Which do you think is less dangerous at 160ft? Open-circuit air or CCR trimix?

[scottgrizzard]

So, in the dive shop, we were having this discussion about a deep cavern dive where the dive plan was 160ft... which is less dangerous?

Is it safer to dive with a little narcosis on a simpler system (sidemount on air with o2 deco), or is it safer to be clear headed on CCR (a rig that is trying to kill you)? (Assume the diver is trained and comfortable with both.)

Why the question: because helium is expensive, so open circuit trimix is not a realistic option.

I could not find statistics about this, so I am asking the question.

 

[Mobulai]

I got follow up questions:
- Is this a dive the diver is familiar with (and can handle well if stress arises), or a mew dive site for them?
- how comfortable is this diver with „lite“ narcosis?
-is current and/or physical exhaustion expected (co2 retention)?
- are we talking mCCR or eCCR?

In a way „been there“, but Air/AirDil no TMX; on a dive site (cavern too) I’m comfortable with; I’m very comfortable with „lite-narc“, even heavier narc (psychologically)
I’m not as comfortable on mCCR as OC (still a noob on that front) so there’s some offset/bias
But trying to be objective, I’d say messing up the predive checks (I know I fumbled it on that specific dive, but caught it with 1st min of dive; closed dil-out tank) is riskier for CC than OC

Edit: I’d say it all comes down to your narcosis tolerance and how well of a team/buddy you have

 

[Angelo Farina]

Well, a deep cavern dive at 160 ft (49 meters) is indeed a quite challenging dive, which definitely should be planned WITH DECO, a short one, say 10-12 minutes max of deco stops.
Being less than 50m, I am certified for doing it with deco using air, and that would be my choice. Simple things are usually safer. No gas switching, no complex setups, just a twin tank, or my 15-liters steel tank plus a pony. All plain air. Deco with an enriched mixture would save you just 3 or 4 minutes of deco stops: it is not worth the complexity and the added risks.
When young, I actually did some dives like this.
Of course, I would NOT like to do it at my age and in my current health conditions, this is something of the past.
My CCR certification is just with pure oxygen rebreathers, so I am not qualified for doing it using a tri-mix CCR. But even if I had got such a qualification, I see no reason for switching to such a more complex and dangerous system, when the dive profile is well within the possibility of being conducted with a much simpler and safer (and way cheaper) apparatus.
If it had been 180 feet, then of course this would not be feasible with air, and the choice would be between open-circuit and closed-circuit trimix.

 

[DiveLikeAMuppet]

Make the dive on OC air. Make the same dive on trimix with END around 25 meters - doesn't matter whether it's CCR or OC. You will never make the same dive on air, ever again, once you realise that you were narked out of your mind and how much you missed on the dive.

I think it depends on whether you can get taskloaded (gas density impacts narcosis), how dark it is and if you expect to solve a problem or just swim around.

 

[Angelo Farina]

Being narked is quite nice, for me. I enjoy the dive much more! Helium is quite depressive, in comparison...

 

[Mobulai]

Being narked is quite nice, for me. I enjoy the dive much more! Helium is quite depressive, in comparison...

I also like „lite“narced; heavy narced is a bit stressful in the dark
I’m yet to start with TMX so

 

[Johnoly]

I got follow up questions:

Ditto............
How long is the dive?
Because there are way more bounce dives to 160 on air/low mix than the "many bottom minutes" type of dives.
In particular, spearos and wreck-tie-in dives in the summer time in florida are almost considered a routine everyday dive. There's thousands of bounce dives to 160ft during the summer.

 

[InWay2Deep]

(a rig that is trying to kill you)

Reframe that as 'a rig that is trying to keep you alive' and then think about the question. A CCR is NOT "trying to kill you", it's a machine of which you are in control of.

 

[inquis]

A CCR isn't trying to kill you. Your lack of awareness does that. Could be a stuck solenoid on CCR, but it could be entanglement or poor navigation, not realizing you're stirring up a silt storm, etc. Since narcosis contributes to a lack of awareness AND difficulty with symptom interpretation and problem solving, my opinion is CCR+Tmx is far safer than OC+air. (This coming from someone who did lots of deep-air "back in the day".)

 

[rjack321]

The CCR doesnt care if you are in an overhead or not, the machine based risk is the same. And generally speaking not particularly extreme.
OC air in an overhead is a big deal. There's a reason this is a contributing factor to MANY cave deaths.

Bottom line... stay out of lower Orange Grove on air.