Go here http://www.scubaboard.com/showthread.php?s=&threadid=32399 Have at it. All those posts that you "claim" are missing are right there.
Breathing off the BC
- Thread starter Genesis
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fins wake
Guest
I actually second that. I disagree strongly with almost all of Popeye's incoherent ramblings on BCD breathing, and I find his debating style questionable to say the least, but I'd rather see his opinion countered by measured argument than to see him run off with no way to respond in kind.
(This isn't about personalities and prestige, although it would appear Popeye does see it that way. It's about dive safety.)
Get that, everyone? Okay?
Now, I will make another effort to discuss the dive points made in the thread.
Absolutely not under the circumstances discussed in this thread! What are you advocating, swimming around on the bottom breathing from the BCD?
You are ascending to free air! It's an OOA situation. If you ascend that gas volume carried will increase! The same, BTW, goes for diving rebreathers and ascending. Again, none of the people who constantly - and erroneously - refer to rebreathers in this thread actually dive them. I do.
But the ambient pressure will vary with depth! And greater ambient pressure makes it harder to suck the air. In fact, at depth it will be impossible. (It's easier to suck a straw at one foot than at two feet. At depth it is virtually impossible.) So again, you really have to ascend as fast as you can. Nothing wrong in my statement to that effect at all.
EDIT: Added comments to Genesis' post.
What did I miss? When and why was Popeye kicked off the board?
Yes, you are ascending to free air. And as you do the gas will expand in your BC. It will do so regardless of whether you are (re)breathing it or not, at exactly the same rate.
The re-breathing of your BC is a nullity in terms of its impact on your buoyancy - the number of molecules of gas involved do not change. You still must vent some of the contents of your BC on the ascent, and whether you do so by exhaling it (not re-breathing) your BC or by releasing it to the water it must be done.
The claim I was responding to was that re-breathing your BC would "change your buoyancy". It will not, as the gas volume does not change as a consequence of that act. A depth change will cause a volume change, but that change will be the same irrespective of whether you are using the BC as a counter-lung or not.
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?
That has been the point all along.
Not material. You are not trying to suck a straw from the surface.
If you have a blown LP hose, you will be unable to get a material volume of air from your regulator. Again, if you do not believe me, do the test.
How tough is it to take a tank that you're about to have refilled, jack on your regulator set, unscrew one of the second stages, and test this for yourself?
Why is it that instead of actually doing the test (I have) that you folks would prefer to argue an (invalid) point?
The claim is that with a burst LP hose you can:
1. Breathe a materially significant amount of gas off the reg (assuming that its not the hose to that reg that has burst)
AND
2. The tank contents will last a materially-significant amount of time (enough to make that emergency ascent and thus provide you that air) with a burst LP hose.
I have pointed out that with a burst LP hose the IP in the remaining LP hoses will be very close to zero, and that while you CAN get air through your reg in that situation, it won't be much - perhaps not enough for you to consider it "air" for the purposes involved.
I have also pointed out that the dump rate of the tank will amaze you in that circumstance with its speed.
Finally, I have given you a very simple and easy-to-duplicate test if you do not believe these assertions.
So instead of performing the test, you choose instead to argue the point - without your own personal evidence in hand.
Why?
Windwalker
Contributor
Genesis, Please excuse me I am trying to understand the LP vs HP venting problem. Let me paraphrase and see if I have the Idea..
The First stage regulator can only allow a finite amount of pressure past in order to maintain ambient. In the event of a LP hose failure, Most if not all that pressure will be vented out the Hole, and very little will be available to the Intact hoses, due to the limitations of the Primary stage and its flow capacity.. where as, when a HP hose burst, Air is still being made available to the LP side while the Ambient pressure is less then the pressure in the tank.
Is this correct?
The First stage regulator can only allow a finite amount of pressure past in order to maintain ambient. In the event of a LP hose failure, Most if not all that pressure will be vented out the Hole, and very little will be available to the Intact hoses, due to the limitations of the Primary stage and its flow capacity.. where as, when a HP hose burst, Air is still being made available to the LP side while the Ambient pressure is less then the pressure in the tank.
Is this correct?
Davey Diverson
Guest
I'm gonna do this. I believe you, for the record. I just like experimental evidence.
It's simpler than that - since the job of the LP side is to deliver plenty of air to breathe even at low tank pressure, the hole is big. On the HP side the job is just to deliver the pressure to the SPG, so the hole is tiny.
