Last Deco Stop Depth, Oxygen Window, and Deco Efficacy

[Superlyte27]

I was thinking about this too tonight so I'm glad you mentioned it. Thinking about this in relation to rebreather divers, I'm under the impression that a set point of 1.3 is common among RB divers. I'm also unaware of any air breaks for RB divers. So, if you're on 1.3 for most of the dive (excluding surface and initial descent, and then catchup on ascent) and aren't taking air breaks, you're effectively at 100% O2 at 10ft right?

So is an air break required if you're on a lower ppO2 like this, or do you guys actually get off the loop for air breaks or lower the ppO2 temporarily? Self-thread hijack!!!

First, it's impossible to get a 1.6ppo2 on a rebreather at 10'. So, no air breaks needed. It's pretty hard to keep at 1.6ppo2 at 20'.

 

[rjack321]

Richard, the empirical concept that George is discussing is not "wrong" per se --it also depends how you tactically AND CORRECTLY apply it. Sometimes even within a prescribed general deco profile, you have to tailor it to your own individual physiology (and George clearly points that out in the lecture above).

For instance some years ago, the GUE rule-of-thumb of diving 30/30 triox like NDL Eanx32 utilizing MinDeco/ascent rules resulted in the "chokes" for me (residual Helium microbubbles overwhelming my lungs' alveolar bed), after two repetitive dives to ave depth 24m with a 90min SIT. I now always use Oxygen for deco on the last repetitive dive when using recreational triox or any leftover Trimix, even on a "no decompression limit" dive above 30m. . .

btw, That other incident of yours Richard in 2008: something about --If you're coming off a bottom mix with an FN2 of 31% ["leftover 19/50 mix"], why did you choose an inefficient deco mix with a higher FN2 of 50% [EANX 50] for the entire decompression profile strategy (or for that matter -even if you did imply adding an Oxygen switch later in the profile)? Do you finally understand the additional dynamic factors of greater Nitrogen Solubility vs faster Helium Diffusivity along with mitigating Oxtox concerns in repetitive decompression diving?

Well George is wrong because about 45 to 90 mins after the dive is when your peak bubbling occurs, so the whole idea that you are "clean" is just a fallacy.

This was not the repetitive dive I got bent on, I just didn't feel that great afterwards. And there was no O2 left on this trip. 50% is what I had.

It was a great dive too, 140ft vis was amazing.

 

[Kevrumbo]

Well George is wrong because about 45 to 90 mins after the dive is when your peak bubbling occurs, so the whole idea that you are "clean" is just a fallacy.

This was not the repetitive dive I got bent on, I just didn't feel that great afterwards. And there was no O2 left on this trip. 50% is what I had.

It was a great dive too, 140ft vis was amazing.

Alright & fair enough . . .such is the nature of the Beast & the Bitch (GI3 and DCS respectively). But a simple DCS type I event can be treatable either with conventional Hyperbaric Oxygen Therapy or In-Water-Recompression with a full face mask O2 regulator; an Ox-tox incident in-situ however always has the risk of DCS/AGE, near-drowning or drowning & death (all acute and/or critically-grave conditions). Better to be conservative and moderate ppO2 & CNS exposure especially during repetitive multi-dive days with mandatory staged decompression.

On my month long Truk & and overseas tech wreck trips, I usually don't do more than two deep deco dives per day, and having at least a three hour Surface Interval in between to degrade that O2/CNS exposure (as well as further unloading/off-gassing all that N2 from my deep air bottom mix ). For local liveaboard charters using double AL80's on techreational repetitive dives with shorter SITs here in SoCal homewaters, I now only do multi-level NDL profiles starting at 39m to 45m max (i.e. on a reef wall or pinnacle and then spend the majority of the total dive time in the shallows of the Kelp Forest around 12m), with the option to do my "Safety Stop" either on air backgas or Oxygen.