running out of air - at what point is it not subtle

[redacted]

Hoppy once bubbled...
how does a reduction in depth increase the amount of air available ?



Hoppy

IP works in relation to ambient pressure. So, on the surface an AL80 has an interior volume of about .4 cubic feet and therefor holds .4 cubic ft of gas at 15psi. Now pump it full and you have 80 cu ft of gas at 3000psi in that .4 cu ft volume. Now take it to 90 ft where the ambient pressure is 60psi and let it "empty". The tank will empty until it hits ambient pressure. So the "empty" tank really contains .4 cu ft of gas at 60psi. Take it to the surface where the ambient pressure is 15 psi and you will ba able to release another 1.2 cu ft. That's where it is coming from. Intermediate Pressure is really pressure in relation to ambient. So the LP circuit on the surface is really containing a pressure of 135 + 15 = 150 psi (or IP plus ambient). At 90 ft that circuit contains a pressure of 135 + 60 = 195.

Its the same thing that makes bubbles get bigger as they ascend.

 

[The Chairman]

The pressure of the air you breathe (not your IP) is essentially the same as the ambient pressure you are in. At sea level you would be breathing 1 atmosphere and at 33 fsw you would be breathing on 2 atmospheres and down we go... with me so far?

Now, if you had an "empty" tank on the surface with the valve just open (forget the regulator for the moment) and tried to breathe from it, what would happen? Well, you wouldn't get much gas as you would have to put the tank into a relative vaccum to get air out. But the tank is not really empty is it??? it still has one atmosphere of gas in it.

Consider that same "empty" tank at 33 fsw. It actually has 2 atmospheres of absolute pressure which would be the ambient pressure for that depth. Just as the tank on the surface, you would have a hard time breathing it because the inner pressure of the tank is equal to the ambient pressure...

BUT, come up 33 feet and you would see half of the air in that tank come right out. The tank pressure would equalize to the ambient pressure by venting the excess gas. There, is that as clear as mud?

 

[Hoppy]

Me being thick I suppose,

Been a long day and theres still loads of it to go, and the weekend

:upset:

Ta

Hoppy

 

[jbd]

in the pool with turning off the air is similar to what big-t has experieced. You get one maybe two breaths that feel harder to draw than normal.

 

[Diver0001]

dlwalke once bubbled...
Let's say, for the sake of argument, that your SPG is stuck on 1200 or so, or maybe you are just spacing out and let your air run down because you haven't been checking your SPG. I assume that breathing effort does not go from normal to sthe feeling I get when an incompletely pureed strawberry gets jammed in the inlet of my smoothie straw. Is this right? If so, at around what point (i.e., how low do you go air in terms of remaining air) before it starts to become quite obvious that you are nearing the end of your tank?

Thanks,
Dave

With a reasonably good reg you won't notice until it's too late. You need to get anal about watching your spg.

R..

 

[Doc Intrepid]

agrees with the above posters. You'll inhale 2-3 breaths that are increasingly harder. Good time to signal your buddy or switch to your bailout. Otherwise, start up Stop exhaling around 30' or so and you'll get another quick inhalation as the gas in the hose expands. Continue to exhale, vent, and slow down the last 15-10' or so.

Heed Diver0001 and don't make it a habit.

Doc

 

[drbill]

Just once did I let my tank "empty" on me, and it was intentional. I was videotaping something I knew I would have trouble relocating and was only at 20 feet. When my tank finally "ran out, I had two or three draws that were more difficult then one more at the surfaced after I slowly ascended.

I would never have gotten this low on my tank were it not an important video opportunity in shallow water.

Dr. Bill