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

[Celt]

Mistake on OC due to nitrogen narcosis is more likely to kill you.
Messed mix switch, O2 taken deep enough, a lot of it.

I haven’t heard of a single case of someone dying at 60m or less because of narcosis.

Has Rebreather Diving Gotten Safer?

The view among many industry insiders, Pre-RF4, was that rebreather diving has gotten safer—an improvement from the estimate that CCR divers were 5-10x more likely to die than their open circuit counterparts, which was put forth at RF3 (2012). Afterall, aren’t most people doing their pre-dive...

indepthmag.com

 

[macado]

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.

I was not going to reply to this thread, I started writing something then deleted my draft but then I figured why not?

Too many variables to honestly fully answer this question. I assume the cavern is Buford based on the location but we can call it at 160ft cavern dive that is technically a cave. I've dove there a number of times. I guess I'll just come out and say it; I feel no need to lug a rebreather there for what amounts to an extended cavern dive but to each their own. Why go to 160ft there? Just because?

How much rebreather experience does the person have? Or rather, how much actual dive experience does this person actually have? How long is the dive? Is it a bounce dive? Are they cave trained? Are they in good health?

Being on a rebreather does not make the dive inherently safer especially if the person in question does not have the proper experience or training. I really hate the phrase "rebreather is trying to kill you." The rebreather does not know anything. It doesn't think or feel. It isn't trying to kill anyone. It's trying to keep you alive based on the parameters set. Most rebreather accidents are user error or poor judgement. The rebreather didn't wake up one day and decide it didn't like you and wanted to kill you.

Why do you consider open circuit helium at 160ft (48m) too expensive and not a realistic option? Compared to what? A $9000 rebreather, $1500 MOD 1 course and $2000 MOD 2 course all to save $100 dollars on a trimix fill?

While I think rebreathers 100% have their place it is not a simple answer. If it's just say you dove 160ft at Buford, then add a little helium to your backgas and call it a day. That was literally what recreational trimix / helitrox courses were designed for. One also does also not need to go 160ft at Buford.

You could literally dive something like 21/25 (a helitrox / recreational trimix) that would not cost you a lot of money at all which has an END of around 100ffw. I'm making this number up but to fill a set of LP85s with 21/25 would probably cost you around $200. You'd need to do a lot of 160ft dives to pay for a rebreather and the training.

I'm saying this as a person that has owned 4 rebreathers. A rebreather is a waste for a cavern dive unless you are training or building hours. I do not think this makes the dive any safer, in fact, it opens a whole other range of issues one could have.

 

[Bowers]

I've dove to the bottom of Buford (meets the description of your cavern) on air and trimix. Trimix is better if you want to remember the dive in detail lol. A 25/25 trimix would only cost just over $100 to fill a couple al80s. Still a much cheaper dive than a rebreather and better prices than any charter dive I'm aware of. So I guess I'm not sure why helium in open circuit is out of the question?

 

[inquis]

I feel no need to lug a rebreather there for what amounts to an extended cavern dive but to each their own.

I'd agree with that. Light trimix, way less of a filter, and I think that's the smart approach for someone who is not a CCR diver for that 160 ft dive. In other words, OC+Tmx is safer than OC+air (similar equipment risks, but overall lower risk due to better awareness & reactions). However, I'd say the CCR is still safer than either (again, assuming competent training/operation). That's intentionally a comparative assessment (and the question posed in the OP), rather than on an absolute scale.

The one that's "less safe" may still be "safe enough" -- that's always going to be a judgement call.

 

[hroark2112]

Here’s my opinion.

OC air - if you run out of air, you’d better hope your buddy isn’t too narced and is paying attention If you get entangled or lost you have a finite time to figure it out.

CCR - if you run out of diluent, you’d better hope can surface easily, you won’t need it anyway. If you run out of oxygen, you’d better hope can use SCR mode to make it to the surface easily. If you really screw up and flood your rebreather, you’d better hope switch to your bailout and head for the surface. If you get lost or entangled, you’d better hope have lots of time to figure it out with your rebreather.

