Required Safety Stop a decompression stop?

[Reg Braithwaite]

I'll try again...

It's impossible to completely reach equilibrium with 1ATM without actually being at 1ATM. In other words, if there is water above you, there's some decompression left to do.

I understand this to be the case for air. The PPN₂ of air on the surface is .79, and my tissues are in equilibrium with this. When I descend below the surface, the PPN₂ in the compressed air I breathe rises above .79 and N₂ is absorbed into my tissues.

What happens if I dive EAN40 to a depth of 13'? The PPN₂ I breathe at 13' is also .79 even though the ambient pressure is roughly 1.3 ata. Do my tissues absorb N₂? Or is there no change?

And what happens if I breathe pure O₂ for an extended 20' rebreather dive like the WWII frogmen? Do my tissues actually off-gas N₂ since the PPN₂ is zero? And do I therefore on-gas N₂ when I return to the surface, the opposite of what usually happens after a dive?

 

[Blackwood]

This could be nitpicky (and wrong), but personally I imagine it's possible to bend yourself on pure oxygen. It may be difficult, but it could probably be accomplished in a dry chamber. All gases dissolve, and your body doesn't make use of oxygen with anywhere near 100% efficiency.

IMO, "done decompressing" is when all the gases are at equilibrium with the surface atmosphere, not just the nitrogen or helium.

 

[TSandM]

I suspect it isn't possible to bend yourself by switching to O2. The whole concept behind O2 deco is that you can keep ambient pressure high(er) while maximizing the gradient. If you could bend yourself on O2, I can't imagine that they would use it for decompression sickness cases in the chambers.

 

[eponym]

It's my understanding that only inert gasses can build up dissolved tension. I would guess that increasing pressure of a reactive gas increases reaction rates and keeps levels constant.

But I think I'll ask Dr. Deco. I do like the question, Blackwood.

-Bryan

 

[Blackwood]

I can't imagine that they would use it for decompression sickness cases in the chambers.

Hyperbaric therapy schedules are extremely well-controlled, particularly with respect to decompression rate, even after long oxygen exposure.

My gut guess is that if you saturate someone with O2 at high pressure before instantaneously switching to 1ATM of air, you could develop DCS.

It's certainly unlikely within practical diving considerations, though. I think you'd have to try to do it.

...which is why I called it nit-picky.

It's my understanding that only inert gasses can build up dissolved tension.

Without kicking this train too terribly far off the track...

I'm not sure why that would be the case, theoretically. We use such a small amount of the gas we inhale, most of it is just along for the ride. Even if it's pure oxygen, it seems that most of it will be just along for the ride, and subject to Boyle. Even if we were perfectly efficient with our use of oxygen, I wonder if given time we couldn't build up (inert) gas tension with the byproducts of the metabolic process.

Anyway, this is a laboratory discussion. Feel free to ping Dr. Deco. I'd be interested in his thoughts, but not enough to start a thread

 

[eponym]

Any gaseous products of metabolism (CO2, for instance) could not build up dissolved tension because their pressure in your breathing gas is nil. So they would offgas continuously rather than accumulate.

Okay, end hijack, will consult the good doctor in a new thread.

-Bryan

 

[ucfdiver]

delete, wrong thread.

 

[CoralSeaMan]

It is worth remembering what the PADI tables (or any other recreational dive tables) are attempting to do. Their purpose is to give a set of rules that will limit your risk of decompression sickness without having to do "decompression stops." That set of guidelines "requires" a stop in some situations. If you do not do that stop, you are not diving under the model that the rules are created for. The effect is that any repetitive dives will not be made under the rules either. Another example is "Limit repetitive dives to 100 foot or shallower." If you do two 100ft. dives in a row, you are not following the rules undlying the tables.
This is simply a question of terminology. For "recreational dives" one does "safety stops." PADI actually call the stop you make if you have exceeded a "no dive limit" a "decompression stop."

 

[Walter]

It is worth remembering what the PADI tables (or any other recreational dive tables) are attempting to do. Their purpose is to give a set of rules that will limit your risk of decompression sickness without having to do "decompression stops." That set of guidelines "requires" a stop in some situations. If you do not do that stop, you are not diving under the model that the rules are created for. The effect is that any repetitive dives will not be made under the rules either. Another example is "Limit repetitive dives to 100 foot or shallower." If you do two 100ft. dives in a row, you are not following the rules undlying the tables.
This is simply a question of terminology. For "recreational dives" one does "safety stops." PADI actually call the stop you make if you have exceeded a "no dive limit" a "decompression stop."

Exactly. PADI is playing word games. If it's required, it's not a "safety stop."

 

[lamont]

I suspect it isn't possible to bend yourself by switching to O2. The whole concept behind O2 deco is that you can keep ambient pressure high(er) while maximizing the gradient. If you could bend yourself on O2, I can't imagine that they would use it for decompression sickness cases in the chambers.

It is possible to get oxygen bends, but not while scuba diving. At something very high ppO2s (something like 3.0, but i can't recall exactly, google might have the exact number), you saturate hemoglobin binding and oxygen begins to simply dissolve just like any other gas, and if you let off the ambient pressure you can get bubbles forming from the dissolved O2.