Imagine a bucket full of water - punching a small hole in the bottom, where the pressure is greatest, will allow the bucket to empty much slower than if you just pour it out the top.
E.
What we're actually solving here is a problem in fluid dynamics.
The test I provided, that of unscrewing the hose at the end of a reg, is the best case, as it provides the most resistance (linear length of hose through which the air flows before it comes out the end of the hose.)
Note that "superflow" hoses are WORSE in such a circumstance, as they have a larger inside diameter!
As an example, a 7' primary that breaks at the regulator will provide much more resistance and more IP than a hose that breaks at the regulator swege.
Unfortunately, the most likely places for a LP rupture are at the regulator swege. This is close to identical in effect to simply removing one of the extra LP port plugs.
The problem is two-fold:
1. The regulator is a feedback device. As the pressure drops below the IP on the secondary side, the metering device opens more widely in an attempt to raise the IP. This continues until the flow device (piston, etc) is fully open - at that point the regulator will deliver gas limited only by the internal resistance to flow through the HP components.
2. Regulator flow rates are rated at their IP. They can deliver higher volumes than their "rated" flow rate (MUCH higher!) into a lower IP.
3. Your second stage requires the IP to help overcome the valve spring resistance, and/or to open the poppet. This is why a stable IP is important to good regulator performance. At a lower IP (especially a drastically lower one) the regulator may not deliver much air at all. You can find out exactly how bad this can get by removing a second stage from its hose and attempting to inhale from the open inlet. Its possible, but I doubt you could get enough air this way to avoid hypoxia - and that's when you're calm. You won't be when a hose explodes.
4. If you stick an IP gauge on the BC hose during the test I posted, you will see pressures that are EXTREMELY low. As in 20psi or less in many cases!
5. This is the BEST CASE test (hose off at the regulator.) If it fails at the SWEGE at the regulator, the effective IP in the rest of the reg will be very close to zero. The most likely place for such a failure is in fact at the regulator swege.
A HP failure is nowhere near as bad because (1) the hoses all have a very small orifice at the end where they screw in, and (2) most regulators, if not all current production ones, have a very small orifice INSIDE the recess where you screw in the hose. Therefore, a HP burst will cause a lot of bubbles BUT as the orifice operates as a "sonic orifice" (once the speed of the gas going through reaches the speed of sound no additional flow occurs, even with higher pressure) the flow rate is severely limited.
You actually have quite a while with a burst HP hose before you run out of air - more than enough to make a clean ascent. Its noisy and there are a lot of bubbles, but the actual flow rate isn't very high. Nor does such a failure impair the operation of the regulator's second stages, at least not until the tank pressure drops below the IP.
With a LP failure you're screwed, as not only is the IP gone (rendering your regulator useless or close to it) but the flow rate is VASTLY higher.
Try it with a tank that you've used and need to refill (say, one that has 600-700 psi in it) and time how long it takes before its very close to empty (don't run it all the way down or your shop may refuse to fill it without a VIP)
You'll be shocked at how fast that needle on your SPG drops, and how difficult it is to breathe off the remaining second stage - if you can draw any meaningful amount of air through it at all.
Hey Genesis... never make explanations simple when they can be made complex and wonderful!
E.
E.
Charlie99
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Genesis has given an excellent, although somewhat technical description.
A simple analogy I've used before when discussing this scenario is to compare the 1st stage to a partially open water faucet, with two sprinklers attached. In normal situation, there is enough pressure to drive both sprinklers. If you cut one of the hoses open, or even just remove the sprinkler head, the pressure at the other one drops dramatically. Same thing for your 2nd stages when a hose blows.
A blown HP hose is more like the pipe that feeds the garden valve springing a very slight leak through a pinhole in the pipe. The slight leak upstream from the valve doesn't materially change the water pressure to the valve, and therefore has no effect on the sprinkler.
I hope my analogy hasn't further muddied the waters.
A simple analogy I've used before when discussing this scenario is to compare the 1st stage to a partially open water faucet, with two sprinklers attached. In normal situation, there is enough pressure to drive both sprinklers. If you cut one of the hoses open, or even just remove the sprinkler head, the pressure at the other one drops dramatically. Same thing for your 2nd stages when a hose blows.
A blown HP hose is more like the pipe that feeds the garden valve springing a very slight leak through a pinhole in the pipe. The slight leak upstream from the valve doesn't materially change the water pressure to the valve, and therefore has no effect on the sprinkler.
I hope my analogy hasn't further muddied the waters.
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