How Much Deco Can you Blow Off?

[lamont]

Clearly when it comes to a T2 dive, blowing off all the deco can kill you (Rouses/Shadow Divers/U-869). When it comes to a T1 dive I'd be uncomfortable blowing off all the deep stops, but might blow off everything above 50 feet in an emergency. With an unconscious diver at the end of a T1 dive on the bottom, I don't think I'd blow off all those stops, and I wouldn't send the diver up to blow their lungs apart on the way up (definitely killing them), so I'd probably shoot for doing 1 minute stops on backgas from where the deep stops start to the surface or something like that. IWR would be an option after that -- which I've thought about -- but I don't know if I'd IWR or just suck down O2 on the boat and head to the chamber (probably depends on how remote the chamber is, what any symptoms i had were, if their was a safety diver to help me, etc).

 

[ianr33]

Lamont, what is the dividing line between T1 and T2 ?

 

[DiverLaura]

not having been in the situation, I would have to say that it would depend a lot on the dive, and when/where i found the dive buddy. If it were Lamont and I on an easy tech 1 dive, and i watched him go limp or start seizing in front of me, I would likely take the gamble and make a controlled ascent, get him on the surface and start sucking O2. If I were on a 'big wreck' with a lot of deco obligation and a heavy helium mix, it would be a more difficult decision.

one of the problems with going back down for IWRC, is that if you are on a charter boat and the boat needs to mobilize to make the run to the dock, or convene with a rescue transport helicopter, having divers in water will complicate issues. if there are still divers on the wreck, and the boat can't cut free, thats a different story, but having run a charter boat, my preference would always be to get all the divers out of the water so i KNEW where everyone was as we start running the emergency evacuation algorithm. If i know someone's blown all their stops, the last thing i want them to do is be underwater hanging out without a healthy safety diver to watch them. I don't need a second fatality or lost diver. Of course some of this changes if you are out in the middle of nowhere, or are cave diving, but out here in the northwest, coast guard can scramble a bird and get to you pretty much anywhere in the sound in well under half an hour, and the one time i saw a scramble from Port Angeles, they were onsite in less than 15.

one thing to consider when thinking about rapid ascents is what helium mix you've been on and for how long. I think in my mind, that becomes part of the dividing line between T1 and T2.

this is something you'll talk about with your team ahead of time though, one would hope. and you'd have a plan in place for the dive du jour.

 

[sea2summit]

What do you base this on?

I base it on being in the water with a support diver who could convey the situation to the surface and when help arrived/gas was depleted I would surface. I'm thinking more cave here than open ocean which would be a far more complicated situation.

 

[lobstah]

What do you base this on?

Bullhman model and intuitive understanding of deco. Deeper portion has faster tissues controlling, shallow portion slower tissues. Fast tissues are ones associated with Type II hits.

 

[sea2summit]

Multi post

 

[JahJahwarrior]

How much risk I would place myself at depends a lot on the probability of there being a benefit to my buddy.

It's a simple concept: optimal behaviour takes place where the marginal benefit equals the marginal cost. If the marginal cost to me is a minor hit, and my buddy is very probably going to live if and only if I take that minor hit, then I would do it. However, if there is almost no chance that my buddy could possibly recover, or that his recovery would be benefited by me taking the risk of a major hit, then marginal benefit is low and marginal cost is high, and the optimal behaviour would be to stay on your deco schedule.

To that end, it is shortsighted to only consider the marginal cost to ourselves. We must also ask, what is the marginal benefit to my injured buddy? Does anyone have any statistics for survival rates of divers unconscious at various depths and times spent unconscious underwater?


Personally, I would be willing to put myself at some risk to help a buddy, even one I didn't know. I

 

[liuk3]

People are talking about how much time to blow-off, but I tend to think about it more in terms of what % of the deco to blow off.

My question would be which would you rather be:
1) 90 minute deco obligation, blow off 20 minutes
2) 30 minute deco obligation, blow off 20 minutes
Or, do you think that they are equally dangerous?

 

[jjones9527]

I don't think this is one of those situations where you can come up with a rule. Each dive is different, and environmental, situational, and physiological differences further complicate it. I would hope that anyone with a bit of experience would know where they stand in their decompression and be able to reasonably weigh the risk/reward factors in an emergency situation.

 

[Dive-aholic]

In an emergent situation, the options you have are to get back in the water and do some IWR, or hang at the surface for a couple of hours breathing O2 and hydrating. I've heard of divers blowing off deco time at 20' stops for a variety of reasons that did just that and didn't require a chamber ride.