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Author Topic: What are the causes of Out Of Air (OOA)  (Read 6294 times)

runez

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Re: What are the causes of Out Of Air (OOA)
« Reply #20 on: 26 Nov 14, 00:14 »
When I first learnt how to dive, I understood that some reserve would be good for emergencies etc. At that time, 50 bar (or 1/4 tank) seemed like a reasonable number and I didn't question further.

When I learned about gas planning - surface consumption rate (SCR) / respiratory minute volume (RMV), rock bottom gas (aka minimum gas) etc, I wondered to myself why the industry adopted "50 bar" as the de-facto gas reserve.

Gathering some inspiration from snickrs's dive planning, but a tad (or a lot..) more liberally, if we assume 2 divers @ RMV=30L/min each, ascending at 9m/min and performing the 3min @ 5m safety stop; the rock bottom gas is approximately:

    10m: 30 bar
    20m: 50 bar
    30m: 70 bar

There's already some buffer catered within these calculations, e.g. the divers' RMV would probably be less than 30L/min, PADI's recommended ascent rate is 18m/min and divers in an emergency may skip the safety stop.

I believe most "deep diver" courses which certify divers to 30m (e.g. AOW, Deep Diver etc) do cover a little bit on gas planning. Probably not enough, as I certainly don't remember much about gas planning from my own AOW course. But interestingly enough, if we consider the rock bottom gas estimations as above but without the safety stop, the rock bottom gas is:

    30m w/o safety stop: 40 bar

IMO, for the recreational scuba diver, 50 bar is a nice compromise between "easy to remember" and "not an excessive amount of reserve". It's also a whole lot easier for the divemaster to brief "Surface at 50 bar on every dive!". Keeping it simple does have its advantages!

Personally, I adopt a more conservative set of rock bottom gas reserves based on a different ascent strategy, but is that to say that the whole industry is wrong with the standard "50 bar reserve gas"?

Anecdotal evidence could prove 50 bar to be sufficient with this question: If 50 bar isn't enough, why aren't we seeing more cases of injured divers who ran out of gas? It does appear that the 50 bar "rule" is keeping most divers safe :)


Edit: Changes made regarding the details about AOW / Deep Diver Specialty for accuracy, thanks to blurblur for pointing it out!
« Last Edit: 27 Nov 14, 23:48 by runez »
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poh6702

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Re: What are the causes of Out Of Air (OOA)
« Reply #21 on: 27 Nov 14, 17:32 »
I would like to put this analogy, most of us have served Army, when the CO say, he wants to inspect the bunk on Friday, OC will pass the instruction to be ready by Thursday, PC will want all to be ready by Wednesday and so for...

When the industry set 50Bar, 18m/Min, 3min safety stop, I am sure there are already safety margin built in. And if we are still putting extra margin ourself, we all end up with very short dive or everyone must carry 2 tanks.

We should follow the industrial recommendation, should not break it, then we are already in the safe zone. Whoever are more Kiasi can add more margin themselves, it is up to individual.
« Last Edit: 27 Nov 14, 21:24 by poh6702 »

blurblur

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Re: What are the causes of Out Of Air (OOA)
« Reply #22 on: 27 Nov 14, 23:19 »
I believe most "deep diver" courses which certify divers to 30m (e.g. AOW, Deep Diver etc) do cover a little bit on gas planning. Probably not enough, as I certainly don't remember much about gas planning from my own AOW course. But interestingly enough, if we consider the rock bottom gas estimations as above but without the safety stop, the rock bottom gas is:

    30m w/o safety stop: 40 bar

Sorry just wanna point out that the "deep dive" portion covered in the PADI AOWD course should be seen as an experience dive.

After passing your AOWD (deep dive portion is mandatory to pass AOWD), PADI recommends a 30m max depth (not 40m) and contingent maximum of 40m.

On the other hand, the PADI Deep Diver Specialty course will allow the diver to dive to a maximum 40m. Among the syllabus for the deep diver specialty is to carry a redundant air source (eg: pony bottle - not spare air).

Cheers.
"Diving is perfectly safe as long as you remember how dangerous it is" Mark Powell.

runez

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Re: What are the causes of Out Of Air (OOA)
« Reply #23 on: 28 Nov 14, 00:05 »
Sorry just wanna point out that the "deep dive" portion covered in the PADI AOWD course should be seen as an experience dive.

After passing your AOWD (deep dive portion is mandatory to pass AOWD), PADI recommends a 30m max depth (not 40m) and contingent maximum of 40m.

On the other hand, the PADI Deep Diver Specialty course will allow the diver to dive to a maximum 40m. Among the syllabus for the deep diver specialty is to carry a redundant air source (eg: pony bottle - not spare air).

