How Long For Freeflow To Empty Tank?

[pufferfish]

Pufferfish, what size tank was that?

AL80
I was very surprised at how fast I drained it and why I started looking into the risk factors for free flows in cold water a lot more closely,....you know dewpoints and all that. Of course the importance of redundancy was clearly highlighted as well.

 

[Rick Murchison]

Is we are assuming 2 different things:

1: the failure is at the tank valve (meaning the first stage isn’t doing anything). This one hurts to think about

2: The failure is after the first stage and at a secondary.

OK?

Assuming the first, I have no scientific idea which would empty before the other, however I think they’d be about the same…….This is just a guess….

If it is a failure of a secondary, the relative volume coming from the leak at the surface or at 100-feet is the same. Because the air/gas is denser at depth (the same volume of gas is heavier) the tank will empty a lot faster.

Truva

I think I’ll go drink more beer now….

No, no, no... it is mass flow rate not volume flow - you're exhausting into the open sea, or the open air - not your lungs. The mass flow rate is the same for the same pressure difference. The gas flowing through the bottleneck doesn't know the nature of its surroundings... all it knows is "how much more pressure do I have on this side than on that side" to determine how much gas it lets by per unit time. The density of the air in the tank - and the air flowing through the restriction - remains the same regardless of depth.
SAC rate principles do not apply to this problem!
Rick

 

[Diver0001]

Would you all agree that putting a regulator, with all its convoluted gas paths and pressure stepdown seats and levers and springs and poppets & other stuff onto a "K" valve cannot increase the flow of gas through the little hole in the "K" valve?
No? You don't agree? Go back to physics 101.
If you do agree, you're right; let's continue...
Now, since the regulator can only slow the flow that is possible through the "K" valve, let's take the regulator and all it's confusing "constant pressures" and such completely out of the problem, and examine what happens when you just open the "K" valve wide open. Gas will flow through the valve from the area of higher pressure to the area of lower pressure based almost completely on only two factors - those are (1) the pressure drop from one side of the valve to the other (this is the same as gauge pressure) and (2) the resistance to the gas flow provided by the valve itself,
*** physics stuff... skip if you like ***
which can be expressed mathmatically as a cross sectional area of a hole (the size of the mathmatical hole is smaller than the actual hole in the valve because the actual hole has length and turbulence and irregularities, but it will *act* like a hole with no length of an easily calculated size based on the valve's flow characteristics) Flow rates will also be effected by temperature and the specific density of the gas involved - He will exhaust faster than O2, for example, but for the purposes of this explanation those effects aren't significant or relevant.
******************************
Now we can't change the size or shape of the hole in the wide open valve, so that restriction to flow is unchanging - the only thing we can change is the pressure difference between the inside of the tank and the outside of the tank.
*** at this point we have to talk a little physics because there is a fundamental misunderstanding in this thread of what happens to the flowing gas under varying ambient pressures... Flow rate is often expressed in liters/minute, because this is a useful measurement above the water... but it lends confusion to this discussion because the flow rate is actually a mass and not a volume flow rate. So (for air) the 1100 liters/minute (approx 39 CFM) cited earlier, for example, is really about 1430 grams/minute, and this flow rate will not increase as a tank is taken to depth. In fact, it will decrease a bit, because the pressure gradient decreases as the ambient pressure rises. And if you wanted to measure the flow rate in CFM or liters per minute at the ambient pressure of, say 5ATA, then that same 1430 grams/minute would yield not 1100 liters/minute, but 220, and the tank would empty at essentially the same rate as it did on the surface.
If this is still not clear, arrange a demonstration to satisfy yourself. You'll find there is no significant difference in the rate of depletion from the tank at any recreational diving depth.
I promise.
Rick

Thank you.

R..

 

[MikeFerrara]

darn it... now i'm confused again...

ok... here's my new question:

why does air go much quicker at depth when you INHALE it but would not go quicker
in case of a free flow?

what's the difference?

It doesn't come out of the tank faster but you use if faster because it takes more gas to fill your lungs with each breath.

 

[dc4bs]

Because when you take a normal breath off your reg, the flow rate is nowhere near the maximum flow rate from the valve. As such, then the limiting factor is the amount of air in each breath, which, as you know, is more at depth than at the surface.

Flow rate of regulator X has nothing to do with using more gas per breath at depth. Regulator Xs flowrate will, however, determine its max usable depth limit though.

-----------
EXAMPLE:

At 33 fsw or 2 ATA (since that keeps math easy) the gas pressure you are attempting to breath in has to be prety close to the pressure on the outside of your body. Too high and you can embolize, too low and your chest muscles can't overcome the outside pressure to draw in a breath. Thus the whole "ambient" pressure addition to the intermidiate pressure of the regulator so the regulator always TRIES to give you air/gas at what subjectively FEELS like normal pressure to your body.

