Is an 80 cuft tank enough air to safely go beyond 80'? My answer is yes but not safely. Please share your opinion.
One cannot generalize as you seem to do. It depends on :
- your Surface Air Consumption (SAC), experience, streamlining and fitness
- your buddies SAC, experience, streamlining and fitness
- conditions prevailing (and notably the current and visibility)
- is the dive an open water dive with mild deco or within the NDL, or is it more severe ?
- ...
Fortunately, experienced buddies with low SACs (eg 12 liters per minute) can often plan and do dives deeper than 30 meters/100 feet safely with an Al80. I wouldn't recommend it though for any kind of "material ceiling" dive (cave or wreck penetration) where one may get stuck for an unpredictable amount of time (contrary to open water deco diving which has a ceiling predictable both in depth and time) and for which, then, one needs redundant tanks, with rules about carried gas that can be much more severe than the "rule of thirds" (see below).
Some remarks :
- It's advisable to set and use rules like, for instance, "when tank is half full one must be at 12 meters/36 feet or shallower".
- Limits of Al80s for deco dives. If your buddies and you are experienced and know well each other, and each of you has a low SAC, and each of you shows consistently prudent attitude ("plans the dive and dives the plan"), and the conditions are OK, and each of you uses an Al80 because you checked it would be enough for the planned dive, and your dive is an open water (no penetration) deco dive with moderate hang time (say less than 10 minutes), even then it's advisable to systematically plan and make sure that each of you has more than one third (of the full tank capacity) of air left in his/her Al80 after completion of the stops. This third is for emergencies, eg sharing air with one buddy during ascent and stops in case of an equipment failure at the end of the bottom time (worst case). If one of you doesn't have this third left, Al80s were too small for the dive.
- To avoid blowing the valve O-Ring after 5 minutes underwater (can happen with O-Rings too soft), use DIN valves and regs. But these valves are not available everywhere.
- Any extra stuff you carry underwater increases (sometimes dramatically) your SAC because it increases your weight/inertia and reduces your streamlining. Make sure that the "plus" of what you wear or bring with you underwater more than compensates the "minus".
- There are physiological limits for how low each individual's SAC can be (eg size of the lungs and global weight) but SAC most of the time reduces significantly with improved training, experience, streamlining and fitness.