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

[broncobowsher]

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

I try to. With a rEvo it is pretty easy.
Rotate the scrubbers and rarely will I get into the 2nd scrubber on a single dive.

 

[rjack321]

I try to. With a rEvo it is pretty easy.
Rotate the scrubbers and rarely will I get into the 2nd scrubber on a single dive.

Maybe in warm water, its pretty tough to have 2hrs of extra scrubber on any unit in Lake Superior at 2C.

 

[Nick_Radov]

Part of your CCR training is to turn off your oxygen and swim around to see just how long it takes for your loop to become problematic. It’s a lot longer than you think it would be.

Sure, at depth with a normal setpoint there's a lot of O2 in the loop and it takes a while to metabolize down to a hypoxic level. But the real risk seems to be starting the dive with the O2 valve turned off and plain air in the loop. Obviously that diver error is easily prevented with proper training and procedures, and yet there have been a number of deaths. Several years ago one local diver jumped in with his O2 off, swam on the surface to the anchor line, passed out, and drowned.

(I'm not arguing in favor of OC deep air.)

 

[justinthedeeps]

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

You're right, current rebreathers we don't know how many more hours each scrubber can be breathed safely in order to finish a prolonged dive. Even rEvo rMS is educated guesswork (but admirably so!)
It would be a correct decision to move onto open circuit deco gas (if available)

That said, at rest (~0.8 L/min O2 metabolism), most scrubbers are going to last far longer than most operational guidelines of 3 to 4 total hours. Hero/legend divers out there have breathed scrubbers calmly for some remarkably long durations, and none of the ~2.2+ kg scrubbers I've taken to 6+ hours have shown any noticeable signs of CO2 intoxication, even in cold-ish water. Onboard CO2 sensors beyond just the rMS promise to make this even easier to gauge. Beyond some thresholds it could end up like the 'narcosis tolerance' debate, except that once there is an expired channel, there is not much time to react safely (reportedly).

But I think we can take heart in that diving a trimix CCR+BO does objectively add additional layers of operational safety and contingency over OC-only diving.