Nitrox gas separation

Just returned from Coz and had a problem with Nitrox fills. Was told it was my analyzer and that if Nitrox sits in a tank for an extended period the nitrogen will separate from the oxygen. Has anyone ever heard of that? BTW, the tanks had only set for about a week.

LOL...No


Diveshop Monkeyitis. "If you don't know...baffle them with bull crap"

ROFL. That's an old one that seems to date from the old days of "devil's gas" (before my diving time). Basically arises from an ignorance of most of modern physics and chemistry, or a lack of competance at gas handling.

Absolutely not true.

It is a mixture not a chemical bond. Of course it is separate parts. That said, O2 does not precipitate out like a solid in a liquid.

To take it even further, it would be similar to saying that if we did not have wind constantly mixing things in our own atmosphere, at any given moment it would fall apart.

Blister

HammerHead05:

Was told it was my analyzer and that if Nitrox sits in a tank for an extended period the nitrogen will separate from the oxygen. Has anyone ever heard of that?

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Have I heard of people saying that? *sigh*... yep, I've heard of it. Is it absurd? Absolutely.

Let's look at it from a basic scientific perspective. The following are the (rounded) molecular weights (atomic weight, for Argon) of the gases that make up "air" and their rounded fractions of air they make up.Consider a disused basement (the scary kind that get kids' adrenaline really running hot). Let's say it has nice 10-foot ceilings (the better to have tall, dusty shelves, eh? :biggrin. If the gases separated, you'd end up with an inch and a quarter of argon on the floor, with just over two feet of pure oxygen floating on top of it, with almost eight feet of nitrogen filling up the rest of the room. If you walked into the basement without stirring it, the nitrogen would asphyxiate you, although when you pass out and fall into the oxygen zone, you may well survive. (You could crawl in the oxygen layer over to the fan... of course, if the fan, socket, or switch were in the oxygen layer, you just might die in a flash fire when you turn it on.)

(Imagine if the atmosphere separated like that. Hehe, you'd be able to get oxygen fills for dirt cheap, but buying nitrogen to dilute it would be expensive.)

Now, admittedly, it *is* possible to do fun things like pouring CO₂ into an empty aquarium to put out a flame, as CO₂'s molecular weight of 44 g/mol (and the usual cold temperature of freshly sublimated or decanted CO₂) yields a higher density than air, but it doesn't just stay there permanently. Even the slightest air movement will stir it up and away, and in quiescent surroundings, you'd still have diffusion to disperse it.

Love ClayJar's basement analogy!!!

As you can see, the dive shop was full of crap. No need to worry about gas separating!

There was a shop in S. Fla. that use to require anyone getting a Nitrox fill to roll the tank on the floor prior to leaving the shop to make sure the gas was properly mixed. This was even if the tank was filled the day before. I always did it with a smile on my face, they gave great fills.

liger,

I guess I will accept your opinion (you being a ChemE) over the "experts" who posted above. But I am curious as to whether or not, in your opinion, 3000 psi has any impact on the possibility of the gas mix separating somewhat ? I got a nitrox fill the other day and personally witnessed the different analyses before and after rolling the tank around on the floor for a few minutes. Is it not theoretically possible for a "mixture" of gasses having different densities to separate ?

boat

Absolutely possible, it can happen under certain circumstances.














However, you really shouldn't be storing your tanks below -297 degrees F, it'll make the metal very brittle.

Zieg:

There was a shop in S. Fla. that use to require anyone getting a Nitrox fill to roll the tank on the floor prior to leaving the shop to make sure the gas was properly mixed. This was even if the tank was filled the day before. I always did it with a smile on my face, they gave great fills.

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Did they give you the proper chants and incantations to do while rolling the tank? If they accidentially gave you the shaker chants by mistake then it would have all been for nothing.

To the orginal poster, I had some trouble with EANx in Coz before myself, and the gas wasn't separated, just not properly blended.