▁bottom ▁gas ▁is ▁appropriate . ▁There ▁is ▁generally ▁a ▁large ▁overlap ▁of ▁depth s ▁where ▁either ▁gas ▁can ▁be ▁used , ▁and ▁the ▁choice ▁of ▁the ▁point ▁at ▁which ▁the ▁switch ▁will ▁be ▁made ▁depends ▁on ▁consider ations ▁of ▁cum ulative ▁to x icity , ▁nar cos is ▁and ▁gas