Anyone who believes a rebreather is trying to kill you is misinformed. I have hundreds of hours on both mCCR and eCCR units and can tell you that unless you make serious mistakes or have significant equipment malfunctions, no unit is doing anything but keeping you alive. 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.

 

[justinthedeeps]

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 [with appropriate trimix diluent]? (Assume the diver is trained and comfortable with both.)

One vote for Yes, CCR with trimix should be safer for divers specifically trained and experienced in its use for that environment.

It's not just a better experience, with advantageous gas characteristics, and better decompression, that avoids wasting large quantities of helium.

It adds a whole additional breathing source, on top of everything that already existed on the OC setup, which is still carried as bailout.
Two is better than one?

CCR is not safe for divers who:

• didn't receive or abide by adequate training,
• exceed limits of training and recoverability,
• are impatient, cut corners, skip checklists and pre-dive checks,
• omit bailouts and failsafe ways of using them,
• don't maintain their equipment,
• screw around with the wrong gases for the dive,
• choose or encounter diving conditions inappropriate for the diver or the config,
• over- or under-trust fallible devices (sensors, regulators, CMFs, solenoids, HUDs, gauges, comps),
• don't adequately monitor their oxygen pressure or ppPO₂,
• fail to acknowledge or act on signs of a compromised rebreather,
• are prone to panic or distractions,
• wouldn't have composure to instantly flush, bail out, or control breathing while sinking or shooting up with an unbreathable or deadly loop (favors drownproofers/freedivers?),
• believe the rebreather lets them massively bend the normal rules and limits,
• dive with other people or mixed groups who might compromise their safety awareness,
• ...

Most of these apply to all diving, only a few contribute specifically more to CCR risk.

Re: stats the InDepth article is good, but where are the corresponding open circuit data for comparison?

 

[crofrog]

assuming properly trained and qualified ccr diver. Oc air is in my opinion significantly riskier

 

[crofrog]

Risk encompasses likelihood of a failure AND severity of consequence. You're correct that there are more things that could go wrong (equipment-wise), but you seem to be missing the fact there are far more options to handle failures if they do. In OC terms, it's like comparing manifolded doubles vs single tank: yes, doubles increases the probability of equipment failure, but the reduction in severity of consequence vastly outweighs that.

Risk also includes the diver, not just equipment. While you may be able to function adequately while narced, that's not the norm. (I also believe even you would handle issues better/more quickly at 60 ft vs 160 ft, lessening the chance of a cascade/spiral of issues.) The self-awareness goes down on deep-air, and the likelihood of (non-equipment) problems goes up. The severity of consequence goes up. That means risk goes up and safety goes down.

It also needs to include the ability to detect and recover from the failure.

In some ways cc gives you more options however the the 3Hs are significantly harder to recover from and detect.

I’m OC if you analyze your gas and do the gas switch correctly. The likelihood of any of the 3Hs is very very low. Sure you could have a freak hyperoxic event at lower than 1.6, or you could skip breath and become hypercapnic. On CC there could be failures that are either easy to miss, not displayed to user correctly, or erroneous that could easily result in hyperoxia or hypoxia, and the chance of being able to recover from one of those failures is very low and the consequences is likely death. Hypercapnia is easier to dectect once you’re far enough down the road, but even then the recovery from it is isn’t easy and might outstrip available OC gas. Those risks only exist on CC

 

[rjack321]

I haven’t heard of a single case of someone dying at 60m or less because of narcosis.

Has Rebreather Diving Gotten Safer?

The view among many industry insiders, Pre-RF4, was that rebreather diving has gotten safer—an improvement from the estimate that CCR divers were 5-10x more likely to die than their open circuit counterparts, which was put forth at RF3 (2012). Afterall, aren’t most people doing their pre-dive...

indepthmag.com

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.

 

[nadwidny]

HPart 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.

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