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

[hroark2112]

Unless you start ascending, especially in the shallows, then it's a lot quicker than you think it would be.

The idea of the exercise is to watch how long your loop lasts, not to observe how PO2 changes with a change in depth, so you’re supposed to maintain a constant depth.

 

[justinthedeeps]

The idea of the exercise is to watch how long your loop lasts, not to observe how PO2 changes with a change in depth, so you’re supposed to maintain a constant depth.

A curious stunt and probably still even tossed into some courses, but it seems like a bad idea in practice. It probably shouldn't be done or taught, not even as a demonstration or curious experiment. Minutes are like seconds, if there are sufficient distractions or task loading.

Idk maybe you are just saying watch it go from 1.3 to 1.0, but why

Just believe your instructor when they tell you there is typically enough oxygen left in the loop to take your time carefully bailing out or doing some other life-saving drill that restores oxygen flow or ppO₂.

Better training and safety point to promptly switch to another breathing source (e.g. BO) if you know that oxygen flow to loop has ceased. There are too many things that could happen on a real dive in between the time you discover this and the time you pass out.

[Manual additions via MAV to maintain setpoint is different, you do not sit there watcing ppO₂ drop]

The whole 'turning off the O2 and swimming around' needs to be scrubbed from courses, and I haven't seen this personally but have heard people have drowned doing this, on courses no less. This is just terrible protocol when divers should be busy MAVing, plugging an offboard O2, or bail out

 

[hroark2112]

A curious stunt and probably still even tossed into some courses, but it seems like a bad idea in practice. It probably shouldn't be done or taught, not even as a demonstration or curious experiment. Minutes are like seconds, if there are sufficient distractions or task loading.

Idk maybe you are just saying watch it go from 1.3 to 1.0, but why

Just believe your instructor when they tell you there is typically enough oxygen left in the loop to take your time carefully bailing out or doing some other life-saving drill that restores oxygen flow or ppO₂.

Better training and safety point to promptly switch to another breathing source (e.g. BO) if you know that oxygen flow to loop has ceased. There are too many things that could happen on a real dive in between the time you discover this and the time you pass out.

[Manual additions via MAV to maintain setpoint is different, you do not sit there watcing ppO₂ drop]

The whole 'turning off the O2 and swimming around' needs to be scrubbed from courses, and I haven't seen this personally but have heard people have drowned doing this, on courses no less. This is just terrible protocol when divers should be busy MAVing, plugging an offboard O2, or bail out

I’ve never seen any report of a diver dying in a course doing that drill. If you have a link to any event like this I’d love to see it.

That being said the instructors I have done the drill with are far beyond your (and my!) expertise. The drill is done in confined water, such as Blue Springs, with the instructor monitoring the drill. At no time is the student’s vision impaired during the drill so the student can monitor their PO2. You never go below a PO2 of .5 and the first thing you do when you approach .5 is turn the O2 back on.

Some students learn well by listening to their instructor. Some students learn better by actually experiencing things for themselves. An instructor telling me “You have a lot of time” means nothing to me, watching it drop with the oxygen turned off gives me actual experience and is a valuable lesson.

 

[justinthedeeps]

Points well given @hroark2112.

In context of explaining how CCRs aren't immediately trying to kill the diver, your original points are well placed

The O2 off experiment seems about as kinky as seeing how long we can actually breathe a loop with no scrubber in it, which should also never be done deliberately, but some of us have done it anyway out of empirical interest. For me that was at least a few minutes into a new/clean BC wing, on land. Would not do such things underwater

 

[Celt]

Probably because all the cavern divers making bad choices on deep air died cause the lost the line, kicked up a silt cloud, wandered into the very much cave overhead, or made a navigation error. None of us were present to see how narcosis contributed to their death. Since you have no experience on CCR and presumably no cave training either, perhaps you might wanna sit this one out instead of opine on matters outside your wheelhouse.

This is the internet, you don’t get to decide who posts. If you want a private debate organise it in your sitting room.

 

[broncobowsher]

O2 off at depth, why?

Education. A sudden O2 failure (solenoid quit in a full eCCR for example) is NOT a sudden problem. Just a slow gradual decline in PPO2. You know what the problem really looks like.
Education. You can stay on the loop with a failed O2 (for a fair bit of time anyway). This isn't like OC where you can't take a next breath when you get a sudden failure.

This is why you get proper CCR training. You get an instructor that teaches, and watches over you while getting trained. This one training exercise is 00.1% of the education you get from an instructor.
You learn what failures can happen and how to work with them.

 

[sea_ledford]

So it seems that all the replies from divers that are cave and CCR trained are saying CCR is safer.

Anyone saying otherwise is conflating the safety of the dive on OC with the training/cost required to become a CCR diver.

The question wasn't - Which is safer, doing a deep cave dive on air on OC that I'm familiar with, or borrow a ccr from someone and do it on trimix, but no other training?

If that was the question, I'm sure the answers would be quite different!

 

[inquis]

seeing how long we can actually breathe a loop with no scrubber in it, which should also never be done deliberately

Disagree, again for the sake of education, and this can be done safely on land. O2 flush and obviously monitor the PO2, add if you get near 0.2. (Doubtful you will get that low before you tap out.) It showed me what the hypercapnic symptoms feel like, and I'm better prepared to recognize them if they ever occur in the future. It also will tell you if you are one of the non-responders.

 

[girlwithbigtanks]

Statistically safer air at 160 in a cavern or ccr tmx ? That’s your question OP?
How many ccr tmx dives vs air to air to 160 in an overhead?
I bet there is currently more active diving on tmx (and OC tmx) vs air dopes in overhead.
Having been on a dive in a cave on OC tmx and buddy on air to 145’, air buddy super doped out.
(Not sure if you are taking the overhead environment into account when considering narcosis, why not throw in some actual cold water to the equation?)

The ccr gives you options to solve problems that OC simply is incapable of. You have to weigh the know risks with the ccr , to the problem solving benefits it has, vs OC , to decide which tool is best for the job.

Once you have a CCR with trimix you have ALL the choices. OC air, OC mix, or CCR. Your dive sounds like a one-and-done. Big Dumb Hole (like Hells Bells or ICBM silos in Wa. Id dive OC sidemount with ean 32. Take my time coming back up. Not worth packing the ccr.

 

[Celt]

People should have a look at some videos of Buford Spring a straight in and out free diving cavern that some need a CCR+trimix. Plenty on YouTube