Where does that extra breath or two come from?
- Thread starter CaptainHornblower
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KeithG
Guest
what is this OOA thing you talk about? Don't dive so deep! Or swim so fast?
We have just returned from 2 weeks of vacation diver diving in Roatan and Utila. We stay shallow and do not swim. Ever.
It took several dives at each dive op for the DM to understand that our response to their air queries were correct. They started out by asking all of us vacation divers to actively signal when we were at 1500psi (half tank, time to turn around?). This quickly produced confusion as nobody signaled the DM. Ever.
The profiles were shallow and none of us were hovers....hoovoers? hooovers? sh*t i wish i could spall.
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LOL giffenk . . .so true about dm waiting for the 1500 signal. 
Jack Hammer
Contributor
A regulator can only deliver down to ambient pressure. This is why you can fully drain the tank via the reg on the surface, but if you tried it at 100' there will still be some gas remaining that is undeliverable. The expansion in the hose exists, it is minimal.
mi000ke
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Trying to make sense of this on a practical level. So, do I have this right?: Say I'm at 100 fsw, then I will be "out of air" at 135 psi (reg IP) + 45 psi (ambient) = 180 psi. And for every 33' I ascend I'll make about 15 psi available via normal breathing. Now let's assume my SAC is 25 psi/min . At 100 fsw my consumption would be 100 psi/min. If I ascend to 66', my consumption will be 75 psi/min and I will have "freed up" 15 psi. So would that give me 15/75 x 60 seconds = 12 seconds of air (or about 2 -3 breaths)? And If I use those breaths to ascend (rather quickly, I know) to 33' then my consumption will fall to about 50 psi/min and I'll have freed up another 15 psi, or about 15/50 x 60 seconds = 18 seconds (or about 3-4 breaths), at which point I could do a quick (but probably not quite CESA) ascent. I'm just thinking worst case where for whatever reason there was no air buddy around.
I think on a practical level you can't count on that kind of precision performance. Any given regulator may or may not be able to deliver that proverbial "extra breath or two." Trying to describe the phenomenon underlying the "extra breath or two" is interesting as a theoretical exercise, but I don't think there is any practical use. As a practical matter, don't breathe your tank down below what you need to be safe.
mi000ke
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Amen to that
LariatAdvance
Registered
Partially from the hose and partially from your lungs.
As you (should have) learned in your OW cert class, the air you breathe at 100 feet is DENSER than the air you breathe at 20 feet. Your reg supplies your air at the ambient density. The average persons lung capacity is 1.6 liters. As you (should have) learned in your OW course, "At a constant temperature, the volume of a confined idea gas varies inversely with it's pressure". So if you fill a balloon with air at 60 feet and tie it off, then ascend up to 10 feet, the balloon will be larger in size (volume). How did that "extra" air get into the balloon as you ascended? It didn't. It was already there.
You won't see this affect inside your SCUBA tank (not anywhere near recreational diving limits, anyway) as it is a rigid capsule not affected by Boyle's law so far as we're concerned. I don't know how many miles down you'd have to go with a SCUBA cylinder to reach it's crush depth. A SCUBA tank with 2,000 PSI at the surface still has 2,000 PSI at 130 feet. Likewise, if you suck your tank empty and you have zero PSI at 130 feet, when they find your body later that day on the surface, your tank is still gonna have zero PSI in it. Your lungs don't work the way the solid steel rigid tank does because your body is not a rigid capsule.
This is why they put dump valves on lift bags. If you filled a lift bag full at 130 feet and connected your 50 pounds of gold bars you just discovered, and sent it up to the surface, the expanding gas in the bag would burst the bag it when it got near the surface and your gold bars would come falling back down on you. When they find your body floating on the surface with a crushed skull, they will be shaking their heads and saying, "He should paid attention to Boyle's Law in class and used a lift bag with vents on it instead of that cheap lift bag without vents." The bag has to have a way to vent off gases as those gases expand in volume. Once the pressure reaches a certain PSI, the spring valves on the bag allow some of the gas to escape. Enough gas escapes to prevent bursting, yet it retains enough gas to remain buoyant.
That "extra breathe" you are talking about didn't get in there. It was already there. The density of the air in your lungs decreased as you ascended, so the air expanded, filling your lungs. At 66 feet it took twice as much air to fill that 1.6 liter space as it did at the surface (this is why you consume air faster the deeper you go). You don't have any more air in your lungs than you did when you started your ascent. The air you have in there is just less dense.
Actually Boyle's law DOES affect your rigid SCUBA tank, just not anywhere near the depths we dive. Submarines have a crush depth, as they are basically steel or Titanium tubes just like a SCUBA cylinder. Moments prior to crush depth failure Boyle's Law will be experienced by the soon-to-be-deceased crew as the outer hull fails and the pressure hull fails. The crush depth of a nuclear ballistic submarine is classified, but I can assure you at that depth Boyle's Law will be so fast and violent you won't realize that you just died.
As you (should have) learned in your OW cert class, the air you breathe at 100 feet is DENSER than the air you breathe at 20 feet. Your reg supplies your air at the ambient density. The average persons lung capacity is 1.6 liters. As you (should have) learned in your OW course, "At a constant temperature, the volume of a confined idea gas varies inversely with it's pressure". So if you fill a balloon with air at 60 feet and tie it off, then ascend up to 10 feet, the balloon will be larger in size (volume). How did that "extra" air get into the balloon as you ascended? It didn't. It was already there.
You won't see this affect inside your SCUBA tank (not anywhere near recreational diving limits, anyway) as it is a rigid capsule not affected by Boyle's law so far as we're concerned. I don't know how many miles down you'd have to go with a SCUBA cylinder to reach it's crush depth. A SCUBA tank with 2,000 PSI at the surface still has 2,000 PSI at 130 feet. Likewise, if you suck your tank empty and you have zero PSI at 130 feet, when they find your body later that day on the surface, your tank is still gonna have zero PSI in it. Your lungs don't work the way the solid steel rigid tank does because your body is not a rigid capsule.
This is why they put dump valves on lift bags. If you filled a lift bag full at 130 feet and connected your 50 pounds of gold bars you just discovered, and sent it up to the surface, the expanding gas in the bag would burst the bag it when it got near the surface and your gold bars would come falling back down on you. When they find your body floating on the surface with a crushed skull, they will be shaking their heads and saying, "He should paid attention to Boyle's Law in class and used a lift bag with vents on it instead of that cheap lift bag without vents." The bag has to have a way to vent off gases as those gases expand in volume. Once the pressure reaches a certain PSI, the spring valves on the bag allow some of the gas to escape. Enough gas escapes to prevent bursting, yet it retains enough gas to remain buoyant.
That "extra breathe" you are talking about didn't get in there. It was already there. The density of the air in your lungs decreased as you ascended, so the air expanded, filling your lungs. At 66 feet it took twice as much air to fill that 1.6 liter space as it did at the surface (this is why you consume air faster the deeper you go). You don't have any more air in your lungs than you did when you started your ascent. The air you have in there is just less dense.
Actually Boyle's law DOES affect your rigid SCUBA tank, just not anywhere near the depths we dive. Submarines have a crush depth, as they are basically steel or Titanium tubes just like a SCUBA cylinder. Moments prior to crush depth failure Boyle's Law will be experienced by the soon-to-be-deceased crew as the outer hull fails and the pressure hull fails. The crush depth of a nuclear ballistic submarine is classified, but I can assure you at that depth Boyle's Law will be so fast and violent you won't realize that you just died.
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