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

[PEDiver]

Based on your location, I'm assuming your example refers to Buford (which has a "cavern" section going down to 110' and a no light section going to 160' which a lot of people still consider a cavern).

Since I've done this dive on both air OC and on CCR with trimix, I would say I felt much more comfortable on CCR

If I had to do that dive over again, it would either be on OC trimix or CCR trimix. I would never do that again on air. Even with a CCR, which I will grant adds complexity, you are probably still better off bailing out to OC trimix in the event of an emergency than doing the same dive on OC air.

 

[sea_ledford]

CCR every time. Also, there is no such thing as a cavern at 160'.

 

[Mobulai]

The input from more experienced divers here made me rethink you question from a slightly different angle, not just the physiological aspect

There’s a "protocol" associated with risky endeavors, be that safety related software, driving a prototype vehicle.. that might be extended/adapted to diving: HARA (hazards and risk assessments)
I used to do that before testing new software in a vehicle (and that’s where I’m extrapolating from)

Think of every aspect of the dive that might go wrong for both OC(air) and CC(tmx) and classify risk levels for these various categories, and the likeliness of that to happen.

Now we make a list of all failure scenarios that are possible; both for the OC and the CC dives, and see for that specific situation (dive, diver, equipment...) what is the likelihood & severity; and we add the weights of that up

the point ofcourse is to create also mitigation actions. eg. a closed Dil tank, or a loop that's installed the wrong way round on a CCR have very high risk; maybe low chance of occurance; but a good predive check + prebrathing + doing 5 minutes at 2-5m checks (IDK just made that up.. but given my previous fumble makes sense to me) are good mitigation strategies against that

templates for risk martices are available online, could be a nice community project to try and adapt one for diving actually

lets try and populate the list of risks for both versions of the dive (non exhaustive, just to demonstrate the idea, filled with arbitrary guesses):
(feedback, additions, and corrections are encouraged not just welcome)

	OC	CC
floodings	N/A	low chance; intermidiate risk
workload @ high gas density	high risk, moderate chance	low x low
thermal losses	mid risk x high chance (depends on deco length & GF choices)	negligiable x low
Nacosis + task loading	high risk x ? chance	low x low
wrong NoTox	intermediate x low(50 deco gas assumed)	n/A (or low x low, equivlence to setpoint awarness)
Entangelment?	...	...
(Other) Equipment failures?	...	..
Total weight (ie total risk assesment)	...	...

But ballparking it from the input of others, sounds like OC version is overall High, CC is moderate to low risk (realative to diving in general )

Edit: I have some suspicion that instructors and divers with a military background might already have some version of this, please enlighten us (as muxh as you’re allowed to)

 

[broncobowsher]

About the only time I would take air over CCR for 160' would be to set or recover an anchor, a bounce dive. And that mostly comes down to not having to deal with PPO2 all the way down and back up.

 

[johndiver999]

Based on your location, I'm assuming your example refers to Buford (which has a "cavern" section going down to 110' and a no light section going to 160' which a lot of people still consider a cavern).

Since I've done this dive on both air OC and on CCR with trimix, I would say I felt much more comfortable on CCR

If I had to do that dive over again, it would either be on OC trimix or CCR trimix. I would never do that again on air. Even with a CCR, which I will grant adds complexity, you are probably still better off bailing out to OC trimix in the event of an emergency than doing the same dive on OC air.

WOW 160 there is a cavern dive?

 

[PEDiver]

WOW 160 there is a cavern dive?

The cavern technically ends around 110’ and the no light zone (cave) extends to around 160’ if you dig in the silt.

 

[PfcAJ]

Go read Exley’s Blue Book.

Avoiding excessive depth is literally on the list of basic cave diving risk mitigation tools, along things like lights, gas management, and line.

 

[AJ]

This one is easy for me. Overhead or deeper than say 30 metres = CCR. I consider my CCR saver than OC under these circumstances. I still do dive less deep or non overhead with OC, but it is becoming more of an exception than an rule.

 

[Celt]

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.

Obviously anyone that dives a CCR is going to say it’s safer.

 

[Celt]

If you put in the time and training to go to 60m on air equivalent to the time and training it would take to go to 60m on CCR+trimix your safer on OC.