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.

 

[Wibble]

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.

The RMS (Revo Monitoring System, aka TempStick on AP Inspiration) can be a little intermittent. The sensors do fail (I've got another failure now which I need to get a replacement)

However, RMS is a "nice to have" feature that shows all's working (OK, when the sensors work) and gives you a warm feeling that the world's all calm. I've learned that they do fail and if I suddenly see the timing drop from 3 hours to zero in red, then I think "am I breathing heavily"; if no then it's a sensor problem.

Good reminder for me to order the replacement sensor now... And a spare cell. And an O-ring service kit. And another $500 bites the dust.

 

[justinthedeeps]

I met four others last season on the wrecks between 55 and 65 meters diving air, something I haven’t seen in 15 years.

Probably still common wherever helium isn't cheap or easy to get.

Easy to say "we'll just stay near 45-50 metres and watch" on the Aikoku Maru, and end up overstaying at 55-60 metres on air, feeling like there is absolutely no problem with that when the narcosis kicks in. And maybe there isn't but it's still not optimal. Might as well swim thru this deck level while we're here eh?

 

[CG43]

Easy to say "we'll just stay near 45-50 metres and watch" on the Aikoku Maru, and end up overstaying at 55-60 metres on air, feeling like there is absolutely no problem with that when the narcosis kicks in.

Years ago I read that the US navy already at 30 m depth measured a reduction of mental abilities on air to 70% compared to 1 bar. . This also coincides more with my experience that it is more of a continuous process that is already present at about 30 m. Let's assume that at a depth of 60 m on air the mental abilities are still 35 % compared to 1 bar.
In order to assess the resulting dangers, one must know which skills are affected and compare this with the requirements of the dive.
Experiments show that the diver on deep air reacts slowly and that his memory functions poorly.
In my experience, balance, spatial orientation, navigation and tactile sensitivity of the hands are reduced. Likewise, the feeling of cold is reduced and there are certainly other abilities that decrease.
Most of the time, the deeps causes a pleasant euphoric feeling, but especially in darkness, cold, poor visibility, high ppCO2 or dangerous situations, the resulting feelings can be intensified.
In the end you have more than one limit for air deep :
Fore exaple :
Under no circumstances ever deeper than 70 m
Outside the Aikoku Maru max. 60 m
Within the Aiko Maruup max. deck 4 = 55m
Today because I slept badly max. 50 m

 

[Celt]

Probably still common wherever helium isn't cheap or easy to get.

Easy to say "we'll just stay near 45-50 metres and watch" on the Aikoku Maru, and end up overstaying at 55-60 metres on air, feeling like there is absolutely no problem with that when the narcosis kicks in. And maybe there isn't but it's still not optimal. Might as well swim thru this deck level while we're here eh?

People who plan dives to 50m and then go to 60m are careless and have no discipline. I’ve never done it and actually never could as I always plan to the deepest part of a wreck. In the case of the Aikoku Maru I’d plan the dive to 64m. I think that poor discipline might be a result of blindly following a computer. I’ve left some nice stuff on the bottom as I ran out of time to get it on a bag.

 

[ginti]

Years ago I read that the US navy already at 30 m depth measured a reduction of mental abilities on air to 70% compared to 1 bar. . This also coincides more with my experience that it is more of a continuous process that is already present at about 30 m.

If you join a project in shallow waters, you'll see how even at very shallow depth our mental abilities massively decrease. I did quite some projects in shallow caves (depth less than 10m) and, really, our problem solving abilities are extremely low compared to ambient pressure. Problem is that rec diving doesn't involve problem solving, so we don't realise it. Until we have an accident and we need to solve the unexpected problem.

 

[Wibble]

For some reason my mental capacity halves as soon as I get into the water. Maybe I’m just thick.

However, knowing that does go some way to helping to diagnose and resolve problems. Training and repetition is the main counter.