▁with ▁the ▁sea ▁level ▁equivalent ▁depth ▁and ▁stops ▁are ▁done ▁at ▁the ▁alt itude ▁stop ▁depth . ▁ ▁The ▁decom pression ▁algorithms ▁can ▁be ▁adjust ed ▁to ▁compens ate ▁for ▁alt itude . ▁This ▁was ▁first ▁done ▁by ▁Bü hl mann ▁for ▁der iving ▁alt itude ▁corrected ▁tables , ▁and ▁is ▁now ▁common ▁on ▁div ing ▁computers , ▁where ▁an ▁alt itude ▁setting ▁can ▁be ▁selected ▁by ▁the ▁user . ▁ ▁F lying ▁and ▁as cent ▁to ▁alt itude ▁after ▁div ing ▁ ▁Ex pos ure ▁to ▁reduced ▁atmos pher ic ▁pressure ▁during ▁the ▁period ▁after ▁a ▁d ive ▁when ▁the ▁resid ual ▁gas ▁levels ▁have ▁not ▁yet ▁stabil ized ▁at ▁atmos pher ic ▁s atur ation ▁levels ▁can ▁in cur ▁a ▁risk ▁of ▁decom pression ▁sick ness . ▁R ules ▁for ▁safe ▁as cent ▁are ▁based ▁on ▁extension ▁of ▁the ▁decom pression ▁model ▁calculations ▁to ▁the ▁desired ▁alt itude , ▁but ▁are ▁generally ▁simplified ▁to ▁a ▁few ▁fixed ▁periods ▁for ▁a ▁range ▁of ▁expos ures . ▁For ▁the ▁extreme ▁case ▁of ▁an ▁exception al ▁expos ure ▁d ive , ▁the ▁US ▁Navy ▁requires ▁a ▁surface ▁interval ▁of ▁ 4 8 ▁hours ▁before ▁as cent ▁to ▁alt itude . ▁A ▁surface ▁interval ▁of ▁ 2 4 ▁hours ▁for ▁a ▁Hel io x ▁decom pression ▁d ive ▁and ▁ 1 2 ▁hours ▁for ▁Hel io x ▁no - de com pression ▁d ive ▁are ▁also ▁specified . ▁More ▁detailed ▁surface ▁interval ▁requirements ▁based ▁on ▁the ▁highest ▁repet itive ▁group ▁design ator ▁obtained ▁in ▁the