High O2 left for long periods...

[neilh]

Originally Posted by scuba steve irl

oxygen can separate in the gas mix and as oxygen is more dense than air the o2 can separate from the mix .

Erm, doesn't air contain oxygen? If this was the case then wouldn't there be a layer of pure O2 near the ground?

[fade]

Originally Posted by neilh

Erm, doesn't air contain oxygen? If this was the case then wouldn't there be a layer of pure O2 near the ground?

There is no wind or convection in a tank, and considering the pressure the gas is at it is nothing like our atmosphere, its getting much more like liquids where the densities come into play much more.

Pete

[neilh]

Originally Posted by fade

There is no wind or convection in a tank, and considering the pressure the gas is at it is nothing like our atmosphere, its getting much more like liquids where the densities come into play much more.

Even so you'd end up with pure Nitrogen at the top and O2 at the bottom, surely - and if that were possible then surely even cylinders destined just for air use should also be O2 clean?

Sorry, just don't buy it myself.

[John63]

There would be no separation.

All gas particles (in fact, all atomic particles ) are in motion and will disperse to fill the volume of the container that they are in. For a mixture of gases, both gases will disperse evenly in a container in proportion to their relative concentrations, so there will not be a layering/separating effect with time; if anything, the mixtures become more evenly dispersed..


This will also apply if both the gases are liquified (gases and liquids are both fluids), irrespective of their densities, so long as they are miscible. Difficult to test at home with liquid O2 / N2 but think about vodka (approx 40% ethanol and 60% water) The realtive densities of ethanol and water are very different, but they do not separate if the bottle is left on a shelf undisturbed (do any of us leave a bottle untouched that long )

Just thought that rather than all of this I could have just hit Wikepedia, so I did (below) First para sums it up


Diffusion is the movement of particles from an area of high concentration to an area of low concentration in a given volume of fluid (either liquid or gas) down the concentration gradient. For example, diffusing molecules will move randomly between areas of high and low concentration but because there are more molecules in the high concentration region, more molecules will leave the high concentration region than the low concentration one. Therefore, there will be a net movement of molecules from high to low concentration. Initially, a concentration gradient leaves a smooth decrease in concentration from high to low which will form between the two regions. As time progresses, the gradient will grow increasingly shallow until the concentrations are equalized.

Diffusion is a spontaneous process. It is simply the statistical outcome of random motion. Diffusion increases entropy, decreasing Gibbs free energy, and therefore is thermodynamically favorable. Diffusion operates within the boundaries of the Second Law of Thermodynamics because it demonstrates nature's tendency to wind down, as evidenced by increasing entropy.[1]

The diffusion equation provides a mathematical description of diffusion. This equation is derived from Fick's law, which states that the net movement of diffusing substance per unit area of section (the flux) is proportional to the concentration gradient (how steeply the concentration changes in space), and is toward lower concentration. (Thus if the concentration is uniform there will be no net motion.) The constant of proportionality is the diffusion coefficient, which depends on the diffusing species and the material through which diffusion occurs. Fick's law is an assumption that may not hold for a given diffusive system (e.g., the diffusion may depend on concentration in addition to concentration gradient), in which case the motion would not be described by the normal (simple, Fickian) diffusion equation. An analogous statement of Fick's law, for heat instead of concentration, is Fourier's law.

The mechanism of diffusion is "Brownian motion" whereby a molecule makes a random walk about a central location since by kinetic theory the mean velocity of a particle is zero if it is not subject to any external forces. Due to collisions with neighboring molecules the motion of the particle is characterized by a mean free path which tends to confine the particle. But since there is no potential field acting to restore a particle to its original position, it is still free to move about the vessel or liquid in which it is located.

[BJ]

You'd do better to worry about any water molecules in the tank that, in the presence of high levels of oxygen, will be eating their way through the steel.

[Richard Mason]

Originally Posted by fade

There is no wind or convection in a tank, and considering the pressure the gas is at it is nothing like our atmosphere, its getting much more like liquids where the densities come into play much more.

Pete

There's still brownian motion though and the gases aren't that dense (unless over about 260bar) that they don't obey the gas laws, such as Boyle. That would be enough to keep it mixed. Unless you are keeping them super-cooled.

[NotDeadYet]

I must remember to roll my bailout bottles before I clip them on as they were filled 18 months ago. All that time I've been risking a gobful of O2 at depth by not doing it...

If corrosion had dropped the O2% dramatically then I'd be more worried about moving the bottle than decoing on it.

[scuba steve irl]

mixes when filled from partial pressure are in a seperated state and mix . but the o2 can will and does not fully mix with the gas untill it is agitated . when you rest a bottle the heaver o2 mocicules sit and rest .

likwise when an o2 cylinder is used in an enclosed space o2 falls to the ground and doesnt readyly mix with air . so this is why it is recommeneded to "air" out a vechical after the use of o2.

so we are told in the ambo any way

[liamm]

Oh goody, we havnt had the "does o2 separate out from a mix" discussion in a while.
Gravity wouldnt be enough of a force to maintain a concentration gradient, the gases will mix in milliseconds, less maybe.

[John63]

Originally Posted by scuba steve irl

mixes when filled from partial pressure are in a seperated state and mix .

Correct, for the initial period when the gasesare first introduced to the cylinder

Originally Posted by scuba steve irl

but the o2 can will and does not fully mix with the gas untill it is agitated .

Incorrect. Gas molecules are in a constant state of motion. the concentration gradient and Brownian motion will cause the gases to fully mix

Originally Posted by scuba steve irl

when you rest a bottle the heaver o2 mocicules sit and rest .

Incorrect, see above

Originally Posted by scuba steve irl

likwise when an o2 cylinder is used in an enclosed space o2 falls to the ground and doesnt readyly mix with air . so this is why it is recommeneded to "air" out a vechical after the use of o2.

so we are told in the ambo any way

I assume ambo is ambulance? The O2 may initially form a lower layer though I doubt this very much due to constant movement within the vehicle(yours, parmedic etc) The more likely reason for "airing" the vehile is that . O2 may have become trapped ( eg in lockers, under beches, in lockers etc) forming pockets of high conc O2 (explosion risk). Because of the restricted nature of these within the vehicle, mixing with the remaining air will be slowed but not stopped.

These pockest do not exist within a cylinder and once mixed there will be no stratification of the gases.

Cheers, John