headhunter:
I don't know if a lift bag or SMB would be an acceptable method of compensating in an emergency from a DIR perspective. I imagine that if the situation can be avoided through proper procedures, then the answer would be to not get yourself into that position in the first place.
This also begs the question of whether or not your buddy should be considered a redundant system for your wing in an emergency. For this last issue, I'm assuming that you can swim your own rig up, but that a little help from your buddy could save some effort.
I'm specifically interested in the DIR answer on this, since I've made a concious decision to learn the system in its entirety.
By the way, I'm not avoiding using common sense here. I'm just trying to get an understanding from a purely academic standpoint.
You're beginning to approach the various debates that continue to keep this issue controversial. Some of them depend on your philosophy, and how willing you are to consider alternatives. At the heart of things, perhaps, is (DIR) guidance that allows different interpretations (Jablonski, 2001, 91-93). Divers need to select an approach that makes sense given their specific circumstances. Jablonski is pretty clear in his conclusion: "The bottom line here...is that divers should be certain that, without any air in their bouyancy compensators, they are capable of swimming against the weight of their configuration with full tanks and all weight in place. This would allow them to verify that they are able to manage their SCUBA configuration in the event of a bouyancy failure" (2000/2001, p.94).
From a purely academic standpoint, that's unambiguous. Reality requires some application of analysis, however. A diver in a drysuit, with a set of steel PST 130s, one 80 and one 40 of deco gas, and an 18 watt HID light, apart from any other V-weight or weightbelt, will be seriously over-weighted should the wing fail at the start of the dive. Tanks, gas in them, and regulators would result in a negative weight of > ~30 lbs. Very few divers could swim up full doubles and two full stage/deco bottles, even using their drysuits to offset some negative bouyancy, unassisted. They'd load up with retained CO2 before they swam up 100'. Even fewer would be inclined to ditch their $1K+ HID and dump the stage/deco tanks full of gas, if other options exist.
Other options include use of the drysuit to offset some negativity, a liftbag and reel or spool, and your buddy - who ostensibly is not unaware that you've suffered a bouyancy failure. An 80 lb liftbag and reel should allow a diver to ascend the line, particularly with a buddy assisting. One cautionary note; a drysuit
alone is often ill-suited for precision bouyancy control - particularly if the diver needs to hold deco stops. While it isn't impossible, if you need to hold the deco stops using a drysuit alone - to offset some 30 lbs of negative bouyancy, it would be a swell idea to try it a few times beforehand so it isn't an 'on-the-job learning experience'.
In a wetsuit your alternative (DIR) options still include dropping weight, using a liftbag/reel/spool, and using your buddy. While it may not be DIR, I'd offer a conclusion that if two stage bottles and decompression stops are in your dive plan, for some wetsuit divers a dual-bladder wing also may make sense. Does it offer greater potential for task loading or failure? Under some circumstances, yes. However, leaving the second LP inflator hose routed but not hooked up eliminates much of the 'bouyancy creep' that an inflator failure might introduce. Should the primary bladder fail, hooking up the second hose ought not to be too much more demanding than donating the hose one is breathing from.
And holding deco stops will likely be easier using a wing than dangling under a liftbag.
In the final analysis, the concept of a balanced rig makes sense. The diver ought to be able - ALONE - to swim up their rig under most failure conditions. In reality, at the start of a challenging dive, that may not always be the case. But of course, if they are diving DIR, ...they won't be alone. There are sufficient alternative forms of contingency bouyancy available, that each diver ought to be able to come up with one or more of the above options that will work for their circumstances.
Hope that addresses some of your questions.
Also hope you enjoy your class.
