Does 100% O2 on deco provide a greater benefit than other deco mixes?

[Beau640]

^^^^

This here. It's a theoretical reason to keep your stop on 100% at 18-20ft rather than ascend to 10ft. Can't do this with a leaner mix. Deco for divers by Powell has some more information about this specifically.

 

[Kevrumbo]

One "special property" of 100% O2 vs others is that you offgass equally quickly regardless of your depth. So once you switch, your deco time will be the same whether you do all remaining time at 20ft vs moving up to the 10ft stop once cleared to do so. The theoretical advantage there is that if there are micro bubbles in the system there's a better chance they will stay micro if you stay at 20ft, whereas moving to shallower depth with lower ambient pressure could trigger them to start growing.

It's not free though- toxicity of course starts to become a concern when staying at PPO2 1.6 for a while.

Here is where depth/ambient pressure while using O2 comes into play.
If you surface with pathological DCS type 1 symptom causing bubbles, going back down to:

3m/10' ~bubble size reduces to 91% of the pathological size at the surface;
6m/20' ~bubble size reduces to 85% of the pathological size at the surface;
9m/30' ~reduces to 80% (max recommended depth for O2 IWR with a ppO2 of 2.0).
And for reference of note:
18m/60' ~reduces to 70% (Recompression Chamber Table 6);
50m/165' -55% (Recompression Chamber Table 6A breathing Eanx50 or Heliox 50/50).

So within the give & take of O2/CNS Toxicity exposure time, nominally staying at 6m/20' on O2 for the entire shallow stop profile to keep those bubbles small and off-gassing efficiently, and then a slow 1mpm/3fpm ascent to surface is a prudent deco strategy.

 

[tbone1004]

One "special property" of 100% O2 vs others is that you offgass equally quickly regardless of your depth. So once you switch, your deco time will be the same whether you do all remaining time at 20ft vs moving up to the 10ft stop once cleared to do so. The theoretical advantage there is that if there are micro bubbles in the system there's a better chance they will stay micro if you stay at 20ft, whereas moving to shallower depth with lower ambient pressure could trigger them to start growing.

It's not free though- toxicity of course starts to become a concern when staying at PPO2 1.6 for a while.

not just toxicity, depending on your dive profile and length you can also burn your lungs with the high PO2's and that reduces offgas efficiency. Huge part of why CCR divers typically run PO2's closer to 1.0 vs the 1.4 that most OC divers use

 

[crookshanky]

...you can also burn your lungs with the high PO2's and that reduces offgas efficiency...

Do you have any resources on this?

 

[tbone1004]

Do you have any resources on this?

I'll dig through rubicon later today to find some papers written on it.

our internet is **** here, but look up stuff on pulmonary oxygen toxicity

 

[The Chairman]

20ft and 70ft for everyone else.

Yeah, I rarely dove 50% (twice?) and it's even less likely now that I dive a rebreather.

 

[kotik]

not just toxicity, depending on your dive profile and length you can also burn your lungs

Pulmonary is a form of toxicity, as well as CNS. I imagine the OP most likely isn't doing that much deco yet

 

[kotik]

I'll dig through rubicon later today to find some papers written on it.

our internet is **** here, but look up stuff on pulmonary oxygen toxicity

Yep, google and wikipedia are to the rescue on this. There is a nice summary with references:
Oxygen toxicity - Wikipedia

 

[The Chairman]

Do you have any resources on this?

This is the basic physiology of pulmonary toxicity. Oxygen irritates your lung tissue causing them to increase their surfactant (like phlegm), making it harder for you to off or on gas all gasses. It doesn't burn it, but it does irritate it. This is problem with neonates that are kept in pure o2 for a prolonged time and that's only at 1 atm.

 

[victorzamora]

at what Po2 ?

Nominally 1.6, the standard for PO2 on deco. Technically O2@20ft is 1.6060 and EAN50 at 70ft is 1.5606.