Triggers of Dive Accidents

[Crush]

To me it's not reasonable being at 70 fsw with 400 psi in the first place. Correct that and everything else falls into place.

In fact, it was less than 400. Perhaps it was 370? At any rate, I have never claimed that this was a prudent or desirable outcome. To the contrary, it was my mistake. I am happy to read your many posts in which you agree repeatedly with me that being at 70 fsw with 400 psi is not a good idea.

My points are that I believe that going near-OOA is a bad idea; and safety stops are optional. However, even if you are low on gas, safety stops are not always contraindicated - it depends upon the situation.

 

[TSandM]

This whole "safety stop when out of gas" thing comes up at regular intervals.

A safety stop is a kludge, designed to slow ascent rates. It is optional (unless you are in the last few pressure groups, in which PADI terms it a "mandatory" safety stop, while failing to explain how this is different from staged decompression). Optional means it can safely be omitted. The tables don't base their NDLs on someone doing a 3 minute stop -- tables say, "From this depth for this amount of time, you may safely ascend at 60fpm to the SURFACE". That means that, when you are running a risk of running out of gas, you leave out the stop.

Sitting at a 3 minute safety stop and wondering whether your pressure gauge is accurate to within plus or minus 50 psi doesn't sound like a good idea, to me.

 

[Crush]

Optional means it can safely be omitted. The tables don't base their NDLs on someone doing a 3 minute stop -- tables say, "From this depth for this amount of time, you may safely ascend at 60fpm to the SURFACE". That means that, when you are running a risk of running out of gas, you leave out the stop.

TSandM, I am not going to pretend to inform you on dive physiology - you know far more on this topic than I do. You know, as do I, that safety stops are optional. I also know a few cave divers who like to just chill on the surface at the end of a dive - a sort of second safety stop - before they trudge out of the water. Neither the 20 ft safety, nor the 0 ft safety, are necessary if you are within NDL and your body perfectly follows a dive table. But, if our bodies always behaved according to tables, no one would ever take an undeserved hit.

Sitting at a 3 minute safety stop and wondering whether your pressure gauge is accurate to within plus or minus 50 psi doesn't sound like a good idea, to me.

I guess that I felt that I could have CESA'd the last 20 if my breathing became laboured.

 

[Mr Carcharodon]

Balance. Getting out of the water with 500 psi, just to get out the water with 500 psi, is not a good idea if you have run close to or past decompression limits are cold and worked hard.

The whole of idea of having a gas reserve is that you may use it, although hopefully planning means this does not happen too often. OOG at 10 fsw is clearly less of an issue than at 100 fsw. Using all of the reserve can be a reasonable thing to do.

 

[spoolin01]

Agreed that it's better not to be in that circumstance in the first place, and certainly that balance is the question at hand. How would you have modified gcbryan's ascent? How much shorter would your ascent have been and what would your air reserve target have been? So far, I'm pretty sure there's almost nothing that would be likely to happen during the latter half of the ascent that absolutely would require bottled air to cope with, so I'd rather let the air run down so that I can slow the ascent, including stops. It's a calculated risk, just my preference to shade it that way.

 

[spoolin01]

This whole "safety stop when out of gas" thing comes up at regular intervals.

A safety stop is a kludge, designed to slow ascent rates. It is optional (unless you are in the last few pressure groups, in which PADI terms it a "mandatory" safety stop, while failing to explain how this is different from staged decompression). Optional means it can safely be omitted. The tables don't base their NDLs on someone doing a 3 minute stop -- tables say, "From this depth for this amount of time, you may safely ascend at 60fpm to the SURFACE". That means that, when you are running a risk of running out of gas, you leave out the stop.

Sitting at a 3 minute safety stop and wondering whether your pressure gauge is accurate to within plus or minus 50 psi doesn't sound like a good idea, to me.

Maybe it's better viewed as a metaphor. Is there no physiological benefit to a safety stop, or two stops, or a 15 ft/sec vs 30 ft/sec ascent rate? Safe, meaning 'no documentation of an increase in immediate DCS manifestation' (my assumption about the state of harm prediction, is this incorrect?) means it's a step function, no point to any more conservative approach? On the other side of the balance, what's the risk from running your tank down while hovering at 15 ft? It's bigger?.

 

[Mr Carcharodon]

Spoolin,

If you read the DAN Fatalities workshop papers you will find that there are ~30x more fatalities from arterial gas embolism (AGE) than from DCS. Of course people can and do embolize from 15 fsw. Given the disproportionate number of fatalities due to AGE not running out of gas seems like a priority when the risk of DCS is low. It is not always low, but it usually is. Anyway I seem to be on both sides of the fence on this topic.

 

[spoolin01]

What do those DAN papers say about DCS morbidity? I stopped riding motorcycles a few years ago, precisely because poor risk management wasn't guaranteed to kill me. I still drive and dive because it's my sense that making a really serious mistake is something a bit more under my control.


And just to beat that horse some more, I plan so as to be even more sure not to run out of gas anytime it wouldn't be (fairly) innocuous. AGE falls outside that scenario, and most others.

 

[lamont]

Balance. Getting out of the water with 500 psi, just to get out the water with 500 psi, is not a good idea if you have run close to or past decompression limits are cold and worked hard.

The whole of idea of having a gas reserve is that you may use it, although hopefully planning means this does not happen too often. OOG at 10 fsw is clearly less of an issue than at 100 fsw. Using all of the reserve can be a reasonable thing to do.

I've recovered a body due to an OOG at 10 feet (resulting in CAGE, and they just can't fix you after you get one of those).

You should not deliberately run OOG, under any circumstances where this is preventable, because it immediately begins to dramatically limit your options. Clearly, you can run down to 400 psi or even 300 psi since your gauge is most likely at least that accurate. But you want to avoid actually sucking the tank dry.

If you are back to 10 fsw with 500 psi, you can do a safety stop and use another 100 psi or less in the process. If you are back to the surface with less than 200 psi you should initiate an air-share with someone with more gas or just get yourself up onto the surface. And the best idea, of course, is to manage your gas so that you're never in this situation.

 

[spoolin01]

How did the poor feller get CAGE, whatever that is, at 10 ft?