Breathing off the BC

[roakey]

Windwalker once bubbled...
Is this correct?

Close enough, but let me explain WHY to make it clearer.

The LP hoses (about 140psi or so above ambient) all come out of a common chamber in the first stage. These ports and hoses are designed to DELIVER gas in large quantities, so if a LP hose bursts, the pressure in the chamber and all the hoses are reduced to ambient.

The HP hose that your SPG is on doesn't have to DELIVER a lot of gas, it just has to pass enough that the gauge can sense the pressure. In all moderen regs, the HP hose is supplied by a very small pinhole that doesn't let much gas through, but the pressure downstream of the pinhole will build up and equalize in a second or two to match the cylinder pressure, which is what the SPG reads.

If the HP hose blows, all the escaping air has to pass through that tiny pinhole, which stops the cylinder from emptying very quickly, and allows HP air in the cylinder to keep the LP side correctly at 140 above ambient. At least until the cylinder pressure falls below that...

Roak

 

[Windwalker]

I got it, Makes sense. The whole concept of High pressure venting slower then low pressure sounded kind of odd, I was wondering how that worked. Thanks for the Excellent explainations everyone.

 

[fins wake]

... with so many different concepts introduced?

BTW, if you're OOA and ascending, and need to vent the contents of the BC, would it not be better to vent them into your lungs, where you can use the oxygen, rather than giving it to the fish?

This is the key issue which, with all respect, Genesis, shows that you don't know what you're talking about. A rebreather diver would never make the above mistake: When you're on the loop (whether BCD or proper rebreather), your lungs are part of it. This is in fact why the bouyancy doesn't change for a rebreather diver staying at the same depth. The diver's lungs are part of the equation.

So, I'll reiterate: When you ascend, the BCD gas content must be vented from the loop, that is, it must be vented into the water. Some less discerning readers may think this is what has been discussed all along. It isn't. Most BCD Rebreather Proponents (BCDRP:s) on this board have consistently argued for rebreathing the BCD on ascent.

There's a lot of waffle spouted about "trying this" and "experimenting with that". Well, when you move away from Internet diving to proper diving with a rebreather, you'll see what I mean. You must vent the surplus gaseous volume (generated according to Boyle's Law) into the water. Period. If you want to experiment with this, go ahead. Just don't blame me for any problems you may encounter.

So instead of performing the test, you choose instead to argue the point - without your own personal evidence in hand.

I'm getting used to this line of argument by now and a little bored by it. Look! I've explained that blown LP hoses underwater are very unusual occurences and I've repeatedly asked you to come with anything (even anecdotal evidence) to show the relevance of this to the thread. No sign of this ...

Instead we're treated to a technical treatise (which I will delve into when I have time, I promise. It may even be correct). The point of the matter is that newbie divers reading this out there do not have to worry about BCD breathing to counter a (virtually non-existent) "threat" of sudden and catastrophic LP hose blowouts whilst underwater.

That's all.

 

[Genesis]

First, to your message...

So, I'll reiterate: When you ascend, the BCD gas content must be vented from the loop, that is, it must be vented into the water. Some less discerning readers may think this is what has been discussed all along. It isn't. Most BCD Rebreather Proponents (BCDRP:s) on this board have consistently argued for rebreathing the BCD on ascent.

Of course gas has to be vented. Duh. You're ascending. If you were doing so OOA, you'd still have to vent gas from your BC. Whether you're breathing it, or rebreathing it, somehow some gas has to go in the water.

Now, given the base conditions of the situation:

1. You are completely unable to get gas from your air supply, and I'll posit a catastrophic failure of some kind, because I agree that if you are simply "out" you're really not, and will get one or two breaths during the ascent from the tank.

2. You are at some degree of depth, and have some volume of gas in your BC.

You get to choose between the following:

1. An ESA, or EBA, with no gas supply WHATSOEVER. That is, you do a "blow and go" - period.

OR

2. You either breathe, or rebreathe, the gas in your BC, releasing the appropriate amount into the water during the ascent.

