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

[old frogman]

The OP (and your quote) contradicts your assumption.

I am not sure what you mean. Are you trying to drag the discussion back to cave diving?

 

[old frogman]

Apologies to you all. It is about cave diving. Misread the first sentence. No further comments from me.

 

[Celt]

Apologies to you all. It is about cave diving. Misread the first sentence. No further comments from me.

What happened is some turned cavern into cave. The OP can correct me if im wrong but I believe he was referring to Buford Spring. The argument that air should not be used past 100 feet is just as ridiculous as arguing it should be used to 400 feet. But that’s what happens when you have people with no experience or training diving air deep condemning it because of what they read or heard somewhere. I’m talking in general not your personal opinion.

 

[iliketopetsharks]

Science moved past deep air decades ago, the only people who still cling to it are chest beating dilettantes. These days, every time you hear someone explaining why they dive deep air, it comes bundled with a healthy helping of mental gymnastics as to why they're choosing to dive what they dive. I think some people are just afraid of change.

Or people that need to rationalize their stinginess, knowing full well trimix would be the better choice...

 

[ofg-1]

Back in the stone age, 1970's, I worked at a commercial diving school. The school was run by ex Navy divers, so a lot of the things we did were right out of Navy training.

One of the exercises, we ran every student to 190' on air, I must have put 80 or so through this. We gave them simple puzzles and math problems to solve asked them to sign their name.

About 20% had no issue with the puzzles or problems. About 20% drooled, laughed or peed themselves rolling around on the floor of the chamber. All of the rest had varying levels of difficulties solving a problem or puzzle. Some could not spell their name. And this was a 10 minute bottom time.

I did Dipolder II once on air, the only thing I vividly remember is my sealed beam light imploding and having to come out on a 3 D cell U S Divers light. The rest is fuzzy except freezing my ass off at deco. We did a lot of of air dives. Most people did not carry O2, we did a metric ass load of air decompression. 160 feet on air was just another Tuesday night after work dive.

I wouldn't do it now for a couple of reasons. First, while I wasn't paying attention, I have turned into a old man. Second. tri mix is still relatively cheap. When you are willing to plop down 15k for a rebreather, 7k for a scooter, more for dry suit, heaters, misc other junk, complaining about the occasional $200 tri mix fill sounds dumb.

 

[CG43]

Science moved past deep air decades ago, the only people who still cling to it are chest beating dilettantes.

There are extreme cases where divers emerge alive with air from 150 m
and there are extreme cases where divers get into difficulties with air in only 30 m.
Both are unusual and should be investigated where possible.
How much is due to the individual diver and what is due to training and behavior that can be influenced by the diver ?
If someone wants to tell me that this investigatio has already happened enough
then I have some questions about examinations !
I guess there is still enough to investigate here.

edit : line 6 addet investigation

 

[old frogman]

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)View attachment 881993

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)

This is really cool.

No disrespect, just asking two questions regarding your risk register:

1. Is the risk register in context to the dive in the cavern or is it a generic risk register, that is, OC versus CCR?
2. Should the list of potential failure points on a CCR be longer than OC? Considering that OC is a simpler apparatus with fewer things to go wrong?
3. What about risk control measures? They would affect each risk rating.

I suspect the overall risk rating may be higher for CCR?

 

[Mobulai]

This is really cool.

No disrespect, just asking two questions regarding your risk register:

1. Is the risk register in context to the dive in the cavern or is it a generic risk register, that is, OC versus CCR?
2. Should the list of potential failure points on a CCR be longer than OC? Considering that OC is a simpler apparatus with fewer things to go wrong?
3. What about risk control measures? They would affect each risk rating.

No offense in asking, and thanks actually for following up — I just provide a framework but it’s far from usable in its current version

1. generic for sure - I treated it as an non overhead dive even, and input from the (entire) community is needed to populate the risks; I’m sure some insurance people (at DAN for eg) have more details on this; if you know a bean counter tag/ask them for info
as I mentioned the „failures“ list (for both OC and CCR) is not exhaustive, they were just examples

2. yes there are far more failures (points) modes in CCR, and it’s also not just the failure mode but the sub aspects: likelihood and severity of impact

3. risk control measures as in risk mitigation? Indeed, the familiarity of a user defines if a mitigate-able risk is high or low (as simple as the ability to clear masks, is that skill stable or not)
But.. it’s more complex than that, and than my ability to guess, some risks/failures are not mitigate-able but still user dependent

I am hoping that the list of failures can be expanded and a realistic range of risk values (likelihood and impact) be made available and defined for users to select from in their cases (for each failure mode); and from there each diver can fill it out (like a questionnaire) so that they can answer this question

I suspect the overall risk rating may be higher for CCR?

For themselves , for a specific dive plan/site

There’s no one result fit all in any case; when I was applying this to vehicle prototype tests, I have filled this risk assessment for the same route (and even sometimes with the same equipment+setup+software+conditions) while reaching varying results (overall risk level)
The same applies for diving, a dive someone did and is familiar with it to death, can still lead to death; it’s all probabilities

 

[inquis]

Should the list of potential failure points on a CCR be longer than OC?

Yes, there are more risks to consider on CCR. Likelihood and severity of consequence would need to be assessed for each, and mitigation put in place for any having an unacceptably high combination.

What about risk control measures? They would affect each risk rating.

Risk mitigation measures can impact the likelihood or the severity of consequence of the original risk. This lowers the risk score (since that is the combination of both). For example, the severity of a blown LP hose would be very high with just a single tank/reg. This is mitigated by running doubles, so the severity goes to low. Mitigation may also add additional risks (that's another hose that could blow), but the severity (and therefore the risk) is still low. Also, the likelihood that both blow is extremely low, but severity would be very high. Which is mitigated by a buddy. Etc. At some point, the likelihood is low enough to override the highest severity.

And example of a mitigation that impacts the likelihood is a depth alarm which reduces the likelihood your PO2 goes toxic on OC. CCRs have PO2 sensors and blinky lights for similar reasons.

 

[inquis]

I suspect the overall risk rating may be higher for CCR?

Depends on the specific scenario. FWIW, my comments early on in this thread were based on this likelihood/consequence view.

Also, don't conflate the number of risks with "overall risk". An airplane has a higher number of risks compared to driving cross country, but flying is much safer overall.