Low air/No air situations

[Rick Murchison]

... But I simply cannot wrap my head around how air will compress and expand as you go up and down when it is in a metal cylinder. Guess it's old age or something.

It's a relative pressure thing.
Let's go to extremes for example.
If you take a full (3000 psi) Scuba tank down to where the water pressure outside the tank is also 3000 psi (about 6600 FSW) and open the tank valve, nothing will come out - there's no pressure difference to cause anything to flow, and the tank is "empty." If you were to put a pressure gauge on the tank, it would read "0" - gauge pressure is the difference in the pressure in the tank and the pressure outside the tank (ambient pressure). As you bring that tank up, the pressure outside the tank decreases and gas will begin to come out of the tank.
The same thing applies when at shallower depths. At 100' the total pressure is 60 psi, and a tank will read "0" when it reaches that pressure. As you ascend, the outside pressure will drop, to about 15 psi by the time you reach the surface. If you gauge it then, the tank will read 45 psi... and you'll be able to get about 1.2 CF out of it (AL 80) before it reads "0" again.
Hope that clears it up
Rick

 

[mikerault]

I didn't actually run out of air, I knew where I was with pressure the entire time, I just kept believing the DM that we were near the boat. At 500 PSI I turned back for the boat solo, at night, on my first night dive. I got about half-way back when I had to surface (luckily we were only in about 30 feet of water) and do the rest on my back (about 50 yards).

Mike

 

[Divin'Papaw]

No ... but I have had it happen to a buddy. Actually, the issue was a closed isolator which he thought was open so he drained one of his two tanks. We handled it by the book and everything went smoothly. We both had the wonderful priviledge of experiencing a real OOA, we both feel good that we handled it correctly and didn't panic, and he learned to always check to make sure his isolator is open on his doubles. I also learned to check during the pre-dive safety check if my buddy is in doubles, or to at least ask. In the end, all good things to learn.

 

[Gil57usa]

How could I ever run out of air on a dive? My air integrated wireless computer keeps me safe by beeping when it is time to go up.

Can you say electronic failure? Murphy's Law? :shakehead

 

[Scott Riemer]

Breath a tank dry sometime. You might be surprised at how little change in breathing effort you notice until the last breath or two.

That was the case in my situation. My first dive after certification, I was diving in Mexico. The DM gave the instruction to signal LOA at 1,000, which I did. We continued the dive for a bit. When I got to 600, I got his attention again and let him know my status. We ascended for the safety stop. While there, I got about 2-3 "warning" breaths and then had to go to his octo. I guess he didn't think I'd burn through my remaining air that fast. Being new at the time and getting nervous about my situation I'm sure didn't help my SAC rate.

I wonder how many instances are related to new divers? I'd assume a large percentage.

 

[Rick Murchison]

How could I ever run out of air on a dive? My air integrated wireless computer keeps me safe by beeping when it is time to go up.

Don't forget your smilie
Someone might think you're serious.

Rick

 

[herman]

Breath a tank dry sometime. You might be surprised at how little change in breathing effort you notice until the last breath or two.

My experience has been just the opposite. I purposely breathed 2 tanks down to less than 50 psi a couple of weeks ago in a quarry (very controlled conditions). This was an experiment just to see exactly how my regs would respond. My freshly serviced Apex/Zeagle regs started to have a noticable breathing resistance at around 200 psi, by the time I got to 100 psi I had to pull pretty hard to get air. The last 30 breaths or so were quite hard. No way could I have got sufficient air in a hard work situation(panic) from the last 75 psi but for an easy and controlled enviroment I got enough air to remain comfortable. Point is, I can't see any way I would not notice the increased resistance of the last few hundred PSI, plenty of air to accend from 50 ft or so or to get to a buddy as long as I kept my cool.

 

[Sideband]

Can you say electronic failure? Murphy's Law? :shakehead

I think (hope) there was a bit of sarcasm there that was missed.

 

[ClayJar]

I've intentionally breathed a cylinder to empty in the pool. I needed to do a visual on it, so I figured why not get some experience with the feeling.

It was almost nothing at all like the feeling of having your air switched off at the valve.

When I had an instructor turn off my valve (with prior warning), it was basically half-a-lung-and-done. Breathing out the bottom of a cylinder, on the other hand, was noticeably different. While diving it, I realized what was happening only a few breaths from empty. By the time the breathing resistance was noticeably increased, there wasn't much air left, but there were several breaths, the last few of which went from "a bit more effort" to "inhaling through a wet towel" to "breathing through a garden hose" to "sucking through a straw" to "trying to suck a breath through a coffee stirrer" to nothing whatsoever. Had I been deeper than 8 feet or so, that last bit of air could've gone much faster (with a bit less of a wall at the end, of course, if I was heading to the surface).

In hindsight, I could see the signs had appeared a bit earlier than that, but I wouldn't want to wager they would be noticeable on a working dive. Even if I would notice, there wouldn't be much time to goof off before the needle parks itself.

 

[Buoyant1]

Which is why when you practice the CESA they tell you to breathe out a long as you can, and if you feel the need to take a breath go ahead, you may get air.

It's a relative pressure thing.
Let's go to extremes for example.
If you take a full (3000 psi) Scuba tank down to where the water pressure outside the tank is also 3000 psi (about 6600 FSW) and open the tank valve, nothing will come out - there's no pressure difference to cause anything to flow, and the tank is "empty." If you were to put a pressure gauge on the tank, it would read "0" - gauge pressure is the difference in the pressure in the tank and the pressure outside the tank (ambient pressure). As you bring that tank up, the pressure outside the tank decreases and gas will begin to come out of the tank.
The same thing applies when at shallower depths. At 100' the total pressure is 60 psi, and a tank will read "0" when it reaches that pressure. As you ascend, the outside pressure will drop, to about 15 psi by the time you reach the surface. If you gauge it then, the tank will read 45 psi... and you'll be able to get about 1.2 CF out of it (AL 80) before it reads "0" again.
Hope that clears it up
Rick