Tank Air Expansion On Ascent?

[fire_diver]

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.

Maybe you need to define "safely reach the surface". Becuase I can't imagine a "safe" ascent rate that is anywhere close to the mad dash for the surface a person would make after sucking the last breath from a tank at 100 feet.

fd

 

[dumpsterDiver]

The J valves worked at more like 300 lbs which provided just enough air to come up from 100 feet without any problem; of course if you had a little problem, it could be a huge problem with that small of a reserve.

The j-valve was one more thing to break and require service, it could very easily be partially or completely pulled without the diver knowing it (which left no reserve). It was an almost unbelievably crude technology, but compared to having no pressure guage.. well it was excellent. My PADI open water training dives were done without pressure gages (or lp inflator or octopus), so the J-valve reserve was pretty important.

As the OP said, you quickly learned to habitualy feel for the pull rod loop and push on it to ensure that it was not accidentally enaged and pulled down. checking it was like a nervous tick, I did this like 20 times a dive (I guess it was actually easier than looking at a pressure gage), because you didn't have to take your eyes off the diving.

 

[Guba]

And that, my friends, is about the best answer possible to the question. Thanks, Rick!

 

[TxHockeyGuy]

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.

Edit: Rick posted a much better explanation than myself. For an illistrated version go check out:

http://entertainment.howstuffworks.com/scuba.htm.

 

[TeddyDiver]

Miketsp is correct.

Your description of unbalanced stage is a unique definition, which differs from how the manufacturers describe how their unbalanced 1st stages operate. A balanced first stage refers to a balance of pressures on each side of the piston seat or diaphram carriers, such that IP is relatively independent of TANK PRESSURE. An unbalanced 1st stage will vary IP more as tank pressure changes. Both balanced and unbalanced 1st stages strive to maintain a constant IP with respect to ambient pressure.

If you don't believe me and Miketsp, you can either go look at manufacturers' literature, or other posts on Scubaboard.

Sorry I was wrong with tank pressure thing. Could anyone explain why they work as I described earlier if the ambient pressure doesn't affect

 

[miketsp]

Sorry I was wrong with tank pressure thing. Could anyone explain why they work as I described earlier if the ambient pressure doesn't affect

It doesn't matter whether it's balanced or unbalanced.
As Rick explained very well in post #20, at extreme depth you don't have any useful air in the tank (unless you can suck it out with a pump ).
As you rise tank pressure becomes higher than ambient and you will get air flow from the tank if the regulator valves are open.

This is the inverse of the old schoolboy experiment.
Imagine an inverted glass at depth. Now fill it with air from your reg so no water remains inside the glass. Now start to ascend. The air will expand and bubble out as ambient pressure reduces.

 

[Devil505]

Maybe you need to define "safely reach the surface". Becuase I can't imagine a "safe" ascent rate that is anywhere close to the mad dash for the surface a person would make after sucking the last breath from a tank at 100 feet.

fd

Perhaps someone who has tried this (as an experiment) & lived to tell us about it would care to enlighten us all?
Without trying to be sarcastic here, some brave/dumb person must have deliberately made such an ascent (from a depth vs time that wouldn't require a decompression stop but would be deep enough to require a few breaths to reach the surface) and would share with us his/her experience?

How about this scenario as a possible test: (please do not try this
1. Take a partialy filled tank (with "J" valve) down to 80'
2. Stay at 80' for less than 40 minutes (the No Decompression limitt for 80') & allow the tank to "run dry."
3. Do not pull the reserve handle but, instead, begin a leisurely but swift ascent directly to the surface while taking whatever breaths you need on the way up. (Making sure you at least continue expeling air from you lungs (to avoid embolism) but trying to breath from your tank without pulling the reserve handle...unless absolutely necessary.)

The danger of this experiment is pretty obvious & I certainly would not recommend anyone actualy try it!! ......I mention it only to point out how simple an experiment it would be & with great certainty that someone has actualy done it, survived , & is willing to tell us about it!<G>
Were you able to breath all the way up without having to pull reserve?
Was the air supply enough, while ascending?

......that type of thing.

 

[neil]

With unbalanced 1st stage the lp remains the same. At surface it releases air to hoses at the same pressure as in depth, ...

That's not correct. Okay, I think this is how it works:
Modern regulators are depth-compensating. This a separate issue from balancing. The first stage delivers air to the second stage at IP+ambient pressure. As someone mentioned, some regs are "over-balanced", meaning the IP goes up higher than that at depth, but this isn't really part of the balancing of the regulator. "Over-compensating" would be a better term.

The way regs compensate is to allow the water pressure to bear on the bias spring in some way. In piston regs it's through that open part in the first stage where there are usually holes, and you can ever sometimes see the spring. In diaphragm regs the pressure bears on the flexible diaphragm or a secondary diaphragm over that one.

 

[captain]

In a unbalanced diaphram first stage as the tank pressure decreases the IP rises but the IP remains at a constant pressure above ambient water pressure.
In a unbalanced pistion first stage the IP decreases as tank pressure decreases but again the IP remains at a constant pressure above ambient.
In a balanced first stage, either piston or diaphragm the IP remains constant no matter what tank pressure is and again the IP remains at a constant pressure above ambient.

 

[plot]

I hope this is one of those "if you know what hits the fan while I'm at a significant depth and my buddy is not around" type of questions, and not one of those "hey, can I expand my bottom time and get more oxygen out of my tank if I do this..." type of questions...

I'm sure this has been discussed somewhere at same time, but on the same token, if you run out of air at depth, could you theoretically drop your weights and breath off of your BC through the oral inflator long enough to reach the surface? The air in it is expanding as you go up as well, how many breaths do you think you could get out of one?