How much air for a CESA
- Thread starter Thanshin
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How come you're running out of air, anyway?!?Surely you learnt, right back in Scuba 101, to check your air gauge - didn't you?!? And to start coming up at 50 bar (or approaching the red mark, whatever system you use)? You did, didn't you? Thought so... So why stop, just because you've done a few dives? The maths is irrelevant - watch your gauge, act your age, and stay alive!
Do I really need to point you out to people with hundreds of more dives than both of us together and who have lived through an OOA situation?
Also, not speaking about situations that shouldn't happen doesn't make sense. It's like removing fire escape signs because you shouldn't start a fire in the first place.
Yhanshin is wondering 'what if the **** hits the fan'. I think that is a good thing. Knowone and Greyhoud have a fair point: don't run out of air.. But seriously, **** happens (blown o-ring, etc) and kudo's to Yhanshin for wondering and asking questions.
To answer the OP's question, in theory the remaining air will become available as the ambient pressure drops - but it is very limited. If you really want to know how much you'll get, take an extra set down to 100ft, breath it down till the last breath (and switch back to your normal air source), close the valve, ascent (make a safety stop) and see how much air you can get out of that cylinder at the surface. In theory it is 12l*3bar=36 liter, but YMMV It won't be 9 'deep breaths'. A deep breath is about 3-4 liter at the surface.. at an average pressure during ascent of 2,5 bar thats 7,5-10 liter each breath, so expect just a couple of shallow sips of air!
Oh, and just don't run out of air!
To answer the OP's question, in theory the remaining air will become available as the ambient pressure drops - but it is very limited. If you really want to know how much you'll get, take an extra set down to 100ft, breath it down till the last breath (and switch back to your normal air source), close the valve, ascent (make a safety stop) and see how much air you can get out of that cylinder at the surface. In theory it is 12l*3bar=36 liter, but YMMV
It won't be 9 'deep breaths'. A deep breath is about 3-4 liter at the surface.. at an average pressure during ascent of 2,5 bar thats 7,5-10 liter each breath, so expect just a couple of shallow sips of air! Oh, and just don't run out of air!
There are several things that have been missed here.
First off, if you are accending those few breaths you will get from the tank are going to last you a lot longer as you exhale....slowly. With the ambient pressure dropping around you, the air in your lungs is going to expand, giving you roughly 2 full lung worth the air to exhale for each 33 feet you accend. At a 30FPM accent rate, thats roughly 1 breath every 30 seconds, that is not too far off (around 20 seconds per breath) of my normal relaxed breathing rate. Rememeber- expanding air is the reason we exhale when doing a CESA. Next and IMO more improtant, it's pure BS (short of a reg failure) that you suddenly "run out of air",that simply does no happen. Your reg was warning you long before you are fully out that you were running low, you chose to ignore it. As soon as the tank pressure drops below the regs IP (135psi +/- a few) the reg gets progressively harder to breathe from with each breath. The last few are like sucking mud through a straw but you can still get them. If you are paying the slightest bit of attention to what your equipment is telling you and keep your cool, surfacing from 100ft with a very low tank is not a big deal.
First off, if you are accending those few breaths you will get from the tank are going to last you a lot longer as you exhale....slowly. With the ambient pressure dropping around you, the air in your lungs is going to expand, giving you roughly 2 full lung worth the air to exhale for each 33 feet you accend. At a 30FPM accent rate, thats roughly 1 breath every 30 seconds, that is not too far off (around 20 seconds per breath) of my normal relaxed breathing rate. Rememeber- expanding air is the reason we exhale when doing a CESA. Next and IMO more improtant, it's pure BS (short of a reg failure) that you suddenly "run out of air",that simply does no happen. Your reg was warning you long before you are fully out that you were running low, you chose to ignore it. As soon as the tank pressure drops below the regs IP (135psi +/- a few) the reg gets progressively harder to breathe from with each breath. The last few are like sucking mud through a straw but you can still get them. If you are paying the slightest bit of attention to what your equipment is telling you and keep your cool, surfacing from 100ft with a very low tank is not a big deal.
Reef_Haven
Contributor
Reef_Haven
Contributor
You will get a few breaths on the way to the surface, if you negligently ran OOA. But if it is from a catastophic air loss, you'll most likely have none.
R
redacted
Guest
Before you go to the bank with those extra breaths, you need to consider what failure mode precipitated the OOA. There are some failure modes (like a failed tank o-ring) that will not hold any pressure in the tank and others (like a catastrophic 1st stage failure) that will not let you access any gas in your tank. For some others (like a failed SPG) your thinking may be correct. If the cause you are thinking of is that you simply failed to monitor and manage your gas supply, then the best solution is to quit diving now.
beautybelow
Contributor
"then the best solution is to quit diving now."
I don't really think he wanted the cause of the OOA situation analyzed as much as he was curious about the physics of expanding air in a tank while ascending. He has less than 50 dives and he is just "wondering out loud" I think.
Maybe the best solution is to keep diving and learning.
I don't really think he wanted the cause of the OOA situation analyzed as much as he was curious about the physics of expanding air in a tank while ascending. He has less than 50 dives and he is just "wondering out loud" I think.
Maybe the best solution is to keep diving and learning.
These "what if" questions present more than an interesting math problem: they can distract for focusing on prevention of the scenario presented. It is obvious that the scenario presented is completely preventable through safe and competent diving practices. For that reason, though I would be a little less sarcastic in communicating it, I fell like Greyhound does. But more importantly, herman makes a great point: you are not going to be "suddenly" out of air. So enjoy the math problem, but also always: plan and follow a safe dive profile, monitor your gauges regularly, stay close enough to you buddy so they can be a source of survival, check your gear thoroughly before every dive trip, and again before every dive. Know and recognize the signs of a low tank- you can feel it. It's fine to know the extremes of an emergency self rescue. It's better to never put yourself in a position where you have to do it.
DivemasterDennis
DivemasterDennis
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