This is a reprint of a post I made on Deco Stop recently. The past few days there have been a couple of different arguments going on on the dive forums concerning how deep people should be diving before they squirt a little Helium in their mix.
I love these "Deep Air" discussions because they usually end up being quite entertaining!
My own experience with deep air requires me to look at each and every diving scenario and make a "hopefully educated" decision as to what is appropriate for the environment and mission in which I will find myself.
Obviously, warm, clear, calm, open water environments are much more forgiving than cold, limited viz, overhead environments. I would suggest that each diver needs to use his own judgement based on experience to gage what is appropriate for himself or the team. Until said diver has sufficient experience to make educated decisions about what is appropriate, dogmatic lines drawn in the sand (END limits) are necessary to keep young divers alive until they learn how to make appropriate decisions.
I really, really, really hate hard fast rules with this stuff, because I consider most of them to be impractical, but I do understand why some instructors approach this in such a dogmatic way with students. It helps keep them alive! What I don't understand is why any student would just accept a rule as the "gospel truth" without having a very good understanding of why it is the rule and then having verified it based upon personal experience. I'm not suggesting that we go out and do something stupid just to verify, but there are lot's of different philosophies out there and sometimes DIR or whatever doesn't necessarily trump DIL (Done it Longer!)
Learn why rules are being suggested and then through personal study and experience, gradually figure out what is appropriate for you and your team in various scenarios. Anything less, is just blindly following something that may or may not be appropriate.
Just my $.02