▁and ▁the ▁mixture ▁may ▁be ▁changed ▁several ▁times ▁during ▁descent ▁and ▁as cent ▁from ▁great ▁depth s . ▁ ▁Cl osed ▁circuit ▁re bre athers ▁are ▁usually ▁controlled ▁to ▁provide ▁a ▁fairly ▁constant ▁partial ▁pressure ▁of ▁o xygen ▁during ▁the ▁d ive ▁( set ▁point ), ▁and ▁may ▁be ▁reset ▁to ▁a ▁rich er ▁mix ▁for ▁decom pression . ▁The ▁effect ▁is ▁to ▁keep ▁the ▁partial ▁pressure ▁of ▁in ert ▁g ases ▁as ▁low ▁as ▁safely ▁pract ic able ▁throughout ▁the ▁d ive . ▁This ▁minim izes ▁the ▁absor ption ▁of ▁in ert ▁gas ▁in ▁the ▁first ▁place , ▁and ▁acceler ates ▁the ▁elim ination ▁of ▁the ▁in ert ▁g ases ▁during ▁as cent . ▁ ▁Sur face ▁decom pression ▁▁ ▁Special ised ▁equipment ▁is ▁available ▁to ▁decom press ▁a ▁diver ▁out ▁of ▁the ▁water . ▁This ▁is ▁almost ▁exclus ively ▁used ▁with ▁surface ▁supplied ▁div ing ▁equipment : ▁ ▁De ck ▁decom pression ▁ch amb ers ▁are ▁used ▁for ▁surface ▁decom pression , ▁described ▁in ▁a ▁previous ▁section . ▁Most ▁deck ▁decom pression ▁ch amb ers ▁are ▁fitted ▁with ▁built ▁in ▁breath ing ▁systems ▁( B IB S ), ▁which ▁supply ▁an ▁alternative ▁breath ing ▁gas ▁to ▁the ▁occup ants ▁( us ually ▁o xygen ), ▁and ▁dis charge ▁the ▁ex h aled ▁gas ▁outside ▁the ▁chamber , ▁so ▁the ▁chamber ▁gas ▁is ▁not ▁excess ively ▁en rich ed ▁by ▁o xygen , ▁which ▁would ▁cause ▁an ▁un accept able ▁fire ▁ha z ard , ▁and ▁require ▁frequent ▁fl ushing ▁with ▁chamber