I have already said (more than once) that the PREFERRED situation is to BREATHE the BC, not REBREATHE the BC. That is, you inhale from the BC, and exhale into the WATER. This accomplishes both giving you some gas AND venting contents as you ascend.

If you are at 99', for example, you have AT LEAST four times the volume of gas in your BC as you need at the surface to make yourself neutral. In reality, diving wet, you have much more, as you are also compensating for suit compression.

The reality of this situation is that you have SEVERAL breaths - not "rebreathes" - breaths - in that BC that you will have to vent during ascent.

Now again, during this ascent, do you wish to have SOMETHING to breathe or NOTHING to breathe? Pick one - those are your choices.

As for the risk of catastrophic failure being so low as to be unmeasurable, if you wish to believe that you're entitled to. Edited by NetDoc for content

I, on the other hand, prefer to give some thought to the various ways that I can buy it while diving and consider the potential "outs" to those problems BEFORE Murphy takes a dive with me.

 

[Otter]

Genesis once bubbled...
... Edited by NetDoc for content ...

And there were complaints about a t-shirt "mock-up" that was in poor taste?

 

[Genesis]

its a simple fact.

As for "poor taste", we have Pete and others here telling us that nobody will ever die from such a cause, since it simply never will happen that an LP hose will burst under water, nor will any other catastrophic failure ever beset anyone without warning, thereby making my comment about dances entirely specious - I'll never get the chance, according to the position of those on the other side of this debate.

A thing cannot be in poor taste if one would never get the chance to exercise it.

But back to these little things called "facts".... here are some more for 'ya.

What's your personal buoyancy shift on your normal tidal volume?

Check it. Its a couple of pounds, right? Maybe three of four, if you have REALLY well-developed lungs, right?

If you have on a measely 3mil full jumpsuit, it, plus a pair of booties, has a buoyancy of about 8-9lbs. I know because I own TWO full 3 mil suits, and they are both about that much positive, as proven by the weight I need .vs. what I need in just a swimsuit.

At least half of that buoyancy is gone at 99'. Heck, probably more like 3/4 of it is gone, but I'll be nice and say its only half.

Ok, so at 99' I have at minimum, not counting the gas shift in my tank, four COMPLETE respirations in my BC. I have the buoyancy at 99' to offset five pounds of (lost) buoyancy from wetsuit compression, and that volume of gas represents four to five FULL inhalations at the surface.

Now if I intend to do an ESA from that same 99', at 30fpm, I will need 3 minutes to get to the surface, right?

I can have one FULL breath every 45 seconds to a minute without (1) blowing my ascent rate significantly, and (2) running out of air.

This is without rebreathing at all - just inhaling from the BC and exhaling to the free water!

ANYONE can make an ESA from virtually ANY depth if they have one breath every 45 to 60 seconds! ANYONE! If you're willing to do that ascent at 60fpm instead of 30, you can have one breath every 15-30 seconds! That's approaching a nice, slow, relaxed and NORMAL respiration rate!

Now add in that air weighs about 1lb for every 19cf, and if this event happens with a half tank you have ANOTHER 4lbs of buoyancy in your wing, which means you have TWICE the amount of breaths I've claimed - in other words, at 60fpm you can have a breath every 10-15 seconds and not run out!

Out of air? WHAT out of air? You have more than enough in your BC to make an slow, reasonable ascent, simply by breathing it in and exhaling to the water.

So how come we find people "out of air" but with gas in their BCs? Probably because people listen to folks like Pete and believe that its all about dive planning and that one should not consider such "radical" ideas like breathing from any gas suplpy they may have - which happens to include gas they're gonig to have to vent on the way up anyway!

If you do not consider your BC a viable source of emergency air during an OOA event, I argue that you have simply lost your mind, and if you are found on the bottom dead with gas in your BC, you've won a Darwin award - and it wasn't due to the OOA incident in the first place.

It was for a dogmatic refusal to consider a perfectly viable gas source as available to you for that emergency ascent.

