There is some small advantage of 80% over 100% in that you can get on it 10 ft deeper at 30 ft rather than 20' but in the end the difference between a 50%, 80% plan and a 50% 100% plan is pretty minimal at the depths and times considered.
Practically speaking it is easier to transfill and partial pressure blend 70% and 80% as no boost pump is required to get a more or less full deco bottle as long as the O2 supply tank is full or nearly full and I suspect that may have something to do with it as well. In comparision to either, 50% is a snap as you can do it with only 1200 psi in the O2 bottle.
Using a second deco gas makes sense in terms of contingency planning for one lost deco gas given the limited backgas deco possible at those depths and times off a pair of AL 80's. But you need to remember that switching to the wrong gas is consistently a top killer of tech divers year in and year out.
Personally, I'd never rely on one gas hanging from an ascent line in an open water situation and it does not make a whole lot of sense to me to do it on a training dive even in what amounts to a confined water spring or sinkhole. It could encourage bad habits in the real world. If a gas is not on you at all times, you need to plan on not having it when you need it.
Also, I do ok on deep air dives as long as I set some firm limits, stay with in them, build up to the max depth over a few weeks early in the season and then maintain a reasonable currency. I am pretty sure if I did a dive to 80 ft and then went to 170-180' without working up to it, I'd be pretty useless at those depths in an emergency. If a goal of the course is to convince you that diving to 180' on air is not real smart, that will probably do it for you.
Practically speaking it is easier to transfill and partial pressure blend 70% and 80% as no boost pump is required to get a more or less full deco bottle as long as the O2 supply tank is full or nearly full and I suspect that may have something to do with it as well. In comparision to either, 50% is a snap as you can do it with only 1200 psi in the O2 bottle.
Using a second deco gas makes sense in terms of contingency planning for one lost deco gas given the limited backgas deco possible at those depths and times off a pair of AL 80's. But you need to remember that switching to the wrong gas is consistently a top killer of tech divers year in and year out.
Personally, I'd never rely on one gas hanging from an ascent line in an open water situation and it does not make a whole lot of sense to me to do it on a training dive even in what amounts to a confined water spring or sinkhole. It could encourage bad habits in the real world. If a gas is not on you at all times, you need to plan on not having it when you need it.
Also, I do ok on deep air dives as long as I set some firm limits, stay with in them, build up to the max depth over a few weeks early in the season and then maintain a reasonable currency. I am pretty sure if I did a dive to 80 ft and then went to 170-180' without working up to it, I'd be pretty useless at those depths in an emergency. If a goal of the course is to convince you that diving to 180' on air is not real smart, that will probably do it for you.
