How Long For Freeflow To Empty Tank?

[Warren_L]

Actually, there probably is some small change in internal volume of a tank as it is pressurized (and hence also when it is depressurized). That is one of the things a hydo would test for.

 

[Diver0001]

Whew!
Alright class, listen up...
There are a couple of factors here, but y'all are all trying to make it too complicated, and you have one bad data point and some misplaced ideas about fluid dynamics from which the rest is flowing. Regardless of what someone remembers, you cannot empty a full 72 through a standard "K" valve in 30 seconds. Not at the surface, not at any recreational diving depth.
As Mike says, flow rate is dependent on pressure differential and flow restriction. Forget the regulator... a "perfect" regulator would provide zero resistance to the gas flow through it and therefore allow flow to proceed at its maximum possible rate as though it weren't there at all. So the two pressures that create the ultimate differential are tank pressure and ambient pressure, and maximum possible flow rate (with an extremely efficient, high performance regulator) actually decreases with depth because the pressure differential between the tank pressure and the ambient pressure decreases. (Before you get your brain in a knot on that one, just think 200 ATM ambient pressure - about 2000 M or 6600 feet deep - at that depth a standard aluminum 80 would never empty (or already be empty, depending on your point of view) because there would be no pressure differential across which the gas could flow; indeed, if you went deeper then opening the tank valve would allow water to flow into the tank, not gas to flow out)
So if you want to see what the worst case could be, just open a tank valve wide open and see how long it takes to empty. At depth it'll take a little longer for the same amount of gas to escape as it does on the surface. But at normal diving depths, the dropoff in pressure differential is very small compared to the flow restriction provided by the valve; at the surface a tank filled to 3000 psig has a 3000 psi "fall" for the gas to drop through the K valve, and taking that tank to 100 feet only reduces the "height" of the fall by 45 psi, and the pressure differential is still 2955 psi initially. It is the restriction of the "K" valve that for all intents and purposes is the limiting factor here, and opening a valve wide open on the surface will give you a very close approximation of how much time you'll have at any recreational diving depth.
Rick

OK,

I'm officially confused. I tried hard no to become confused but I admit defeat.

What you say sounds completely logical but it also seems completely logical that a free flow at depth will empty your tank faster (just like breathing "the same amount" of air at depth drains the tank faster).

Something about the rigid container making a difference strikes a bell but what confuses me is this. If your tank expells 1100 litres of air a 5 ata then aren't those bubbles going to expand on the way up? Aren't they going to turn into 5500 litres of bubbles? Doesn't that mean that you must have ejected 5500 litres of air at depth under pressure to make this happen?

My head hurts.

R..

 

[Big-t-2538]

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

Yes...technically, I am wrong...the internal volume of the tank does not significantly change...I was referring to the gas in the tank, which aslo does not change if it does not leave the tank....pardon my oversight.

However, as soon as the gas in the tank is expelled through the 2nd stage of the regulator, or dropped back down to the ambient via some sort of leak, physics is easily applied. You now have the same physics action (I believe this is Boyel's law) happening to the gas in your tank as you would if you took a balloon containing the equivalent inital volume of gas and compressed it down with you at depth. The flow rate is the same in Cu.ft per minute or whatever you want to measure it in regarless of depth.

This is not theory...this is simple physics.

 

[Diver0001]

Yes...technically, I am wrong... <snip>

That's been happening a lot lately.....Do you want to talk about it? LOL

R..

 

[Big-t-2538]

That's been happening a lot lately.....Do you want to talk about it? LOL

R..

That's what happens when you see bogus like adding mass to a closed system...you become frustrated and yearn for beer. All will be good after a few pints and some tossed spikes this evening.

 

[Diver0001]

That's what happens when you see bogus like adding mass to a closed system...you become frustrated and yearn for beer. All will be good after a few pints and some tossed spikes this evening.

pfffff. After the day I had I wish I could join you.

R..

 

[weekender]

The flow rate is the same in Cu.ft per minute or whatever you want to measure it in regarless of depth.

This is not theory...this is simple physics.

ok, that makes sense to me. for that matter so does tossing back some pints. so in short, we are in complete agreement.

 

[truva]

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.

Where are you getting closed system? We are talking about a secondary free flow and would it empty a tank faster, slower or the same at depth. The density of the gas flowing out of the first stage increases with depth.....

Truva

 

[H2Andy]

ok guys... what we need to do is summarize our answers into one liners to see
how many possible answers we have.

my answer is:

a cylinder at 5 atms will empty roughly five times faster than at the surface, all other things being equal (same cylinder, same regs).

let's hear the rest. and no obfuscation

 

[adder70]

ok guys... what we need to do is summarize our answers into one liners to see
how many possible answers we have.

my answer is:

a cylinder at 5 atms will empty roughly five times faster than at the surface, all other things being equal (same cylinder, same regs).

let's hear the rest. and no obfuscation

I can't answer it in one line. It depends on the reg, the tank, and the point of constriction. If the tank hits choked flow, then it will definitely not empty 5 times as fast. My guess is that most tanks and regs will not drain 5 times as fast. Even if the flow isn't choked, I think it would be more like 50% faster at depth.