When you breath in at 33 fsw you are breathing in about 2 times the amount of gas you do at the surface becuase the gas is at 2 times surface pressure as it is released from the 2nd stage and into your lungs. Basicaly, air under 2 ATA pressure takes about 1/2 the space it did at 1 ATA pressure. This assumes ideal gas laws (which we can do here because at these pressures, air acts prety much like an ideal gas). So, at 2 ATA, each full breath you take requires an amount of gas equivalent to two normal, surface lungfulls to make it feel 'normal' to your body and thus you use up two times as much gas as you do at the surface to breath normaly (5 x as much at 5 ATA, etc...).

Any regulator has a maximum flowrate that it can deliver. As you go deeper and you require more and more gas to fill your lungs with each breath and the flowrate you are demanding from the regulator approaches the maximum flowrate of the regulator, it begins to get harder to breath in. This can also happen at shallower depths if you are working hard or are panicked and breath heavily. The regulator simply can't keep up with the gas demand you put on it (this is known as 'overbreathing a regulator" and can contribute towards a panic inducing situation - not good).

You always want to keep from getting too close to a regulators maximum depth in order to maintain a buffer in case you do need to breath harder for a bit. If you are already at its maximum and your breathing rate increases for any reason, there simply won't be any extra gas available from the reg to make up the difference.

This is why divers who go to deeper depths spend lots of money on 'high performance' regulators while divers who dive no deeper than 3 ATA (66fsw/20m) in warm tropical waters can use almost any regulator on the market and dont have any trouble getting plenty of gas from that lower performance regulator.

Hmmm... I seem to have hijacked the thread. sorry.

 

[adder70]

Points to consider:

1. The pressure (and density) of the gas when it is out of the tank is only relevant in how it affects the pressure of the gas when passing through the restriction. WHo cares if it 1 atm or even 20 atm when it is gone. If it's 150 atm as it passes through the restriction, you will have the same density and mass flow rate will be the same at the same volumetric flow rate (for the same gas).

2. Flow resistance is based on fluid properties and volumetric flow, not mass flow. Flow rules of thumb that use mass flow rates are for constant density fluids. Most show m={rho}AV, where m is the mass flow rate and rho is the density. If the density is different, the mass flow rate changes. Gases with variable densities can be considered incompressible at the restriction (density is relatively constant during this process) and have different densities at other times. Actually, gases will have near identical volumetric flow rates at the same pressure, but heavier gases (where each molecule has a higher molecular weight) will have a higher mass flow rate, nearly equivalent in relationship to the average molecular weight of the gas. Helium may in fact have a higher volumetric flow rate than O2, but that is based on fluid properties, not density and constant mass flow rate. The mass flow rate for O2 will be significantly higher. But then there is much less mass in the same volume and pressure of He than in O2.

So what you have is a situation where the volumetric flow rate is the same in both instances as well as the mass flow rate since the pressure gradient in each case is so much higher than the difference in low pressure for each case.

For the air consumption, your body requires a certain volume of fluid for each breath, where that volume is measured at the ambient pressure. In that case the volume of 2 liters at 4 ata (atmosphere absolute) will consist of 4 times the mass of the same volume at 1 ata, you will need 4 times the mass at depth. Since the density of the gas flowing through the restrictions doesn't change in porportion nearly as much, the required volumetric flow rate through the restriction goes up. This in turn calls for a higher performance regulator for use at greater depth.

 

[Warren_L]

Flow rate of regulator X has nothing to do with using more gas per breath at depth. Regulator Xs flowrate will, however, determine its max usable depth limit though.

Exactly my point.

 

[Big-t-2538]

You'll find there is no significant difference in the rate of depletion from the tank at any recreational diving depth.
I promise.
Rick

I don't follow how there could not be a difference. If that was true, our SAC rate at depth would be meaningless.

I agree that the flow rate would not change, but why wouldn't the amount of gas required to "empty" a tank at depth be less?

 

[Warren_L]

I don't follow how there could not be a difference. If that was true, our SAC rate at depth would be meaningless.

I agree that the flow rate would not change, but why wouldn't the amount of gas required to "empty" a tank at depth be less?

There is a slight difference, but not significant (due to differences in the ambient pressure). The amount of gas required to empty a tank at depth is the same as at the surface. The volume of that gas would be different though.

 

[Big-t-2538]

There is a slight difference, but not significant. The amount of gas required to empty a tank at depth is the same as at the surface. The volume of that gas would be different though.

Wait a tick....I just re-read Rick's response....it all just clicked.

I now agree with Rick and others...it isn't a volumetric issue like I thought it was...It has everything to do with flow through the restriction....which like warren said results in a different volume on the other side of the restriction.

I now also see why the SAC principles do not apply here.