Cheers.

Thanks for pointing it out! I've edited my original post, hopefully it's more accurate now! I didn't mention 40m though..

I did a quick search, and it seems that the standards for PADI's Deep Diver Specialty course do not require a redundant air source? (but maybe instructors can choose to include it if they want?) I'm basing my information off here and here.
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lovelyinn

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Re: What are the causes of Out Of Air (OOA)
« Reply #24 on: 18 Feb 15, 01:01 »
Thanks for pointing it out! I've edited my original post, hopefully it's more accurate now! I didn't mention 40m though..

I did a quick search, and it seems that the standards for PADI's Deep Diver Specialty course do not require a redundant air source? (but maybe instructors can choose to include it if they want?) I'm basing my information off here and here.

when I did my deep dive, we did not have any pony tank.
The only tank we had was the safety stop tank which was hanging by the boat at 5m.

normally when I am diving 30-40m, when I hit 100 bar or NDL, I would start to move to 20m range.

also most of the time, I will be diving ard 20m range. I used to like to dive deep, but not now already... unless there is a wreck, then it is a diff case.

normally I would listen to the dive brief and get to know the dive site and etc, then would discuss with my buddy for the dive plan.

moonyeap

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Re: What are the causes of Out Of Air (OOA)
« Reply #25 on: 02 Apr 15, 17:27 »
Thanks for all the information in this thread !

globalcookie

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Re: What are the causes of Out Of Air (OOA)
« Reply #26 on: 21 Sep 16, 15:59 »
Old but interesting thread.  I thought of reviving this thread to share my own experiences.  We learn through mistakes of others or ourselves.

I agree with Alvin Ho when he mentioned, "I would say most OOA situations are due to diver error. equipment failure is seldom the issue when you read dive accident reports. ".

There was 1 incident of equipment failure that I came across.  The hose burst during the dive, and air escape rapidly.

Coming to my personal experiences, I had 2 OOA situations in the last 2 years.  Sounds stupid but here's the experiences.

1.  Cebu Moalboal (Feb 2015)
It was my own mistake.  One word, STUPID!

In short, I was down to about 50 bars, when we came to this small cave.  There were 2 teams ahead of us, and instead of signalling to the guide that i'm down to 50 bars, I was stupid.  In short, after that cave, I was down to about 20 bars.  I should have immediately told the guide, again, stupidity.  We were going shallow, so I presumed we are going for safety stop.

At safety stop, I was OOA.  First thought, worse case scenario, shoot up.  My buddy happened to face me when I was OOA and I signalled.  Due to the initial panic, I took the spare, stuffed it into my mouth and breath!  All the precaution learnt were forgotten, I drank 2 mouths of water.  Threw that spare, took back my own (very very limited air at most) and drank another mouth of water.  Yes, I panicked, I freaked out, I was scared!  Luckily, the guide was facing me then, and quickly took his spare and purge water lodge in it, and passed to me.  I was on deco too!

That was the stupidest dive!  But that was the dive that I learn diving all over again!  This could be the scariest moments in my life.  I was almost dumbfound after that dive for the next 2 hours (but I did a last dive for that trip after that).

2.  Bali, Padangbai (Sep 2016)
This one was fully aware!  The dive was at Padangbai, Jetty.  It was shallow dive, about 14 meters at most.  The whole dive lasted (including safety stop) 84 mins.

From the first bad experience, all dives thereafter, were filled with precautionary.  I do constant gauge checks (every 5-10 minutes), dive computer checks, etc.  As I approached 50 bars, I was consciously checking gauge every other minute.  During safety stop, OOA happened again. 

Well, experience... so I was a lot more calm, took the spare regulator of my buddy and complete the safety stop.  Throughout the safety stop sharing of a single take, my eyes were glued on the gauge and my dive computer.

------------------------

In theory, OOA feeling is, we need some effort to breath (air doesn't flow out readily).  But in actual, if one haven't experience it, how does it feel like?  Try obstruct your lips/mouth with your fingers and breath.  You should feel certain resistance.

------------------------

Early this year, while doing Deep Diver course, my instructor had highlighted the importance of Deep Stop.  I wonder if many divers are aware of this (experienced divers, or those who really read/learn would know)?  So far, I have not encounter any dive where were were consciously doing deep stop.  Perhaps the guides had conducted deep stop but not told divers.  It is us, divers, who should be conscious than expect the guide to do it.

Any dives beyond 20 meters, should come with a deep stop (half way mark) for 1 minute.  This is a habit I practice since my Deep Diver course.  When I did 40m, my instructor then, told me, we will do 2 deep stop, before doing the usual safety stop.  So yes, anyone who isn't aware, do practice deep stop in your future dives.