Emergency Descent During Deco

[TSandM]

I think it's a good question, and there can be other reasons for having to descend, too. I had a situation on a staged decompression dive in Canada where I lost my buddy in poor viz, and got vertigo looking for him. Knowing that the end result of midwater vertigo for me was usually a trip to the surface, I made the decision to dump everything I had in my wing and suit and sink to the bottom, which I knew was at roughly 50 feet. (Luckily, I had not yet done my gas switch to my O2 bottle.) The descent was very brief, though, because the vertigo resolved when I reacquired a visual reference, so I didn't adjust my deco at all.

If I'd had to go to 100 feet, I would have had to think about it, especially if I'd had to remain there for some time. By the time you are at a 20 foot stop, you should have offgassed enough in the faster compartments to be ongassing with a descent to 100 feet, and there is also the scarier issue of possible bubble-pumping. I would definitely want to make the subsequent ascent slower than 30 fpm, and doing 10 fpm from 50 or so would seem to make sense. Then you are left with how much additional deco to do in the shallows. If I'd had to sit at 100 for, say, five minutes, I'd probably add that to my deco, gas permitting, or even double it if gas, thermal considerations, and O2 clock allowed it. Nobody ever got hurt from doing too much deco . . .

I do think this is a place where a decompression-capable computer could be quite useful. It's a great example of how a combination of an unforeseeable deviation from profile AND significant diver stress could result in difficulty making solid decisions about how to change the deco.

 

[dumpsterDiver]

With this situation.. the ship is coming very fast, probably 30 mph. By the time you realize you must descend, the ship is close and it will have passed by rapidly as well. I don't think the time at depth would need to be more than 2 minutes and it would be over (one way or another).

 

[peterbj7]

Interesting points are coming out. Like others I did all of my early trimix dives on tables, but not since I acquired my first VR3 when they came out. I now make every dive with a dive computer. It would be two, but I've had two computers die on me in recent months (they don't seem to be very well made these days).

This scenario is unlikely to occur in most places. But there are places where it most certainly can occur. There are several dive sites around Britain which are directly under busy shipping lanes (presumably how the wrecks got there in the first place!) and a straight descent from the surface is not possible. In effect there's a glass ceiling, and it has to be treated a bit as a cave dive. On the French side of the English Channel for example (I'm thinking of the part where it's narrow, getting up towards Calais), there are many WW2 wrecks which lie directly under the main shipping channel. Some of the biggest ships in the world pass through there, and even if they see you they won't be able to slow or avoid you. Small boats are in the same danger there. To dive those wrecks you have to descend well to the side, and after the dive return along the bottom to a point where you can safely ascend. There aren't of course fixed markers on the bottom, and if you get it wrong you nay find yourself ascending into the path of shipping. These ships are so giant that a descent to 100ft is the minimum you can safely do, then continue the course you should have been on in the first place. You need to have lots of gas or a CCR available.

If you're getting towards the end of considerable decompression and then have to drop abruptly to 100ft you are likely to suffer ill effects, and should do all you can to stave off the effects of deco sickness. But rather that than be mashed by the biggest propeller you will ever (briefly) see.

 

[MalibuJerry]

been tech diving for 15 years without a computer to rely on for deco. A well trained tech diver will view this as a multi-level dive and can do deco on the fly to ascertain the appropriate deco from the second bottom to the surface. It is not that hard if you understand deco, it is really difficult if you understand reading a computer screen but don't know why it gives you the amount of deco and you have not planned your dive and contingencies ahead of time.

 

[Adobo]

With this situation.. the ship is coming very fast, probably 30 mph. By the time you realize you must descend, the ship is close and it will have passed by rapidly as well. I don't think the time at depth would need to be more than 2 minutes and it would be over (one way or another).

I don't have an answer for this scenario as I have never thought about it. If I felt my life depended on it, I imagine I would descend back to 70 ft or 100ft or whatever the appropriate depth would be.

I do want to add that the decision doesn't seem trivial to me in that:

• You might be breathing O2 when it dawns on you that a big ship might be bearing down on you. Obviously, the first step to would have to be switch back to backgas.
• For those of us who only do team oriented dives, keeping the team together in this scenario doesn't sound trivial. Meaning, the entire team needs to be aware of the approaching danger and all need to quickly conclude on what next steps would be. (switch to backgas, expedited descent to whatever depth, etc.)
• By the time you get to the desired depth, you could conceivably be dealing with time pressures due to how much backgas you have. That is, originally, your dive was called because either you or your buddy were at or close to min gas. By the time you are at 100ft, you could be well below min gas.
• Of course, it is likely that you did not clean up your O2 reg and it is still dangling around your neck. Be sure to clean that up so that you don't inadvertently put it in your mouth or donate it to your buddy if another "situation" arises.
• Hopefully there are multiple SMBs with the team at the start of the dive. Likely, the SMB you had, you had to let go when you re-descended. You'll have to send up another SMB to tell the boat where you are... 4 knots of current and all can separate you and your dive boat pretty significantly if you still have a bunch of deco to do.

My feeling is that it is best to put the contingency plans in place long before you get into the water. That is, you know where you are diving and you know that there is a possibility of drifting into shipping lanes. Reserve the extra gas and talk about what the boat and the divers are expected to do if something like this comes up.

 

[MalibuJerry]

All good points Adobo. You have to agree that if one of you signals a descent you both spit your O2 go to back gas and descend. One has to be the designated to calculate the new deco and that might have to be on back gas or 50/50 if you lost another gas. These are things that go into the planning of a tech dive. Just having two computers does not a tech diver make.

 

[Tortuga68]

I would go one step further... If you can't execute a technical dive without a computer, you should take a serious look at your training

 

[peterbj7]

Who said anything about "can't"? I for one can make any dive safely without a computer, and years ago we all did anyway. But with a computer it's easier to be safe and more flexible. Your personal description says you have no more than 999 dives - I have more technical dives than that. Probably 500 on trimix, and certainly 300 on CCR. I have made perhaps 10 dives to below 500'. I see too much of people not believing what others can do because they can't do it themselves.

 

[Tortuga68]

Good for you, but my comment was a general statement so no need for you to feel the need to justify yourself

 

[Centrals]

It would be interesting to note how the tec computer would behave if one is decompressing at 6m with 100% O2 and suddenly descent to 30m on back gas for a short time and then ascent. A lot of frantic tapping.