How Long For Freeflow To Empty Tank?

[ianr33]

Any chance somebody with a spare tank could do this experiment to settle this discussion?
Take a full tank,hit the purge and time how long it takes to drop to ,say ,300 psi.
Thanks

 

[truva]

Assuming this was a Second stage failure or a cut hose or something like that. The density of the gas is what matters and here’s why:

The pressure stays the same within reason; the total amount of volume that the regulator can flow remains the same but at plus one atmosphere the mass of the gas has doubled. So much the same way that you use more gas per breath the deeper you go, the tank should empty faster.

I am not even going to think about the fact that the extra mass will take more force to move, which is why even the highest performing regulator will seem to breath harder the deeper you go, your lungs have to move a lot more mass than they are use too. But I am not going to think about that now.


Truva

 

[ScubaRon]

Your tank will not run dry if you kink your second stages hose and allow air through only as you need it. This way a freeflow can be safely managed until you reach the surface using a proper asent rate.

ABSOLUTELY NOT!
You're hose will blow open and now your air is lost even faster. Kinking your LP hose can stop a 2nd stage freeflow, but since it is difficult to distinguish it is better to never do this.
ScubaRon

 

[Falcon99]

Is this a good reason to easily be able to shut off the tank valve? I'm thinking that the last quarter turn or so before shutting the tank off would be very effective to conserve some air for a safe, breathable ascent.

Any ideas from the experienced folk?

 

[adder70]

Everyone here seems to be missing 1 very important factor, and a possibly important second factor.

The first, and likely insignificant factor is that air in this situation may not behave as an ideal gas. The temperature and pressure ranges are too wide to be sure that the air won't exhibit some deviation from the accepted ideal gas laws. It probably will, but it would be worth checking. I don't have the proper reference material here to check.

The more important factor: You are most likely dealing with flow approaching supersonic speeds. This will cause the fluid to behave far differently than an incompressible fluid. (Yes, for most fluids, even gases, you can use incompressible assumption; compressibility for flow is usually a function of Mach, although heat transfer during the flow can affect this a great deal.) I don't have the time (or the info handy) to do the calcs, but I bet you will have choked flow for at least part of the time, where the Mach number stays at 1 at the tightest constriction (Mach 1 = speed of sound, and speed of sound goes up with pressure (usually), so constant Mach number is not constant speed) Once you hit choked flow, increasing the pressure gradient were have far less effect on the flow that when in unchoked flow.

I just have to resist the urge to measure my tank valve and calculate this out.

Must...fight...the...geekiness

 

[MikeFerrara]

Yes. I would expect that the flow through a wide open valve (even more so with a reg attatched) would be turbulant flow as apposed to laminar meaning that flow will not change linearly with the pressure differential and won't be the same throughout the cross section of the flow path.

 

[Big-t-2538]

Assuming this was a Second stage failure or a cut hose or something like that. The density of the gas is what matters and here’s why:

The pressure stays the same within reason; the total amount of volume that the regulator can flow remains the same but at plus one atmosphere the mass of the gas has doubled.

Stop...hault...desist...What school did you go to? How does increasing pressure add mass to a closed system? This has me baffled....let alnose how it in any way relates to the problem at hand.

Second, what Rick mentioned is completely true. What matters for flow is the pressure differential. The opening and restriction of flow of gas through the reg is all that is involved with how fast the amount of gas can flow out of the tank...this will not change with depth.

what does change is the internal volume of the tank with Depth...so yes, this is a linear conversion. If you drain an AL80 on the surface in 4 minutes, at 33 feet, it will take you 2 minutes....at 66 feet it will take you 1 minute 20 seconds, (or 4 minutes divided by 3). at 99 feet, it will take you 1 minute...etc., etc. What you have to consider at 99 feet however is this...if my gauge is now reading 1500psi on a 3000psi rated tank...now I only have 30 seconds b/c my starting volume is 1/2 as much.

 

[Big-t-2538]

The first, and likely insignificant factor is that air in this situation may not behave as an ideal gas. The temperature and pressure ranges are too wide to be sure that the air won't exhibit some deviation from the accepted ideal gas laws. It probably will, but it would be worth checking. I don't have the proper reference material here to check.

You are correct...to be exact air going from 3000 psi down to 50 psi does exibit a small change in denstiy, but it has nothing to do with temperature change.

This is shown on the PST tank ratings in this chart

However, the change is essentially negligible for all intensive pruposes, do the math if you like....I did once and came to that conclusion.

 

[Big-t-2538]

Yes. I would expect that the flow through a wide open valve (even more so with a reg attatched) would be turbulant flow as apposed to laminar meaning that flow will not change linearly with the pressure differential and won't be the same throughout the cross section of the flow path.

Mike -

I agree, but this has nothing to do with the flow rate changing, we're just stating that the flow rate (laminar or turbulent) will not change during the entire depletion of the tank...

Sheesh...and I thought I'd never use this school stuff.

 

[weekender]

Big-t-2538 stated"what does change is the internal volume of the tank with Depth..."

you cant change the internal volume of a rigid tank, right? all changes that occur due to pressure occur when the air leaves the tank and meets with the pressure around it. for the volume inside to change wouldnt the tank have to be crushed? i could be wrong but never noticed my tank getting smaller as i decend. or is my 6th grade physics completely off? if so, ignore me. but if im right Bigt's theory is off.