Question for the masters - TriMix / Manual CCR

[BlueTrin]

Do you really think the value of the lower PO2 as emergency diluent offsets the benefit of the higher PO2 when bailing out from a deco situation? Firstly if you are in a situation where you are out of diluent you might consider ascending.

I was wondering: if you really need the higher PPO2 to deco faster, shouldn’t you just bring another stage for accelerated deco?

 

[nickbutcher]

Please explain why you say that. I am not referring to the cave part because I get it but open water where direct ascent is feasible? Extraordinary claims require extraordinary evidence.

Having survived one ox-tox, 13 years ago, I'm in no rush to put myself in a situation where it might happen again.
I am more likely to bailout due to CO2 than any other reason and if I do, I want a low gas density and do not want to spike my PO2.
I will worry about deco when I'm shallow.

 

[wedivebc]

Having survived one ox-tox, 13 years ago, I'm in no rush to put myself in a situation where it might happen again.
I am more likely to bailout due to CO2 than any other reason and if I do, I want a low gas density and do not want to spike my PO2.
I will worry about deco when I'm shallow.

So was your tox situation a result of exposure to 1.6 PO2 for a short durations, or something else?

 

[Wibble]

Do you really think the value of the lower PO2 as emergency diluent offsets the benefit of the higher PO2 when bailing out from a deco situation? Firstly if you are in a situation where you are out of diluent you might consider ascending.

Need to compare two identical bailout plans, differing only in the PPO2 of the bottom bailout gas.

Example. CCR dive to 67m/220ft with 35 mins bottom time using setpoint 1.3 with 14/55 diluent on GF50:80 planned with MultiDeco. Bailout at end of bottom time.

MOD for 14% is 0.14 x 7.7ata = 1.08 PPO2
MOD for 18% is 0.18 x 7.7ata = 1.39 PPO2

• Standard CCR dive runtime is 129 mins
• Bailout using 14/55 and 60% — runtime is 173 mins
• Bailout using 18/55 and 60% — runtime is 166 mins

There’s 7 minutes difference with the higher 18% oxygen mix compared with the lower 14% mix. That is significant. However, that represents a surface gradient factor of ~84%, i.e. not much.

Using 18% will be too 'hot' for using the bottom bailout as offboard diluent on the bottom (PPO2 nearly 1.4).

Using 14% will match your diluent for offboarding and be less risky should you be bailing out due to high PPO2 and better if you’ve a CO2 hit, especially if caught early

One other issue with deep bailout being picked for PPO2 of 1.4 at the bottom is what about the scar below the wreck? The charts may say it’s at 67m, but the propellers/whatever may be in a scour a couple of metres lower.


Caveat: the above is for discussion only. The exact runtimes and gasses may vary according to the specific circumstances (solo, team, lazy shot, diver and other's skills, task, location, etc., etc.

(Chose 67m as that's 1.4 for the 18/45 standard gas. The depth of 2+:1 exaggerates the PPO2 differences.)

 

[wedivebc]

I think we are arguing a rather pedantic point. I prefer my dil to have a bottom PO2 of 1.0, and that is why I like a hotter bottom baailout. rjack321 claimed bailing to 1.6 PO2 was a bad idea. While your strategy is different than mine I am not convinced mine is a bad idea.
Can anyone cite a credible example (sport diving) of anyone toxing at 1.6 or less? I know of one case and if you look at it there were extenuating circumstances.

 

[lostsheep]

Why is the recommended PO2 1.4 for the working portion of a dive vs 1.6 for the resting/deco portion of a dive?

If you are on CCR and take a hypercapnia hit, are you working or resting?

I don’t think that a po2 of 1.6 is a problem by itself , it’s the other factors that led to the bailout, I.e., hyperventilating , exertion, etc that all combine to increase the likelihood of an ox tox event to unacceptable levels. You’ll probably get away with it but you’re decreasing your odds unnecessarily.

 

[Wibble]

Why is the recommended PO2 1.4 for the working portion of a dive vs 1.6 for the resting/deco portion of a dive?

Oxygen clock driving up the CNS during the working phase as you may be breathing far more deeply.

"Some people believe"** that you want to keep the CNS low when you're deep, but you can exceed the 100% CNS at deco.


** Have read that and inferred it from some people's comments including (if I remember correctly) some instructors.

 

[nickbutcher]

So was your tox situation a result of exposure to 1.6 PO2 for a short durations, or something else?

It was a result of some poor decisions.

I bailed-out due to hypercapnia and that was subsequently followed by too much open-circuit O2 at 6m (1.6 PO2). It's not something I would repeat now.

I have no idea what might have happened at depth if I'd bailed out to a high PO2 but my fate might not have been as fortunate as it was when I realised I was toxing at 6m.

I certainly would not have reduced my deco obligation by any substantial amount by spiking the PO2 in my deep bailout.

 

[Merida Cave Diver]

It was a result of some poor decisions.

I bailed-out due to hypercapnia and that was subsequently followed by too much open-circuit O2 at 6m (1.6 PO2). It's not something I would repeat now.

I have no idea what might have happened at depth if I'd bailed out to a high PO2 but my fate might not have been as fortunate as it was when I realised I was toxing at 6m.

I certainly would not have reduced my deco obligation by any substantial amount by spiking the PO2 in my deep bailout.

Hey Nick, what you said about poor decisions interests me as I think we all have experiences we learn from and don’t want to repeat.

What is it you do differently now? Was it related to dive planning, over exertion, etc? In my personal experience each dive I’m learning something new despite all the planning I do.

 

[nickbutcher]

Hey Nick, what you said about poor decisions interests me as I think we all have experiences we learn from and don’t want to repeat.

What is it you do differently now? Was it related to dive planning, over exertion, etc? In my personal experience each dive I’m learning something new despite all the planning I do.

The initial CO2 breakthrough was caused by over-exertion swimming along the shot line to the wreck (at 66m). There was a lot of current below 50m.

Although the actual dive on the wreck was OK, I was aware of the heavy breathing and CO2 on the ascent and bailed-out. With hindsight I should have realised it was just breakthrough from over-breathing the scrubber and not a complete exhaustion of the lime. I should've returned to the CCR once I'd got my breathing back under control and started my staged deco.

However, I stayed on OC and I caned the O2 for about 30 minutes at 6m with no air breaks. It was at this point that I realised I was toxing. With 13 years more knowledge and experience, If I'd had to stay on open circuit I would now creep-up shallower and take some air breaks to reduce the PO2 and the CNS load.

At the time, I wasn't aware of the risk of high PO2 after a hypercapnic event. I subsequently discussed the ox-tox with Simon Mitchell at the 2010 Eurotek conference and he was pretty sure that the initial CO2 breakthrough I'd experienced was a key factor.

With all that extra knowledge, I'd have done lots of things differently.