Nitrox PPO2 blending waiting period for homogenisation of mix

[Doc]

Ye reckon?
Never can tell... we may have been operating without essential life support equipment for decades!
I want pictures!

Rick

Go old school, good for pony tanks:

 

[Brian Sharpe]

Makes me glad my LDS has a membrane compressor and keeps EAN32 banked - fills while you wait (unless they're backlogged on a weekend)

 

[Arizona_SVO]

The need to wait is far overstated in the industry as the concept of the gases stratifying in the cylinder as laughable given the gases we are working with... after all, the atmosphere is made up of these gasses and we don't seem to have layers of oxygen and nitrogen hanging around. (The argument that the turbulence keeps them mixed also works in our favor as the very act of injecting oxygen at a higher pressure into a cylinder causes that same turbulence and thus, mixes the gases). Some folks suggest something called 'tumbling' to help them mix - go ahead and tumble your cylinders if you want, but I agree with Rick who says he'll sign the book now and re-do at dive time... if they change excuses, then you know you are getting the run-around.

If anybody can show me the science behind these gases NOT mixing almost instantaneously and this having an effect on the analysis after about 10 seconds, I'll buy them a spare air so they can be extra safe.

 

[charlesml3]

Agreed. I'd like to see any kind of science (and I'll even accept Mythbusters) that shows how gas can stratify in a tank. This business of "shaking" or "rolling" a tank is just nonsense.

Paint will shake and mix because there's an air gap at the top of the can to give the paint some room to slosh around inside the can. Same for spray paint.

Mixed gases in a tank do not have that property.

-Charles

 

[TSandM]

I have to say that I have analyzed tanks right after an O2 topoff and had them rich -- ten minutes later, they have settled down to what they really are.

 

[bleeb]

Agreed. I'd like to see any kind of science (and I'll even accept Mythbusters) that shows how gas can stratify in a tank. This business of "shaking" or "rolling" a tank is just nonsense.

Ok.



I couldn't find a YouTube video of the segment where Jamie and Adam actually reproduce the trick themselves, but other people have done the demonstration too.



To be clear, this is not spontaneously stratifying, but slowly pouring a heavier gas to avoid turbulent mixing. It's been pointed out here on SB before that going this slowly may not be unlike what might be sometimes accidentally achieved where 100% O2 is involved. However, the density ratio of air/He is about the same as that of SF6/air used in the demo, so we'd be talking about trimix and not nitrox.

 

[charlesml3]

I assume you're just joking here.

The gas they're using in the aquarium is very, very dense and WAY heavier than Oxygen and Nitrogen. And of course they're all heavier than Helium.

Cute demo, but does nothing to refute my point.

-Charles

 

[Seaduced]

A little hijack here...

I work in high voltage/power electronics and we use SF6 regularly. You know how helium makes your voice high and squeaky? I got a mouthful of SF6 a while back, the voice that came out of mouth sounded like something from a demon possession movie. I nearly passed out laughing at the look on my coworkers face. He drew back and his eyes were as wide as could be.

SF6 is so much denser than air, it refracts differently so you can see it, like a heat wave.

 

[fnfalman]

Just consider it a good time for physical training (PT). Pick up the cylinder, keep it above your head, tilt from side to side, then hop around a bit, pumping the cylinder up and down to your chest.

 

[bleeb]

The gas they're using in the aquarium is very, very dense and WAY heavier than Oxygen and Nitrogen. And of course they're all heavier than Helium.

What I was trying to get across was that the gas they were using (sulfur hexafluoride) is about 5-6x denser than oxygen or nitrogen, just like oxygen and nitrogen are about 5-6x denser than helium, and the tendency for gases to stratify or remain stratified is driven partly by the density difference.

Gases that are already mixed are going to have to sit undisturbed for a really long time to settle out. Without hauling out the calculator, I'd guess months or years. But if they're already somewhat separated because they've been carefully placed in the same container, the rate at which one diffuses upwards into the other will depend partly on the relative molecular weights of the gases. Keep in mind we're only talking about this actually happening with mixes that include helium, and not nitrox, which was the original point of this thread. Will it happen all the time, or even often? Seems unlikely, since it doesn't take much to stir things up. But it is a possibility if everything lines up just right and the moon is in the right phase, which could really ruin your day.