OUT OF AIR - Last ditch effort breath from you BCD

[Lead_carrier]

Consider that if you run out of air at 99 fsw (4 atm). By the time you rise up to 66 fsw (3 atm) you will have about 0.4 cubic feet of air to breathe. This is about a breath for most people. SOOOOOOO, every atmosphere that you ascend you will have yet another breath from that "empty" tank.

NetDoc, are you saying that the air will expand in a tank on an ascent? I respectfully disagree. We have a "sealed" aluminum cylinder, designed to hold 3000psi. I honestly can't grasp ascending 1atm allowing the air to expand in the tank. If you figured the volume of the hoses, I can believe that but not in a tank. If you can explaing how that one works I'd definitely like to learn more.

 

[dherbman]

Are you familiar with the incident that killed Sheck Exley? His tanks contained over 900 PSI when retrieved, but at his max depth (over 900') they were at 0.

 

[Sideband]

NetDoc, are you saying that the air will expand in a tank on an ascent? I respectfully disagree. We have a "sealed" aluminum cylinder, designed to hold 3000psi. I honestly can't grasp ascending 1atm allowing the air to expand in the tank. If you figured the volume of the hoses, I can believe that but not in a tank. If you can explaing how that one works I'd definitely like to learn more.

That's not what he is saying and you are correct that the air in the tank is not being compressed by depth. What is changing is the ambient pressure and the amount of pressure needed to deliver it at a given depth. As you get shallower each breath requires less air. A tank that appears to be empty at depth still has air in it. It is just at a pressure equal to the ambient pressure and is therefore at equallibrium. Even at the surface if a tank is "empty" it is not truely at 0 Psi. It is at 14.7psi. If you placed that tank into a vacuum and opened the valve, air would be forced out because the pressure outside the tank is lower than inside the tank. Basically the same thing is happening at depth. I hope I said that the way I mean it and that it makes sense to someone.

Joe

 

[redacted]

NetDoc, are you saying that the air will expand in a tank on an ascent? I respectfully disagree. We have a "sealed" aluminum cylinder, designed to hold 3000psi. I honestly can't grasp ascending 1atm allowing the air to expand in the tank. If you figured the volume of the hoses, I can believe that but not in a tank. If you can explaing how that one works I'd definitely like to learn more.

When your tank turns up "empty" at 4 ATM and you move to a depth of 3 ATM, there is then another 1 ATM that can be accessed with your regulator (tank is still 4 atm and reg is 3 atm). Repeat until you surface or die.

You got to admit, it makes one of those silly little, too small to do any good, Spare Airs look like a possibly desirable option.

 

[all4scuba05]

lets face it....every once in a while, sh_t happens to someone somewhere...when it does, the guy with the least panic, help of the Gods, and the most redundancy, the most expensive equipment(yeah right), luck on the way up, and most choices, tends to live and tell about it. So when it happens to any of you, could one of you please do the BC thing so that the rest of us can learn something new...

 

[Chaseh]

James Bond once breathed out of a car tire. PADI should teach that.

 

[Scububbs]

I'm very new to diving in general, but one issue I didn't see addressed in this thread is the ability to actually inhale a lung full of air from your BC when that air isn't pressurized as it is in your tank. Can it even be done is my question, and if not, isn't this a moot point? Personally I prefer the "always have 500 pounds" rule as most likely to keep you out of trouble.

 

[Diver Dennis]

I'm very new to diving in general, but one issue I didn't see addressed in this thread is the ability to actually inhale a lung full of air from your BC when that air isn't pressurized as it is in your tank. Can it even be done is my question, and if not, isn't this a moot point? Personally I prefer the "always have 500 pounds" rule as most likely to keep you out of trouble.

You can do that out of the water and as soon as you get in the water, the ambient pressure increases. So yes you can.

 

[Scububbs]

That makes sense, thanks!

 

[The Chairman]

NetDoc, are you saying that the air will expand in a tank on an ascent? I respectfully disagree. We have a "sealed" aluminum cylinder, designed to hold 3000psi. I honestly can't grasp ascending 1atm allowing the air to expand in the tank. If you figured the volume of the hoses, I can believe that but not in a tank. If you can explaing how that one works I'd definitely like to learn more.

As other have pointed out... we breathe air due to the "tension" (difference) between the ambient pressure and the tank pressure. You have a TOUGH time breathing on a snorkle that is longer than 18 inches because the air in your lungs is at 1 atm while the ambient pressure surrounding your chest at 18 inches is 1.045 atms which is less than a single PSI.

If the pressure OUTSIDE of the tank is 4atm, then you can not breathe your tank LOWER than 4 atms. However, bring it up to 33 fsw (2 atm) and you now have a 2 atm tension (4 atms in the tank and 2 atms outside). Breathing is now possible until you breathe down the tank to the ambient pressure again.

This is why we teach students to KEEP their regs in their mouths! You don't want to waste ANY air trying to clear the regulator so KEEP IT IN!

For those of you who have taken up the sport within the past 20 years, remember that for MANY of us during the 70s, running out of air was the way we ended our dives. We did not OWN an SPG or a depth guage for that matter. When we felt the "pucker" we would reach back and trip our J-Valve for a couple of hundred PSI to get to the surface. Often, we would find that the J-Valve had already been tripped and we just had to KICK HARD to the surface. Even then, we could always count on at least one breath on the way up.