OOA Ascents and expanding air

[dflybldr]

This is probably a question related to breathing air in your BC during an emergency OOA accent, but different enough I thought a new thread was in order.

Anyway, I was tought in my BOW that if I suck my tank dry and the surface is the closest reliable air source, that I should keep the reg in my mouth on the way up. I think I understand all the reasons to do that, (taking a breath of nothing is better than lungs full of water ...) but the comment about the air in my tank, lines and reg expanding enough for one or two more breaths on the way up has not sunk in yet.

Is the air in my tank really expanding?, or is the reletive pressure on the way up at some point now enough to overcome the "ambient" outside pressure and fill my lungs when it wasn't 30f deeper?

I would think that the air in my tank being compressed by the water pressure, thus allowing expansion on the way up would mean an tank that would probably not pass hydro next time.

I know it might sound like I'm splitting hairs so to speak, but I have found that truly understading my environment increases comfort level, and then reduces the chances of panic in an emergency.

 

[lamont]

Is the air in my tank really expanding?, or is the reletive pressure on the way up at some point now enough to overcome the "ambient" outside pressure and fill my lungs when it wasn't 30f deeper?

Yeah, its all about the pressure differential between ambient, the LP pressure and tank pressure. As ambient decreases as you ascend, you've got more pressure differential and you might get an extra breath or two. The air in your tank does not expand, but you get another 14.7 psi of differential between ambient and tank pressure for every ATA that you go up...

At 204 atmospheres, or 6700 fsw, you'd be at 3000 psi of ambient pressure and you couldn't even breathe off of a full scuba tank.

 

[dflybldr]

Soooo, at 60 ft or so it's really impossible to "empty" your tank? Small consolation though, not really a safe reserve.

Thanks

 

[H2Andy]

well... yes, you can empty your tank at 60 feet

if there is no air left in the tank to expand, there is no air left in your tank to expand

however, if there is "some" air left in your tank, it will expand as you ascend

 

[JeffG]

well... yes, you can empty your tank at 60 feet

if there is no air left in the tank to expand, there is no air left in your tank to expand

however, if there is "some" air left in your tank, it will expand as you ascend

You might want to re-think that one.

 

[H2Andy]

ok... fun fun... thought experiment:

perhaps because of the minimal pressure needed to operate the regulator,
you can never "run out of air" under pressure?

but if you crank the valve open and let it bleed out, you can empty it.

how's that?

 

[JeffG]

but if you crank the valve open and let it bleed out, you can empty it.

how's that?

Thats the lawyer in you trying to wiggle out of a mistake.

 

[Damselfish]

well... yes, you can empty your tank at 60 feet

if there is no air left in the tank to expand, there is no air left in your tank to expand

however, if there is "some" air left in your tank, it will expand as you ascend

not without a pump. There will be air in there at ambient pressure.

 

[H2Andy]

Thats the lawyer in you trying to wiggle out of a mistake.

man... whatever

just having fun

 

[DBailey]

well... yes, you can empty your tank at 60 feet

if there is no air left in the tank to expand, there is no air left in your tank to expand

however, if there is "some" air left in your tank, it will expand as you ascend

The tank is a rigid, closed container; therefore, the air is not expanding within the tank due to depth changes.