How Long For Freeflow To Empty Tank?

[-hh]

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

The condition of maximum flow will exist at _maximum_ pressure differential, which means the _minimum_ ambient pressure. This test has been done by Undercurrent and it was found that a bare AL80 takes nearly 2 minutes to drain at the surface.

(FWIW, the test was done because of Spare Air advertising claims that an HP hose failure could drain a tank in 30 seconds...wrongo!)

On the question of the flow rate at a depth of 5atm's, it cannot be greater than at the surface, where the ambient pressure is lower. However, it may not be substantially different because of the gas dynamics principles of "choked flow", which is what limits the maximum flow rate. Basically, this is a mass flow limit, as the location of the minimum cross section within the nozzle throat cannot ever exceed local Mach=1). As such, the "outside" pressure of the gas at depth is irrelevant until the insideutside pressure ratio's lowers so that the nozzle is no longer choked. But once the nozzle is unchoked, this means that it is flowing at less than absolute maximum, so the total time to drain must increase. Note that once unchoked, the flow will then be proportional to the pressure ratio's, which means that as the tank pressure approaches outside ambient pressure, the flow rate will slow down, eventually stopping when it reaches equilibrium.

When you add a regulator to the above tank, you'll probably change the cross-sectional size of the "bottleneck" that is the source of the choked flow. As such, the max flow rate can change (eg, go down), but as to what value depends on the specific regulator.


-hh