Tank Air Expansion On Ascent?

[Devil505]

This kinda goes along with my other thread (Do Yo feel the need to breathe) but this time does not involve air expansion in the lungs on ascent. I have always dived with the knowledge/understanding that, since your useable tank air will increase as you ascend, you would still have enough air to breathe, even if you totaly ran out of air at depth. I was trained to use the old "J" valve (reserve) & still feel that it provides a bit of additional safety that can't hurt. (of course, you must make a habit of checking that the operating rod is cocked a few times during your dive to be safe, but that is very easy to do...... The few times I have used the "K" valve
I have to fight the habit of reaching back (every so often) with my left hand to check the position of the operating rod!<G>) My understanding has always been that if you feel that ...getting hard to inhale....signal (that means you are running low on air) that ascending will enable you to keep breathing normaly, all the way to the surface. (The "J" valve reserve merely enables you to have enough air to breathe should you have to swim level or descend a bit (if you are in say a wreck or cave) before you can ascend. Isn't this correct?

 

[Mike Veitch]

this should get interesting... will enjoy the answers when i check in in the morning....

 

[knotical]

Back in the J-valve days, I once bumped my reserve and ran out of air at depth. I then experienced the truth of the following: while ascending, the ambient pressure reduces, allowing you to get a few more breaths from a nearly empty cylinder. However, I would not quite describe my breathing as &#8220;normal&#8221;. It took a noticeable effort at depth and got easier as I neared the surface.

 

[Paco]

I don't believe the air in the cylinder expands. It is in a rigid container and is not affected by ambient pressure. As you ascend, however, your lungs need less air from the cylinder, therefore making you think the air in the tank is expanding, but it is not. You are simply needing less volume on each breath as you ascend, making the little are left in the tank last longer.

 

[scott&sara]

The air in your tank does not expand as you ascend! The tank is a rigid body so the volume of gas and thus the pressure remain contant at any depth. In the old days the first stage reulators were not balanced and therefor became hard to breath at depth because they were unable to deliver the demanded air... but upon ascent the amount of air one demands becomes less and therefore will continue to supply a limited amount of gas upon ascent. This should be covered in an OW text as basic gas law principals (boyles law, charles law).

 

[1_T_Submariner]

The Air in your tank and or the tank it self does not significantly expand as you change depth.

The reason can get more air from your tank as it runs low is that the pressure on the out side is reducing as you go up allowing your regulator to have a positive pressure differential which allows air to flow out of you regulator.


I'm not familiar with the internal construction of the J valve. I'm sure someone will answer your question on that.

 

[1_T_Submariner]

Wow took me long enough to type that reply that 4 others replied in that time.

 

[RumBum]

I have always dived with the knowledge/understanding that, since your tank air will expand as you ascend, you would still have enough air to breathe, even if you totaly ran out of air at depth... ?

I have not tried this myself, and I sure don't want to. I am guessing that if you were out of air at say 100' and started swimming upwards you might get about 1 breath every 33 feet... so 2-3 breaths between 100' and surface. Not enough air for a leisurely ascent.

 

[Devil505]

Hmmm....Interesting about the air not expanding in the tank (if true).......BUT.......since all the responses (so far) indicate that (for whatever the reason) a diver would still be able to breathe (even if not quite "normaly") enough air to allow him/her to safely reach the surface, my original understanding remains the same.

 

[Meng_Tze]

...My understanding has always been that if you feel that ...getting hard to inhale....signal (that means you are running low on air) that ascending will enable you to keep breathing normaly, all the way to the surface.

Dont know about JValve... what I do know is that 'getting hard to inhale' should not be a measure of how much gas you have and definitely not be be a surprise.....

Gas planning and regular checking of spg should be standard process when diving. Gas does expand a little when ascending, not so much because of difference in pressure, but more because of the temperature change....... If you slowly ascend/descend (allowing for the temperature to affect the gas) you will see the difference.

So yes in theory you 'get more breaths' going up, but I would not plan on it.