Thought experiment: Diffusion between tanks

[tridacna]

Dang. Seems so counterintuitive. Now you got me thinking the efficiency of a partial pressure fill. Hyperbolic geometry seems quite simple compared to this.

 

[huwporter]

Heck, I witnessed FIRST HAND a shop pump O2 into a tank, then air into that, and then analyze only EAN23. After some rolling of the tank, it analyzed at EAN30. The different gas mixtures almost act like fluids of different densities...which they shouldn't do. It's very counter-intuitive.

Agree - I wouldn't have believed it if I hadn't witnessed it myself. Helium pumped on top of trimix, then persistently and thoroughly analysed at something like 8/75. Tanks rocked and shaken, then analysed spot on at 18/45.

When the gases are thoroughly mixed, they won't then stratify out again, but until they are mixed they do behave weirdly. And obviously, counter intuitively, filling tanks alone doesn't create enough turbulence to thoroughly mix in all cases.

 

[bl6394]

Agree - I wouldn't have believed it if I hadn't witnessed it myself. Helium pumped on top of trimix, then persistently and thoroughly analysed at something like 8/75. Tanks rocked and shaken, then analysed spot on at 18/45.

When the gases are thoroughly mixed, they won't then stratify out again, but until they are mixed they do behave weirdly. And obviously, counter intuitively, filling tanks alone doesn't create enough turbulence to thoroughly mix in all cases.

Not just trimix. I mixed a batch of EAN80 is an Al40. Oxygen first slowly - then added air slowly to 3000. Waited 12hours - and the analyzer gave me 72%. Rolled the tank for 10 minutes - and remeasured at 80%.

Can take quite a while to become a homogeneous mix - depending upon how you fill. When I'm not doing really rich mixes - I usually give the cylinder a sharp blast of air to create some turbulence and movement. When it's a rich mix - all I can do is roll and wait.

 

[cool_hardware52]

Assume the following:

Three gas cylinders each filled to half working pressure.

Tank 1 has EAN25.
Tank 2 has EAN50
Tank 3 has EAN75

All three are connected together with a standard whips. Assume ID of 1/8".

Whips are turned on simultaneously.

How long before the gas diffuses between all three tanks until EAN50 is achieved in all three? Obviously the diameter and length of the whips are the most important variables here. If the ID of the whip is the same as the ID of the tank, equilibrium will be established almost immediately. But we're trying to contemplate a real life working situation.

I'm a mathematician and my buddy is an engineer. We've struggled unsuccessfully trying to establish some kind of formula to establish diffusion patterns. Anyone out there in "Fluid Dynamic land " have any ideas?

It's been done. One tank in a set of doubles was filled with 100 O2 and the other with air to the same pressures. There was *no* appreciable mixing after many days. Remember the "mixing front" in a typical manifold is on the order of .100' diameter and there is no pressure differential. Search the archives of the deco stop. Russel Edge, who is certainly well qualified, did the testing. All you have is "Brownian Motion" and across a tiny mixing front and loooog conduits connecting the two gas volumes I would expect near zero mixing.

Tobin

 

[redacted]

You guys really need to meet some women.

 

[tridacna]

It's been done. One tank in a set of doubles was filled with 100 O2 and the other with air to the same pressures. There was *no* appreciable mixing after many days. Remember the "mixing front" in a typical manifold is on the order of .100' diameter and there is no pressure differential. Search the archives of the deco stop. Russel Edge, who is certainly well qualified, did the testing. All you have is "Brownian Motion" and across a tiny mixing front and loooog conduits connecting the two gas volumes I would expect near zero mixing.

Tobin

Tobin, my question is not whether or not they would mix: I think that everyone agrees that eventually they must. Is there some way of describing this predictably? This is only a thought experiment with zero practical use...

 

[lowviz]

...//... I think that everyone agrees that eventually they must. ...//...

Bit of a problem with that too. How about saltwater in saltwater mixing:
At the moment, I'm kinda partial to awap's suggestion...

 

[jseyfert3]

How long before the gas diffuses between all three tanks until EAN50 is achieved in all three? Obviously the diameter and length of the whips are the most important variables here. If the ID of the whip is the same as the ID of the tank, equilibrium will be established almost immediately. But we're trying to contemplate a real life working situation.

I'm a mathematician and my buddy is an engineer. We've struggled unsuccessfully trying to establish some kind of formula to establish diffusion patterns. Anyone out there in "Fluid Dynamic land " have any ideas?

As a ME senior graduating in December, my gut feeling is years. I wouldn't be suprised if they didn't come anywhere close to EAN 50 after a hundred years (assuming all are kept together and heat/cool at the same rate).

I have no interest in trying to calculate it, though I'm sure you could. How close the calculation would be is anyones' guess though, cause experimentally nobody probably has time to do it (and who would want to tie up three perfectly good scuba tanks for this?).

Edit: May be a good question to have Randall Monroe ponder over on What-If? Well...may not be absurd enough for him, actually.http://what-if.xkcd.com/

 

[tridacna]

As a ME senior graduating in December, my gut feeling is years. I wouldn't be suprised if they didn't come anywhere close to EAN 50 after a hundred years (assuming all are kept together and heat/cool at the same rate).

I have no interest in trying to calculate it, though I'm sure you could. How close the calculation would be is anyones' guess though, cause experimentally nobody probably has time to do it (and who would want to tie up three perfectly good scuba tanks for this?).

Edit: May be a good question to have Randall Monroe ponder over on What-If? Well...may not be absurd enough for him, actually.

Exactly. I don't want to see any molecules of steel or air harmed in this experiment. Let me be clear. I'' looking to describe this with a series of equations. I don't care whether it takes a thousan or ten thousand years. It''s the equation to describe the behaviour that we'd like to describe.

Or maybe me and him should just go pick up some women. Prob of solving tank prob > mixing with women tho. No proof reqd.

 

[BenjaminF]

You guys really need to meet some women.

Maybe this behaviour is because they HAVE met some women?