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

[justinthedeeps]

Rather than continue to rehash the deep air debate - at what depth does CCR become safer than OC Trimix?

On a trimix rebreather, if you got stuck (or overstayed) on the bottom for twice as long as planned, you in most ordinary cases would still never run out of gas or runtime, even if you needed to do 2 more hours of deco.

Similarly, if you needed to re-descend to alleviate minor DCS symptoms, or handle some issue, or give assistance, there is almost no risk of out-of-gas when repeating or extending stops on nearly unlimited runtime.

One could completely lose all of the 'deep' offboard/open circuit bailout gas in regulator/manifold failures etc, and still have a chance to safely, slowly ascend, decompress, and finish the dive normally (provided overly buoyant empty cylinders are jettisoned)

On a long CC deco ascent, maintaining a high/deco setpoint during the ascent in many cases assures that you are always breathing the optimal deco mix at every single deco stop, instead of spending many of the stops breathing a suboptimal fixed mix on a limited number of open circuit cylinders. (But intermediate diluent switch/flushes may be required)

 

[justinthedeeps]

Another nice analysis in that Fock paper is an estimation that it is ~23 times more likely to experience some kind of CCR equipment issue, versus an OC twinset.

But since CCR divers carry bailout, the CCR+BO system is nevertheless about ~7 times [mechanically] 'safer' from all-systems / dive-ending failure

 

[rjack321]

Rather than continue to rehash the deep air debate - at what depth does CCR become safer than OC Trimix?

Probably more that the CCR risks become smushed together with OC risks and there is no "safer". It's not like Doc deep would have survived on CCR either.

 

[rjack321]

On a trimix rebreather, if you got stuck (or overstayed) on the bottom for twice as long as planned, you in most ordinary cases would still never run out of gas or runtime, even if you needed to do 2 more hours of deco.

2 hours... Twice as long as planned could have you in the water 4, 6, or more hours extra in the water.

ain't no big deal when you ran out of scrubber an hours ago and don't have the BO either.
@Dsix36 overstayed in Eagle's Nest once for a lost buddy and it was not a good time.

 

[justinthedeeps]

2 hours... Twice as long as planned could have you in the water 4, 6, or more hours extra in the water.

ain't no big deal when you ran out of scrubber an hours ago and don't have the BO either.
@Dsix36 overstayed in Eagle's Nest once for a lost buddy and it was not a good time.

You're right about what you are saying here, but my post specifically says 2 more hours of deco though

 

[Celt]

Air to 50m - why? Because some time ago people used to do it; now technology, knowledge, etc., have evolved, but those who started like that do not want to adapt - and I am sure they won't. Again, I believe it is mostly fine if they understand the risk(s); not really if they underestimate them though...

It’s not used to do it anymore, you just won’t read it on Scubaboard, the story “I tried a bounce to 200 feet and I was narked out off my head “ is becoming common. You can’t dive air like that, it has to be done gradually and consistently deeper and longer. The term, you plan the dive and dive the plan is the backbone of deep air diving. To me bouncing to 200 feet at the weekend is exactly the same as someone buying a rebreather on the internet and saying it tried to kill him when used at the local quarry.

 

[Angelo Farina]

Rather than continue to rehash the deep air debate - at what depth does CCR become safer than OC Trimix?

According to my CMAS training and certification, the boundary is 50 meters.
For depths less than 50 m, air is recommended (it is not "deep", for CMAS deep air is 50 to 60m, still allowed, but not recommended anymore).

 

[ginti]

It’s not used to do it anymore, you just won’t read it on

Let me rephrase it. Some time ago it was the only way.

Now more modern approaches exist, they are widely recommended by scientists, but are more expensive - trimix. Those who are still following the old school (NOT recommended by scientists) often don't want to adapt to avoid the extra cost.

Agree on the other part of your post, but it still is unreasonable to me. PS I actually hate being narced and I avoid air and nitrox below 30 if I can dive with my rebreather. I never go beyond 35 without helium - never, really... But that's just me

 

[Celt]

Let me rephrase it. Some time ago it was the only way.

Air was never the only way, Heliair, Heliox and Trimix but the price of helium and availability has made them unaffordable, that leaves a rebreather, people who are already diving to 45m on air won’t spend the time or money on a rebreather to get them to 60m for a handful of dives in the summer so they go there on air. I met four others last season on the wrecks between 55 and 65 meters diving air, something I haven’t seen in 15 years.

 

[rjack321]

You're right about what you are saying here, but my post specifically says 2 more hours of deco though

Do you always have a couple spare hours on your scrubber?