Low on air in deco

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Actually, the original question is an interesting one. Let's change the circumstances slightly, keeping the scenario within a properly planned recreational dive:

We are engaged in a hypothetical dive at a maximum depth of 100'. Approaching our planned NDL of 17 minutes, I have 1400psi remaining. My buddy unexpectedly becomes entangled in some monofilament hooked up on a log on the bottom. He turns to see what he is caught on and gets further tangled. A bit of tugging on his part and suddenly there is tangled line everywhere. Seeing he is in difficulty, I move in to assist, but do so cautiously to ensure I do not become ensnared. I carefully cut away the line with my shears and line cutter, but this takes, oh, say 8 minutes. Meantime, my buddy is consuming air at an elevated rate due to increased effort and stress. In fact, so am I, so now I have 700 psi left and he is down to, say, 300 psi.

At 30fpm, we will each consume 200 psi or so during the ascent, if we can calm down and get our breathing back to normal. My tables (NASDS) give me a 6 minute deco obligation at 15 feet. My buddy will run out of air before then and will be breathing from my octo. If everything goes right during the ascent, my buddy will be OOA and I will have less than 200 psi left when we surface.

That's cutting it awful close and it depends on getting our air consumption rates down to normal. Should I stick to the 30 fpm ascent rate and be prepared to blow off some deco time if we run out at 15'? Increase the ascent rate to, say, 60fpm to reduce air consumption and ensure we meet the full deco obligation? Or is there some other strategy that should be considered?
 
derwoodwithasherwood:
Actually, the original question is an interesting one. Let's change the circumstances slightly, keeping the scenario within a properly planned recreational dive:

We are engaged in a hypothetical dive at a maximum depth of 100'. Approaching our planned NDL of 17 minutes, I have 1400psi remaining. My buddy unexpectedly becomes entangled in some monofilament hooked up on a log on the bottom. He turns to see what he is caught on and gets further tangled. A bit of tugging on his part and suddenly there is tangled line everywhere. Seeing he is in difficulty, I move in to assist, but do so cautiously to ensure I do not become ensnared. I carefully cut away the line with my shears and line cutter, but this takes, oh, say 8 minutes. Meantime, my buddy is consuming air at an elevated rate due to increased effort and stress. In fact, so am I, so now I have 700 psi left and he is down to, say, 300 psi.

At 30fpm, we will each consume 200 psi or so during the ascent, if we can calm down and get our breathing back to normal. My tables (NASDS) give me a 6 minute deco obligation at 15 feet. My buddy will run out of air before then and will be breathing from my octo. If everything goes right during the ascent, my buddy will be OOA and I will have less than 200 psi left when we surface.

That's cutting it awful close and it depends on getting our air consumption rates down to normal. Should I stick to the 30 fpm ascent rate and be prepared to blow off some deco time if we run out at 15'? Increase the ascent rate to, say, 60fpm to reduce air consumption and ensure we meet the full deco obligation? Or is there some other strategy that should be considered?
Yeah... an what if it takes 32 minutes to get him untangled instead of only eight? What will you do then???
This reminds me of reporters asking Alan Shepherd back in the Mercury days "what if this happens" and "what if that happens" until Alan finally said "Well, some days you eat the bear... and some days the bear eats you. If all of those things happen at once then I guess I'm just dead."
Rick
 
derwoodwithasherwood:
Actually, the original question is an interesting one. Let's change the circumstances slightly, keeping the scenario within a properly planned recreational dive:

We are engaged in a hypothetical dive at a maximum depth of 100'. Approaching our planned NDL of 17 minutes, I have 1400psi remaining. My buddy unexpectedly becomes entangled in some monofilament hooked up on a log on the bottom. He turns to see what he is caught on and gets further tangled. A bit of tugging on his part and suddenly there is tangled line everywhere. Seeing he is in difficulty, I move in to assist, but do so cautiously to ensure I do not become ensnared. I carefully cut away the line with my shears and line cutter, but this takes, oh, say 8 minutes. Meantime, my buddy is consuming air at an elevated rate due to increased effort and stress. In fact, so am I, so now I have 700 psi left and he is down to, say, 300 psi.

At 30fpm, we will each consume 200 psi or so during the ascent, if we can calm down and get our breathing back to normal. My tables (NASDS) give me a 6 minute deco obligation at 15 feet. My buddy will run out of air before then and will be breathing from my octo. If everything goes right during the ascent, my buddy will be OOA and I will have less than 200 psi left when we surface.

That's cutting it awful close and it depends on getting our air consumption rates down to normal. Should I stick to the 30 fpm ascent rate and be prepared to blow off some deco time if we run out at 15'? Increase the ascent rate to, say, 60fpm to reduce air consumption and ensure we meet the full deco obligation?

Or is there some other strategy that should be considered?

This is an interesting hypothetical, and it results in what we normally call a C-F.

I would suggest the following, for your hypothetical.

First, do not dive with air deeper than 50 ft. GUE already tells you not to dive with air at all, rather with nitrox. JJ is not stupid, he is actually quite bright. If I am diving between 50 and 100 ft, I definitely will be diving with nitrox. Dial your nitrox mix into your diving computer, and you will see that your deco times for accidental unplanned deco are shorter.

Second, dont spend forever cutting your buddy out of an entanglement. There are other alternatives, none of them terribly safe, but all of them better than running out of air during a deco obligation.

Third, use the rule of thirds for deep dives. For you, deep dives would be anything in the range of 100 ft. Deep is relative. If you are an NDL diver, then 100 ft is definitely deep. That means you turn around after 1/3 of your breathing gas is gone, not when you have only 1/3 left.

When a C-F happens to you, it will depend on how badly you violated rules of conservatism as to whether you live to dive another day. Because sometimes the bear gets you, while other times you get the bear. You will just never know for sure. :)
 
Rick Murchison:
...This reminds me of reporters asking Alan Shepherd back in the Mercury days "what if this happens" and "what if that happens" until Alan finally said "Well, some days you eat the bear... and some days the bear eats you. If all of those things happen at once then I guess I'm just dead."
Rick

I have always wondered where that time honored Navy expression came from!

Heard it spoken many times. Never knew before today why it was so popular.

When Alan Shepherd was riding in space rockets, I was riding my bike as a kid in the neighborhood. :)
 

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