Old air in tanks.

[Peter69_56]

This has nothing to do with complacency. We hydro and VIP because tanks can and do blow up when filling them, not because "air goes bad".


@Peter69_56 , I suspect you meant to say testing for CO and not CO2 ?

LOL yes, damn the typos

 

[oldschoolto]

VIP's are the biggest ripoff of the diving public.... Taking a tank that is one year and a day old from the date it was made then hydro'd and saying it needs a VIP is a joke... A bad joke at that...

JIM....

 

[Centrals]

Oxygen molecule is slightly heavier than Nitrogen molecule and CO2 is a lot heavier. Anyone remember Lake Nyos disaster in 1986?
I wonder what would happen if you let a tank of air, not under pressure, to stand over a period of time without disturbing. Would the individual component settle out according to their molecular weight eg. CO2 only at the bottom?

 

[TrimixToo]

Oxygen molecule is slightly heavier than Nitrogen molecule and CO2 is a lot heavier. Anyone remember Lake Nyos disaster in 1986?
I wonder what would happen if you let a tank of air, not under pressure, to stand over a period of time without disturbing. Would the individual component settle out according to their molecular weight eg. CO2 only at the bottom?

As I understand it, air is a solution, not a mixture. (Same for trimix and nitrox.) Nothing "settles out" when it's just sitting there.

 

[Storker]

I wonder what would happen if you let a tank of air, not under pressure, to stand over a period of time without disturbing. Would the individual component settle out according to their molecular weight eg. CO2 only at the bottom?

tl;dr version: No.

For one, there's this thing called diffusion, which is caused by the molecules' Brownian motion. Secondly, separating a mixture decreases the mixture's entropy. That will never happen spontaneously, as stated in the second law of thermodynamics. To separate a mixture, you have to spend some energy. So when something is mixed, the mixture won't separate into its individual components just by itself.

 

[Storker]

As I understand it, air is a solution, not a mixture.

What do you mean by this? A solution is simply a mixture at the molecular level. The term is normally used when a gas or a solid is dissolved in a liquid, but there are also solid solutions and gaseous solutions.

Wikipedia has put it very well:

a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent.

Solution - Wikipedia

If the solvent is a gas, only gases are dissolved under a given set of conditions. An example of a gaseous solution is air (oxygen and other gases dissolved in nitrogen). Since interactions between molecules play almost no role, dilute gases form rather trivial solutions. In part of the literature, they are not even classified as solutions, but addressed as mixtures.

Solution - Wikipedia

 

[dberry]

The Lake Nyos disaster was the result of a huge release of CO2, and it did initially smother people in lower lying areas. But eventually diffusion (and the entropy of mixing) won out and the CO2 dispersed into the atmosphere. That's why it was safe for investigators to enter the villages later (and why the cause of the deaths was initially a big mystery.)

 

[Centrals]

tl;dr version: No.

For one, there's this thing called diffusion, which is caused by the molecules' Brownian motion. Secondly, separating a mixture decreases the mixture's entropy. That will never happen spontaneously, as stated in the second law of thermodynamics. To separate a mixture, you have to spend some energy. So when something is mixed, the mixture won't separate into its individual components just by itself.

1. Mixing two common occurring gases eg. N2, O2 does not require any input/output of energy unlike dissolving a solid in a solvent(endothermic eg. dissolving salt in water or exothermic eg. dissolving NaOH in water).
2. There is no interaction between gas molecules of two different gases eg. He and O2. Unlike eg. hydrogen bonding in some liquid solutions eg. acetic aicd which has "unusual" higher boiling point.
3. Gases do stratify by molecular weight in mesosphere or higher.
4. Helium, the lightest molecule, escape from earth for good.
5. I am not talking about gas under pressure in a confined space(tank). It is about FREE moving one.

 

[Centrals]

The Lake Nyos disaster was the result of a huge release of CO2, and it did initially smother people in lower lying areas. But eventually diffusion (and the entropy of mixing) won out and the CO2 dispersed into the atmosphere. That's why it was safe for investigators to enter the villages later (and why the cause of the deaths was initially a big mystery.)

The MW of CO2 certainly plays a part in this tragedy. Would the outcome be different if it was eg CO which is lighter?

 

[TrimixToo]

What do you mean by this? A solution is simply a mixture at the molecular level. The term is normally used when a gas or a solid is dissolved in a liquid, but there are also solid solutions and gaseous solutions.

Wikipedia has put it very well:

Solution - Wikipedia


Solution - Wikipedia

I meant what Wikipedia says. That the O2 and trace elements are essentially dissolved in the nitrogen. I am not a chemist, but my lay understanding is that solutions do not "settle out by density" but remain dissolved under some ranges of temperature, concentration of the soluble parts, and pressure, essentially forever. I will go further out on the limb and opine that 3000 PSI is in the range within which parts of a gas solution remain in solution once they go into solution in the first place (which is not instant). I can ask a chemist tomorrow if he happens to be available when I'm free and I think of it at work, neither of which is at all assured..