It's time for some SCUBA questions

[Sdiver]

Okay, I know that alot of the SF guys here aren't scuba qual'ed, but I do know that there are some here that are certified and seeing that Scuba is an SF fieldcraft, I thought I'd post this here.

Okay, here goes....

----How is it possible to do two seperate recreational dives, where your 2nd dive is your deeper dive and not run the risk of DCS (Decomperssion Sickness)?

***For those of you that aren't certified, It's ALWAYS taught that you do your DEEPER dive first, then move in progressivly shallower.***

Here's what you have to work with.....

1st Dive is at 80 feet
2nd dive is at 70 feet

***Remember, your 2nd dive is the deeper one.***

Both dives are for 30 minutes.
You have a 1 hour and 30 minute (1:30) serface interval.
You are diving on AIR only. No NITROX or mixed gases.
No computers can be used.
No protable chambers.

You must SHOW YOUR WORK !!!! I want to see dive profiles.