How Long For Freeflow To Empty Tank?

[Big-t-2538]

Density: If you have a liter of a gas at one atmosphere and you have a liter of the same gas at 5 atmospheres the gas at 5 atmospheres is 4 times the density.

You're right, but why does it matter? I never argued that the density changes....just that it makes no difference in this problem at all.

Back to a first stage being able to keep up with a free flowing secondary, keeping up with only means that the set IP pressure set on the regulator remains there. Now this only matters to the regulator right at the diaphragm, that is where the regulator “measures” it, not at the low-pressure ports or at the end of the hoses.

Why does this matter....if it takes me 4 minutes to do this on the surface, how in the world does that change at depth? We don't care how it is happeneing....just how long it is going to take. This changes if it were a neck seal leak b/c you've changes what resistance the gas in the tank is seeing as it flows out of the tank.

 

[perpet1]

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.

I totally agree but realistically how significant is this on the overall dynamic.

 

[Warren_L]

Density: If you have a liter of a gas at one atmosphere and you have a liter of the same gas at 5 atmospheres the gas at 5 atmospheres is 4 times the density.

Actually, not quite. At 5 ATA the gas is 5 times the density than the same gas at 1 ATA.

 

[truva]

You're right, but why does it matter? I never argued that the density changes....just that it makes no difference in this problem at all.


Why does this matter....if it takes me 4 minutes to do this on the surface, how in the world does that change at depth? We don't care how it is happeneing....just how long it is going to take. This changes if it were a neck seal leak b/c you've changes what resistance the gas in the tank is seeing as it flows out of the tank.

I can't help then sorry

ruva

 

[perpet1]

Moral to all the story. It just does not matter. It will takES the time it takeS and then it will all be gone.

If you stay awake at night because of this it is time to buy a Pony Bottle.

Pete

 

[Diver0001]

So... you are measuring the time it takes to drain a tank as a function of a specified flow rate (lpm) through the K valve "wide open". This is interesting because last I checked flow rate was dependent on Delta P (Pressure Differential) and flow resistance (head loss)

Thank you once again for the physics lesson. It's been covered repeatedly in this discussion already but since so many people didn't/don't get it, going over it again won't hurt. What is especially relevant, in my opinion, to this discussion is the conclusion that a tank doesn't empty any faster at depth than it does on the surface.

R..

 

[mac_diver]

I think that the problem understanding why the tank would empty slightly slower at depth than on the surface is that this seems to be opposite of our experience. We all know that the tank is emptied quicker at 99 feet than at 33 ft or on the surface. ( who breathes their tank on the surface?) Lets assume that our lung volume is 2.5 liters. At 99 ft our lung voulme is still 2.5 liters but we breath in an equivalent of 10 liters. Since the flow rate is essientially the same it actually takes longer to fill our lungs (this was noted by the guy or gal who noticed that it takes longer to fill the bc). However, since the flowrate of the regualtor is much higher than our lung capacity, the time is very short. Most people wouldn't notice the additional time. What we do notice is that we are using 10 liters of air ve per breath versus the 2.5 liters of air on the surface. This is why see our tank empty faster at depth. During freeflow, the flow would be constant, not based on our breathing cycle, therfore the tank would empty slightly slower at depth.

I hope this helps to clear things up.

 

[dc4bs]

So... you are measuring the time it takes to drain a tank as a function of a specified flow rate (lpm) through the K valve "wide open". This is interesting because last I checked flow rate was dependent on Delta P (Pressure Differential) and flow resistance (head loss)

In fact flow rate = Delta P / Head loss (looks a lot like ohms law)

Sooooo.... your assumption is 100% accurate if you had a constant pressure source (like a compressor) but flawed when you look at the fact that the Delta P is going down as the tank is drained.

You could look at it like you have your stated 1000 lpm when the tank is full but I am thinking you will have 0 lpm when you are at 1ata in the tank. In fact all things being consistant the change in flow rate and pressure will be linear.

Now back to the simplification as stated in a few other posts.

Assume head loss is consistant for the purpose of this discussion.

The flow rate will be proportional to Delta P. So the time it takes to drain a tank is proportional to Delta P and since, as stated in a previous post, the pressure difference in between the tank and its surroundings is less at depth------> the tank will drain slower at depth. Although not significantly so, also as stated in a previous post.

Well, to a certain extent you CAN assume constant flow for most of the time the tank drains.

Crack a valve open and it will only allow a certain lpm flow through reguardless of what the pressure is inside/outside the tank. Once the pressure diferential is reduced enough that the flowrate drops below the maximum for the valve, then it will begin to slow down.

Charting the flowrate over time would look more like a parabolic arc than a straight line
Prety straight (constant flow rate) for a while and then dropping off to 0 only towords the end.
kind of like taking a teapot off the stove. Constant whistle till you do and then it fades out as the pressure in the pot drops to ambient.

 

[truva]

There are two types of free flows here

One is with a first stage free flow, or just opening a tank valve, the other is with a secondary.

They will behave differently more so with the hi-performance regulators.

Truva

 

[H2Andy]

i just want to thank everyone, and Rick Murchison in particular, for a very educational
thread.

basically, there is no significant difference in the rate at which at tank free-flows to empy at the surface or under pressure. SAC issues don't have any relevance here.

again, thanks