Tank Air Expansion On Ascent?

[Devil505]

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.

My understanding of how the "J" valve works is that it mechanicaly prevents the last few hundred lbs of tank air from being released until the operating handle is pulled down by the diver.... thus providing a short (depending on your depth) amount of "reserve" air.

 

[The Kraken]

The air in a tank doesn't "expand" in the normal sense of the work.
The air is under pressure. The pressure of the gas exerts itself in all directions equally. As the gas in the tank is depleted the density of the remaining gas changes, becoming less dense.

One cannot create more gas by depleting gas.

the K

 

[TeddyDiver]

I like this kind of thread
First, what kind of 1st stage is used?
With unbalanced 1st stage the lp remains the same. At surface it releases air to hoses at the same pressure as in depth, and becouse of it you don't get any air from the tank after it stops giving gas.
With balanced 1st stage the lp difference with the environment stays the same. At depth (sorry now using metric units) at 30m (4bar external pressure) it gives absolute lp about 12bar, and at surface about 9bar (1bar external pressure). With 10l (about AL80) tank the difference 3bar*10l=30l of air gives a few breaths while ascending...

PS corrected 13bar to 12 and
Second, The type of valve has nothing to do with this when the "spare"air with J valve is used

 

[Devil505]

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.

You are quite right. My previous mention of the "getting hard to inhale" feeling was not meant to disclose the way a diver should learn about the amount of air he has left in his tank, or ever come as a surprise. Experience, a depth guage, a good dive watch & (now) the SPG should now & have always been ways the safe diver moniters his/her reserves of air. I mentioned the feeling only to point out (to any new divers who have not experienced it) that your air supply is not isuddenly "cut off"....with no warning when you "run out" at depth.
(I would often finish my dives by waiting until I needed to pull the reserve handle before I would start to head up to the surface. I always planned my dives so that I would know when I was close to "hitting reserve" & be close to my boat/beach at that point.... Doing this allows for catching another lobster or two!<G>)

 

[trigfunctions]

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.

More precisely, you need fewer air molecules on each breath, since the air in your lungs is less compressed the shallow you are. You still would need the same volume of air in your lungs.

 

[Shepman]

You are quite right. My previous mention of the "getting hard to inhale" feeling was not meant to disclose the way a diver should learn about the amount of air he has left in his tank, or ever come as a surprise. Experience, a depth guage, a good dive watch & (now) the SPG should now & have always been ways the safe diver moniters his/her reserves of air. I mentioned the feeling only to point out (to any new divers who have not experienced it) that your air supply is not isuddenly "cut off"....with no warning when you "run out" at depth.
(I would often finish my dives by waiting until I needed to pull the reserve handle before I would start to head up to the surface. I always planned my dives so that I would know when I was close to "hitting reserve" & be close to my boat/beach at that point.... Doing this allows for catching another lobster or two!<G>)

Wow, a discussion involving J-Valves, I almost getting nostalgic.

The previous discription of the J-Valve was accurate. Your air got hard to breathe, so you pulled the J-Valve. That meant you had 200 lbs. Dive training at the time dictated an ascent immediately when engaging the J-Valve.

When I say hard to breathe, I mean it. You got essentially 3 breaths. One that dragged a little, one that was tough, and one baby breath that made you pull that lever right now. You had to really plan your dive, watch your time and plan to be back at the boat, etc., etc..

In spite of being more diligent, I certainly sucked more tanks out of air than I ever have now. I did have one OTA experience in the Keys. I felt the "tough breath" and pulled the valve on the 3rd one. Nothing. Somehow (I got my suspicions) the J-Valve was already pulled. Fortunately it was a shallow dive (30 fsw).

Still when I got to the surface, I took the worlds biggest suck of wind!

 

[miketsp]

I like this kind of thread
First, what kind of 1st stage is used?
With unbalanced 1st stage the lp remains the same. At surface it releases air to hoses at the same pressure as in depth, and becouse of it you don't get any air from the tank after it stops giving gas.
With balanced 1st stage the lp difference with the environment stays the same. At depth (sorry now using metric units) at 30m (4bar external pressure) it gives absolute lp about 12bar, and at surface about 9bar (1bar external pressure). With 10l (about AL80) tank the difference 3bar*10l=30l of air gives a few breaths while ascending...

PS corrected 13bar to 12 and
Second, The type of valve has nothing to do with this when the "spare"air with J valve is used

This is a misleading post.
All modern day 1st stage regulators are balanced for ambient pressure, whether diaphragm or piston.
IP will be x ATM above ambient pressure.
Some are over-balanced by design - IP will increase slightly with depth to improve efficiency when the gas is denser.
However in general the term "balanced 1st stage" is intended to indicate that there is a compensating mechanism to maintain IP constant (or nearly) over the working range of tank pressures. ie Minimal IP variation as the cylinder drops from 200bar down to 50bar.

 

[TeddyDiver]

This is a misleading post.
All modern day 1st stage regulators are balanced for ambient pressure, whether diaphragm or piston.
IP will be x ATM above ambient pressure.
Some are over-balanced by design - IP will increase slightly with depth to improve efficiency when the gas is denser.
However in general the term "balanced 1st stage" is intended to indicate that there is a compensating mechanism to maintain IP constant (or nearly) over the working range of tank pressures. ie Minimal IP variation as the cylinder drops from 200bar down to 50bar.

Last time I looked around there were also unbalanced 1st stages available (how ancient design they are or not).. The point is as I wrote from balanced you get a bit of air while ascending and from unbalanced not.. And balanced in general indicates compensating IP(=LP) with the ambient pressure NOT with cylinder pressure (HP)

 

[Charlie99]

.. And balanced in general indicates compensating IP(=LP) with the ambient pressure NOT with cylinder pressure (HP)

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.

 

[Rick Murchison]

Hmmmm.... some basic misunderstanding of the physics involved here.
For the sake of example, let's use some extremes.
An AL 80 at 3000 psi, taken to about 6600 feet, will be "empty." That is, the pressure outside and inside the tank will be equal and if you open the valve... nothing comes out, and a pressure gauge would measure zero psig.
Now, let's say you run out of gas at 33'. The pressure inside and outside the tank will be 2ATM (0 psig). If you then bring that tank to the surface, the pressure outside the tank will be 1ATM and gas will come out of the tank. How much? For an AL80 about .4 CF, five or six breaths for most divers. Likewise, if you were to "empty" a tank at 100', close the valve and bring it to the surface, it would now register about 45 psig and deliver about 1.2 CF before again being "empty" and measuring zero psig.
However...
Since your consumption is also directly related to pressure, the effective useable gas on ascent is Delta-P/P times the internal volume of the tank. That is, if you commenced an ascent from 100' on an "empty" tank, and then "emptied" it again at 33', you'd only get 0.4 CF at 2 ATM, ( ((4ATM-2ATM)/2ATM) X 0.4CF ) and another 0.4 CF if you then took it to the surface. In other words, the deeper you run out of gas, the more vertical distance you must ascend to get a breath due to the decreasing pressure around the tank.
The type regulator you use may affect the work of breathing, but the available gas will be the same for all regs and is, once again, Delta-P/P times the internal volume of the tank.
Rick