Since about half of the DAN accidents reported arise from OOA events, it seems obvious to me that telling people not to consider such a gas supply in an emergency is crazy at best.

 

[Otter]

Genesis once bubbled...
A thing cannot be in poor taste if one would never get the chance to exercise it.

Ok, So I can wear a t-shirt that says I dance upon my dead daughter's grave. Since I have no daughters, I would never get the chance to excercise it and therefore it wouldn't be in poor taste....

In any case, I don't want to go far off-topic on the BC breathing. Its an interesting concept that merits consideration. I suspect at 99' and OOA for whatever reason, I would try just about anything including sucking the air out of a fish's air bladder.

I pray I not get myself into that situation.

Otter

 

[The Chairman]

But in regards to;

As for "poor taste", we have Pete and others here telling us that nobody will ever die from such a cause, since it simply never will happen that an LP hose will burst under water, nor will any other catastrophic failure ever beset anyone without warning,

I gave it anywhere between a 0.5% to 0.001% chance of happening. That's a far cry from never. As for my poor taste, I am rather found of lawn flamingos... so what?

Since about half of the DAN accidents reported arise from OOA events,

and the major cause of these were what? Equipment failure? Blown hoses will probably make up the majority of OOAs on their list. What did you come up with?

 

[Genesis]

and the major cause of these were what? Equipment failure? Blown hoses will probably make up the majority of OOAs on their list. What did you come up with?

have pointed out, at the point you find yourself OOA at 100', it is rather irrelavent to make recriminations as to exactly why it has happened.

The more important point at that moment seems to be the question as to whether you will be shortly atoning before St. Peter (or Neptune, or for that matter, Beelzebub if you prefer) or not.

Since I prefer the option to be "not", I (and others, including the late Popeye) have pointed out that you have a quite-usable supply of air in your BC.

I have further pointed out that even in one of the most conservative of situations - that of a very thin wetsuit - you have at least four full breaths without rebreathing the BC between 99' and the surface.

This is if your tank is actually empty (that is, you've been dumping to adjust for a lightening tank over the course of the dive.)

If the cause of the problem is a failure when you had gas in the tank (e.g. first stage lockup, LP hose failure, etc) then you will have more gas in the wing - perhaps as much as 8-10 breaths.

This assumes you do begin ascending immediately, of course. In open water, why would you not?

Now Pete, let's be honest here.

Would you rather face an OOA from 100' with:

1. Zero breaths.

OR

2. 4-10 breaths.

Those are you options at the point the OOA occurs.

All the bandying about of how "all OOAs are avoidable" are pointless at the point you actually experience an OOA.

Its very nice to take the position you have, but its irrelavent to the actual discussion, which is that at the point an OOA has occurred, you have options available to you that the agencies have not taught you, and that some teachers (including yourself, specifically) will vehemently deny exist at all!

 

[The Chairman]

If I am at 100' depth and neutral would you say that I had 10# worth of lift in my BC??? Probably excessive for what I dive with but... 10/64= .15 cf ( I rounded) or...

(80/3000)*14.7= .39 cf. .39/.15= 2.6

Sooooooooooooooooooooooooooo, 2.6 times the amount of gas in my tank than I could even hope for in my BC, even if I have a generous 10# of lift to deal with (I wear no wet-suit, or a shorty at best).

By the time I reached the surface... the most I could get from my BC would be .45cf... but I could get a gauranteed 1.17 cf from my tank. I don't see the reasoning to get the "possible" .45cf instead of the definite 1.17cf.

And please... I didn't ban Popeye. I would have liked to but I didn't. I left THAT decision to another. If you want to play the pity party go to the appropriate thread, but your continual reference to it here has no bearing on the question in hand.

BTW, there is a slight flaw in my reasoning (can you find it?) and there is actually only twice as much air available from the tank as the bladder... (IF I had a full 10# of lift in my BC, which is doubtful).

To put it your way...

Would you rather face an OOA from 100' with:

1. a possible 0.6 cf. (bladder)

OR

2. a definite 1.2 cf (tank) which is already conveniently set up for breathing.

???